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Frequently Asked Questions


  • What is involved with Grounds or Grounding?
  • What is a Grounding Transformer?
  • What is an Isolation Transformer?

      Practically all transformers (with the exception of Autotransformers) are, in fact, Isolation Transformer.  This is due to the fact that their primary and secondary windings are PHYSICALLY isolated from each other (they are not physically connected to each other).  The transformation in voltage and current between primary and secondary windings occurs as a result of the shared magnetic field in the core (Mutual Inductance).

  • What does ANC mean?

      “Air Natural Convection Total Enclosed” – The dry-type transformer is enclosed in a non-ventilated enclosure.  Heat is transferred through the sides of the enclosure to the air to cool the unit.

  • What is the abbreviation UL?
  • What does the abbreviation NEMA refer to?
  • What are Static Synchronous Compensators (STATCOMs)?
  • When is Sound Level an issue in the design?

      Sound needs to be considered when transformers are located in close proximity to occupied areas. All energized transformers emanate sound due to the alternating flux in the core. This normal sound emitted by the transformer can be a source of annoyance unless it is kept below acceptable levels. There are ways of minimizing sound emission as discussed in the HPS “Field Service Guide”. HPS  Transformers are built to meet the latest ANSI, CSA and UL standards. These standards are outlined in the accompanying table.

  • What is a Solid State Device?

      It is a device that contains components that do not depend on electronic conduction in a vacuum or gas. Semiconductors or the use of otherwise completely static components such as resistors or capacitors performs the electrical function of a solid-state device.

  • How are single phase and three phase load amps and load kVA calculated?

      Single phase Amps = (kVA x 1000)/Volts

      Three phase Amps = (kVA x 1000)/Volts x 1.73

      Single phase KVA = (Volts x Amps)/1000

      Three phase KVA = (Volts x Amps x 1.73)/1000

  • Should low voltage system be assessed for arc-flash hazards?

      IEEE 1584-2018 provides mathematical models for designers and facility operators to apply in determining the arc-flash hazard distance and the incident energy to which workers could be exposed during their work on or near electrical equipment.

      It generally indicates that systems with an available short circuit current of 2000 Amps or higher should be assessed for arc-flash potential. A rule of thumb would indicate that most systems fed by a 45 kVA or larger transformer will need to be assessed if impedance (%Z) of 45 kVA is less than 6%, 30 kVA if %Z is less than 4% or 15 kVA if %Z is less than 2%.

  • What is Series Compensation?
  • How are Seismic Units Rated?

      Three criteria are typically defined for seismic units:  Sds, Ip, z/h.

      Sds = Design earthquake spectral response acceleration parameter at short periods (ASCE 7-16 Section 11.4.4 Design Spectral Acceleration Parameters).  The required motion coefficient is dependent on the facility’s location and soil type. Most of the United States requires Sds = 0.05 to 1.5g. Specific regions require an Sds = 2.0g such as along the Missouri state line south of Illinois and parts of California.

      Ip = Component Importance Factor (ASCE 7-16 Section 13.1.3 Component Importance Factor).  Ip is dependent on the function of the building in which the transformer is installed. Typically, an Ip is assumed to equal 1.5 for transformers expected to function continuously through and after an earthquake.

      z/h = A ratio of the height in the structure that the component has been anchored, to the overall height of the structure.  A value of z/h of 1.0 states that the component is capable of being installed anywhere within the structure (ASCE 7-16 Section 13.3.1 Seismic Design Force).

      z/h - a ratio of the height of the structure

  • What is Seismic Certified?

      A fair amount of construction projects require components to be “Seismic Certified.” A Seismic Certification ensures the component will withstand and operate after an event such as an earthquake. In addition to requiring structural components to meet specific seismic regulations, most jurisdictions also require non-structural components – including electrical systems – to be “Seismic Certified.”

      Seismic requirements are defined by the International Building Code 2018 and the California Building Code (2019). ASCE 7-16 is the base standard for many building codes, and is referenced by both IBC and the CBC.

      OSHPD, the Office of State-wide Health Planning and Development, requires actual “shake-testing” of products prior to allowing products to be specified for construction or retrofit projects anywhere in the state of California. This testing must be reviewed by a California state certified structural engineer. Without a widespread nationwide approval process, many other jurisdictions require the OSHPD Special Seismic Certification Preapproval (OSP) for projects.

  • What is a Secondary Winding?
  • What is Secondary Voltage Rating?
  • What is a Rectifier Transformer?
  • What is a Ratio Test?

      A test that is used to ensure the correct number of turns on the primary and secondary windings.  When the resulting ratio of turns between the primary and secondary windings is applied to the secondary winding phase voltage, you should arrive back at the primary phase voltage.

  • What is Ratio in terms of Voltage?

      In terms of voltage, the ratio of a transformer can be viewed from the primary side (the ratio of the primary voltage to the secondary voltage), or inversely from the secondary side (the secondary voltage to the primary voltage).

  • What are Static VAR Compensators (SVCs)?
  • What is a Primary Winding?
  • What are Primary Taps?

      Primary taps are additional terminals added to the primary winding that allows the customer to apply different supply voltages.  Primary taps can be provided as FCBN (Full capacity Above Nominal), or FCBN (Full Capacity Below Nominal).  The taps are generally below or above the nominal rated primary voltage in specific percentage increments provided by the customer.

  • What is Power Factor or True Power Factor?

      The ratio of real power to apparent power and is:  PF = (Power actually delivered to load) ÷ (RMS Voltage x RMS Current).  Waveform distortion caused by harmonics is included in this calculation.  The worse the phase shift between voltage and current and/or the worse the harmonic distortion, the worse the power factor.  Low power factor cause by either harmonic currents (and a distorted sine wave) or reactive power can increase transformer heating.  If PF is low but DPF is not, adding power factor correction capacitors may not help

      Displacement power factor (DPF) is different.  DPF is the cosine of phase angle between the current and voltage fundamental sine waves.  Low power factor is typically caused by inductive loads such as motors.  Fundamental power factor only looks at the 60 Hz sine wave and does not take into effect harmonic currents.  DPF is most useful for sizing and measuring the effectiveness of power factor correct capacitors.

      If PF is low but DPF is not, harmonics may be causing the problem and adding power factor correction capacitors may not improve either PF or DPF.  Solutions such as harmonic mitigating transformers or line reactors should be considered.

  • What is a Potential (Voltage) Transformer?
  • What is preferred for a neutral grounding transformer

      It is up to the user to specify this. Typically, The most common magnetic grounding device is a zig-zag autotransformer. This design offers greater flexibility at a cost and size smaller than a comparable Wye-Delta isolation transformer.

  • What are Motor Starting Autotransformers?

      Motors have a large inrush current upon energization that can stress the electrical system and cause low voltage conditions. Motor Starting Autotransformers (MSAT’s) are used in reduced voltage starters to temporarily reduce the voltage being applied to the motor. This will extend the time it takes the motor to reach full speed and reducing the overall startup current to the motor.

  • Why do non-linear loads have low power factors and why is it important to have a high power factor?

      Power factor is a measure of how effectively a specific load consumes electricity to produce work. The higher the power factor, the more work produced for a given voltage and current. Figure 3-1 shows the power vector relationships for both linear and non-linear loads. Power factor is always measured as the ratio between real power in kilowatts (kW) and apparent power in kilovolt-amperes (kVA).

      For linear loads, the apparent power in kVA (S = V•I) is the vector sum of the reactive power in kVAR (Q) and the real power in kW (P). The power factor is P/S = CosΦ, where Φ is the angle between S and P. This angle is the same as the displacement angle between the voltage and the current for linear loads. For a given amount of current, increasing the displacement angle will increase Q, decrease P, and lower the PF. Inductive loads such as induction motors cause their current to lag the voltage, capacitors cause their current to lead the voltage, and purely resistive loads draw their current in-phase with the voltage. For circuits with strictly linear loads (a rare situation) simple capacitor banks may be added to the system to improve a lagging power factor due to induction motors or other lagging loads.

      For non-linear loads, the harmonic currents they draw produce no useful work and therefore are reactive in nature. The power vector relationship becomes 3 dimensional with distortion reactive power, H, combining with both Q and P to produce the apparent power which the power system must deliver. Power factor remains the ratio of kW to kVA but the kVA now has a harmonic component as well. True power factor becomes the combination of displacement power factor and distortion power factor. For most typical nonlinear loads, the displacement power factor will be near unity. True power factor however, is normally very low because of the distortion component. For example, the displacement power factor of a personal computer will be near unity but its total power factor is often in the 0.65 – 0.7 range. The best way to improve a poor power factor caused by non-linear loads is to remove the harmonic currents.

      Most Utilities charge their customers for energy supplied in kilowatt-hours during the billing period plus a demand charge for that period. The demand charge is based upon the peak load during the period. The demand charge is applied by the utility because it must provide equipment large enough for the peak load even though the customer’s average power may be much lower. If the power factor during the peak period (usually a 10 minute sliding window) is lower than required by the utility (usually 0.9), the utility may also apply a low PF penalty charge as part of the demand charge portion of the bill.

      More Harmonic Mitigating Transformer Frequently Asked Questions

  • What is an Autotransformer?

      It is a transformer that has only one winding per phase, part of which is common to both the primary and secondary circuits.

      Transformers wired in a “Buck-Boost” configuration are autotransfomers. Autotransformers are designed to adjust the supply voltage when isolation from the line is not necessary and where local electrical codes permit. An autrotransformers can be used in either a step-up or step-down application unlike isolation transformers. Autotransformers can also be used as part of a reduced voltage starter to reduce motor inrush currents.

  • What is the difference between a buck-boost transformer and an autotransformer?

      A Buck-Boost transformer is typically a small single-phase low voltage lighting transformer that can be wired as an autotransformer to provide small voltage corrections for single and three phase applications. An autotransformer is a transformer with a direct connection between the primary and secondary and does not act as an isolation transformer. Autotransformers can also include wider classes of products including buck-boost, dedicated three coil distribution style units, motor starting autotransformers and solar grid tie transformers.

  • When a Buck-Boost transformer is connected as an autotransformer, what is the procedure for determining the current rating of the over-current protective device, such as the fuse or circuit breaker?

      The NEC Article 450-4 outlines over-current protection for autotransformers. It is reproduced as follows: “NEC 450-4 – Autotransformers 600 Volts, Nominal, or Less

      (a) Over-current Protection. Each autotransformer 600 volts nominal, or less shall be protected by an individual over-current device installed in series with each ungrounded input conductor. Such over-current device shall be rated or set at not more than 125 percent of the rated full load input current of the autotransformer. An over-current device shall not be installed in series with the shunt winding.

      Exception: Where the rated input current of an auto transformer is 9 amperes or more and 125 percent of this current does not correspond to a standard rating of a fuse or non-adjustable circuit breaker; the next higher standard rating described in our section shall be permitted. When the rated input current is less than 9 amperes, an over-current device rated or set at not more than 167 percent of the input current shall be permitted.

      (b) Transformer Field-Connected as an Autotransformer. A transformer field-connected as autotransformers shall be identified for use at elevated voltage.”

      Example: A 1kVA transformer, Catalog No. Q1C0ERCB, is rated 120 x 240 to 12 x 24 volts. It is to be connected as an autotransformer to raise 208 to 230 volts single-phase. When connected as an autotransformer in this application, the kVA rating is increased to 9.58 kVA, or 9,580 VA. This is the rating to be used for determining the full load input current and the corresponding size of the over-current protection device, either a fuse or breaker.

      Full load input amps = 9,580 Volt Amps = 46 Amp, 208 Volts.

      When the full load current is greater than 9 amps, the over-current protection device (usually a fuse or nonadjustable breaker).  Current rating can be up to 125 percent of the full load rating of the autotransformer input current.

      Max. current rating of the over-current device = 46 amps x 125% = 57.5 amps.

      The National Electrical Code, Article 450-4 (a) Exception, permits the use of the next higher standard ampere rating of the over-current device. This is shown in Article 240-6 of the N.E.C.

      Max. size of the fuse or circuit breaker = 60 amps.

  • Why is the isolation transformer kVA rating shown on the nameplate instead of the autotransformer kVA rating?

      Shipped as an isolating transformer, the nameplate is required to show the performance characteristics accordingly. Additionally, as an autotransformer, the eight different combinations of voltages and kava’s would be impractical to list on the nameplate. A connection chart, listing the various connections, is included with each unit.

  • Does HPS build secondary unit substation transformers?

      Secondary Unit Substation Transformers feed low voltage switchgear or switchboards. They typically range from 150 kVA to 2,500 kVA three-phase. The Primary voltage ranges from 2400 VAC to 34,500 VAC and the secondary from 600VAC to 480 VAC. Taps are typically manually changed while the unit is de-energized. The primary connections are typically delta connected while the secondaries are usually wye connected.

      The units are typically installed inside a building and are either dry (VPI) or cast coil because of safety and installation concerns. These units are often connected directly close coupled to the switchgear. HPS can provide both VPI and Cast Coil Secondary Unit Substation Transformers.

  • Does HPS build substation transformers?

      Substation transformers are located outside and typically range from 750 kVA to 5000 kVA single-phase and 25,000 kVA three-phase. The Primary voltage ranges from 2400 VAC to 46,000 and the secondary from 480 VAC to 15,000 VAC. Taps are typically manually changed while the unit is de-energized.

      While they are typically oil, HPS dry-type transformers can offer similar performance with a lighter weight and fewer environmental concerns.

  • What is Coil Hot-Spot Temperature?

      It is the absolute maximum temperature present in the transformer. This number is equal to the sum of the ambient temperature, temperature rise and a variable.

      T Hot Spot = T ambient + T rise + (10-20) °C.

  • VAR compensator reactor – definition
  • What is a Unit Substation Style Transformer (USST)?
  • What is U.L. 1562?

      U.L. 1562 covers medium voltage dry-type transformers:

      1.1 These requirements cover single-phase or three-phase, dry-type, distribution transformers, including solid cast and resin encapsulated transformers. The transformers are provided with either ventilated or non-ventilated enclosures and are rated for a primary or secondary voltage from 601 to 35000 V.

      1.2 These transformers are intended for installation in accordance with the National Electrical Code, ANSI/NFPA 70.

      1.3 These requirements do not cover the following transformers:

      1. Instrument transformers
      2. Step-voltage and induction voltage regulators
      3. Current regulators
      4. Arc furnace transformers
      5. Rectifier transformers
      6. Specialty transformers (such as rectifier, ignition, gas tube sign transformers, and the like)
      7. Mining transformers
      8. Motor-starting reactors and transformers

      1.4 These requirements do not cover transformers under the exclusive control of electrical utilities utilized for communication, metering, generation, control, transformation, transmission, and distribution of electric energy regardless of whether such transformers are located indoors, in buildings and rooms used exclusively by utilities for such purposes; or outdoors on property owned, leased, established rights on private property or on public rights of way (highways, streets, roads, and the like).

  • What is U.L. 1561?

      UL1561 covers 600 Volt Class Transformers:

      1.1 These requirements cover:

      1. General purpose and power transformers of the air-cooled, dry, ventilated, and non-ventilated types to be used in accordance with the National Electrical Code, ANSI/NFPA 70. Construction types include step up, step down, insulating, and autotransformer type transformers as well as air-cooled and dry-type reactors


      1. General purpose and power transformers of the exposed core, air-cooled, dry, and compound-filled types rated more than 10 kVA to be used in accordance with the National Electrical Code, ANSI/NFPA 70. Constructions include step up, step down, insulating, and autotransformer type transformers as well as air-cooled, dry, and compound-filled type reactors.

      1.2 These requirements do not cover ballasts for high intensity discharge (HID) lamps (metal halide, mercury vapor, and sodium types) or fluorescent lamps, exposed core transformers, compound-filled transformers, liquid-filled transformers, voltage regulators, general use or special types of transformers covered in requirements for other electrical equipment, autotransformers forming part of industrial control equipment, motor-starting autotransformers, variable voltage autotransformers, transformers having a nominal primary or secondary rating of more than 600 volts, or overvoltage taps rated greater than 660 volts.

      1.3 These requirements do not cover transformers provided with waveshaping or rectifying circuitry. Waveshaping or rectifying circuits may include components such as diodes and transistors. Components such as capacitors, transient voltage surge suppressors, and surge arresters are not considered to be waveshaping or rectifying devices.

  • What is the “Efficiency” of a transformer?
  • dV/dt reactor – definition
  • What are dust filters?

      Dust filters are placed over ventilation openings to mitigate dust accumulation within the transformer’s enclosure. Filters are typically made to be removable and washable. Care must be taken for regular maintenance since accumulating dust will limit airflow and reduce air cooling. For this reason, dust filters are typically avoided through using non-ventilated designs or moving the transformer’s location. Because of reduced airflow, transformers cannot be retrofitted with dust filters without derating or using fan forced venting.

  • What is a “Dual Winding”?
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  • What is a Cast Coil transformer?
  • Capacitor switching reactor – definition
  • What does the term BIL mean?
  • What is an Applied Voltage test?
  • What does ANSI stand for?
  • What is ANSI C57.12.91?
  • What is ANSI C57.12.51?

      IEEE Standard for Ventilated Dry- Type Power Transformers, 501 kVA and Larger, Three-Phase, with High- Voltage 34.5 kV to 601 V and Low- Voltage 208Y/120 V to 4160 V covering General Requirements. The current standard was updated in 2008.

      This standard is intended to set forth characteristics relating to performance, limited electrical and mechanical interchangeability, and safety of the equipment described, and to assist in the proper selection of such equipment. Specific rating combinations are described in the range from 750/1000 to 7500/10 000 kVA inclusive, with high-voltage 601 to 34 500 volts inclusive and low-voltage 208Y/120 to 4160 volts inclusive. Part I of this standard describes certain electrical and mechanical requirements and takes into consideration certain safety features of 60-Hz, two-winding, three-phase, ventilated dry-type transformers with self-cooled ratings 501 kVA and larger, generally used for step-down purposes. Part Il describes other requirements or alternatives which may be specified for some applications and lists forced-air-cooled ratings for certain sizes.

  • What is an Ampere?
  • What is Ambient Noise Level?
  • What is an air terminal chamber (ATC) or line terminal compartment?

      This is an air filled terminal compartment, typically 12″-24″ wide that is bolted to one or both sides of a substation transformer. This typically contains either the primary or secondary connections with a steel barrier separating it from the larger chamber containing the actual transformer core and coil. The ATC may also contain additional connections for loop feeds and/or lightning arresters.

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  • What is an Air Cooled transformer?

      It is a transformer that uses “air” as the cooling medium. This term is abbreviated with the ANSI designation AA, indicating open, natural draft ventilated construction.

  • What does 50/60 Hertz mean?

      Transformers that are designed to specifically run at 60 Hz can’t be run at 50 Hz or in some cases only with significant derating. Magnetic flux is proportional to frequency so a 50 Hz transformer has a core 20% larger to handle 20% more magnetic flux than a 60 Hz unit. A 50 Hz transformer will simply run cooler at 60 Hz given the proper voltage is applied. Transformers cannot change frequency, the primary frequency equals the secondary frequency.

  • What is a Type 4X enclosure?
  • What is a Type 4 enclosure?

      Type 4 is a non-ventilated indoor or outdoor enclosure designed primarily to provide a degree of protection against windblown dust and rain, splashing water, hose-directed water, and damage from external ice formation.

      It is suitable in areas where exposure to large amounts of water from any direction. (Note: not submersible)

  • What is a Type 3R enclosure?

      Similar to the Type 3, it is also intended for outdoor use. It provides a greater degree of protection against rain, sleet, falling snow or dirt and damage from external ice formation. It is typically used for outdoor installations where no blowing snow or conductive dust exists.

  • What is a Ventilated enclosure?
  • What is a Type 3R-E enclosure?

      Although similar to the Type 3R enclosure, a Type 3RE also provides added protection against snow and particulate materials. It is more suitable for outdoor installations where snow or other particulate materials are present. In certain conditions, where sustained high wind conditions exist (typically > 75-80 km/hour), external measures to reduce the flow of snow and/or particulate materials may be required. Please consult HPS for assistance with applications where sustained high wind conditions exist.

  • What are Distributed Static VAR and Synchronous Compensators (D-SVCs/D-STATCOMs)?
  • What defines the audible noise levels in a dry type transformer?

      NEMA ST-20 (2014) defines the noise level in transformers up to 1.2 kV and up to 1000 kVA.

      NEMA TR-1 defines the sound level for medium voltage transformers above 1.2 kV class up to 7500 kVA.

  • What makes up a Coil?
  • What is the International Building Code

      The International Building Code (IBC) is a model building code developed by the International Code Council (ICC). It has been adopted for use as a base code standard by most jurisdictions in the United States. The IBC addresses both health and safety concerns for buildings based upon prescriptive and performance related requirements. The IBC is fully compatible with all other published ICC codes. The code provisions are intended to protect public health and safety while avoiding both unnecessary costs and preferential treatment of specific materials or methods of construction.

  • What is Subtractive Polarity

      The relative polarities of voltages on single phase transformers is important when using the two units in parallel or connecting two or three units to create a three phase bank. Most single phase transformers are wired in Additive Polarity. Visualize two primary terminals H1 and H2 on top of a square representing a single phase transformer. Now visualize two secondary terminals, X1 and X2. If X2 is on the left size and X1 is on the right side, (reading X2 – X1 left to right) then this would be additive polarity. If the terminal X1 is on the left side and X2 is on the right side (reading X1-X2 left to right) then this would be subtractive polarity.

      Most single phase wiring diagrams have a dot on both the primary and secondary sides, typically on the H1 and X1 terminals. This dot represents the matching polarity on the primary and secondary sides of the single phase transformer. When wiring single phase units in parallel or banking, the polarities of the transformers used must be kept consistent.

  • Why is the insulation rating for some distribution transformer set at 220°C and for others the rating is 200°C?

      Most standard ventilated distribution transformers use a 220°C insulation system. This insulation system provides a 150°C temperature rise over ambient, a 30°C hot spot and is meant to be installed in a 40°C ambient temperature.

      However, if a transformer is wound using copper wires, a few of the smaller frame sizes (15kVA and 30kVA three phase, 15kVA and 25kVA single phase) utilize a 200°C insulation with a 130°C temperature rise over ambient, a 30°C hot spot and is meant to be installed in a 40°C ambient temperature.

      The differences in these smaller kVA sizes using copper wire are the result of U.L. ratings of the wire’s insulation temperature rating. Copper wire has a significantly smaller diameter than an equivalent ampacity aluminum wire.

      Small copper wire with 220°C insulation is not always available, so insulation systems are limited to 200°C (Note: U.L. does not cover 200°C insulation systems for units greater than 1.2kV, they will use 180°C insulation systems).

      Small control and potted transformers have insulation systems well below 220°C because of the resin used. As a result, transformer manufactures generally rate transformers using copper conductors 30kVA and below as having a 200°C insulation system.

      Manufacturers compensate for this by building the transformers to run cooler at full load and as a result have a lower 130°C temperature rise and can operate in a 40°C environment.

      Because both units can be operated in a 40°C ambient, we say copper transformers 30kVA and below with a 130°C temperature rise are equivalent to larger units with a 150°C temperature rise.

  • What is the difference between impregnation and encapsulation

      Impregnation is the complete penetration of the process materials (generically called varnish or epoxy) into the windings of the transformers coils. This is often accomplished by using a Vacuum Pressure Impregnation (VPI) cycle.

      Encapsulation is the complete encasement by the process materials (generically called varnish or epoxy) onto the windings of the transformers coils. In simple terms, the process materials have a thicker, more complete coating than an impregnated transformer. Encapsulation can be done by performing two or more VPI cycles or using a higher viscosity process material. Sometimes “potted” will be used for “encapsulated”.  A Cast Coil transformer would be considered to be both impregnated and encapsulated.

      There is not an industry standard for encapsulation. A winding can be encapsulated, it can be impregnated or it can be encapsulated and impregnated.

  • What is NFPA (National Fire Protection Association)?

      The National Fire Protection Association (NFPA) is a United States trade association, albeit with some international members, that creates and maintains private, copyrighted standards and codes for usage and adoption by local governments.

      NFPA 70E covers the Standard for Electrical Safety in the Workplace.

  • What is Dielectric Material in a transformer?
  • What is meant by “Class” in insulation?

      The insulation rating is the maximum allowable winding (hot spot) temperature of a transformer operating at an ambient temperature of 40°C. Insulation systems are classified by the temperature rating. The following table summarizes the different insulation systems available.

      Insulation Rating

      Insulation Class

      Average Winding Temperature Rise

      Hot Spot Temperature Rise

      Maximum Winding Temperature

      Class 105 A 55 degree C 65 degree C 105 degree C
      Class 150 or 130 B 80 degree C 110 degree C 150 degree C
      Class 180 F 115 degree C 145 degree C 180 degree C
      Class 200 N 130 degree C 160 degree C 200 degree C
      Class 220 H 150 degree C 180 degree C 220 degree C
      Note: the maximum acceptable temperature rise based on an average ambient of 30 degree C during any 24 hour period and a maximum ambient of 40 degree C at any time.

  • What is IR drop

      IR drops relates to Ohm’s Law: Voltage = Current x Resistance. Transformers utilize conductors which have resistance and can cause a slight voltage drop. Typically manufacturers compensate the winding ratio to mitigate any effects of IR voltage drop in the secondary voltage.

  • What is IEEE 1584-2018?

      IEEE 1584-2018 provides mathematical models for designers and facility operators to apply in determining the arc-flash hazard distance and the incident energy to which workers could be exposed during their work on or near electrical equipment.

      It generally indicates that systems with an available short circuit current of 2000 Amps or higher should be assessed for arc-flash potential. A rule of thumb would indicate that most systems fed by a 45 kVA or larger transformer will need to be assessed if impedance (%Z) of 45 kVA is less than 6%, 30 kVA if %Z is less than 4% or 15 kVA if %Z is less than 2%.

  • What is an audio transformer

      Audio transformers are typically electronic transformers used in microphone and amplifier applications. Audio transformers are typically rated by power (VA), frequency response and distortion. They are often essential in impedance (power) matching functions.

      HPS does not manufacturer audio transformers and is often confused with Hammond Manufacturing at

  • What is Additive Polarity

      The relative polarities of voltages on single phase transformers is important when using the two units in parallel or connecting two or three units to create a three phase bank. Most single phase transformers are wired in Additive Polarity. Visualize two primary terminals H1 and H2 on top of a square representing a single phase transformer. Now visualize two secondary terminals, X1 and X2. If X2 is on the left size and X1 is on the right side, (reading X2 – X1 left to right) then this would be additive polarity. If the terminal X1 is on the left side and X2 is on the right side (reading X1-X2 left to right) then this would be subtractive polarity.

      Most single phase wiring diagrams have a dot on both the primary and secondary sides, typically on the H1 and X1 terminals. This dot represents the matching polarity on the primary and secondary sides of the single phase transformer. When wiring single phase units in parallel or banking, the polarities of the transformers used must be kept consistent.

  • What is a vault room

      A vault room is a reinforced concrete structure used for the purpose of housing liquid cooled transformers, switchgear and other electrical distribution equipment. Requirements for vault rooms for liquid filled transformers are defined by many specifications including NEC 450.26. Vault requirements are outlined in NEC 450, Part III, beginning with 450.4. Typical requirements might include:

      • Ventilation with outside air via a dedicated ductwork
      • Three hour fire-resistant construction including a minimum 4″ concrete floor
      • Oil containment capable of containing the entire liquid contents of the largest transformer
      • Some exceptions will allow lower construction standards such as the use of sprinklers, carbon dioxide or halon systems and/or the use of “less flammable” liquids.

  • What is a Non-ventilated enclosure

      A non-ventilated enclosure is constructed to restrict unintentional circulation of external air through the enclosure. Common industry descriptions would include Type 12, 4 and 4X in North America.

  • What is a Epoxy Cast transformer?
  • What is a Cast Resin transformer
  • What does Two-Phase Circuit mean?

      Two-phase electrical power was an early 20th-century polyphase alternating current electric power distribution system. Two circuits were used, with voltage phases differing by one-quarter of a cycle, 90°. This has been entirely replaced by three-phase power in a modern power grid.

  • What does Readily Accessible mean?

      Per 1.2.2 of NEMA ST-20, Capable of being reached quickly for operation, renewal, or inspections, without requiring those to whom ready access is requisite to climb over or remove obstacles or to resort to portable ladders, chairs, etc.

  • What does MSR stand for?
  • What does MSAT stand for?
  • What does LV stand for
  • What does Eddy Current mean?

      Eddy currents (also called Foucault’s currents) are loops of electrical current induced within conductors by a changing magnetic field in the conductor according to Faraday’s law of induction. Eddy currents flow in closed loops within conductors, in planes perpendicular to the magnetic field.

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  • What does Dry-Type Self-Cooled Transformer Class AA mean?
  • What does Dry-Type Self-Cooled Future-Forced-Air-Cooled Transformer Class AA FFA mean?

      Per of NEMA ST-20, a dry-type transformer that has a self-cooled rating with cooling obtained by the natural circulation of air and which contains the provision for the addition of forced-air-cooling equipment at a later date.

  • What does Dry-Type Self-Cooled Force Air-Cooled Transformer Class AA FA mean?

      A dry-type transformer that has a self-cooled rating with cooling obtained by the natural circulation of air and a forced-air cooled rating with cooling obtained by the forced circulation of air. This is sometimes referred to as fan-cooled. Fan cooling can increase a transformer’s kVA rating by 25% to 50% depending on the type and size of the transformer.

  • What does Dry-Type Forced-Air-Cooled Transformer Class AFA mean?
  • What does DIT stand for
  • What does Combustible Materials mean?
  • What does Ampacity mean?

      Ampacity is defined as the maximum current, in amperes, that a conductor can carry continuously under the conditions of use without exceeding its temperature rating.

  • What does a One-Line Diagram or Single-Line Diagram mean

      It is a simplified notation for representing a three-phase power system. The one-line diagram has its largest application in power flow studies. Electrical elements such as circuit breakers, transformers, capacitors, bus bars, and conductors are shown by standardized schematic symbols.

      Click here for more information

  • What are Voltage Fluctuations?

      Under voltages and over voltages are caused by faults on power lines, and the subsequent actions of fault clearing devices. High current loads which temporarily exceed the FLA of a transformer, such as across the line motor starts or contactor soil energiztion, can cause voltage drops of 20% or more. This can result in expensive and time consuming errors, loss of information, downtime, recovery and rerun costs and possible equipment damage. Voltage fluctuation on the input of a transformer or autotransformer will be passed through to the secondary.

      These high inrush currents need to either be mitigated (e.g. reduced voltage starters) or have the transformer sized large enough to handle. Control transformers are recommended for loads with magnetic coils such as contactors.

      Snubber circuits should be considered in medium voltage applications which also use vacuum breakers.

  • What is a Type 3 enclosure?

      This is a general purpose ventilated enclosure for outdoor use designed primarily to provide a degree of protection against rain, sleet, wind blown snow or dust and damage from external ice formation. It is considered ideal for construction sites, subways etc.

  • What is a Dielectric System in a transformer
  • What do I need to specify a neutral grounding transformer

      Neutral grounding transformers are not sized by kVA. To properly specify a grounding transformer, the following parameters must be known:

      1. System Voltage & System BIL
      2. Continuous Neutral Current
      3. Fault Current & Duration
      4. Impedance
      5. Connection Type: Zig-Zag Autotransformer or Wye-Delta
      6. Enclosure Type
      7. Ambient Temperature
      8. Other Environmental Concerns

  • What are the Advantages and Disadvantages to Using a Fan-Cooled Transformer?


      • Smaller size; fans may add some height but may reduce width and depth
      • Lower costs for larger units (generally above 1000kVA) to add fans instead of conductor and core
      • Potentially better low-load efficiencies



      • Increased complexity and maintenance
      • Increased cost as fan packages may cost more than just adding material in smaller units
      • Additional energy losses and noise when fan motors are operated in higher loads

  • What is a Type 2-S enclosure?
  • What are Important Specifications for a Marine Duty Transformer?

      Marine Duty is often left to the manufacturer to define.  HPS provides American Bureau of Shipping (ABS) approved transformers which include:

      • Copper or aluminum windings
      • Dry-type convection-cooled
      • Standard NEMA taps
      • 150ºC, 115ºC or 80ºC temperature rise available
      • 220ºC, 200ºC or 180ºC insulation system available
      • VPI Impregnation for salt environments
      • Fungus resistant
      • Braced for marine applications
      • Enclosures:  NEMA 2 or NEMA 3R (per ABS requirements)
      • Stainless steel hardware (on units over 1000kVA)
      • UV protection for outdoor enclosures
      • UL50 frames, channels, etc.
      • CSA, UL and ABS approval (Lloyd’s Register and DNV approval available upon request)

  • What is a Type 2 enclosure?
  • What are some of the solutions to Dirty Power?

      The solutions are as wide ranging as the problems. So are the prices. This Table summarizes some solutions and their price ranges.

      Table for dirty power

      Unfiltered Surge Fuses are very inexpensive, and may provide damage protection from lightning strikes or other surges, but they do not filter out adverse noise.

      Filtered Surge Suppressors are inexpensive solutions to noise suppression and surge protection. The better units inhibit surges above 5000 volts, 200 amps. They should also provide noise filtration of 10dB or more to cover average power disturbances.

      Computer Regulators or Line Voltage Conditioners protect equipment from both noise and voltage fluctuations. They are an inexpensive solution, available in both portable and hardwired models. They provide ideal protection in high noise areas where voltage fluctuations exceed the regulating range of the computers power supply.

      Super Isolation Transformers provide inexpensive protection against frequency variation or noise related disturbances. This is adequate where voltage fluctuations are not a serious problem. Most high-end computers have built-in voltage regulation, but still require protection from line noise.

      U.P.S. Systems are in effect self-contained power centers. They provide backup power for a period of time when utility power is interrupted. Most U.P.S. systems also provide noise filtration and surge suppression.

  • What is an incoming line interrupter switch (electrical disconnect)?

      This is typically a two position, three phase switch designed to disconnect a transformer on the line side. The switch may or may not also have fuses. The switch assembly is typically attached directly to the transformer enclosure and electrically connected through close coupled bussing.

  • What are Impedance Voltage and Load Loss tests?

      The voltage required to circulate the rated current under short-circuit conditions when connected on the rated voltage tap, is the impedance voltage. Rated current is circulated through the windings with the secondary short-circuited. The impedance voltage and load loss is measured. They are corrected to rise +20°C reference temperature.

      Note: This is a standard test only on units over 500kVA. It will only be carried out on lower kVA units when specifically requested.

  • What are the types of shielded isolation transformers?

      Shielded Isolation Transformers with Single Electrostatic Shield

      This is the simplest type of shielded transformer with one grounded shield extending from top to bottom between the primary and secondary windings. This will typically supply 60dB of common mode noise attenuation from 100Hz through 1MHz. Up to 100dB of TMNA and 40dB at 1000kHz of CMNA can be obtained with effective close coupling and low capacitance.


      Shielded Isolation Transformer with Double Electrostatic Shields

      This has two grounded shields extending top to bottom between the primary and secondary windings and between the secondary windings and the core. This will typically supply 60-80dB of common mode noise attenuation from 100Hz through 1MHz.


      Shielded Isolation Transformer with Triple Electrostatic Shields

      This has three grounded shields extending top to bottom between the primary and secondary windings and between the secondary windings and core and covering the outer winding. Little benefit is gained by having the third shield. This will typically supply 65-80dB of common mode noise attenuation from 100Hz through 1MHz.


      Note that recent testing may indicate that electrostatic shields have little to no benefit in typical applications where the secondary is grounded.

  • What is the maximum surface temperature of a transformers enclosure

      Per NEMA ST-20 (2014):

      • Transformers <= 10 kVA can be up to 65C above ambient with a typical maximum ambient of 25C.
      • Transformers > 10 kVA can be up to 50C above ambient with a typical maximum ambient of 40C.
      • Transformers with lower than standard temperature rises will have lower maximum enclosure temperatures.

  • What are Harmonics?

      Harmonics, in an electrical system, are currents created by non-linear loads that generate non-sinusoidal (non-linear) current waveforms. These current and voltage wave forms operate on frequencies that are in multiples of the fundamental 60hz frequency. That is, the fundamental frequency is at 60 hertz, the 2nd harmonic is at 120hz frequency (60 x 2), the 3rd at 180 hertz, and so forth. Harmonics are principally the by-product of switch-mode power supply technology where AC is rectified to DC, and back again. In the process, a capacitor is charged in the first half-cycle, and then discharged in the next half-cycle, in supplying current to the load. This cycle is repeated. This action of recharging causes AC current to flow only during a portion of the AC voltage wave, in abrupt pulses. These abrupt pulses distort the fundamental wave shape causing distortion to the various harmonic frequencies.

  • What is Flux Density?
  • How are HPS transformers designed to shield against voltage transients?

      Electrostatically shielded transformers may help minimize or limit the effects of voltage transients. Common Mode noise is measured from line to ground and is usually the most troublesome. Transverse Mode noise is measured from line to line. Attenuation is the difference of an incoming transient on the primary of the transformer to the secondary side.

      An electrostatic or Faraday shield is simply a thin piece of grounded non ferrous metal (generally copper foil) placed between the primary and secondary windings of a transformer. The shield extends from the top to the bottom of the windings. Some manufactures use shields that don’t extend the full length of the coil face. While less expensive, they will not offer as much protection as a full shield.

      There is no national standard that gives test methods for measuring CMNA and TMNA. Hence, in the industry, various companies have different claims that they have succeeded in getting into customer or consultant specifications. A lot of the confusion for shielded transformers results from differing claims made by various manufacturers and experts. Some recent reports indicate that and electrostatic shield may have little to no benefit where the secondary is grounded which is in most applications. The difference in the claims results from many variables:

      1. Standard single shielded distribution and drive isolation transformers may theoretically provide typical values of CMNA =60 dB and TMNA = 10 dB.
      2. A single shielded transformer with a low capacitive coupling of less than 30 pF may theoretically provide typical values of CMNA = 100 dB and TMNA = 40 dB.

      A manufacturer should be willing to share their testing procedures and test circuit to verify their

      Note attenuation ratings vary by frequency. As the frequency increases, dB ratings go down. The ratings given above may be best case for a wide range of frequencies from 100Hz to 1MHz. Actual attenuation might be significantly higher at the lower frequencies. Some manufactures may claim higher dB’s attenuation by using much lower frequency ranges.

      There may be a large difference between calculated dB and actual dB due to real-life inconsistencies in material and manufacturing. Manufacturers should have actual test data to back up their attenuation claims.

  • What does the abbreviation FCBN stand for?

      “Full Capacity Below Normal.” This designates that a transformer will deliver its rated kVA when connected to a voltage source that is lower than the rated voltage.

  • What salt spray rating test have Hammond’s painted enclosures passed?
  • Can transformers be operated above a 1000m/3300′ altitude?

      There are two main considerations for operating transformers at altitudes above 1000m/3300′. Current standards state designs must be valid to these heights. Above this height, the density of air no longer works as effectively to remove heat. As a result the functional kVA of the transformer must be reduced at higher altitudes, typically about .3% for every 100m/330′. The second issue is the dielectric constant of air is reduced at higher altitudes. Dry type transformers use air gaps as an important component of the electrical insulation properties. At higher altitudes, this lower insulation values, typically in medium voltage BIL levels. Ideally, if transformers will be installed above 1000m, inform the manufacturer and the design can be adjusted to meet all requirements at the higher altitudes.

  • What does the abbreviation FCAN stand for?

      “Full Capacity Above Normal.” This designates that a transformer will deliver its rated kVA when connected to a voltage source that is higher than the rated voltage.

  • What is a Low Voltage General Purpose Transformer?

      HPS’s low voltage general-purpose transformers provide a safe, long lasting, highly reliable power source. They are designed for general lighting and other low voltage applications. They are UL listed and CSA certified.

  • What is a Fan Cooled transformer?
  • Explain Frequency?

      On AC circuits, designates the number of times the polarity alternates from positive to negative and back again . . . such as 60 cycles per second. Measured in Hertz.

  • Can I increase the kVA rating of an existing transformer?

      The most common way to increase the available kVA rating of an existing transformer is to add additional fan cooling. This typically requires modifications including raising the transformers core, adding fans and fan brackets, a motor power supply and controls to start fan cooling when the transformer’s components reach a preset temperature. Fans should never be added without contacting and following specific instructions from the manufacturer. Low temperature rise transformers (115C and 80C rise with 220C insulation) can maintain higher loads in lower ambient transformers. Always follow proper ventilation and clearance instructions.

  • What is an exciting or excitation current?

      A transformer exciting current is the current or amperes required to energize the core. Even with zero load, a transformer will draw a small amount of current due to internal loss. The excitation current is made up of two components. The real component in the form of losses that are commonly referred to as no-load losses. The second form is reactive power measured in KVAR.

  • Do dry-type, potted or cast resin transformers contain PCB’s?
  • What is “Exciting Current (No-Load Current)”?

      Current which flows in any winding used to excite the transformer when all other windings are open-circuited. It is usually expressed in percent of the rated current of a winding in which it is measured.

      A transformer will always consume a small amount of current to energize the windings even if there is no load. This energizing current can create a large inrush several times the transformer’s rated current for a few cycles during initial energization before it reduces to a much lower steady draw.

  • How do you choose the correct, most cost-effective Clean Power Solution?

      Not everyone has the same power problem. Finding the most cost-effective solution requires some analysis of your equipment, the power system and the available solutions in the market. The table below lists causes and effects of many common power problems. You or your electrician can determine the most likely cause of power problems based on knowledge of your location, the kinds of equipment you operate in that location, and the kind of power distribution system in your building.

      The following table lists the types of Clean Power products available from HPS to solve your power problems.

      Table for clean power


      HPS offers the following products for clean power solutions:


  • What are Energy Efficient (TP1) Transformers?

      TP1 was often used to generically refer to the minimim efficiency levels that were originally require in Canada in 2005 and in the U.S. in 2006 for low voltage ventilated transformers. Specifically, the TP1 specification covers energy efficiency in transformers based on the NEMA Standards Publication, TP-1-1996, “Guide for Determining Energy Efficiency for Distribution Transformers”. The TP1’s recomendations did consider the total owning cost of ownership unique for industrial or commercial installations. TP1 is measured per TP2 and displayed on the nameplate per TP3.

      The TP1 specifcations have now been replaced by higher efficiency specifications:

      • U.S.A.: DOE 2016 efficiency levels (January 1st, 2016)
      • Canada: NRCan 2019 efficiency levels (May 1st, 2019)

  • How do you calculate the Total Losses in a transformer?

      It is the transformer electrical losses, which include no-load losses (core losses) and load losses (winding losses). There is not a specific formula to calculate total losses since they vary by size and manufacturer. Manufacturers will often provide charts that show losses at one or more load points.

      HPS does offer an efficiency calculator for distribution transformers.

      HPS Efficiency Calculator

  • What is the ambient temperature specs for a standard transformer?

      A standard transformer with 220°C insulation and a 150°C temperature rise, will be rated to run full load in an average 30°C ambient environment over 24 hours with a maximum 40°C ambient temperature.

      Other magnetics may have lower or higher ambient ratings depending on the design and application.

  • What are Encapsulated Transformers and where are they used?

      Encapsulated units are covered in a thicker coating of insulation than typical. Often the coils are completely encased in epoxy or an epoxy and aggregate mixture. Sometimes they are referred to as “potted” or “cast coil”.

      The encapsulated design is especially suited for installations in harsh environments where dust, lint, moisture and corrosive contaminants are present. Typical applications include: pulp and paper plants; steel mills; food processing plants; breweries; mines; marine and shipboard installations.

  • What is an “Electrostatic Shield”?

      Electrostatically shielded (Faraday Shield) transformers provides a copper electrostatic shield between the primary and secondary windings. The shield is grounded and thus shunts some noise and transients to the ground path rather than passing them through to the secondary. Transformers having a K-Rating are required to have an electrostatic shield.

      Electrostatically shielded transformers often preferred for electrical installations where electronic circuitry operating at low voltage DC is present and is very sensitive to ‘noise’. Recent testing of electrostatically shielded transformers has questioned their perceived effectiveness where the transformer’s secondary is grounded which would cover most applications.

  • What is electrical noise?

      Noise is a very broad term that can be applied to a number of AC power line disturbances. Lightening surges or any other sudden changes in load, such as switching motor loads or power factor correcting capacitors can produce voltage spikes and ringing. Phase controlled rectifier loads and arcing devices produce continuous noise unless adequately filtered. Noise sources are either common mode, which appears between both sides of a power line and ground or of transverse mode, which appears from line to line. HPS Clean Power products, such as our Computer Regulators remove these noise sources.

  • What is a “Dry Type” transformer?
  • What are Drive Isolation Transformers and where are they used?

      Drive Isolation Transformers (DIT) are designed to supply power to AC and DC variable speed drives. The harmonics created by SCR type drives requires careful designing to match the rated hp of each drive system. The duty cycle included is approximately one start every 2 hours. The windings are designed for an over-current of 150% for 60 seconds, or 200% for 30 seconds.

      DIT’s are covered by NRCan efficiency regulations in Canada but are exempt from efficiency regulations in the U.S.A.

  • Does HPS have NRTL certification?
  • What is Dirty Power?

      Dirty power is caused by a number of things. Simply put, dirty power is what causes your radio or telephone to ‘crackle’ during an electrical storm; or what causes ‘snow’ on your TV when someone is using a power tool, sewing machine or other appliances in your house. This dirty power, or electrical noise, is a nuisance when it appears on your radio, TV or telephone. When it gets into your computer, it can cause serious errors; improper readouts, printing problems, or even damage your computers circuit.

  • What are Dielectric tests?

      The purpose of dielectric tests is to demonstrate that the transformer has been designed and constructed to withstand the voltages associated with specified insulation levels.

  • DC smoothing reactor – definition
  • What is a Current Transformer?
  • Current limiting reactor – definition
  • What is the Core of a transformer?
  • What is Core Loss?

      Losses in watts caused by magnetization of the core and its resistance to magnetic flux when excited or energized at rated voltage and frequency. Also referred to as excitation loss or no-load loss.

  • What is the term Continuous Rating?
  • What is a Compensated Transformer?

      It is a transformer with a turn’s ratio is adjusted to compensate for the natural voltage drop caused by the coils resistance from the wire. Typically large compensation factors at only found in smaller transformers below 5 kVA.

      Winding compensation can cause lower output voltages if a transformer is backfed.

  • What is an Induced Voltage test?

      The induced voltage test is applied for 7200 cycles or 60 seconds whichever is shorter. The voltage applied is twice the operating voltage, and confines the integrity of the insulation

  • Interphase reactor – definition
  • Iron core filter reactor – definition
  • Iron core motor starting reactor – definition
  • What is a Type 12 enclosure?

      This is a non-ventilated indoor enclosure designed primarily for providing a degree of protection against circulating dust, falling dirt, and dripping non-corrosive liquids. This enclosure is both oil and rust resistant suitable for applications such as oil refineries where oil or other chemical liquids may be prevalent. (Note: not watertight)

  • What is a Type 1-N enclosure?

      This is a general-purpose non-ventilated enclosure for indoor use primarily designed to provide a degree of protection against limited amounts of falling dirt. It is ideal for normal factory environments.

  • What is a Type 1 enclosure?

      This is a general-purpose ventilated enclosure for indoor use primarily designed to provide a degree of protection against limited amounts of falling dirt. It is ideal for normal factory environments.

  • What is a triplex transformer?

      A triplex transformer is composed of three separate single phase transformers which are banked and connected directly together to form a single three phase unit in a common enclosure. While slightly larger than a dedicated three phase unit. a triplex design can be broken down into three significantly smaller and lighter components when there are size and/or weight restrictions for transporting the transformer. These are often used in mining or high rise building installations where the unit must be transported in an elevator.

  • What is the maximum surface temperature of a transformers terminals wire leads or connections points

      Per NEMA ST-20 (2014):

      • Transformers <= 10 kVA can be up to 50C above ambient with a typical maximum ambient of 25C.
      • Transformers > 10 kVA can be up to 35C above ambient with a typical maximum ambient of 40C.
      • Transformers with lower than standard temperature rises will have lower maximum connection point temperatures.

  • Does HPS Have 5% Impedance Centurion® R Reactors at 690 Volts?
  • What is the lowest temperature transformers can be energized

      Dry-type transformers with the exception of Cast Coil (HPS Endura Coil) can be stored to temperatures of -50 degress C. Cast Coil Transformers can only be stored to temperatures of -20C. If a standard cast coil transformer is stored below -20C, the epoxy coils can crack. Transformers which are energized before 0C can be run in ambient temperatures to -40C.

      There are two main concerns with low temperatures occur during storage when the units are not energized. Energized transformers have load and no-load losses which will keep the core and coils operational to -40C:

      • Contraction and Expansion of the core and coil during low temperatures can crack or damage the insulation.
      • Cold temperatures can cause condensation to form on the transformer which can result in short circuits and insulation damage.

      The minimum ambient temperature designed for is -40C based on the Environment Canada data Extreme Minimum Temperatures for Wiarton, ON (-36.4C on 18-Jan-1977) and Lucknow, ON (-36.7C on 05-Feb-1918). If the temperature of a transformer’s core and coil is below -25C (typically during unenergized storage), please consult HPS for the cold start procedure before any energization. If the temperature of a transformer is below 0C, please follow the HPS Cold Start Procedure. It is recommended that the transformer be meggered to make sure it has a minimum value of 100 Megaohm before energization after storage.

  • Where is a Scott-T Transformer used

      Typical applications for a Scott-T transformer include:

      • Used in an electric furnace installation where it is needed to operate two single-phase feeds and draw a balanced load from the three-phase supply.
      • Used to supply the single phase loads such as traction power. This helps to keep the load on the three-phase system as nearly balanced as possible.
      • Used to link a 3-phase system with a two–phase system with the flow of power in either direction.

      The Scott-T connection permits conversions of a 3-phase system to a two-phase system or vice versa. Since 2-phase generators are not available, the conversion from two phases to three phases is not a practical application.

  • What problems can occur if I undersize the short circuit protection on a transformer?

      Nuisance tripping is a concern when short circuit protection is undersized from the National Electric Code recommendations in NEC 450.3.  All transformers experience an inrush current during any energization. The inrush current results from the transformer establishing the initial electromagnetic field and is not linked to load. If short circuit protection is undersized, there is a chance for nuisance tripping of fuses and circuit breakers anytime the transformer is energized.

  • What is the term Load?
  • What is the lowest temperature transformers can be stored in

      Dry-type transformers with the exception of Cast Coil (HPS Endura Coil) can be stored to temperatures of -50 degrees C. Cast Coil Transformers can only be stored to temperatures of -20C. If a standard cast coil transformer is stored below -20C, the epoxy coils can crack. Transformers which are energized before 0C can be run in ambient temperatures to -40C.

      There are two main concerns with low temperatures occuring during storage when the units are not energized. Energized transformers have load and no-load losses which will keep the core and coils operational to -40C:

      • Contraction and Expansion of the core and coil during low temperatures can crack or damage the insulation.
      • Cold temperatures can cause condensation to form on the transformer which can result in short circuits and insulation damage.

      The minimum ambient temperature designed for is -40C based on the Environment Canada data Extreme Minimum Temperatures for Wiarton, ON (-36.4C on 18-Jan-1977) and Lucknow, ON (-36.7C on 05-Feb-1918). If the temperature of a transformer’s core and coil is below -25C (typically during unenergized storage), please consult HPS for the cold start procedure before any energization. If the temperature of a transformer is below 0C, please follow the HPS Cold Start Procedure. It is recommended that the transformer be meggered to make sure it has a minimum value of 100 Megaohm before energization after storage.

  • What is a Transformer?

      It is a static electrical device, which, by electromagnetic induction transforms energy at one voltage and current to another voltage and current at the same frequency.

  • What is a SCR?
  • What is the induction principle of transformers

      A transformer consists of laminated silicon steel cores on which one or more coils of wire have been wound. The two windings are electrically isolated from each other (with the exception of autotransformers) and usually have widely different numbers of turns.

      If the transformer primary is connected to an A.C. power source of suitable voltage, a small no-load current called the exciting current will flow into the coil and produce a magnetic flux in the iron core. Since the source is A.C., the flux will also be alternating. This alternating magnetic flux links the secondary turns and induces a small voltage in each turn. The induced volts per turn of the secondary windings adds to appear across the secondary terminals. It should be understood that the flux induces a voltage in each primary turn equal to that in each secondary turn. The difference between the total induced primary voltage and the applied voltage is approximately equal to the IR drop. The ratio of turns between the primary and secondary coils determines the output voltage.

  • What is the hot spot allowance of a transformer?

      The coils of a transformer core do not evenly heated during energization. Parts of one coil will be hotter than the surrounding areas because they are farther from any ventilated openings or closer to the core which also produces heat. The hot spot allowance is a set number as defined by industry standards and is associated with the insulation class. Typically the smaller the transformer, the lower the insulation class and the more uniform the heating.

      • 105C Insulation System: 10C Hot Spot Allowance
      • 150C Insulation System: 30C Hot Spot Allowance
      • 180C Insulation System: 25C Hot Spot Allowance
      • 220C Insulation System: 30C Hot Spot Allowance

      The Hot Spot Allowance is added the expected ambient temperature and full load temperature rise to get the total expected temperature rise of a transformer.

  • What is Primary Voltage Rating?
  • Do any performance issues arise during low ambient temperatures?

      Generally low ambient temperatures do not affect an energized transformer. No-load losses on an energized transformer typically generate enough heat to operate effectively in temperatures to -20°C or lower.

      The main issue with lower temperatures is when the unit is not energized. Extremely low temperatures or if the transformer heats up too quickly may cause welds and insulation to become brittle and crack, especially if the transformer experiences any mechanical stresses.

      More importantly, low temperatures can cause moisture (dew, frost) to form on the unit. This can be absorbed into the insulation system and not be apparent.

      If ambient goes below -30°C, special designs and cold start procedures may be necessary. Care should be taken to store transformers in dry areas with temperature control. Installation manuals typically suggest that transformers be tested (meggered), brought above 0°C and/or go through a dry-out process if moisture is suspected to be present.

      Damage and injury can result from energizing a transformer which has had its insulation system compromised by moisture.

  • What is a Step-Down Transformer?

      It is a transformer where the high voltage winding (primary) is connected to the input or power source and the low voltage winding (secondary) to the output or load.

  • What is Temperature Class?

      It is the maximum temperature that the insulation can continuously withstand.

      The classes of insulation systems in a transformer are rated as follows:

      Class 105°C

      Class 150°C

      Class 180°C

      Class 220°C

  • What does the abbreviation NEC refer to?
  • What is Reactance?
  • What is RCBN – Reduced Capacity Below Nominal

      For transformer equipped with primary or secondary taps designed for RCBN, the customer is permitted to connect to a lower voltage tap (BELOW NOMINAL), provided that the customer load capacity (KVA) is also REDUCED so as to ensure that the rated winding current does not exceed its nominal value.

  • What is Polarity?
  • What is Overload?

      Overload is when a transformer is subjected to voltages and/or currents that exceed its design specifications. Transformer voltages and current limits are typically governed by their nameplate specifications.

  • What is NFPA 70E?

      NFPA 70E is the titled Standard for Electrical Safety in the Workplace, is a standard of the National Fire Protection Association (NFPA). The document covers electrical safety requirements for employees. The NFPA is best known for its sponsorship of the National Electrical Code (NFPA 70).

      NFPA 70E addresses employee workplace electrical safety requirements. The standard focuses on practical safeguards that also allow workers to be productive within their job functions. Specifically, the standard covers the safety requirements for the following:

      • Electrical conductors and equipment installed within or on buildings or other structures, including mobile homes, recreational vehicles, and other premises (yards, carnivals, parking lots, and industrial substations)
      • Conductors that connect installations to a supply of electricity
      • Not covered are – electrical installations in marine, aircraft, auto vehicles, communications and electrical utilities.

      Key principles covered are JSA/JHA/AHA procedures to ascertain shock protection boundaries, arc flash incident energy expressed in calories/cm2, lockout-tagout, and personal protective equipment. While the various OSHA, ASTM, IEEE and NEC standard provide guidelines for performance, NFPA 70E addresses practices and is widely considered as the de facto standard for Electrical Safety in the Workplace.

  • What is Inrush Current?

      It is an abnormally high transient current, caused by residual flux in the core, which is drawn for a short period when a transformer is energized.

  • What is Inductance?

      Inductance is the property of a conductor by which when exposed to alternating current flow, the change in current flow induces a voltage across the conductor.

  • What is Impedance?

      It is the apparent resistance in a circuit to the flow of an alternating current analogous to the actual resistance to a direct current.

  • Transformer Short Circuit Considerations

      Short circuits or faults can and do occur on electric power and distribution systems.  When a fault occurs on the load side of a transformer, the fault current will pass through the transformer.  As components on these systems, transformers need to be able to withstand these fault currents.

      Fault currents flowing through transformers are significantly higher than the rated currents of the transformers.  In worst case, the current would be as high as the current that would flow if system voltage was applied to the primary terminals while the secondary terminals are shorted – limited by the transformer impedance only.  These currents produce both mechanical and thermal stresses in the transformers.

      Forces resulting from the currents passing through the transformer act on the conductors.  The forces are a function of the peak asymmetrical current (the highest peak value of any cycle of the current), which is usually at its highest during the first half cycle of the fault.  The duration of the fault is not normally a concern for mechanical withstand because the peak value of each cycle of the current reduces as the fault persists.  The transformer manufacturer needs to ensure these forces do not damage the transformer.

      The thermal stress is caused by the high current causing heating in the transformer.  Both the RMS symmetrical current magnitude and duration of the fault contribute to the heating of the transformer.  The transformer manufacturer needs to ensure the components of the transformer do not become hot enough to be damaged.

      General purpose dry type transformers are typically designed to withstand the mechanical and thermal stresses caused by a short circuit occurring on the secondary terminals of the transformer with rated voltage applied to the primary terminals for a maximum of 2 seconds, provided the current does not exceed 25 times rated current.  The fault current magnitude is a function of the transformer impedance.  The table below shows the fault current for selected impedances and applies to both line currents and phase currents.

      Transformer Impedance Fault Current (times rated)
      4.0% 25.0
      5.0% 20.0
      6.0% 16.7
      7.0% 14.3
      8.0% 12.5

      The maximum of 25 times rated current is listed so that transformers with an impedance below 4% need only be able to withstand 25 times rated current, although the fault current could be higher than this.  It does not mean that all transformers are able to handle a fault current up to 25 times rated current – with rated voltage applied to the primary a transformer impedance above 4% will not allow 25 times rated current to flow.

      Many specifications indicate a fault level at the primary terminals of the transformer.  Some customers will ask for a transformer to be braced for the primary fault level.  A general purpose transformer is suitable to be connected to a system with the specified fault level, but the transformer impedance will limit the fault current through it to well below the available fault level.  As an example, the customer requests a 2500 kVA transformer, 13.8  kV delta to 480Y/277 V, 5.75% impedance to be braced for 750 MVA fault level at 13.8 kV.  In this case, the available fault current at the primary terminals is 31.4 kA.  For a fault on the secondary side of the transformer, the transformer impedance will limit the fault current that flows in the primary to 1.8 kA in the lines and 1.1 kA in the coils – significantly lower than the available fault level.  There is no need for the transformer primary conductors to be sized and braced to handle a 31.4 kA fault current.

      Some operating conditions need special attention.  Some customers specify the transformer to operate continuously with load at higher than rated voltage.  If a fault occurs when the transformer is operating at higher than rated voltage, the resulting fault current would be higher than a typical transformer is designed for.  This will increase both the forces in the transformer and the heating of the transformer.  Some customers specify a fault duration longer than 2 seconds without specifying a higher voltage.  This does not affect the forces but does increase the heating in the transformer.  In these cases, a special design may be required.

      One case in particular requires special attention – transformers directly connected to a generator.  When a generator is supplying a load and the load is suddenly disconnected, the output voltage of the generator rises significantly for a short time until the excitation system decreases the voltage.  If a fault occurs on the secondary of a transformer at this time, the fault current can be significantly higher than a typical transformer is designed for.  Some applications may not have any overcurrent protection between the generator and the transformer primary winding, resulting in an increased duration.  In such cases, it is recommended that IEEE C57.116 IEEE Guide for Transformers Directly Connected to Generators be reviewed to determine the short circuit withstand requirement for the transformer.

      General purpose transformers have short circuit withstand capabilities that are sufficient for many applications.  The transformer manufacturer needs to be informed of cases where a fault could occur on the secondary of the transformer when the transformer is supplied above rated voltage or the fault duration is longer than 2 seconds to ensure the transformer is suitably designed to withstand the possible secondary faults.

  • What is Transverse Mode Noise Attenuation (TMNA)?

      It is attenuation of electrical noise or voltage disturbances that occurs between phase and neutral (between lines), or from spurious signals across the metallic hot line and the neutral conductor. A transformer’s impedance, isolation properties and electrostatic shield may all contribute to the total TMNA level. This is typically expressed in decibels.

  • What are some of the Tests performed on transformers?

      Normal, routine production tests include:

      1. core loss;
      2. load loss – winding or copper loss;
      3. Impedance;
      4. hi-pot – high voltage between windings and ground;
      5. induced – double induced two times voltage.

      Optional special tests include:

      1. heat run – temperature testing;
      2. Noise tests – sound level measurement;
      3. impulse tests – BIL tests:
      4. partial discharge

  • What are the overload capacities of cast coil transformers?

      Cast coil transformers typically come with two insulation systems. 155C is common for IEC rated transformers while 185C is required for U.L. listed units. The 185C insulation system offers additional overload ratings over the 155C system. Assuming a maximum ambient of 40C and average of 30C, a 185C insulation system is typically rated at 115C temperature rise. When built with a 100C temperature rise, the unit will have an overload capacity of 6%. At an 80C temperature rise, the unit will have an overload capacity of 17%.

      A cast coil transformer using a 155C base insulation system has a base 80C temperature rise. So, at 80C, there would be no additional overload capacity.

  • Why Are There Physical Clearance (Distance) Requirements on the Nameplate?

      A ventilated transformer’s physical clearance requirements are designed to provide adequate clearance for airflow cooling. Generally, the larger the transformer the more airflow and clearance is needed. The more important areas are the front and back of ventilated transformers where the air may enter in the bottom and exit at the top. Since the sides of distribution transformers generally don’t have ventilation openings, side clearance is less important. There must also be no obstructions that limit airflow into the bottom vents and top clearance from ceilings must also be maintained.

      All smaller clearances should be reviewed by the manufacturer to verify they are adequate. Also note that electrical codes require minimum front panel clearances to allow safe and easy access to the wiring area. Units supplied with factory installed wall-mounting brackets may also have the back closer to the wall than the nameplate requirements; this is acceptable.  These statements may not apply to non-ventilated and/or potted transformers which don’t have ventilation slots and/or may have zero clearance when mounted to wall suing supplied brackets.

  • Which applications are best addressed by an Active Harmonic Filter

      AHF’s are used where a significant portion of the load consists of VFD’s or other three-phase non-linear sources such as large three-phase DC power supplies, electric vehicle chargers or UPS’s. VFDs are defined as non-linear loads which generates an enormous amount of harmonics in a system. Harmonics cause a host of electrical problems. AHF’s are great candidates to mitigate harmonics from a system where multiple VFD loads that represent a significant portion of the total load.

      Active filters are designed to reduce harmonics from three-phase sources. For single-phase harmonic sources, solutions such as harmonic mitigating transformers should be considered.

  • What communication options come with the HPS TruWave Active Harmonic Filter
  • What are the main functions performed by an Active Harmonic Filter
  • Can I use the 690 Volt Centurion® R Reactors on the output of my drive?
  • What is the Intended Use of UL Type 3R Enclosures for Centurion R?
  • What are the benefits of an active filter over a passive filter

      Here are some of the advantages that Active Harmonic Filters can provide over the Passive Filters.

      • Active Harmonic Filters provide far superior flexibility and performance over passive filters.
      • Not all Passive filters can achieve the 8% or 5%THD IEEE-519 specification even at full load. The HPS TruWave AHF will achieve less than 5% THD even until 10% loaded. Passive filters typically provide less overall mitigation as the load decreases.
      • AHF will not cause a leading power factor at no load while passive filters do
      • AHF can be installed anywhere in the lineup, while the passive filters must be installed at each VFD
      • Active filters are cost and space effective with the use of multiple VFD loads compared to passive filters

  • How many CT do I need to use for the Active Filter

      CT’s are used with the HPS TruWave AHF to continuously monitor the load and harmonic currents. Typically, if the system only has three-phase loads downstream to the AHF, two CT’s can be used; the TruWave software will calculate the third phase current. If the system has any single-phase loads, a third CT is required.

      Here are some the installation considerations for the current transformers (CT’s) with the AHF:

      • Must be located upstream of VFD loads requiring correction
      • Two CT’s are required for the correction of three-phase loads
      • A third CT is only required if there are also single-phase (line to neutral) loads
      • The CT’s are sized based on the current rating of the bus

  • What is the Intended Use of UL Type 1 Enclosures for Centurion® R?
  • Does the VFD have to be equipped with a DC link choke to work with Active Harmonic Filter
  • When Using a Two or Three Contactor Bypass with a Variable Frequency Drive, Where Should the Input and Output Line Reactors be located?
  • Do I need to use a line reactor with VFDs to work with an Active Harmonic Filter

      All non-linear loads must have an input line reactor (minimum 3%) or a DC link choke to achieve the desired system performance. While an AHF can correct harmonics without line reactors, issues can occur if there is not sufficient impedance between an AHF and a load.

      Using line reactors is also cost effective since reactors mitigate some of the harmonics and a smaller AHF can be deployed.

  • What is the Short Circuit Current Rating for Centurion® R Reactors?
  • Which Safety Standards do Centurion® R Reactors Meet?
  • Can the TruWave be used on a 600V system

      Typically the TruWave is rated for 480VAC. However, through the use of a 600V to 480V autotransformer between the TruWave and the load, the TruWave can be used on 600V systems. This same setup can also be used on voltages if needed.

  • How does a HPS Reactor reduce Line Notching?

      Whenever a rectifier converts AC power to DC, using a nonlinear device, such as an SCR, the process of commutation occurs. The result is a notch in the voltage waveform. The number of notches is a function of both the number of pulses and the number of SCR’s in the rectifier.

      Reactors are used to provide the inductive reactance needed to reduce notching, which can adversely effect equipment operation.

  • Can neutral currents such as the 3rd harmonic be reduced by the use of 3rd harmonic blocking filters?

      Some manufacturers are promoting the use of 3rd harmonic (180 Hz) blocking filters for the treatment of high neutral currents caused by non-linear loads such as personal computers. These devices are parallel L-C filters tuned to 180 Hz and are connected in the neutral of 4-wire systems between the transformer secondary and the neutral-to-ground connection.

      Their high impedance to the flow of 3rd harmonic current forces all connected equipment to draw current that does not contain the 3rd harmonic. Although their use will result in a significant reduction in 3rd harmonic current, it is achieved at the risk of rather severe consequences.

      1. The installation raises questions with respect to NEC 2002 compliance. NEC 250.30(A)(2)(a) states that “a grounding electrode conductor for a single separately derived system … shall be used to connect the grounded conductor of the derived system to the grounding electrode…” In addition, “the grounding electrode conductor shall be installed in one continuous length without a splice or joint…” [See NEC 250.64(C)].

      If a simple splice connection is not allowed, then certainly the L-C circuit of the 3rd harmonic blocking filter should not be allowed either. Also, the installation results in an impedance grounded wye system rather than a solidly grounded system. The only reference in NEC that allows for the introduction of impedance between the neutral and the grounding electrode is found in Section 250.36, High-Impedance Grounded Neutral Systems. However, these systems are permitted only at 480V and higher and only if they do not serve line-to-neutral loads. They also require the use of ground fault detectors. None of these requirements is met in the normal application of the 3rd harmonic blocking filter where the loads are primarily 120V, phase-to-neutral connected computer or other power electronic equipment.

      2. Although tuned to 180 Hz, the L-C circuit will introduce some impedance at 60 Hz as well. The consequences are:
      a. Line-neutral short circuit current will be reduced which will limit a circuit breakers ability to clear a line-neutral fault. This can be very dangerous because an uninterrupted fault (commonly referred to as an arcing fault) will often result in an electrical fire.
      b. The neutral point at the transformers wye secondary can shift. This can result in 120V line-neutral voltages that rise and fall unpredictably as the load balance between the phases varies.

      3. High impedance to the flow of 3rd harmonic current will produce voltage distortion in the form of flat-topping – a dramatic reduction in peak to peak voltage. This will:
      a. Significantly reduce the ride-through capability of switch-mode power supplies (SMPS) since the DC smoothing capacitors will not be allowed to fully charge.
      b. Reduce the SMPS DC bus voltage, thereby increasing the current demand the associated I2R losses. Component reliability will be reduced due to higher operating temperatures.
      c. Often cause Single Phase UPS systems to switch to battery back-up.
      d. Force connected equipment to operate without a 3rd harmonic current – an operating mode for which they have not been intended or tested.

      At first, when loading is light, problems may not be extremely obvious. However, as the load increases, voltage distortion and flat-topping will also increase until problems do arise. Although neutral current can be reduced, it is often achieved at the expense of a tremendous increase in voltage distortion. At 30%, the voltage distortion can be up to 4 times the maximum limit of 8% recommended by IEEE std 519. In addition, the measured crest factor can be significantly below the normal sinusoidal crest factor of 1.414.

      4. The 180 Hz L-C blocking filters requires the use of capacitors and it is well known that capacitors are less reliable than inductors and transformers. Failure of the capacitor or its protection could result in a very high impedance ground at the neutral over the full frequency range. This would have a dramatic effect on 60 Hz unbalance and fault currents.

      5. At frequencies above the resonant point (180 Hz), the parallel L-C circuit becomes capacitive which could result in a resonant condition at some higher harmonic frequency.

      More Harmonic Mitigating Transformer Frequently Asked Questions

  • What does AHF stand for
  • What is the Peak Voltage for the 690 Volt Centurion® R Reactors?
  • Can I use PFCC with Active Harmonic Filters

      Power Factor Correction Capacitors can be used on systems with AHF’s. AHF’s harmonic mitigation may even be required to protect PFCC from excessive heating and failure caused by harmonics. PFCC cannot be installed on the load side of AHF current sensors. PFCC should be installed between the AHF and the utility point of common coupling (PCC).

  • If an output load reactor or dV/dT filter is installed on a VFD, should VFD Cable be installed from the drive to the motor?
  • Can equipment manufacturers design their products to be free of harmonics

      Yes they can, but lowering the current distortion levels at the input to the SMPS in a computer will add to the cost of the computer. This is not a step that computer manufacturers wish to take because of the continuous and intense cost cutting in the computer industry.

      Actually it is less costly overall to provide a harmonic mitigating transformer to feed several hundred computers than it is to improve the operation of the SMPS in each computer. This is especially true when we consider that the added cost of the improved SMPS will reappear every three years when a new computer system is purchased.

      More Harmonic Mitigating Transformer Frequently Asked Questions

  • Can a non-inverter duty rated motor be used if load reactor or dV/dT filter is installed?
  • Can and Active Harmonic Filter improve power factor
  • How do HPS Line Reactors handle Heat Dissipation?
  • Can an Active Harmonic Filter be used to protect a specific circuit or machine in a building

      AHF’s are used to mitigate harmonics in a total circuit. Let’s set up an example. A company has 5 work cells and each is producing 20 amps of harmonics. A sixth work cell is introduced, it’s a different machine and it produces 30 amps of harmonics. Some operational issues on the new sixth machine have been determined to be caused by Power Quality issues. Can an AHF be deployed to just protect the new machine?

      • If the new machine is fed from a dedicated transformer, it’s possible an AHF designed to handle at least 30 amps could be installed to mitigate harmonics on the secondary of the transformer feeding this machine.
      • If all six machines are fed by one large distribution transformer with no other isolation transformers between, then an active filter large enough to handle the entire harmonic amperage ((20 x 5) + 30 = 130 amps) must be installed.

  • Are there standards that can help in addressing harmonics?
  • How does a Line Reactor minimize harmonic distortion?

      Nonlinear current waveforms contain harmonic distortion. By using a HPS line reactor you can limit the inrush current to the rectifier in your drive. The peak current is reduced, the waveform is rounded and harmonic distortion is minimized. Current distortion typically is reduced to 30%. Severe Harmonic current distortion can also cause the system voltage to distort. Often, high peak harmonic current drawn by the drive, causes “fl at-topping” of the voltage waveform. Adding a reactor controls the current component, and voltage harmonic distortion is therefore reduced.

  • Are there voltage drop concerns when using a load reactor or dV/dT filter for long lead lengths?
  • What is IEEE 519-2014

      IEEE is the Institute of Electrical and Electronics Engineers. IEEE 519-2014 is a document that establishes levels of voltage and current harmonic distortion acceptable to the distribution system based on the input transformer characteristic and the loads on a customer’s facility. Many electrical consultants are including compliance with IEEE 519-2014 in their design specifications to help reduce harmonic problems and avoid penalties that can be imposed by electrical utilities. More information about the levels of harmonics can be found on the IEEE website.

      • The IEEE 519-2014 also outlines the Point of common coupling (PCC) as the point where the utility meets the facility
      • The current and voltage harmonic limits set by IEEE and followed by many specifiers are clearly outlined in the following IEEE tables shown below:

      Voltage Distortion Limits & Maximum Harmonic Current Distortion

  • What is the Impedance of the Centurion® R Reactors?
  • What is an active harmonic filter?

      Due to the increasing usage of non-linear loads such as VFDs, harmonics are being introduced into the power grid which is contributing to poor power quality and leads to overheating of equipment and nuisance faults. Active Harmonic Filters are parallel devices that are used to mitigate harmonics to the levels defined by IEEE-519.

      HPS TruWave AHF utilizes high frequency current sensors to continuously monitor the load and harmonic currents. By utilizing highly sophisticated software and a powerful DSP microcomputer, the system is able to instantaneously inject a corrective current from its IGBT based inverter to dramatically reduce harmonic distortion. The corrective current is equal to but 180 degrees out of phase with the existing harmonic currents to cancel their effect.

      Active filters work on the same principle s as noise cancelling head phones except they cancel harmonic currents and reduce distortion.

  • how Line Reactor eliminate Nuisance Tripping

      Transients due to switching on the utility line and harmonics from the drive system can cause intermittent tripping of circuit breakers. Furthermore, modern switchgear, equipped with solid-state trip sensing devices, is designed to react to peak current rather than RMS current. As switching transients can peak over 1000 volts on a 600 volt system, the resulting over-voltage will cause undesirable interruptions.

      A reactor added to your circuit restricts the surge current by utilizing its inductive characteristics and mitigates nuisance tripping. The impedance of a transformer will have similar effects.

      Learn about HPS Centurion R Reactors

  • What information is needed to size an Active Harmonic Filter

      The following information is all required in order to correctly size an active harmonic filter:

      • One-line diagram of the system. Location and size of VFD’s and Power Factor Correction Capacitors is very useful.
      • Detailed equipment lists can also be used, especially in conjunction with one-line diagrams.
      • VFD information: Horse Power or Current size.
      • Are line reactors being used with each VFD? If so what is the impedance?
      • Will the active filters operate on generator?
      • Are there any large soft start loads located downstream of the AHF?
      • Local Environmental Conditions

  • Why do 3rd harmonic currents overload neutral conductors?

      Sinusoidal currents on the phases of a 3-phase, 4- wire system with linear loads sum to return on the neutral conductor. The 120° phase shift between the sinusoidal load currents causes their vector sum to be quite small. In fact it will be zero if the linear loads are perfectly balanced.

      The instantaneous sum of the currents in the three phases taken at any moment will also be zero if the linear loads are perfectly balanced. If they are not, then there will be a small residual neutral current.

      With linear loads, the neutral conductor can be the same size as the phase conductors because the neutral current will not be larger than the highest phase current. Unfortunately, this is definitely not true for non-linear phase-to-neutral loads.

      120VAC non-linear loads like the SMPS used in computers and in monitors draw current in two distinct pulses per cycle. Because each pulse is narrow (less than 60 degrees), the currents in the second and third phases are zero when the current pulse is occurring in the first phase. Hence no cancellation can occur in the neutral conductor and each pulse of current on a phase becomes a pulse of current on the neutral.

      Even if the phase currents of the SMPS loads are perfectly balanced in RMS amperes, the RMS value of the neutral current can be as much as √3 times the RMS value of the phase current because there are 3 times as many pulses of current in the neutral than in any one phase. If the phase current pulses do overlap because they exceed 60 degrees in width, then there will be some cancellation so that the neutral current will be less than √3 times the phase current. Overlapped or not, because there are 3 times as many pulses in the neutral than in a phase, the predominant component of the neutral current will be the 3rd harmonic (180Hz for a 60Hz system). The linear current completes only 2 cycles in the same time period that the non-linear neutral current completes 6 cycles or 3 times the fundamental.

      Often, in new construction this situation is addressed by simply doubling the neutral conductor ampacity. In existing facilities however, it is most often very difficult and too costly to implement this solution, therefore an alternate method is usually necessary. Question 11 describes how Zero Sequence Harmonic Filters can be used very effectively to reduce 3rd harmonic currents in the neutral conductor.

      More Harmonic Mitigating Transformer Frequently Asked Questions

  • Will a HPS Line Reactor extend the life of your motor?

      Line reactors, when selected for the output of your drive, will enhance the waveform and virtually eliminate failures due to output circuit faults. Subsequently, motor operating temperatures are reduced by 10 to 20 degrees and motor noise is reduced due to the removal of some of the high frequency harmonic currents

  • Can I use Active Harmonic Filter for single phase loads

      An active harmonic filter cannot be used to correct harmonics from single-phase harmonic sources. AHF’s correct the harmonics from three-phase sources and therefore are also only designed to run on three-phase systems.

      Isolation transformers and line reactors can mitigate some of these harmonics from single-phase sources. Three-phase system with large loads of single-phase harmonic sources can also use Harmonic Mitigating Transformers (HMT).

  • How does a Line Reactor extend the life of switching components?
  • What Type of Enclosure do I need for my Centurion® R Reactor?
  • What is the Difference between UL Listed and UL Recognized?

      The UL Listed Mark on a product is the manufacturer’s representation that samples of that complete product have been tested by UL to nationally recognized safety standards and are found to be free from reasonably foreseeable risk of fire, electric shock and related hazards.  UL’s Component Recognition Service covers the testing and evaluation of component products that are incomplete or restricted in performance capabilities.  These components will later be used in complete end products or systems Listed by UL.

  • How do the Centurion® R Reactors Have a 600 Volt Rated Insulation System but can be used as a 690 Volt Reactor?
  • Which Centurion® R Reactors are available as UL Listed Products?
  • What information does the TruWave Active Harmonic Filter display give
  • benefit of vacuum pressure impregnation

      All HPS transformers and reactors are vacuum impregnated with a “Polyester Resin” and oven cured which seals the surface. Impregnating the entire unit provides a strong mechanical bond and offers protection from environmental conditions.

      Using a vacuum during this process eliminate moisture and provides deeper penetration of insulation into transformer cavities.

  • Why are the Centurion R® Enclosures so much larger than the Reactor?
  • Can You Use a Control Transformer Connected in Reverse

      Theoretically, yes. However, the output voltage will be less than its rating, due to the voltage compensation factor of the windings. Care must be taken in sizing fuses due to higher inrush when back-feeding. Back-feeding may also violate the 2014 NEC Code unless it is specifically allowed on on the nameplate.

  • What is the difference between an Air Core Reactor and an Iron Core Reactor?

      Air Core:

      They are used primarily as current or voltage limiting devices, particularly where large currents can enter a system that uses small amounts of power. An example is the telephone system, which uses very small voltages where the current in a fault condition needs to be kept to a minimum.

      Iron Core:

      An iron core reactor provides the same current or voltage control on a system as its air core counterpart. Iron core units tend to be used on smaller applications where the variables need greater or more sensitive control.

  • What is the Advantage of having a UL Listed Centurion® R Reactor?

      The UL Listed mark provides a higher level of acceptability.  Centurion® R Reactors that are UL Listed meet the same safety standards but are viewed differently by Underwriters Laboratories.  They consider UL Listed products as being complete end-products, versus components that will be used as part of a larger system.  While a UL Recognized Reactor may adequately address a system’s needs, the field inspection may require UL Listed products in a given installation.  UL Listed Reactors meet a broad range of installation requirements.  UL Recognized products may require an addition to a user’s UL file, whereas UL Listed products may not.

      Learn more about HPS Centurion R

  • What Type of Fuses are Recommended for HPS Control Transformers?
  • 690 Volt Centurion R Reactors UL Listed
  • What is a Control Transformer?

      A control transformer is an isolation transformer that provides good voltage regulation, and is also designed to provide a high degree of secondary voltage stability (regulation) during a brief period of overload condition (also referred to as “inrush current”). Control transformers are also known as Machine Tool Transformers, Industrial Control Transformers or Control Power Transformers.

  • What is ANSI NETA ATS-2017
  • What Will the Impedance be at 690 Volts?
  • As an autotransformer, how can a Buck-Boost transformer supply loads significantly higher than its nameplate rating as a low voltage lighting isolation transformer?

      With an autotransformer, only a portion of the current acts as a load on the transformer.  This portion is roughly proportional to the voltage change.  If you increase the voltage from 100VAC to 120VAC, you are roughly adding 20% (20/100).  As a result, only about 20% of the current acts as a load on the transformer.  This would mean that a 500VA low voltage lighting transformer used in an autotransformer (Buck-Boost) application like this could provide a 2500VA (2.5kVA) load even though the nameplate is only rated for 500VA (.5kVA)

      This is a function of changing the voltage by a small amount. For example, if the transformer is connected in such a way that 22 volts is added to a 208 volt primary, a 230-volt output will result.  Only a portion of the current goes through a buck-boost autotransformer roughly equivalent to the voltage change.  As a result, if a buck boost transformer changes the voltage by 10%, only 10% of the current (kVA) go through the unit.  Therefore a transformer rated for 1 kVA when used as an isolation transformer could handle a 10 kVA load if it adjusted voltage by 10% because only 10% of the total load would go through the unit.

      Using this example, the calculation for autotransformer kVA is as follows:

      KVA = (Output Volts x Secondary Amps)/1000

      KVA = (230V x 41.67 Amps)/1000 = 9.58 KVA

  • What is the base temperature rise of a transformer

      The base temperature rise of a transformer is the maximum temperature rise at the expected full load capacity. Transformers can be built to run cooler than the base temperature rise, these are typically referred to as lower temperature rise transformers which can either operate in higher ambient temperatures or have additional service factor.

      • 105C Insulation System: 55C Base Temperature Rise
      • 150C Insulation System: 80C Base Temperature Rise
      • 180C Insulation System: 115C Base Temperature Rise
      • 220C Insulation System: 150C Base Temperature Rise

      The Hot Spot Allowance is added the expected ambient temperature and full load temperature rise to get the total expected temperature rise of a transformer.

  • Buck-Boost transformers are almost always installed as autotransformers. Does the National Electrical Code (NEC) permit the use of autotransformers?

      Autotransformers are very common and recognized by all the safety and standard authorities.

      You can refer to N.E.C. Article 450-4, “Autotransformers 600 Volts, Nominal, or Less”, as a reference publication. Item (a) details over-current protection for an autotransformer, and Item (b) covers an isolation transformer being field connected as an autotransformer for a Buck-Boost application.

  • Do Buck-Boost transformers present a safety hazard compared to conventional autotransformers?

      Buck-Boost transformers only change voltage by a small amount, such as 208 to 240 volts. This small increase does not represent a safety hazard. Conventional autotransformers, manufactured as single winding transformers, change much higher magnitudes of voltage, e.g. 480 to 240 volts. In a system where the line is grounded, it is possible to have 480 volts to ground when the expectations are that 240 volts is at the output. For this reason, qualified personnel only should maintain conventional autotransformers.

  • Do I need to connect the neutral and ground my HPS three-phase autotransformer?

      If the application needs a neutral (including 3 phase 4 wire systems), the autotransformer must be ordered with the optional neutral terminals (“3L0U” suffix).

      This option will provide the customer with a common (H0/X0) neutral connection point that is connected by the factory to the middle point of the Y winding configuration.

      When selecting this option, both the Line and Load side neutral cables must be connected to the respective neutral terminals in order to ensure the proper operation of the autotransformer.

      HPS does not recommend that the transformer H0/X0 point be grounded locally.

      When an autotransformer without neutral connections is selected, typically the neutral is grounded at the source transformer secondary and is properly referenced throughout the whole installation and carried through to the end load downstream the autotransformer.

      When installing an autotransformer with neutral connections problems can occur when the X0 point of the autotransformer is grounded locally. In such cases a multiple grounding situation may occur which would be against the electrical codes in North America.

      In the above case typically the upstream transformer secondary is grounded at the X0 point of the Y secondary (GND1), in the meantime grounding the X0 point of the autotransformer would create a secondary ground (GND2). Since the two grounds are typically in two different locations, likely far away from each other they will be at different ground potentials.

      This situation can create a number of issues including:

      With the two grounds at different potentials, if the autotransformer center point (X0) is used as a neutral, the line voltages compared to that local neutral would be unbalanced. The extent of the unbalance would depend on the extent of the potential difference between the two grounds (GND1 and GND2). This unbalance could cause issues with the equipment connected to the autotransformer.

      Grounding the X0 of the autotransformer will force the center point of the Y to be always at a certain potential, defined by the local ground. However the voltages of the lines coming into the autotransformer are referenced to the ground point of the upstream transformer. The likely scenario is that the two grounds will be at different potentials which will result in conflicting reference points at the autotransformer. The autotransformer and the electrical system will try to resolve the conflict and equalize the two ground points. The only way that can happen is by having ground current flowing between the two grounds.  Depending on how much of a difference in voltage potential there is between the two grounds and also depending on the ground resistances, there can be a significant current flow through the wye center points.  Adding to this fact that the impedance of an autotransformer is typically low, there could be enough current through the autotransformer to burn out one or more coils of the autotransformer.

      The effects and resulting problems that occur due to improper grounding can be unpredictable and manifest themselves differently in time. Ground potentials can greatly vary depending on environmental conditions.  After installing an autotransformer and grounding the center point of the wye (X0) problems may not surface initially.  However, there is a chance that after a rainstorm or some other event, all of a sudden the user experiences high ground currents just because the grounding conditions have changed.  These problems could be very intermittent in nature and hard to diagnose.

      When an autotransformer with neutral connections is requested, we do not recommend the grounding of the X0 point and recommend that the customer and installing contractor should refer to the local electrical code requirements for grounding and the short circuit protection of a three phase autotransformer.

  • How do I use a Grounding Transformer

      Three-phase grounding transformers provide an artificial neutral for grounding. The main requirement is a specific zero-sequence impedance from the Zig-Zag or the Wye/Delta transformer in addition to the fault current withstand rating. For grounding purpose, only the Zig-Zag or a Wye/Delta connected transformer can be used. Autotransformers will have a high zero-sequence impedance and hence, cannot be used for grounding. Air-core reactors can be normally connected between the artificial neutral and ground to provide some additional current-limiting impedance.

      Grounding transformers are often required by the utility to attach a load generator such as solar, wind or a generator to the power grid.

  • What is a Lamination?
  • What is a Harmonic?

      A sinusoidal component of a periodic wave having a frequency that is a multiple of the fundamental frequency. A component whose frequency is twice the fundamental frequency is referred to as the 2nd harmonic, (120 Hz is the 2nd harmonic of 60 Hz).

      It is recommended that transformers which experience high harmonic levels be specified with a K-rating.

  • What is a Step-Up Transformer?

      A transformer in which the low voltage winding (secondary) is connected to the input or power source and the high voltage winding (primary) is connected to the output or load.

  • What does the term Hertz (Hz) mean?
  • What are the advantages and disadvantages of an open delta transformer configuration ?

      Open-delta configurations are typically deployed by utilities, typically to loads with little single-phase requirements.


      • Open-deltas only require the utility to install two transformers.
      • Future Capacity can be increased by simply installing a third similar sized transformer verses installing 2-3 larger transformers.


      • While the line to line voltages will be equal, the line to neutral voltages will have two phases being equal and one phase being 1.732 times larger.
      • Unbalanced single phase loads can cause voltage fluctuations and additional, uneven transformer heating.
      • An open delta connection only has 58% of the capacity of a full set of three transformers, that is a 42% decrease in actual capacity event though the installed capacity only drops by 33%.


  • What are non-linear loads and why are they a concern today?

      A load is considered non-linear if its impedance changes with the applied voltage. The changing impedance means that the current drawn by the non-linear load will not be sinusoidal even when it is connected to a sinusoidal voltage. These non-sinusoidal currents contain harmonic currents that interact with the impedance of the power distribution system to create voltage distortion that can affect both the distribution system equipment and the loads connected to it.

      In the past, non-linear loads were primarily found in heavy industrial applications such as arc furnaces, large variable speed drives, heavy rectifiers for electrolytic refining, etc. The harmonics they generated were typically localized and often addressed by knowledgeable experts.

      Times have changed. Harmonic problems are now common in not only industrial applications but in commercial buildings as well. This is due primarily to new power conversion technologies, such as the Switch-Mode Power Supply (SMPS), which can be found in virtually every power electronic device (computers, servers, monitors, printers, photocopiers, telecom systems, broadcasting equipment, electric vehicle chargers, etc.). The SMPS is an excellent power supply, but it is also a highly non-linear load. Their proliferation has made them a substantial portion of the total load in most commercial buildings.

      More Harmonic Mitigating Transformer Frequently Asked Questions

  • What are Knockouts and what are they used for?
  • Does it Matter Which Way I Wire Through the Centurion® R Reactor?
  • Does HPS build primary unit substation transformers?

      Primary Unit Substation Transformers are located outside and typically range from 750 kVA to 10,000 kVA three-phase. The Primary voltage ranges from 2400 VAC to over 100,000 VAC and the secondary from 2400 VAC to 34,500 VAC. Taps are typically manually changed while the unit is de-energized. The primary connections are typically delta using bushings, throats or air terminal chambers.

      While often oil, HPS dry-type transformers can offer similar performance with a lighter weight and fewer environmental concerns.

  • What are Insulating Materials?

      Those materials used to electrically insulate the transformer’s windings, turn-to-turn or layer-to-layer, and other assemblies in the transformer such as the core and busswork.  Nomex, polyester, epoxy, rubber, Glastic and plastic are commonly used as insulating materials in the electrical industry.

  • What is Voltage Regulation?

      Voltage regulation is the difference between the transformer secondary No-Load and Full-Load voltage with respect to its Full-Load voltage.  Essentially, every transformer has a voltage drop caused by its own impedance (which is composed of its winding resistive and inductive properties).  Therefore, at different voltages and loading conditions, this internal voltage drop across the transformer windings will vary and ultimately will affect the final secondary output voltage.

  • What is Volt-Amperes (VA)?

      The VA or volt-ampere output rating designates the output that a transformer can deliver for a specified time at its rated secondary voltage and rated frequency, without exceeding its specified temperature rise.  It is the current flowing in a circuit multiplied by the voltage of the circuit.

  • Define Vacuum Pressure Impregnation?

      A vacuum and pressure impregnation process using a resin that is then oven cured to completely seal and protect the surface of a transformer and provides a strong mechanical bond. This process is standard on all HPS transformer products.

  • How does a UPS transformer’s efficiency affect the overall system efficiency?