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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 Output Problems Can Occur with Variable Frequency Drives (VFD or VSD) and How Can You Mitigate These Issues?

      A voltage-sourced Variable Frequency Drive (VFD) uses Insulated-Gate Bipolar Transistors (IGBTs) to rapidly switch voltage on and off to form a Pulse Width Modulated (PWM) voltage source for the motor. The PWM simulates a sine wave voltage source to the motor and it operates as if it was being powered by a sine wave.  The PWM wave allows the VFD to change the fundamental frequency of the PWM waveform and simulate sine waves.  Since the speed of a motor is directly related to the fundamental frequency of the sine wave, a VFD can control speeds from a fraction of a hertz to hundreds of hertz.

      Output reactors, dV/dT filters or drive isolation transformers can be used to help mitigate some issues caused by the PWM output.  PWM outputs cause rapid switching transitions which can cause over-voltages due to parasitic capacitance and inductance in the motor’s leads. The parasitic currents and voltages can be determined by the equation of V = L × (Δi/Δt).  VFD’s switching frequencies (the amount of pulses used to simulate the sine wave) generally range from 1,000-20,000 pulses per second.  IGBT’s produce an almost perfect square wave which produces a very high Δv/Δt.   High Δv/Δt can cause higher surge currents in the leads. This then causes high voltage pulses across the parasitic inductances.  Therefore the faster the pulses switch, the greater the impact of cable capacitance and inductance. These voltage pulses stress the motor’s windings causing higher audible noise, heat and possibly premature failure of the insulation. There is also capacitance in the motor’s bearings.  The combination of lubrication and air gaps prevent direct and continuous contact of the bearings to the metal traces that contain them.  Parasitic currents [I = C × (Δv/Δt)] causes current to flow through the bearings.  The amount of current will increase as the VFD output switching speed increases. These currents can cause micro pits to form in the bearings and eventually will lead to premature bearing failure.

  • 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.

  • Line reactor – definition
  • 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.

  • dV/dt reactor – definition
  • 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.

  • How bad can the Dirty Power problem get?

      One form of dirty power usually called a surge can burn out computer, audio, video or nay other electronic circuitry in seconds. A surge is a high voltage pulse riding the normal power wave. Surges will commonly measure 600 to 2500 volts. Even though they occur for only mille-seconds, this is enough time to melt down circuits.

  • How does Dirty Power affect my electronic equipment?

      Your computer operates by reading electronic impulses. Dirty power contains a great number of random pulses riding on the normally smooth surface of a power wave. As these random pulses enter the circuits, your computer ‘reads’ them as data. This can cause a whole range of problems. You may suddenly get garbled numbers or letters in a readout or printout.

      You could loose files, skip program steps, have trouble loading programs or have connection problems while on the Internet.

  • 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.

  • Are there voltage drop concerns when using a load reactor or dV/dT filter for long lead lengths?
  • Can a non-inverter duty rated motor be used if load reactor or dV/dT filter is installed?
  • 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?
  • What is EMF (Electric and Magnetic Fields)?
  • What Effects Does EMF (Electric and Magnetic Fields) Have on Equipment?
  • What is the difference between a Variable Frequency Drive’s (VFD) fundamental frequency and the VFD’s carrier (switching) frequency?

      A VFD’s fundamental frequency is the frequency of the output current, typically >0 to 400HZ for most VFD’s. This frequency determines the speed of the motor and is simulated by the drive’s higher carrier frequency output.

      The VFD’s Carrier (Switching) frequency is the frequency of the PWM (Pulse Width Modulation) pulses, i.e. the frequency at which the VFD’s output transistors; usually IGBTs (Insulated Gate Bipolar Transistors), are switching. This typically ranges from 2 to 20 kHz. This high frequency PWM current output simulates a lower frequency sine wave and provides the drive’s variable fundamental frequency output.

  • 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

  • Does the VFD have to be equipped with a DC link choke to work with Active Harmonic Filter
  • 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

  • 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.

  • 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.

  • 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).

  • Can and Active Harmonic Filter improve power factor
  • 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 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 are the main functions performed by an Active Harmonic Filter
  • What communication options come with the HPS TruWave Active Harmonic Filter
  • What does AHF stand for
  • 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 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

  • What information does the TruWave Active Harmonic Filter display give
  • common transformer installation issues

      Improper Secondary Ground
      If the secondary of the transformer is not grounded properly, the output voltage will look ok between the phases but it will float and not be referenced to earth ground.

      Back-Feeding Delta Primary/Wye Secondary Transformers
      While a base wye secondary transformer can be field modified to backfed, the field modifications may violate U.L., NEC or local code and the transformer’s warranty. Don’t back-feed delta/wye transformers.

      Back-Feeding Transformers above 1 kVA
      Back feeding larger transformers can result in high inrush currents upon transformer energization and nuisance tripping of circuit breakers and fuses. Due to a number of factors which affect inrush, this issue is difficult to predict and costly to fix. The best way to handle this is to purchase transformers wound as step-up. If this isn’t feasible, transformers should be sized to the maximum amperage protection allowed by code, the larger the transformer, the more potential for this to occur.

      Power Wires Routed over the core and coils
      The are being ventilated through the core and coils can be very hot, in excess of 100oC. This can cause wire insulation failure.

      Power Wires terminated in the bottom of the transformer compartment
      Conduit should not be terminated in the bottom of the transformer with a grated floor. The grated floor is needed to provide airflow to cool the transformer but the grates provide a poor surface to mount a coupling and may also violate NEC code.

      Missing Vibration Pads or Vibration Isolators
      All transformers vibrate at 120 hz because of the electromagnetic field in the core. These vibrations and audible noise can transfer through the floor, vibration pads and isolators help to minimize this issue in commercial applications.

      Missing Drip Shields
      While all outdoor applications need a minimum of a NEMA 3R enclosure, even indoor applications near sprinklers would require a minimum NEMA 2S enclosure and therefore drip shields.

      Transformer Harmonic Heating
      Due to the prevalence of non-linear loads and the harmonics they produce, transformers can overheat if not specified properly. As a rule of thumb, if a load contains 25-50% non-linear sources, use K=4, if a load exceeds 50% non-linear sources use K=13.

      Transformer Ambient Heating
      Transformers need to be placed in locations that allow proper ventilation to remove the heat they produce during normal operation.

      More troubleshooting

  • 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:


  • Iron core filter reactor – definition
  • Air core filter reactor – definition
  • 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%.

  • VAR compensator reactor – definition
  • 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 are Static Synchronous Compensators (STATCOMs)?
  • What are Static VAR Compensators (SVCs)?
  • What is Series Compensation?
  • 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.

  • Why is Clean Power so critical?

      Your computer is a delicate electronic instrument. When you use the keyboard, you’re sending a series of tiny electronic impulses through the computers circuits. The computer ‘reads’ these electronic impulses and makes calculations or performs tasks according to your programmed instructions. If the electrical power feeding your computer is smooth and clean, your computer will behave normally. However, if the power fed into your computer is “dirty”, you could be in for many unpleasant surprises.

      Practically all electronic devices are sensitive to fluctuations in voltage, therefore clean power is vital in order to ensure uninterrupted performance of modern-day electronic equipment.

  • Do I need a Wye-N connection to supply my solar inverter if it does not need or check for a balanced phase to ground voltage?
  • Which side of the transformer is primary and which side is secondary?

      From the transformer manufacturer’s perspective, the side on the transformer that is initially energized should be considered primary. HPS solar duty transformer identifies the primary side on its nameplate. Energizing the transformer from the secondary side may cause elevated energizing inrush currents that can cause nuisance faults.

  • How do you properly size a distribution transformer?

      Distribution transformers need to take several items into consideration when sizing including:

      • Maximum Load
      • Potential future load growth (typical is 25%)
      • Load Inrush and voltage regulation
      • Harmonics and Power Factor
      • Ambient Temperature
      • Additional Service Factor

      For reference, NEC Article 210, Branch Circuits, and NEC Article 230, Services is used to select panelboards and the size of branch circuits. Typically a transformer must be sized to support the load requirements of the switchgear, panelboards and branch circuits. For drive isolation transformers, it is suggested to take sizing charts provided by manufacturers into consideration due to derating for harmonics. In addition to sizing a transformer, the general types including general purpose, K-Rated, Harmonic Mitigating and Drive Isolation also need to be chosen.

      Distribution transformers are often sized from loads based on NEC Article 220. NEC Article 220.87 does allow transformers to be sized based on peak-load data over a 1-year period. NEC also allows loads to be sized using metered data over 30 days if the additional maximum anticipated heating and/or cooling load is also factored in. This often allows a transformer to be sized lower than the base calculations from NEC Article 220. Peak efficiency for 600V class distribution transformers is typically at 35%. Peak efficiency for medium voltage transformers is typically at 50% load.

      Additional capacity for future loads can be obtained by A) specifying a lower temperature rise (15%-30% for dry type) or B) utilizing fans (25%-50% for dry type).

  • What is ANSI NETA ATS-2017?
  • 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 are Electrostically Shielded Transformers and where are they used?

      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.

      Learn more about HPS Sentinel Series