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How do Harmonic Mitigating Transformers save energy?

Harmonic Mitigating Transformers reduce harmonic losses in the following ways:

1. Zero phase sequence harmonic fluxes are cancelled by the transformer’s secondary windings. This prevents triplen harmonic currents from being induced into the primary windings where they would circulate. Consequently, primary side I2R and eddy current losses are reduced.

2. Multiple output HMTs cancel the balanced portion of the 5th, 7th and other harmonics within their secondary windings. Only residual, unbalanced portions of these harmonics will flow through to the primary windings. Again I2R and eddy current losses are reduced.

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  • 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.

  • 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.

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  • 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.

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  • Are there standards that can help in addressing harmonics?
  • Capacitor switching reactor – definition
  • 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.

  • Line reactor – definition
  • Iron core filter reactor – definition
  • 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.

  • Shunt reactor – definition
  • Air core filter reactor – definition
  • Neutral grounding reactors – definition
  • What is NEMA ST 20?
  • DC smoothing reactor – definition
  • VAR compensator reactor – definition
  • 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

      OR

      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 ANSI C57.12.01?
  • What are K-Factor Transformers and where are they used?

      K-factor transformers are designed to withstand the extra heating and higher neutral currents caused by harmonics created by non-linear loads such as VFD, DC power supplies and LED lighting. K-factor distribution transformers in North America are subject to minimum efficiency regulations in both the U.S.A. and Canada. K-Rated transformers must have:

      • Operate at specific K-rated harmonics without overheating
      • 200% rated neutral
      • Electrostatic Shield

  • 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?
  • 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.

  • 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?
  • How do Harmonic Mitigating Transformers save energy?

      Harmonic Mitigating Transformers reduce harmonic losses in the following ways:

      1. Zero phase sequence harmonic fluxes are cancelled by the transformer’s secondary windings. This prevents triplen harmonic currents from being induced into the primary windings where they would circulate. Consequently, primary side I2R and eddy current losses are reduced.

      2. Multiple output HMTs cancel the balanced portion of the 5th, 7th and other harmonics within their secondary windings. Only residual, unbalanced portions of these harmonics will flow through to the primary windings. Again I2R and eddy current losses are reduced.

      More Harmonic Mitigating Transformer Frequently Asked Questions

  • How do Harmonic Mitigating Transformers reduce voltage distortion?

      Delta-wye transformers, even those with a high K-factor rating, generally present high impedance to the flow of harmonic currents created by the non-linear loads. Non-linear loads are current sources that push the harmonic currents through the impedances of the system. Any voltage drop across the impedance of the transformer at other than the fundamental frequency (60 Hz) is a component of voltage distortion.

      Because of its higher impedance to harmonic currents, the voltage distortion at the output of a delta-wye transformer often reaches the 8% maximum voltage distortion limit recommended by IEEE Std. 519-2014 by the time that the secondary side load has reached just one-half of full-load RMS current. At closer to full-load, these transformers can produce critically high levels of voltage distortion and flat-topping at their outputs and at the downstream loads.

      To minimize the voltage distortion rise due to the transformer itself, Harmonic Mitigating Transformers (HMTs) are designed to reduce the impedance seen by the harmonic currents. This is accomplished through zero sequence flux cancellation and through phase shifting. The secondary winding configuration of the HMT cancels the zero sequence fluxes; those produced by the 3rd, 9th, 15th (triplen) current harmonics, without coupling them to the primary windings.

      This prevents the triplen current harmonics from circulating in the primary windings as they do in a delta-wye transformer. The flux cancellation also results in much lower impedance to the zero sequence currents and hence lower voltage distortion at these harmonics. In addition, the reduced primary winding circulating current will lower losses and allow the transformer to run cooler.

      The remaining major harmonics (5th, 7th, 11th, 13th, 17th & 19th) are treated to varying degrees through the introduction of phase shifts in the various HMT models.

      Single output HMTs are offered in 0° and -30° models to provide upstream cancellation of 5th, 7th, 17th and 19th harmonic currents on the primary feeder.

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  • What is a SMPS and how does it generate harmonics?

      The Switch-Mode Power Supply (SMPS) is found in most power electronics today. Its reduced size and weight, better energy efficiency, and lower cost make it far superior to the power supply technology it replaced.

      Electronic devices need power supplies to convert the 120VAC receptacle voltage to the low voltage DC levels that they require. Older generation power supplies used large and heavy 60 Hz step-down transformers to convert the AC input voltage to lower values before rectification. The SMPS avoids the heavy 60 Hz step-down transformer by directly rectifying the 120VAC using an input diode bridge. The rectified voltage is then converted to lower voltages by much smaller and lighter switch-mode dc-to-dc converters using tiny transformers that operate at very high frequency. Consequently the SMPS is very small and light.

      The SMPS is not without its downside, however. The operation of the diode bridge and accompanying smoothing capacitor is very nonlinear in nature. That is, it draws current in nonsinusoidal pulses at the peak of the voltage waveform. This non-sinusoidal current waveform is very rich in harmonic currents.

      Because the SMPS has become the standard computer power supply, they are found in large quantities in commercial buildings. Acting together, the multitude of SMPS units can badly distort what started out as a sine wave voltage waveform.

      Twice per cycle every SMPS draws a pulse of current to recharge its capacitor to the peak value of the supply voltage. Between voltage peaks the capacitor discharges to support the load and the SMPS does not draw current from the utility. The supply voltage peak is flattened by the instantaneous voltage drops throughout the distribution system caused by the simultaneous current pulses drawn by the multiple SMPS units. The expected sine wave with a peak of 120 x √2 = 169.4V instead starts to resemble a square wave. The flattened voltage waveform contains a lowered fundamental voltage component plus 3rd, 5th, 7th, 9th and higher voltage harmonics.

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  • 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.

  • Do different types of non-linear loads generate different harmonics?

      By far the majority of today’s non-linear loads are rectifiers with DC smoothing capacitors. These rectifiers typically come in 3 types:
      (i) single phase, line-to-neutral
      (ii) single phase, phase-to-phase
      (iii) three-phase

      Single-phase line-to-neutral rectifier loads, such as switch-mode power supplies in computer equipment, generate current harmonics 3rd, 5th, 7th, 9th and higher. The 3rd will be the most predominant and typically the most troublesome. 3rd, 9th and other odd multiples of the 3rd harmonic are often referred to as triplen harmonics and because they add arithmetically in the neutral are also considered zero sequence currents. Line-to-neutral non-linear loads can be found in computer data centers, telecom rooms, broadcasting studios, schools, financial institutions, etc.

      208V single-phase rectifier loads can also produce 3rd, 5th, 7th, 9th and higher harmonic currents but if they are reasonably balanced across the 3 phases, the amplitude of 3rd and 9th will be small. Because they are connected line-line, these loads cannot contribute to the neutral current. The largest current and voltage harmonics will generally be the 5th followed by the 7th. Typical single phase, 208V rectifier loads include the switch-mode power supplies in computer equipment and peripherals.

      Three-phase rectifier loads are inherently balanced and therefore generally produce very little 3rd and 9th harmonic currents unless their voltage supply is unbalanced. Their principle harmonics are the 5th and 7th with 11th and 13th also present. They cannot produce neutral current because they are not connected to the neutral conductor. The rectifiers of variable speed drives and Uninterruptible Power Supplies (UPS) are typical examples of three-phase rectifier loads.

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  • 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 a Harmonic Mitigating Transformer and how is it different than a K-Rated Transformer?

      Harmonic Mitigating Transformers, or HMTs, are specifically designed to minimize the voltage distortion and power losses that result from the harmonics generated by non-linear loads such as personal computers. The accomplish this through the use of a zig-zag winding.

      K-rated transformers, on the other hand, are simply designed to prevent their overheating when subjected to heavy non-linear loading, but do very little to reduce the harmonic losses themselves. And as for voltage distortion, K-rated transformers perform the same as conventional general purpose delta-wye transformers.

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  • 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).

  • 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.

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  • Are electrical systems safe from harmonics if K Rated transformers?

      K-Rated transformers made their appearance several decades ago as a means of preventing transformers from failing when subjected to heavy non-linear loading. They are essentially ‘beefed up’ transformers with extra steel in their cores and conductors in their windings to allow for better dissipation of the excessive losses produced by harmonic currents. K-Rated transformers are not designed to cancel harmonics or their fluxes and therefore, do nothing but protect themselves from overheating.

      Harmonic losses are normally not significantly reduced and voltage distortion will typically remain quite high under more heavily loaded conditions. To improve power quality in the form of reduced voltage distortion and to save energy costs, the use of a transformer designed to cancel harmonics is necessary.

      Over-sizing neutrals, on the other hand, can be a reasonably low cost method for the prevention of neutral conductor overheating. It is important to remember that the non-linear loads are the source of the harmonic currents. They must flow from the loads back to the transformer. Because the 3rd and 9th current harmonics created by the 120 VAC switch-mode power supplies are flowing back on the neutral, the neutral current is usually larger than the phase currents. This is of minimal consequence provided the neutral has suitable ampacity to carry the extra current and the 120/208V 4-wire run length is not too long.

      When selecting phase and neutral conductor sizes in a non-linear load application, the electrical code requires that an ampacity adjustment or correction factor be applied. This is because the neutral conductor is considered to be a current carrying conductor along with Phase A, Phase B and Phase C. With more than 3 current carrying conductors in a conduit or raceway, a 0.8 factor must be applied.

      To minimize harmonic problems in new installations, avoid the old approach of using a large central transformer with a 120/208V secondary and long 4-wire risers or radial runs through the building. The impedances of these long runs are high so that harmonic currents flowing through these impedances will create high levels of voltage distortion and neutral-to-ground voltage. To prevent these problems, an effective rule of thumb is to limit each 120/208V run length to that which would produce a 60Hz voltage drop not greater than 1/2% to 3/4%. For a typical 200 amp feeder this would be < 50 ft.

      Combining the use of Harmonic Mitigating Transformers with short 120/208V feeder runs and double ampacity neutrals will ensure compatibility between the distribution system and the non-linear loads. Generally this will keep voltage distortion safely below the maximum of 5% as recommended for sensitive loads in IEEE Std 519-2014.

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  • What is a dual output Harmonic Mitigating Transformer?

      Like a standard HMT, a dual put HMT minimizes the voltage distortion rise due to the transformer itself. Harmonic Mitigating Transformers (HMTs) are designed to reduce the impedance seen by the harmonic currents. This is accomplished through zero sequence flux cancellation and through phase shifting. The secondary winding configuration of the HMT cancels the zero sequence fluxes; those produced by the 3rd, 9th, 15th (triplen) current harmonics, without coupling them to the primary windings.

      This prevents the triplen current harmonics from circulating in the primary windings as they do in a delta-wye transformer. The flux cancellation also results in much lower impedance to the zero sequence currents and hence lower voltage distortion at these harmonics. In addition, the reduced primary winding circulating current will lower losses and allow the transformer to run cooler.

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