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Active Harmonic Filter


  • 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.

  • 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 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

  • 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 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 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).

  • Are there standards that can help in addressing harmonics?
  • 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
  • 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