Dr.InPhase Edu Series – Active Harmonic Filter

Howdy!! Everyone

Active Harmonic Filter Education seriesIt is Dr.InPhase Power again. Our today’s edu series will focus on educating our readers better about Active Harmonic Filter. Our team has taken efforts to list down the 16 mostly asked question about Active Harmonic filter and the corresponding answers. I hope it gives you better understanding of harmonic filters. Should you have more questions we are happy to answer it mail us to info[@]inphase.in. I will be happy to answer.


  1. What are harmonics?

In an electrical circuit, when a fundamental AC sine wave gets distorted, it causes additional frequency which are integer multiple of fundamental frequency. As per Indian standards, fundamental frequency is 50Hz. So the frequency of the harmonics are as follows 150Hz (3rd order), 250Hz (5th order) and so on.

  1. Effects of Harmonics
  • Conductor over heating
  • Overload and premature ageing of Power Factor Correction capacitors
  • False or spurious operations of fuses and circuit breakers
  • Increase in iron and copper losses or eddy current due to stray flux losses at transformers
  • Operation instability at generators
  • Energy meters may record incorrectly resulting in excess billing to customers
  • Computer and its peripherals may experience interference or failure
  1. Which order harmonics has to be mitigated and monitored?

The most significant harmonic orders in three-phase distribution networks are the odd orders (3, 5, 7, 9, 11, 13 ….)
Triple-n harmonics (order multiple of 3) are present only in three-phase, four-wire systems, when single phase loads are connected between phase and neutral.
Utilities are mainly focusing on low harmonic orders (5, 7, 11, and 13).Generally speaking, harmonic conditioning of the lowest orders (up to 13) is sufficient. More comprehensive conditioning takes into account harmonic orders up to 25.Harmonic amplitudes normally decrease as the frequency increases. Sufficiently accurate measurements are obtained by measuring harmonics up to order 30.

  1. What are the different harmonic mitigation techniques?
    • Isolation transformers
    • Line reactors
    • Harmonic filters (Active harmonic filter and Passive harmonic filters)
  1. What is a  Active Harmonic Filter / Passive Filter? 

Passive Filters:
Based on the harmonic analysis or report.  A LC circuit is tuned and connected parallel to each non-linear loads to remove the respective harmonic orders. Thus preventing the harmonics to flow in the distribution network

Active Filters:
Power electronic conditioners are connected across each non-linear loads that produces harmonics. This filters injects in opposite phase the harmonics drawn by the non-linear loads resulting in pure sinusoidal current.

  1. Why active harmonic filter are preferred instead of passive filter
  • Active Harmonic Filter analyses the load by itself and one need not know the harmonic in the load as in case of passive harmonic filter.
  • Active Harmonic Filter is immune to the main frequency variation whereas the passive harmonic filter is very much sensitive to the frequency of the mains power.
  • Active Harmonic Filter can take care of the changes in the non-linearity of the load current whereas Passive Harmonic Filter can’t take care of the changes in the load current pattern.
  • Active Harmonic Filter can filter the harmonics up to 31st harmonic. The Passive Harmonic Filter is very difficult to tune as the harmonics beyond 11th. And it is ineffective in case of harmonics beyond 11th harmonic.
  1. A) How to detect harmonics in the electrical system

Occurrences like flickering of lights, alarms going off, or MCB’s, MCCB’s, RCD’s and Earth Leakage devices tripping for no apparent reason.

Cables getting hot, hot switchboards or overheating motors. Replacing motor’s bearings & insulation often is a strong indication of the presence of harmonics.

  1. B) What are the essential indicators of harmonic distortion and measurement principles?

Power Factor, Crest Factor, Harmonic spectrum and R.M.S

  1. Do Active power filter help in improving the power factor?

Real power is used to perform real work, but Inductive loads require Real and Reactive power of their operation. Utilities provide apparent power (Apparent Power = Real Power + Reactive Power) Reactive power performs no work. However, the flow of reactive current, a component of reactive power, does consume energy as it passes through resistive elements of the power system, thus reducing overall system efficiency. This reactive power is used to generate magnetic fields within motors, transformers, and other magnetic devices. Reactive power, combined with harmonic currents, contribute to poor power factor in electrical systems. The capacitors inherent in harmonic filters design, supply the necessary reactive power so the utility doesn’t have to. The reduction in harmonic currents further improves the ratio of active power to apparent power. This overall improvement to true power factor assists in the efficient operation of facilities and the avoidance of possible poor power factor.

  1. Why to improve Power Factor?
  • Reduction in cost of electricity
  • Reduction in cable size
  • Reduction in losses
  1. Can I use Active Harmonic Filter on multiple connected systems

Harmonic filters are designed to focus on drive-applied applications but can be sized for multiple motor systems as well. Caution must be taken to size the unit properly as well as to take all of the power system variables into consideration.

  1. Can I use Active Harmonic Filter when the loads are powered by the Generator?

You can use on generator power; however it is more critical to size the device properly. If the nonlinear load represents a great portion of the total generator load and the drive is very lightly loaded, the filter capacitance may cause the generator to experience voltage regulation issues. You may also contact the generator provider for their guidelines on the use of harmonic mitigation and capacitive equipment on their systems.

  1. Does Active harmonic filter available for 240V systems?

One can use a three phase reactor in a single phase circuit that will perform limited mitigation

  1. Why power factor analysis has to be done before installing capacitors?

Harmonic analysis has to be performed before installation of power factor improvement capacitor to ensure that resonance frequency (Harmonic Resonance) do  not correspond with prominent harmonics contained in current and voltage. It would affect / damage capacitor banks when offending frequency is large enough during resonant condition.There is high probability that other loads would also get damaged.

  1. Where to place the Active Harmonic Filter for harmonic mitigation?
    • At the load
    • At the transformer
    • At the primary distribution

The best place to remove harmonic currents is often at the nonlinear loads that generate harmonic currents or, alternately, at the sub-panels that supply the branch circuits. The object is to reduce harmonic current at a point that is as-close-as-possible to the harmonic generating nonlinear loads and also it depends on different applications. The reduction of harmonic current will result in the reduction of ‘penalty’ losses.

  1. What are the factors to consider when specifying an active harmonic filter?

Losses, harmonic compensation capacity, response speed, Inter harmonics, EMC, De-rating, physical footprint, modularity, commissioning software, HMI, Smart grid functionality, IP/NEMA class and water cooling, voltage

  1. What does K-Rating of a transformer mean?

K-Factor rating, applied to transformers, is an index of the transformer’s ability to supply harmonic content (%THDI) in its load current while operating within its temperature limits. K-Rated transformers are only intended to survive in a harmonic rich environment. They do not mitigate harmonic currents or voltages.

Meet you again with a interesting read. until then bye bye!! from Dr.InPhase Power. Your Power Quality Partner.Doctor welcome








At InPhase Power we are very serious about improving the power quality of the nation. Our products provide unmatched improvement in Power Quality by effectively curbing harmonics and maintaining steady power factor. Explore our Products Active Harmonic Filter(IPC150-AHF) and STATCOM (IPC150-SCOM) to find how they can improve your power quality.

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