Power Quality & Harmonics
An active harmonic filter is a power-electronic compensation device. Unlike a passive filter built from fixed inductors, capacitors and resistors, it measures the distorted current in real time and injects an equal-and-opposite current so the source current becomes nearly sinusoidal. This final part of the series covers how it works, how it is sized and configured, and why CT location and network impedance still matter.
An active harmonic filter (AHF) reduces harmonic current distortion by measuring the distorted current in real time and injecting a compensating current into the system. Where a passive filter relies on fixed RLC components, the active filter is a controlled current source.
Section 1
If the nonlinear load current is \(i_L(t)=i_1(t)+i_h(t)\) — fundamental plus total harmonic current — the filter injects the opposite of the harmonic part, and the supply current is left with only the fundamental:
In practice compensation is not perfect — measurement delay, control bandwidth, switching frequency, CT accuracy, background distortion and rating limits all matter — but the concept holds: a controlled current cancels the unwanted harmonic components of the load current.
Section 2
A passive filter is a fixed impedance tuned by \(L\) and \(C\); its behaviour depends on system impedance and it can resonate if poorly coordinated. An active filter measures the waveform, computes the unwanted components and injects compensating current through an inverter:
| Feature | Passive Filter | Active Filter |
|---|---|---|
| Main principle | Fixed impedance at selected frequencies | Real-time current compensation |
| Main components | Capacitor, reactor, resistor | IGBT inverter, DC link, controller, CTs, output filter |
| Tuning | Fixed by \(L\) and \(C\) | Programmable and adaptive |
| Resonance risk | Possible if poorly coordinated | Normally non-resonant |
| Best for | Known dominant harmonics and reactive power | Variable loads and changing spectra |
| Losses / cost | Usually lower | Higher (power electronics) |
| Extra functions | Limited | Harmonics, reactive, unbalance, neutral current (model-dependent) |
Neither is always better. Passive filters can be the best choice for large, stable industrial sources with a known spectrum and a reactive-power need; active filters are preferred where the load is variable, several sources exist, future growth is expected, or passive resonance risk is hard to manage.
Section 3
The most common arrangement is the shunt active filter, connected in parallel with the nonlinear load. It measures the load (or source) current and injects compensating current at the connection point, so \(i_S(t)=i_L(t)+i_{AF}(t)\). With the load expressed as a Fourier series, the filter generates the negative of the harmonics and the source keeps only the fundamental:
A series active filter (much less common) is connected in series and mainly compensates voltage distortion to protect the load from a distorted supply:
| Arrangement | Compensates | Typical Purpose |
|---|---|---|
| Shunt active filter | Harmonic current | Protect the network from nonlinear loads |
| Series active filter | Harmonic voltage | Protect sensitive loads from a distorted supply |
Most industrial AHFs — for drives, UPS, EV chargers, rectifiers and mixed loads — are shunt-connected.
Section 4
| Component | Function |
|---|---|
| Current transformers | Measure source or load current |
| Digital controller | Calculates harmonic and reactive current components |
| DC-link capacitor | Provides the internal DC energy buffer |
| IGBT inverter | Generates the compensating current |
| Output LCL / interface filter | Smooths the inverter current and reduces switching-frequency components |
| Protection and monitoring | Guards against overload, temperature and abnormal operation |
The controller decomposes the measured current into the parts to keep and the parts to compensate:
and forms an output current command from the components selected for compensation:
Depending on settings and rating, the filter may compensate harmonics, reactive current, unbalance and sometimes neutral current or selected harmonic orders.
Section 5
Active filters run in one of two modes:
| Mode | Description | Typical Use |
|---|---|---|
| Overall | Reduces all detectable harmonics within the bandwidth | Mixed loads, general THD reduction |
| Selective | Targets selected harmonic orders | Known dominant harmonics, limited rating, capacitor coordination |
Selective compensation is valuable when the rating is limited or capacitor banks are present — it focuses the available current on the most important orders. A six-pulse rectifier produces \(h=6n\pm1\) (orders \(5, 7, 11, 13, 17, 19, \ldots\)), so selective compensation usually prioritises \(5, 7, 11, 13\), which dominate voltage distortion and resonance. A twelve-pulse converter produces \(h=12n\pm1\) (orders \(11, 13, 23, 25, \ldots\)) and is configured differently.
Section 6
The filter has a finite current rating \(I_{AF,\text{rated}}\). The current it would need to compensate everything at once is:
If \(I_{AF,\text{req}} > I_{AF,\text{rated}}\) the filter cannot compensate everything at once, and the priority setting decides how the available current is allocated — harmonic-first (then reactive with any spare capacity) or reactive-first (then harmonics). The rating must therefore be selected from the required compensation current, not from transformer kVA or load kW:
A margin should be added, because load conditions change and the spectrum is rarely constant:
| Factor | Reason |
|---|---|
| Load growth | Additional nonlinear loads may be connected later |
| Operating diversity | Several loads may operate simultaneously |
| Background distortion | The filter may absorb existing system harmonics |
| CT error | Measurement accuracy affects compensation |
| Temperature / altitude derating | Cooling capability reduces the usable rating |
| Generator operation | Higher source impedance may increase voltage distortion |
| Capacitor-bank operation | Resonance and current sharing may change |
Section 7
Compliance is checked at the point of common coupling (PCC). The aim is not perfectly sinusoidal internal currents but acceptable distortion at the PCC and protected internal equipment. The active filter reduces \(I_h\), so the harmonic voltage falls — but network impedance still governs the result:
If the filter fully compensates the harmonic current, \(I_{h,\text{AF}}\approx I_{h,\text{load}}\) and \(V_{h,\text{new}}\approx 0\); in practice a residual remains. Where the system resonates near a harmonic, even a small residual current can produce significant voltage, so a harmonic study may still be needed. The same \(V_h=Z_h I_h\) explains why a load fed from a generator can show higher voltage distortion than from a transformer — the generator's subtransient reactance gives a higher \(Z_h\):
| Supply Mode | Check Required |
|---|---|
| Utility transformer supply | Normal PCC harmonic limits |
| Generator supply | Higher voltage distortion from higher source impedance |
| Parallel transformers | Lower impedance but different harmonic sharing |
| Single-transformer outage | Higher impedance and possible resonance shift |
| Weak network | Increased voltage-distortion sensitivity |
For installations with standby generation, the AHF should be checked in both utility and generator modes.
Section 8
Correct CT location is critical — the filter can only compensate what it measures:
| CT Location | Meaning |
|---|---|
| Source-side / PCC-side | The filter tries to make the source current sinusoidal |
| Load-side | The filter measures the load current and injects the opposite harmonics |
If CTs are on the wrong side, or a load sits outside the measurement zone, compensation is poor; if capacitor current is wrongly included, the filter may respond in an unintended way. CT polarity, phase sequence, ratio, burden and accuracy all matter — reversed polarity can make the filter inject in the wrong direction. Active filters can work with capacitor banks, but the arrangement needs care, because a non-detuned bank can resonate with the source inductance and the filter is not a cure for a severe capacitor resonance:
| Requirement | Reason |
|---|---|
| Use detuned capacitor banks with nonlinear loads | Reduces resonance risk |
| Check the capacitor tuning frequency | Keep it out of the AHF compensation range |
| Use selective compensation where needed | Prevent unwanted interaction |
| Place CTs so capacitor current is not compensated | Avoid hunting or unintended operation |
| Frequency scan for all combinations | Confirm no dangerous resonance |
| Check capacitor RMS current and voltage | Ensure the capacitor is not overloaded |
Power-factor-correction capacitors should not be added blindly to a system with drives, rectifiers or active filters — their effect on harmonic impedance must be checked.
Section 9
Decomposing the load into fundamental active, fundamental reactive and harmonic parts, \(i_L(t)=i_{1p}(t)+i_{1q}(t)+i_h(t)\), shows that harmonic compensation and power-factor correction are not the same. With harmonics only, the source still carries the reactive current; with harmonics and reactive together, the source is left with active current alone:
As a concrete example, take a distorted current with a fundamental and four harmonics. The filter injects the negative of the harmonic part, leaving a clean fundamental:
The filter does not remove the fundamental active current — it removes the unwanted harmonic components. This is the core idea behind every active-filter waveform.
Section 10
| Advantage | Explanation |
|---|---|
| Adaptive compensation | Responds to changing load conditions |
| No fixed LC tuning | Lower risk of passive resonance |
| Parallel installation | Often retrofitted with limited system rework |
| Selective control | Can target selected harmonic orders |
| Combined functions | Harmonics, reactive compensation and load balancing may be possible |
| Modular expansion | Modules can be added for future growth |
| Good for variable loads | Suits VSDs, UPS, chargers, mixed electronic loads |
| Limitation | Explanation |
|---|---|
| Higher cost | Power electronics and controls are expensive |
| Limited current rating | Cannot compensate beyond its rated current |
| CT-dependent performance | Wrong CT location or polarity causes poor operation |
| Switching losses | Heat dissipation and ventilation required |
| Control bandwidth | Very high-order harmonics may not be fully compensated |
| Not a substitute for poor design | Severe resonance or unsuitable capacitors still need correction |
| Requires commissioning | Settings, priorities and CT configuration must be verified |
The headline advantage is flexibility; the headline caution is that an AHF is engineered equipment, not a plug-in device.
Section 11
The rating should be based on measured or calculated harmonic current, with reactive and unbalance added if required, then a margin applied:
For small LV jobs, manufacturer sizing tools and measurements may suffice; for larger or MV / utility-facing installations a harmonic study is needed, reviewing the inputs and running the study items below.
| Study Item | Purpose |
|---|---|
| Existing harmonic measurements | Establish background distortion |
| Harmonic source spectrum | Estimate injected harmonic currents |
| Frequency scan | Identify resonance and impedance peaks |
| Harmonic load flow | Calculate harmonic voltages and currents |
| Active-filter and capacitor models | Represent compensation limits and detuning |
| Source and generator impedance | Assess source strength |
| Operating scenarios | Normal, outage, generator, minimum fault level |
| PCC compliance and equipment duty | Verify limits and equipment ratings |
A frequency scan is useful even with an active filter: it reduces current injection, but the network impedance still determines voltage distortion and possible amplification.
Section 12
Harmonic limits are normally referenced to the PCC: IEEE 519 for voltage and current distortion at the user interface, IEC TR 61000-3-6 for emission allocation on MV/HV/EHV systems, and CIGRE guidance for frequency-domain network modelling. In short:
At commissioning, the most common real-world problem is not the filter technology but incorrect CT installation or a wrongly defined compensation zone. The key checks are:
| Check | Purpose |
|---|---|
| CT ratio, polarity, phase sequence | Correct scaling, direction and phase reference |
| CT location setting | Source-side or load-side compensation |
| Compensation mode and priority | Overall/selective; harmonic or reactive priority |
| Individual harmonic settings | Target the dominant orders |
| Capacitor-bank interaction | Avoid unintended response |
| Current rating and loading | Confirm the filter is not at its limit |
| Source-current waveform and PCC THD | Confirm improvement and compliance |
| Thermal performance, alarms and trips | Confirm ventilation, derating and protection |
An active harmonic filter is a controlled current source in parallel with nonlinear loads: it measures the unwanted harmonic current and injects an equal-and-opposite current, \(i_{AF}(t)=-i_h(t)\), so the source current \(i_S(t)=i_L(t)+i_{AF}(t)\) becomes nearly sinusoidal — and with reactive compensation enabled, more closely in phase with the supply voltage. It cancels current, not “magic distortion”; its rating must come from harmonic current rather than kW; CT location defines what it can do; and \(V_h=Z_h I_h\) means network impedance and capacitor banks still matter. A good AHF design must satisfy PCC limits, internal equipment duty, CT correctness and network frequency-response requirements together.
Four-Part Technical Series
A four-part guide to harmonic filters — passive filter arrangements, single-tuned filter design, the second-order damped filter, and the active harmonic filter for adaptive current compensation.
The active filter that cancels harmonic current in real time — shunt vs series, selective compensation, current rating, CT location and PCC compliance.
