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Reducing Harmonics with Acrel Active Power Filters for Better Power Quality
Harmonic distortion is one of the most common power quality problems in modern electrical installations. It often builds quietly as more variable speed drives, LED lighting, UPS systems, rectifiers, chargers, and switch-mode power supplies are added to a site.
The result can be distorted voltage, overheated equipment, nuisance tripping, transformer losses, neutral conductor stress, capacitor failures, and lower system efficiency. In severe cases, harmonics can shorten the life of assets that should otherwise run reliably for years.
Acrel Active Power Filters address this problem by measuring harmonic currents in real time and injecting an equal but opposite compensating current into the electrical system. The aim is simple: restore cleaner current waveforms, reduce distortion at the point of connection, and support better power quality across the installation.
Harmonics are a waveform problem caused by non-linear loads
In an ideal AC system, voltage and current follow smooth sine waves at the fundamental frequency. In Australia, that fundamental frequency is 50 Hz. Harmonics are frequency components that occur at integer multiples of that base frequency, such as 150 Hz for the third harmonic, 250 Hz for the fifth harmonic, and 350 Hz for the seventh harmonic.
These harmonics do not usually come from the supply itself. They are mostly created by non-linear loads. A linear load, such as a simple resistive heater, draws current in proportion to the voltage waveform. A non-linear load draws current in pulses or chopped segments. That current shape is no longer sinusoidal, even when the supply voltage looks reasonably clean.
Once distorted current flows through the impedance of cables, transformers, switchboards, and upstream supply equipment, it can also distort the voltage waveform. That matters because voltage distortion affects other equipment connected to the same electrical network.
Common harmonic indicators include:
Total Harmonic Distortion of current
Often written as THDi, this shows how distorted the current waveform is compared with its fundamental component.
Total Harmonic Distortion of voltage
Often written as THDv, this shows how much the voltage waveform has been distorted.
Individual harmonic levels
These show the size of specific harmonic orders, such as the 5th, 7th, 11th, or 13th.
Power factor and displacement power factor
These help separate phase shift issues from waveform distortion issues.
A site can have acceptable displacement power factor but still have poor power quality due to high harmonic current. That is one reason harmonic analysis should look beyond simple kW, kVA, and power factor readings.
Common sources of harmonics in electrical systems
Most commercial and industrial sites now include several harmonic-producing loads. One small device may not cause a major issue, but the combined effect across a switchboard can be significant.
Harmonic source | Why it creates distortion | Typical concern |
Variable speed drives | Rectifier front ends draw pulsed current | 5th and 7th harmonics are common in six-pulse drives |
UPS systems | Power electronics convert and condition supply | Distortion can appear during charging and load support |
LED lighting drivers | Switch-mode circuits draw non-sinusoidal current | High density installations can affect neutral loading |
EV chargers | AC to DC conversion creates non-linear demand | Harmonics vary with charger type and loading |
Solar inverters | Power electronics interface DC generation with AC systems | Harmonic behaviour depends on inverter design and site conditions |
Welding equipment | Arc characteristics and power electronics vary current rapidly | Flicker and harmonics may occur together |
Computer and server power supplies | Switch-mode supplies draw current in peaks | Large quantities can add up across distribution boards |
Rectifiers and DC supplies | Diode or thyristor bridges create characteristic harmonics | Higher distortion under heavy loading |
The harmonic profile depends on equipment type, loading, network impedance, transformer size, cable lengths, capacitor banks, and whether other filters already operate on the system.
For example, a manufacturing plant with many variable speed drives may show strong 5th and 7th harmonic currents. A building with many single-phase electronic loads may show triplen harmonics, such as 3rd, 9th, and 15th, which can accumulate in the neutral conductor of a three-phase four-wire system.
This is why harmonic mitigation should not rely on guesswork. Two sites with similar load ratings can have very different harmonic behaviour.
How Acrel Active Power Filters measure distortion
Acrel Active Power Filters work as shunt-connected power quality devices, installed in parallel with the load or at a relevant distribution point. They monitor the electrical system continuously, then respond to the harmonic current detected.
A typical active harmonic filter arrangement includes:
Current transformers installed on the supply or load conductors
Voltage sensing connections for waveform reference
A control system that analyses current and voltage signals
A power electronic inverter stage
Coupling inductors and protection devices
A display or communication interface for monitoring and settings
The current transformers are central to the measurement process. They feed real-time current signals to the controller. The controller compares the measured waveform with the ideal fundamental component. From that comparison, it determines which part of the load current is useful fundamental current and which part is unwanted harmonic current.
In practice, the controller tracks the harmonic spectrum and phase relationship of the distortion. It then calculates the compensation current needed to cancel the unwanted harmonic components.
This measurement happens dynamically. That is a key advantage of active filtering. Load conditions rarely stay fixed. A production line starts and stops. A lift motor accelerates. A bank of chargers ramps up. A chiller drive changes speed. The filter must respond to these changes as they occur.
Passive filters, which use tuned inductors and capacitors, can work well in stable systems with known harmonic frequencies. Active filters suit sites with changing loads because they can adapt their output in real time.
How compensating current improves the waveform
An active harmonic filter does not remove the non-linear load. The drive, rectifier, charger, or UPS still draws a distorted current. Instead, the filter supplies a compensating current that offsets the harmonic part of that load current.
Think of the load current as a combination of two parts:
The fundamental 50 Hz current needed to do useful work
Harmonic current that distorts the waveform and adds stress to the network
The active filter targets the second part. Its inverter produces current at the required harmonic frequencies, but in the opposite phase to the detected harmonic current. When the filter current and load harmonic current meet at the connection point, they cancel or greatly reduce each other.
The upstream supply then sees a cleaner current waveform.
This process can be summarised as:
The load draws non-sinusoidal current.
The filter measures the distorted current.
The controller identifies harmonic components.
The inverter generates opposing harmonic current.
The supply current becomes closer to a sine wave.
The result is usually seen as lower THDi upstream of the filter and, where network impedance allows, reduced voltage distortion as well.
Many active power filters can also support other power quality functions, depending on model and configuration. These may include reactive power compensation, phase balancing, and neutral current reduction in three-phase four-wire systems. The exact capability should always be checked against the selected Acrel model and project requirements.
Why system assessment matters before filtering
Effective harmonic filtering starts with a proper site assessment. Installing a filter without measurement can lead to poor results, incorrect sizing, or compensation at the wrong point in the electrical system.
A good assessment should answer several technical questions.
Where is the distortion coming from
The first task is to identify the main harmonic-producing loads. Measurements may be taken at the main switchboard, distribution boards, motor control centres, drive panels, or specific feeders.
This helps distinguish between site-wide distortion and local distortion. A harmonic issue at one motor control centre may not need filtering at the main switchboard. By contrast, distortion that affects the whole installation may call for compensation near the main low-voltage distribution point.
What harmonic orders are present
The harmonic spectrum matters. If the site mainly has 5th and 7th harmonic current from six-pulse drives, the filtering approach may differ from a site with high triplen harmonics from single-phase loads.
Knowing the dominant harmonic orders also helps assess risks such as capacitor resonance. Power factor correction capacitor banks can interact with network inductance. In some systems, this can amplify certain harmonic frequencies. Any existing capacitor bank should be reviewed before adding harmonic mitigation equipment.
How the load changes over time
A short snapshot may miss the real problem. Harmonic levels can vary across shifts, production cycles, seasons, or occupancy patterns.
For example:
A cold storage site may show different behaviour when refrigeration compressors ramp together.
A commercial building may have higher distortion during peak lighting and HVAC operation.
A factory may show short periods of severe distortion when several drives accelerate at once.
A site with solar may behave differently across the day as generation changes.
Power quality logging over a representative period gives a clearer picture of normal and worst-case conditions.
What point of common coupling needs improvement
Power quality limits are often assessed at the point of common coupling or another agreed connection point. Filtering near the source of harmonic current may protect internal equipment, while filtering near the main switchboard may reduce distortion exported to the supply.
The correct location depends on the goal. A plant may need to protect a sensitive process line. A building may need to reduce distortion at the main incomer. A site with multiple non-linear feeders may benefit from a central filter, local filters, or a combination of both.
Proper sizing is critical for reliable harmonic reduction
Sizing an active harmonic filter is not the same as sizing a circuit breaker or transformer. The filter rating must match the harmonic current it needs to compensate, not simply the total load current.
If the filter is too small, it may reach its output limit during peak distortion periods. Once that happens, it cannot inject enough compensating current, and harmonic levels remain above the desired target. The filter may still help, but it will not deliver the expected result.
If the filter is far larger than needed, the installation may carry unnecessary cost and take up more switchboard space than required. Good sizing balances performance, headroom, future load changes, thermal conditions, and installation constraints.
Key sizing factors include:
Measured harmonic current
Use real site data where possible. Estimate only when measurement is not practical, and allow suitable design margin.
Target THDi or THDv improvement
The required outcome affects the rating. Reducing severe distortion to acceptable levels may need more capacity than modest improvement.
Load diversity
Not all non-linear loads run at full output at the same time. Diversity can reduce the required filter rating, but it must be based on realistic operation.
Future expansion
Planned drives, chargers, production equipment, or solar inverters should be included in the design allowance.
Three-wire or four-wire systems
Three-phase four-wire systems may need neutral harmonic compensation, especially where single-phase electronic loads dominate.
Ambient temperature and enclosure design
Switchroom ventilation, panel layout, and heat dissipation affect long-term performance.
Connection voltage and supply configuration
The selected filter must match the site voltage, frequency, earthing system, and installation arrangement.
Short-circuit level and protection coordination
The filter installation must suit the available fault level and integrate correctly with upstream and downstream protection.
Current transformer selection and placement also matter. Incorrect CT orientation or location can cause poor compensation or unstable behaviour. The filter must measure the right current path. In multi-source systems, such as generator-backed supplies or sites with embedded generation, CT placement deserves extra care.
What to expect after installing an active filter
After commissioning, performance should be verified with measurements. The main indicators are waveform shape, THDi, THDv, individual harmonic orders, filter loading, and any change in equipment behaviour.
A well-applied active filter can help:
Reduce harmonic current seen by upstream transformers and cables
Improve current waveform shape
Lower heating caused by harmonic losses
Reduce neutral current where triplen harmonics are addressed
Improve compatibility with sensitive equipment
Support compliance with site or utility power quality requirements
Reduce stress on power factor correction equipment
The strongest results come from matching the filter to the actual electrical system. Acrel Active Power Filters can play a valuable role in reducing harmonics, but their performance depends on sound engineering around measurement, placement, sizing, protection, and commissioning.
Better power quality starts with measured decisions
Harmonics are not just a waveform curiosity. They represent real current flowing through real equipment, creating heat, losses, interference, and reliability risks. As more power electronics enter commercial and industrial sites, harmonic control becomes a practical design issue rather than a specialist afterthought.
Acrel Active Power Filters reduce this distortion by measuring harmonic current in real time and injecting compensating current that restores a cleaner upstream waveform. That makes them well suited to installations with changing non-linear loads, such as drives, UPS systems, LED lighting, EV charging, rectifiers, and inverter-based equipment.
The best results come from a disciplined process: assess the system, locate the sources, measure the harmonic spectrum, define the target, size the filter correctly, and verify performance after commissioning. When those steps are followed, active filtering can deliver a clear improvement in power quality and help electrical systems run with less stress.
