Even-Order Harmonics: The Distortion Standard Filters Miss

Even-order harmonics are current and voltage components at even multiples of the fundamental frequency — the 2nd, 4th, 6th and higher orders — that appear as asymmetric distortion in a three-phase power system. They behave differently from the familiar 3rd, 5th and 7th odd harmonics: balanced six-pulse rectifiers do not generate them, but asymmetry does. Half-controlled bridges, DC-offset loads, saturated magnetics, unbalanced rectifier legs and lopsided thyristor firing angles all produce them.

Here is the failure that triggers most harmonic investigations: a plant installs a detuned passive filter bank, the THDi at the main breaker looks acceptable, yet transformers still run hot and motors still pit their bearings. Passive filters are tuned to specific odd orders, so the 2nd and 4th orders pass straight through to the transformer, the neutral and the point of common coupling (PCC).

An active harmonic filter (AHF) answers this differently. Instead of absorbing energy at fixed frequencies, it measures the load current through current transformers, calculates the harmonic content order by order, and injects an equal-and-opposite current so the distortion cancels at the measurement point. YT Electric builds this product category for industrial, commercial and infrastructure power systems.

Cause: Why Even-Order Harmonics Appear

The standard assumption is that a balanced six-pulse rectifier produces only odd harmonics. That fails whenever the positive and negative half-cycles are not identical. Common causes include:

• Half-controlled (semi-converter) bridges and asymmetric thyristor firing angles
• DC components from welding sets, battery chargers or half-wave loads
• Transformer or reactor saturation that produces an asymmetric magnetising current
• Mismatched rectifier legs, failed diodes or unequal phase loading

Once asymmetry exists, the 2nd harmonic appears directly and feeds the 4th, 6th and higher even orders through the same non-linear elements.

Consequence: Transformer Heat, Motor Stress and Zero-Sequence Current

Even-order harmonics do not cancel the way balanced odd harmonics do. Their practical consequences are:

• Transformer overheating — even and zero-sequence components circulate in delta windings and the neutral, adding copper and stray losses so the transformer runs hot even below rated load.
• Motor and generator stress — negative- and zero-sequence components create counter-rotating fields, shaft voltages and bearing currents.
• Neutral overload in four-wire systems — the neutral, not the phase, is often the first conductor to fail.
• Nuisance tripping and interference — UPS systems, drives, protection relays and sensitive medical or data-centre loads can misread or trip.

Measurement: How to Confirm Even-Order Harmonics

Before specifying any filter:

• Log THDi and the individual harmonic orders (at least the 2nd to the 25th) across a full production cycle, not a ten-minute snapshot.
• Place analyser CTs at the PCC and, where possible, at the transformer secondary, and confirm CT placement and ratio.
• Record reactive power, power factor, negative-sequence, zero-sequence and neutral current together.
• Check for resonance — capacitor banks can interact with system inductance and amplify specific orders.

Solution: How an Active Harmonic Filter Cancels Even Harmonics

An AHF is a voltage-source converter connected in parallel at the PCC. Its controller samples the load current, isolates each harmonic order and injects a compensating current in real time. Verified performance for the YT Electric AHF includes:

• Three-level topology with IGBT switching at 25.6 kHz, which reduces switching ripple and dv/dt stress;
• Harmonic reduction of at least 97%, achieving THDi of 5% or less at rated load;
• Filtering from the 2nd to the 51st harmonic order;
• Target power factor adjustable from -1.0 to +1.0 (leading and lagging reactive power);
• Three-phase balancing within 5%, mitigating negative- and zero-sequence components;
• Neutral filtering capacity up to 3x rated filter current in four-wire systems;
• Initial response of 50 microseconds or less and overall response of 5 ms or less;
• Active loss of 2.5% or less, with compliance to IEEE 519 and ERG5/4.

Because the correction is closed-loop and order-by-order, it adapts as the load changes — something a fixed passive bank cannot do. See how YT's active harmonic filter works for a fuller explanation.

Comparison: AHF, Passive Filters, SVG and Hybrid Systems

A Static Var Generator (SVG) shares the same converter platform but targets reactive power and voltage support rather than order-by-order harmonic cancellation.

Selection: Sizing and Configuring an AHF

Selection is a current-budget exercise, not a THDi percentage. Use the measured spectrum to decide how much harmonic current the filter must cancel, then choose a model whose rated current covers it with margin. The matrix below lists the verified parameters that drive the choice.

For plants that expect to grow, a modular AHF design allows capacity to be added in steps, keeping spare parts common and letting the installation follow the load.

Limitations and Engineering Considerations

• The AHF rating must exceed the harmonic current to be filtered; beyond that, multiple units or a hybrid scheme are required.
• Over-compensation can raise voltage, particularly on lightly loaded or generator-backed systems, so the target power factor must be set deliberately.
• Existing capacitor banks and their resonance points must be analysed before commissioning.
• At 45 °C full load the thermal margin narrows, so cabinet airflow and derating must be planned.
• The AHF cancels distortion but does not remove its root cause; asymmetric loads, unbalanced phases and DC-offset sources should be corrected where practical.
• In four-wire systems, verify neutral and zero-sequence capacity (up to 3x rated filter current).

Frequently Asked Questions

How do I size and procure an AHF?
Size from the measured harmonic current, not from the transformer rating alone. Log the individual orders and THDi at the PCC, sum the harmonic current the filter must cancel, and select the model whose rated current and voltage cover it with margin. The AHF product page lists the available voltages and current ratings.

Where are the CTs installed and where does the AHF connect?
The AHF connects in parallel at the PCC, and its CTs are installed on the load side of the point where harmonics are to be removed, so the controller measures the true load current. Match the CT ratio to the controller rating during installation.

What maintenance does an AHF need?
Maintenance is mostly passive: keep cabinet airflow clear, check fans and filter mats, monitor DC-bus capacitor aging and ESR on a schedule, and re-verify THDi annually. There are no tuned reactors to drift.

How does an AHF deliver ROI?
The return comes from three sources: avoiding utility power-factor and harmonic penalties, recovering transformer thermal margin and extending equipment life, and reducing losses (active loss of 2.5% or less). Because the AHF also performs continuous power-factor correction from -1.0 to +1.0, it can replace separate capacitor banks.

Next Step: Characterise the System Before You Commit

Even-order harmonics are not a niche problem. As more non-linear loads — drives, rectifiers, chargers and inverters — are added, the 2nd and 4th orders grow, and passive filters cannot follow them. An AHF cancels the distortion dynamically, corrects power factor and balances the phases in one device. New to the technology? Start with what an active harmonic filter is, then bring your measurements to a specialist.

YT Electric provides pre-installation site surveys and harmonic studies so that the correct AHF rating is chosen from the start. Contact our engineering team at sales@yt-electric.com for a power quality assessment and a tailored proposal.