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Harmonics and Variable Frequency Drives

Variable frequency drives and other power electronic loads can improve efficiency and process control, but they also change how current is drawn from the electrical system. Harmonic current can interact with system impedance to distort voltage, heat equipment, reduce usable capacity, contribute to nuisance trips, and increase stress on transformers, cables, generators, and capacitor banks.

Power quality testing shows whether distortion originates within the facility or upstream, identifies the operating conditions that make it worse, and supports a practical mitigation plan. IEEE Std 519 evaluates the combined steady state harmonic effect of a facility at the point of common coupling, or PCC. It does not certify an individual VFD, and it is separate from evaluating the motor side PWM waveform.[1]

How Harmonics Affect Your Power System

In many facilities, conventional three phase six pulse VFDs produce a recognizable line side signature. The fifth and seventh current harmonics are usually the most prominent, followed by the eleventh and thirteenth.[4][5]

The same drive can have very different effects on two electrical systems. A stiff utility source may hold voltage distortion to an acceptable level, while a long feeder, small transformer, generator, or other weak source can allow the same harmonic current to create substantially more voltage distortion. Capacitor banks and system resonance can amplify the effect further.

How Drives Create Line Side Harmonics

A diode or SCR input bridge conducts when the instantaneous line to line voltage can charge the drive's DC bus. Instead of drawing smooth sinusoidal current throughout the electrical cycle, the bridge draws current in pulses. Those pulses can be represented as the 60 hertz fundamental plus a series of harmonic currents.

When harmonic current flows through the impedance of the electrical system, it creates harmonic voltage drop:

Vh = Ih × Zh

where Vh is the voltage at harmonic order h, Ih is the current at that harmonic order, and Zh is the system impedance at that frequency.

This relationship explains why identifying the load is only part of the investigation. Source strength, transformer impedance, conductor impedance, capacitors, generators, and operating configuration can all change the result.

Characteristic Harmonic Orders

For an ideal balanced p pulse line commutated converter, the expected characteristic harmonic orders are:

h = k · p ± 1

In this expression, h is the harmonic order, p is the converter pulse number, and k is any positive integer beginning with 1. For a six pulse converter, the first pairs are the fifth and seventh, then the eleventh and thirteenth, followed by the seventeenth and nineteenth.

This rule predicts where characteristic harmonics are expected; it does not predict their exact magnitude. Supply unbalance, transformer phase shift, firing angle, DC link design, background distortion, loading, and resonance can create residual or noncharacteristic components.

Typical Harmonic Patterns by Drive Type

The patterns below are useful starting points for interpretation. Measured data, equipment information, and the electrical one line should be evaluated together before assigning a source or recommending corrective equipment.

Three Phase Six Pulse Diode VFD

Typical line side orders 5th, 7th, 11th, 13th, 17th, 19th, 23rd, 25th, 29th, 31st, 35th, and 37th
Usually prominent 5th and 7th
What the pattern means This is the most common industrial front end. A line reactor or DC choke can reduce harmonic magnitude, but it does not change the characteristic harmonic orders.[4][5]

Three Phase Six Pulse SCR Front End

Typical line side orders 5th, 7th, 11th, 13th, 17th, and 19th
Usually prominent 5th and 7th
What the pattern means The ideal orders are the same as those of a six pulse diode bridge. Firing angle and commutation notching affect the magnitude and waveform.

Twelve Pulse Converter

Typical line side orders 11th, 13th, 23rd, 25th, 35th, and 37th
Usually prominent 11th and 13th
What the pattern means Two six pulse bridges with a nominal 30 degree phase shift cancel much of the fifth and seventh harmonics. Imperfect voltage balance, phase shift, or bridge loading can leave residual fifth and seventh content.[5]

Eighteen Pulse Converter

Typical line side orders 17th, 19th, 35th, 37th, 53rd, and 55th
Usually prominent 17th and 19th
What the pattern means Three phase shifted bridges ideally cancel characteristic orders below the seventeenth. Through the fiftieth harmonic, the seventeenth and nineteenth and the thirty fifth and thirty seventh are expected.[5][6]

Twenty Four Pulse Converter

Typical line side orders 23rd, 25th, 47th, and 49th
Usually prominent 23rd and 25th
What the pattern means Phase shifted bridges ideally cancel the lower characteristic orders. Transformer design, voltage balance, and load balance determine the residual spectrum.

Thirty Six Pulse Converter

Typical line side orders 35th, 37th, 71st, and 73rd
Usually prominent 35th and 37th
What the pattern means The lower characteristic orders are ideally cancelled. This approach is used where very low line current distortion is required.

Active Front End and Regenerative Drives

Typical line side orders No single low order sequence dominates; residual low orders may appear with switching frequency components and sidebands
Usually prominent Usually low line side current distortion
What the pattern means An IGBT front end shapes the input current. The remaining spectrum depends on the filter design, control method, switching frequency, operating point, and source impedance.[6]

Six Pulse Drive with Passive Filter

Typical line side orders The underlying six pulse orders remain, while targeted orders commonly including the 5th and 7th are reduced
Usually prominent Depends on filter tuning and operating load
What the pattern means The residual spectrum changes with load, voltage unbalance, source impedance, and the condition of filter and capacitor components.

Single Phase Diode Capacitor Input

Typical line side orders 3rd, 5th, 7th, 9th, 11th, and 13th
Usually prominent Often the 3rd
What the pattern means This pattern is common in small electronic power supplies and single phase drives. Triplen harmonics can add in a shared neutral and contribute to neutral heating.

Cycloconverters and Large Controlled Converters

Typical line side orders Characteristic harmonics plus subharmonics and interharmonics related to supply frequency, output frequency, and firing pattern
Usually prominent Application specific
What the pattern means Measured spectra and manufacturer data are essential because the pulse number alone may not predict every component.

Motor Side PWM Inverter Output

Typical line side orders Carrier frequency and sidebands, common mode components, and fast edge energy rather than a simple 5th and 7th line side pattern
Usually prominent Switching frequency region
What the pattern means This waveform is relevant to voltage rise time, reflected wave effects, insulation stress, and bearing current. It requires a different investigation from line side IEEE 519 testing at the PCC.

How to Interpret Harmonic Measurements

• A harmonic order does not uniquely identify a source. The thirty fifth and thirty seventh harmonics, for example, can be higher order components of a six pulse converter or the second characteristic pair of an eighteen pulse converter.

• A strong eleventh and thirteenth pair is not proof of a twelve pulse converter. Six pulse drives also produce these orders. Confirm the front end topology and transformer connections.

• Multipulse cancellation depends on the real installation. Transformer phase shift, voltage balance, equal bridge loading, and correct phasing all affect performance. Residual lower order harmonics do not automatically indicate defective equipment.

• Multiple drives do not always add arithmetically. Differences in loading, impedance, and harmonic phase angle can create diversity or partial cancellation.

• Capacitors can amplify a narrow frequency range. Capacitor banks and system inductance can create parallel resonance that magnifies a harmonic that was not especially large at the source.

• Current THD can be misleading at light load. Current total harmonic distortion, or THD, often rises when the fundamental current falls. Review harmonic amperes and total demand distortion, or TDD, before concluding that conditions have become more harmful.

Common Diagnostic Clues

A measured spectrum is a clue, not a final diagnosis. The recommended follow up should confirm both the equipment source and the system conditions that shape the result

Observed patter

Possible e

Recommended follow up

5th and 7th dominant

Typical three phase six pulse rectifier or VFD signature.

Correlate the spectrum with drive loading, then inspect line reactors, DC chokes, and source strength.

11th and 13th dominant

Possible twelve pulse characteristic pair or the next pair from six pulse loads.

Verify transformer phase shift, bridge balance, and drive topology.

17th and 19th dominant

Possible eighteen pulse characteristic pair; these orders also appear at lower magnitude in six pulse spectra.

Verify the front end pulse number and compare harmonic current in amperes.

23rd and 25th dominant

Possible twenty four pulse characteristic pair or higher order six pulse content.

Check the multipulse transformer and the effectiveness of lower order cancellation.

3rd 9th and 15th with neutral heating

Single phase nonlinear loads or other zero sequence and triplen behavior.

Measure neutral current and review phase loading and transformer connection.

Even harmonics or DC offset

Unbalance, asymmetric conduction, half wave behavior, sensor error, or an equipment problem.

Verify the instrument setup, then investigate abnormal converter operation.

One narrow amplified order

Possible resonance involving capacitors and source inductance.

Compare capacitor switching states and perform an impedance or resonance study.

Broad or noninteger sidebands

Interharmonics, variable frequency interaction, modulation, or active front end switching.

Use appropriate Class A measurement methods and inspect the full spectrum and event data rather than only integer THD. [2][3]

Why Some Power Systems Are More Susceptible

Harmonic severity is determined by both the nonlinear loads and the electrical system that supplies them. Facilities are more likely to experience problems when one or more of the following conditions are present:

• VFDs, UPS systems, rectifiers, electronic lighting, computer power supplies, chargers, or other nonlinear loads make up a substantial share of the total load.

• Long feeders, small service transformers, or limited available fault current create a relatively high source impedance.

• Standby generators, on site generation, or microgrid operation provides a weaker source than the normal utility connection.

• Drive speed, process loading, or the number of operating converters changes throughout the day.

• Power factor correction capacitors, harmonic filters, or cable capacitance interact with system inductance and create resonance.

• Intermittent or unattended operation makes trips, overheating, or equipment alarms difficult to correlate without time synchronized data.

What Power Quality Testing Provides

A well planned harmonic assessment connects electrical measurements with equipment status and operating conditions. The results can:

• Separate current injection from upstream voltage quality problems at the PCC.

• Identify the harmonic orders, loads, and operating states that drive the problem.

• Quantify exposure of transformers, conductors, capacitors, generators, and other equipment to distorted current and voltage.

• Support selection of line reactors, DC chokes, passive filters, active filters, multipulse converters, active front end drives, or system level changes.

• Provide before and after commissioning evidence that the selected mitigation changed the intended harmonic quantities.

• Create an objective baseline for troubleshooting nuisance trips, overheating, capacitor failures, and poor generator performance.

Screening Survey and IEEE 519 Assessment

A harmonic screening survey can identify likely sources, reveal operating trends, and guide troubleshooting. A formal IEEE 519 assessment requires an agreed PCC, system short circuit information, a defensible maximum demand current, appropriate measurement intervals, statistical evaluation, and documented operating conditions. The scope should be established before monitoring begins so the final report answers the right question.

We identify how VFDs and other equipment affect your power system and recommend practical solutions.