A variable frequency drive with a six-pulse diode rectifier draws current from the supply in pulses, not as a sine wave, and that current contains harmonics of order 5, 7, 11, 13 and higher. Harmonics heat transformers and cables, overload and resonate capacitor banks, distort the voltage and cause trips and equipment failures. They are controlled with reactors, higher-pulse or active rectifiers, passive or active filters and detuned banks, depending on what the measurement shows against IEEE 519.
Why a drive generates harmonics
Most low-voltage variable frequency drives have a six-pulse diode rectifier at the input that charges a DC bus with capacitors. The diodes conduct only when the supply voltage exceeds the bus voltage, so the input current is not a sine wave. It is a set of pulses near the peak of each half cycle.
That waveform breaks down into a fundamental plus harmonics, currents at multiples of the supply frequency. In a six-pulse rectifier the characteristic harmonics are of order 6k ± 1: 5th, 7th, 11th, 13th, and so on, with the 5th and 7th the largest. The more drives and the more power, the larger the harmonic current the plant injects into its network.
Current distortion is measured as THDi (total harmonic distortion of current) or, to compare with IEEE 519, as TDD (total demand distortion), referred to maximum demand. As these currents flow through the network impedances, they also distort the voltage (THDv), which is what affects the other equipment.
What damage they cause
| Effect | Why it happens |
|---|---|
| Hot transformers | Harmonic currents increase eddy-current losses in the windings; a transformer can overheat without being overloaded in kVA |
| Failing capacitors | Resonance between the bank and the network, and overload from harmonic currents |
| Nuisance trips | Breakers and relays that misread distorted waveforms; capacitor fuses that blow |
| Electronics failures | Power supplies, PLCs and sensitive equipment fed with distorted voltage |
| Hot cables and connections | More RMS current for the same useful power |
| Low power factor | Distortion lowers the true power factor, even if the displacement power factor is good |
The costliest case in industry is a capacitor bank without detuning in a plant full of drives. The two guides on the reactive power cost problem are power factor in industry and capacitor bank.
One effect is often overlooked: the drive harmonic currents are also “seen” by on-site generation. A standby generator set has a much higher impedance than the grid, so the same harmonic current produces more voltage distortion when the generator runs in island mode. Drives that work well on the grid can cause problems with the generator.
What IEEE 519 says
IEEE 519-2022 sets voltage and current distortion limits at the point of common coupling (PCC), the point where the plant connects to the grid and where its harmonics can affect other users. Voltage limits depend on the voltage level at the PCC. Current limits depend on the ratio between the available short-circuit current and the plant maximum demand current: the weaker the grid compared with the load, the stricter the limit.
Two practical consequences. First, compliance is assessed at the PCC, not at each drive. Second, the same drive can be acceptable in a plant with a strong grid and problematic in another with a weak grid or on-site generation. The tables and the method are in harmonic study under IEEE 519. Further detail on the IEEE 1531 criteria for applying and specifying passive filters: harmonic filters and the IEEE 1531 criteria.
The solutions
| Solution | How it works | When it fits |
|---|---|---|
| Line reactor or DC choke | Smooths the current pulses and reduces distortion | First step, low cost, on almost any drive |
| 12- or 18-pulse rectifier | Cancels low-order harmonics with phase-shifted transformers | Large drives |
| Active rectifier (AFE) | Draws almost sinusoidal current | New, regenerative drives or demanding networks |
| Tuned passive filter | Gives a low-impedance path to one specific harmonic | A few large drives with a stable load |
| Active filter | Injects currents that cancel the measured harmonics | Many drives, variable load, several orders |
| Detuned bank | Compensates reactive power with a series reactor that moves resonance below the 5th harmonic | Plants that need compensation and have drives |
There is no universal solution. Choosing without measuring usually ends in a filter that falls short or one that is oversized.
Harmonics are not the only drive effect
On the motor side, the drive output produces fast voltage edges that, with long cables, cause overvoltages at the terminals and bearing currents. That is a different problem with different solutions (dv/dt or sine filters, suitable cables and grounding). For protection of a motor fed by a drive, see motor protection relay.
Signs that your plant has a harmonics problem
- Capacitor fuses that blow or capacitors that swell.
- A transformer that runs hotter than its kVA load would justify.
- Breakers or relays that trip for no apparent reason when the drives start.
- PLCs, computers or drives that reset or fail without explanation.
- Differences between what different instruments measure at the same point.
- A power factor that does not improve even though compensation was added.
How it is diagnosed
- Measure with a Class A power quality analyzer at the main switchboard and at the boards with drives, over a representative operating cycle. See power quality analyzer.
- Identify the sources and the dominant harmonic orders.
- Model the network: transformer, cables, capacitors and the external grid impedance, to find resonances.
- Compare with IEEE 519 at the PCC and decide the solution.
How we help
Our team carries out harmonic and power quality studies for industrial plants. We measure, model the network and its resonances, compare against IEEE 519 and recommend the technically and economically suitable solution, with the specification to buy it. We do the analysis and design remotely, worldwide. In Colombia, or elsewhere on request, we also measure and verify on site after installation.
See our power quality service or write to us telling us how many drives you have and what is failing.
Related guides
- Harmonic study under IEEE 519
- Power factor in industry
- Motor protection relay
- MV and HV Capacitor Banks: Unbalance and Testing
- Power Quality Analyzer: Class A, Harmonics and Flicker
- Nuisance Tripping: Causes in Protection Systems
- Harmonic Filters: IEEE 1531 Criteria
References
- IEEE 519-2022, Standard for Harmonic Control in Electric Power Systems.
- IEC 61000-4-7, measurement techniques for harmonics and interharmonics.
- IEC 61000-4-30, power quality measurement methods (Class A).
- IEEE C57.110, Recommended Practice for Establishing Liquid-Immersed and Dry-Type Power and Distribution Transformer Capability When Supplying Nonsinusoidal Load Currents.
- IEEE 18, Standard for Shunt Power Capacitors.
Frequently asked questions
Which harmonics do variable frequency drives generate?
A drive with a six-pulse rectifier mainly generates harmonics of order 6k ± 1: 5th, 7th, 11th, 13th and higher, with the 5th and 7th the largest. With twelve-pulse rectifiers the dominant ones become the 11th and 13th, and with an active front end (AFE) the current distortion is much lower.
Why do capacitors burn out in a plant with drives?
Because the capacitor bank and the network inductance form a resonant circuit. If its resonant frequency falls near a harmonic the drives inject, that harmonic is amplified, overloads the capacitors and raises the voltage. It is avoided with detuning reactors or by changing the type of compensation, as the study indicates.
Which is better for drive harmonics: a passive or an active filter?
It depends on the load. A tuned passive filter is robust and economical for one or a few large drives with a stable load; an active filter adapts to variable loads and to several harmonic orders at once. Sometimes the simplest option, a line reactor or a drive with more pulses, is enough. The choice comes from measurement and study.