Control Harmonic Distortion

While variable frequency drives (VFDs) offer many advantages for critical motor speed control and optimizing energy consumption, they are known for producing significant levels of harmonic distortion in adjacent power distribution systems. Utility systems can absorb some of that distortion, but when VFDs are on a circuit driven by a generator, the disturbances can threaten reliable operation.


While variable frequency drives (VFDs) offer many advantages for critical motor speed control and optimizing energy consumption, they are known for producing significant levels of harmonic distortion in adjacent power distribution systems. Utility systems can absorb some of that distortion, but when VFDs are on a circuit driven by a generator, the disturbances can threaten reliable operation.

Water treatment plants often are equipped with VFDs, ozone generators and other loads that produce harmonic distortion. In many cases, these facilities also have emergency standby generators to operate critical electrical loads during extended electric utility outages. One such facility, a reverse-osmosis desalination plant was concerned about the long term effects of harmonics, and the ability of its 938 kVA standby generator to operate reliably in the presence of harmonic distortion injected by large VFDs serving pump motors.

Facility operators were concerned about potential generator failure during extended periods of emergency operation. To determine the extent of the problem, operators compiled harmonic measurements, compared distortion levels on both utility and generator sources, and performed engineering analysis to assess harmonic mitigating techniques dictated by the measured harmonic levels.

Monitoring distortion

Schneider Electric performed testing at the line (source) side of the switchboard serving the facility to measure harmonic distortion. Testing was performed with a portable circuit monitor capable of measuring more than 200 power system parameters. Harmonic distortion measurements were taken at a sampling rate of 512 points per cycle to provide accuracy to the 250th harmonic.

Power consumption increases in visible increments as additional equipment goes into operation.

Power factor characteristics are similar whether running on utility or generator supplied power. The 94% full load power factor precluded the use of one mitigation strategy.

Load measurements indicated that the onsite standby generator was loaded to approximately 53% of its rated capacity under the conditions at the time. (See Average current graphic.) Load levels peaked at 403 kW and 431 kVA during the test. The average RMS current during the testing showed the effects of various machines operating.

Voltage levels were slightly lower when running on generator power, but both utility and generator sources were able to maintain acceptable steady-state voltage—100-103% of 480 V—for the duration of the testing. Voltage unbalance was also within acceptable limits at less than 1%.

The load test demonstrated that the power factor was lowest during times of lightest load and washigh with all drives operating. (See Power factor graphic.) This made it difficult to use conventional harmonic filters to reduce harmonics on this circuit due to their inherent power factor improvement characteristics.

The load testing also showed that voltage distortion at the 480 V bus peaked at approximately 6.5% on the utility source and about 10% on the generator source. (See Harmonic distortion graphic.) The current distortion dropped when 60 hp drives were operated with 250 hp drives, due to the cancellation effects of the combination of delta-wye transformers and line reactors. Current total demand distortion (TDD) is based on the harmonic current divided by 80% of the generator rating, or 900 amps.

How much distortion is too much?

Institute of Electrical and Electronics Engineers’ (IEEE) Standard 519-1992, Recommended Practices and Requirements for Harmonic Control in Electric Power Systems , offers guidelines on how much harmonic distortion is tolerable. While this standard was developed primarily as a recommended practice for electric utilities and their customers, it can be used as a guide for assessing existing harmonic current levels for equipment within a facility.

Harmonic distortion peaks with all three 250 hp drives running, but adding the 60 hp units mitigates it through beneficial cancellation effects.

Testing on both electric utility and generator sources at the desalination plant showed that key power system parameters, including voltage regulation and unbalance, as well as current unbalance, were within acceptable limits. While harmonic distortion levels did not cause noticeable operating problems for the facility loads, it was still important for the facility to consider harmonic reduction. Operators were concerned about the long-term effects of harmonics, and, more importantly, about the reliability of the generator during extended periods of emergency operation since IEEE Standard 519-1992 harmonic limits were being exceeded when operating on the standby generator. Moreover, harmonic reduction techniques also maximize equipment life and system reliability.

The harmonic limits table (graphic) shows testing results compared to the harmonic limits of the IEEE Standard 519-1992 typically applied to power generation equipment. As demonstrated, these limits were exceeded during the onsite measurements. In addition to comparing test results to the standard, Schneider Electric performed computer simulations of different mitigating techniques.

Simulations and mitigation analysis

Schneider Electric performed harmonic simulations to estimate the reduction in harmonic current that could be achieved by making various modifications. As mentioned previously, some harmonic cancellation of 5th and 7th harmonic currents was already being achieved when 250 hp and 60 hp drives were operated at the same time. The system required about 27% and 16% less current at the 5th and 7th harmonics, respectively, despite a 19% increase in total RMS current. The “worst case,” or highest harmonic level, occurs when only the 250 hp drives are operating. A total of four modifications were considered:

Bypass one delta-wye isolation transformer —Each 250 hp drive has a delta-wye isolation transformer. Bypassing one of these had a beneficial effect, reducing the amount of harmonic current distortion. Bypassing added to the cancellation effects from combining the 250 hp and 60 hp drives mentioned above, reducing the 5th and 7th harmonic currents. 5th and 7th harmonic currents on the bypassed unit remained unchanged, and once these currents were added to the other one or two 250 hp drive currents, additional cancellation occurred. This technique, however, is only appropriate for temporary improvement until more effective means are installed.

Replace one delta-wye isolation transformer —A more effective harmonic mitigation technique was to replace one of the delta-wye isolation transformers, instead of bypassing it, with a delta-zig-zag-wound transformer. This modification provided increased cancellation effects of the 5th and 7th currents because harmonic currents are not phase-shifted through delta-zig-zag-wound transformers. It also maintained the beneficial effects of harmonic attenuation.

Passive harmonic filter —A 5th passive harmonic filter could have been added at the 480 V main, but this option wasn’t practical because passive harmonic filters also increase fundamental power factor. Since power factor at the facility was already high (94% at full load), the system couldn’t tolerate much parasitic capacitance without putting the facility into a leading power factor condition.

Active harmonic filter —The overall best solution for this plant was to reduce harmonic current by installing an active filter at the 480 V main. Active filters measure the amount of harmonic current required by the load and inject current 180 degrees out of phase. This technique reduces the harmonic distortion levels considerably and often is used where stringent harmonic limits are applied. In addition, Schneider Electric recommended installing onsite power monitoring equipment to track the facility’s performance in harmonic distortion, voltage quality, disturbances and costs.

Temporary, permanent solutions

By temporarily bypassing an isolation transformer, the facility was able to reduce its harmonic distortion levels by increasing harmonic cancellation. This unusual technique helped the facility operate through its peak summer season. Later, they installed an active harmonic filter at its main switchboard. This device virtually eliminated the harmonic distortion concerns. Measurements performed after installation of the active filter showed current distortion below 8%, and voltage distortion below 2%.

Harmonic limits table





IEEE “limit”





Worst case




All drives





w/o isolation transformer




Author Information

Eddie Jones, PE; Larry Ray, PE; and Tim Shuter, PE; work for Schneider Electric’s Water Wastewater Competency Center. Reach them at ;

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