Parallel Active Harmonic Filters

There is a common perception in the utility industry that power quality should be dealt with at the service entrance. As a general rule, however, managers of commercial and industrial facilities know that the real problems are inside the plant–their own (heavy) equipment is usually the cause of disruptions to their own sensitive loads.

There are greater benefits when harmonic and displacement power factor correction solutions are placed close to the causes of the problem. A facility’s internal electrical system can be better utilized, chances of harmonic related mysterious shutdowns are eliminated, and resonance cannot occur.

Parallel active harmonic filters (AHFs) use fast switching semiconductors to inject harmonic current onto ac lines. Microprocessor logic circuits monitor the total current of the load(s); determine the harmonic content; and control the switching sequence of the power semiconductors to inject a current waveform to exactly cancel and thus remove the bulk of the harmonic current.

AHFs are installed parallel to nonlinear loads, and use current transducers to monitor the load current. They do not use power factor capacitors. The power semiconductors are insulated gate bipolar transistors (IGBTs) that move power from the ac lines to a dc bus capacitor circuit and back to the ac lines in a prescribed manner. (Before the advent of IGBTs, active harmonic filters were ineffective and quite expensive.) Some AHFs can also measure and inject reactive current for system displacement power factor correction.

Historically, AHFs have been viewed as either did not working well or costing far too much. If only the initial equipment cost is considered, a 5th harmonic tuned filter will be cheaper. However, when all costs are included, AHFs are the most economical and the most complete solution.

Products are available commercially for use 1) on utility distribution lines, 2) at the point-of-common-coupling (PCC) for facilities to the utility, 3) within the three phase distribution system inside facilities, 4) on the three phase four wire facility distribution system, and 5) to remove neutral harmonics on three phase four wire systems.


Electronic Power Conditioning Inc. (EPC) is offering a series of compact AHF devices called AccuSine, in 50/100/300 amps sizes for placement close to loads. AccuSine can also be paralleled (up to 5 units) for larger requirements such as the utility interface when appropriate. It is designed with the fastest response possible — spectrum cancellation responds in 8 milliseconds (about 1/2 cycle). This approach will cancel interharmonics (noninteger multiples of the fundamental) above 60 Hz, where FFT cannot. This permits AccuSine to correct for “all” types of loads (eventually to include welders and arc furnaces).

With rapid switching of the IGBTs (20 kHZ), AccuSine cancels all harmonics up to the 50th order. Customers do not need to choose which specific orders or put up with limited range of orders to be canceled (as with products from other suppliers).


AccuSine is a flexible product, and can be used in many applications, for power factor correction only, harmonic cancellation only, or both simultaneously. It can be applied to single loads or on a bus for many loads.

Some frequent applications include:

VFD — Variable frequency drives are the largest group of harmonic polluters in use today. AccuSine eliminates the distortion of one or many VFDs depending upon the installation.

WWTP — Water and wastewater plants use many VFDs for pump and fan applications. With AccuSine the current harmonics can be limited to any amount desired, even to

Office Buildings — AccuSine will protect the tenants from VFDs on air handling units and DC drives on elevators. TDD can be kept

Generators — Generators are unstable and overheat when small amounts of the total load are nonlinear. Placing AccuSine between the generator and the loads converts the loads into linear loads. Thus full capacity can be used when PFC and harmonic suppression are both used.

UPS – Input & Output — UPSs create significant harmonic distortion on the AC lines. AccuSine will maintain TDD to

UPSs do not operate well with a high content of nonlinear loads on their output. AccuSine on the UPS output is the answer.

Plant Entrance Systems — IEEE 519-1992 requires harmonic current distortion emissions be held to defined levels. AccuSine on the low voltage system provides absolute compliance regardless of the load dynamics.

DC Motor Controls — DC drive applications have rapidly changing loads that create poor power factor and large amounts of harmonic current due to the SCR phase converter. Only AccuSine can remedy both the power factor and harmonic current pollution simultaneously and dynamically.


In June ’98, EPC announced the successful installation of an AccuSine Power Correction System at a wastewater treatment plant in Oregon, where variable frequency drives (VFDs) had been installed to improve control of raw sewage pumps and to decrease operating costs through variable speed operation. Interference with the computer management system was an unexpected complication. Noise pickup caused multiple logic errors. Sometimes erroneous warnings were sounded. Additionally, depending upon the total load of the VFD/raw sewage pump system, the circuit breaker protecting the management control system would trip thus shutting the plant down. If partially treated sewage is dumped into the adjacent river as a result of computer failure, substantial government fines occur.

After costly and time consuming on-site studies, harmonics from the VFD were determined to be the cause. The initial solution was to install an input line reactor (believed to be the lowest cost solution), which was partially successful. However, at near full load and speed conditions, the problem persisted. AccuSine has eliminated the problem.

This installation required an AccuSine rated at 50 amperes and 480 volts performing only harmonic cancellation to correct the harmonic current caused by a VFD rated at 125 horsepower. AccuSine has successfully reduced the total harmonic current distortion from 39% to 4% insuring trouble free operation of the plant.


EPC has a website with more information and a national list of reps.
(Available March 1 — in the meantime,
temporary site is

Contact: Jim Johnson, VP Sales 541-753-7220,
Electronic Power Conditioning, Inc. (Corvallis OR)

Additional ref: “Parallel Active Harmonic Filters: Economically Viable Technology”, Charles Gougler and James R. Johnson, IEEE PES Winter Power Meeting, Feb ’99

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