Cost-Effective Electrical Harmonic Reduction

by Jim Lousier  hardware development manager

Electronic variable-speed drives have made a major contribution to the electrical efficiency of oil and gas production. However, the increased popularity of electronic drives has contributed to a form of electrical power-line pollution called harmonics.

Harmonic currents are those that deviate from a pure sine wave. They are generally expressed as a percentage of the fundamental current or as a distortion power factor. The percentage of harmonic current is generally referred to as Total Harmonic Distortion (THD).

High harmonic currents cause unfavorable distortion power factor, poor energy efficiency, and voltage distortion that may affect other equipment. Electrical utilities and users of electrical power are becoming increasingly concerned over harmonic issues. That concern is often expressed as a requirement to meet harmonic limits, such as those prescribed in the IEEE 519 standard.

A properly designed variable-speed drive system can minimize harmonic pollution and maximize overall energy efficiency. The current traces below illustrate the nature of the harmonic problem and cost-effective remedies to eliminate that problem.

Basic variable-speed drives rectify fixed-frequency (50 or 60 Hz) line power into a DC bus voltage. Using a process called pulse width modulation (PWM), the bus voltage is converted into variable-frequency, variable-voltage power for controlling AC motors. Variable-speed drives, in this basic form, create substantial harmonic distortion in the shape of "rabbit ears."

Many manufacturers of variable-speed drives have added a link choke in the DC bus to reduce harmonic distortion by smoothing out the "rabbit ears." The configuration is sometimes referred to as a six-pulse drive based upon the nature of the three-phase rectification process.

A few manufacturers, including Unico, offer 12-pulse drives that provide an additional reduction in harmonics by providing more sinusoidally shaped currents. This type of drive can use a delta wye transfomer of half the total drive capacity and a small balance reactor to achieve a considerable reduction in harmonics.

This configuration also has the benefit of higher limits under the IEEE 519 standard. The IEEE 519 standard prescribes maximum harmonic current limits that are based on the softness of the power line. That softness is defined as the ratio of the line short-circuit current to rated line current. In other words, the greater the short-circuit current relative to the rated capacity, the stiffer the power line.

The 12-pulse drive provides a nearly sine-wave-shaped input current compared to a six-pulse drive. The IEEE 519 standard recognizes this improved waveform by allowing higher harmonic limits for 12-pulse versus six-pulse configurations.

Six-Pulse Drive Harmonic Limits
IEEE 519 Standard

12-Pulse Drive Harmonic Limits
IEEE 519 Standard

Harmonics generally are multiples of the power line frequency (5th, 7th, and so forth) with amplitudes that depend upon the pulse number of the drive. The IEEE 519 standard specifies limits on individual harmonic components as well as Total Demand Distortion (TDD) as a percentage of maximum load demand current.

To learn more about reducing harmonics, please contact us.