HARMONIC-MITIGATION TECHNOLOGY

Reducing the magnitude of peak current values and managing current waveforms are important to mitigating harmonic distortion. Methods of reducing harmonic current include the use of AC line reactors, DC bus chokes, 12-pulse and 18-pulse AC drives, and devices that generate AC sine waves. These methods have cost and benefit trade-offs. Specifying engineers often request a harmonic-mitigation solution that unnecessarily increases construction and installation costs. The best solution is a trade-off between acceptable harmonic distortion and acceptable cost.

Recent innovations have made AC drives with reduced harmonics and improved performance possible. Advances in the materials used in long-life plastic-film capacitors for the diode-bridge-rectifier power-conversion sections of AC drives are reducing the amount of capacitance across DC buses. New AC drives have 3 to 5 percent of the capacitance of previous-generation drives.

New variable-torque AC drives have powerful motor-control microprocessors that are programmed with motor-control algorithms designed to produce sinusoidal waveforms for centrifugal-fan and pump applications. New technology also has decreased the size of variable-speed drives and reduced dependence on external line filters, such as AC line reactors and DC chokes, for the reduction of harmonics.

Figure 1 shows typical voltage and current waveforms of a 100-hp six-pulse AC drive without an AC line reactor. The input voltage is the orange waveform, while the input current is the purple waveform. Large spikes in the current waveform are attributed to capacitors charging and discharging. The magnitude of the current waveform peaks at about 300 amps, as the capacitors charge. The large, double-humped current waveform significantly contributes to harmonic distortion. The total harmonic-distortion current (THDI) in this example is 80 percent.

Figure 2 shows typical voltage and current waveforms of a 100-hp six-pulse AC drive with a 3-percent AC line reactor. The input voltage is the orange waveform, while the input current is the purple waveform. The peak current reaches 190 amps and is lower because of the use of the 3-percent AC line reactor. The double-humped waveform contributes to harmonic distortion, but is reduced in comparison with Figure 1. The THDI in this example is 38 percent.

Figure 3 shows typical voltage and current waveforms of a 100-hp six-pulse AC drive with new reduced-harmonics technology. The input voltage is the orange waveform, while the input current is the purple waveform. The peak current is 190 amps; however, the current waveform is square-shaped and more closely resembles a sinusoidal waveform because of the reduced capacitance value. The new square-shaped current waveform produces less harmonic distortion. The THDI is 33 percent. An AC drive with new reduced-harmonics technology performs as well as an AC drive with a 3-percent line reactor.

An AC drive with reduced-harmonics technology is more efficient in handling AC power and does not require an external AC line reactor or DC bus choke.

Removing the AC line reactor or DC bus choke from a drive:

• Reduces the size and weight of the drive enclosure.

• Reduces the cost of the drive.

• Improves ease of installation.

• Reduces mechanical/electrical-room wall-space requirements.

An AC drive with new reduced-harmonics technology is a cost-effective AC-drive solution.

For past HPAC Engineering feature articles, visit www.hpac.com.

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Harvey Eure is a product manager for enclosed variable-frequency drives for Schneider Electric. He can be reached at harvey.eure@us.schneider-electric.com.

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