The optional installation of a separate 5 percent impedance input line directly in front of the

VFD ," Because it reduces voltage distortion, it therefore reduces THD. This option is effective as long as the drive load does not exceed 20 percent of the power distribution system's capacity. The stiffer the line (ratio of system, kilovolt-amperes [KVA]/drive KVA), the more likely the drive system will meet THD guidelines of 5 percent or less.

The addition of a DC-link inductor to the drive between the rectifier and the inverter ," This helps to reduce voltage distortion, minimizing THD in much the same way as an external line reactor. Some newer VFDs include a built-in DC-link inductor as a standard feature in models with ratings above 5 hp.

The addition of an AC line inductor between the AC line and the rectifier ," Although this provides slightly better performance than a DC-link inductor, it can be used in conjunction with a DC-link inductor.

The use of 12-pulse configurations, which reduce current harmonics per phase by taking advantage of dual-diode bridge connections ," Instead of current pulses seen 60 degrees apart, pulses are seen 30 degrees apart, reducing AC line harmonics. Drives offering this feature are typically in the 350-hp-to-1,000-hp range and also can be found on commercial compressors that represent a large percentage of the system load.

The addition of synchronous rectifiers on AC drives ," Whether integrated into the drive or added as a separate component, synchronous rectifiers use an IGBT configuration, similar to the inverter section of an AC drive, to replace the diode bridge. By controlling the operation of the transistors in each incoming phase at a high frequency between 10 kHz and 20 kHz, the drive eliminates the pulses normally seen in a diode bridge rectifier. Although this technology to reduce current harmonics per phase originally was designed to allow AC drives to regenerate energy back to the AC line, the benefit of near-zero harmonics has become an even greater benefit.

Total Harmonic Distortion (THD)

 

A mystery solved

In the Texas plant's "Twilight Zone" case, the engineers discovered a shared-neutral wire configuration within the VFD and surrounding conductors in the control unit that were responsible for the overloads and failures. The neutral wire could not accommodate the increased amperages. These amperages were created by harmonic currents fed by the multiple hot circuits converging on a single neutral wire.

The engineers not only installed an upgraded neutral wire to accommodate these higher amperages, but also added neutral wires for the individual hot circuits. At the same time, they located several low-voltage VFDs without any harmonic filters. They installed harmonic filters and line reactors to filter the incoming voltages on the VFDs. An upgraded neutral wire now accommodates these higher amps.

With minimal effort and expense, the Texas petrochemical plant moved out of the Twilight Zone and the Bermuda Triangle and back into the real world of efficient operating facilities. CP

Graf is a Jackson, Miss.-based industry manager for Applied Industrial Technologies, Cleveland. Contact him at dgraf@ait-applied.com.

Illustrations courtesy of Rockwell Automation/ Reliance Electric.