Variable-frequency-drive- (VFD-) generated harmonics largely is a perceived, rather than real, issue. In 27 years of applying VFDs in HVAC and other applications, this author has experienced only a handful of actual harmonics problems, with all but one stemming from high levels of voltage distortion, not the current distortion that has been getting so much attention lately.

Most of the VFD-interference problems this author has encountered have been the result of poor installation — particularly, poor wiring and grounding. In the majority of cases, radio-frequency interference (RFI) or electromagnetic interference (EMI), not harmonics, was the culprit. RFI/EMI issues stem from noise in the 50-Khz-to-low-megahertz range, not the 300-Hz fifth or 420-Hz seventh harmonic range.

HISTORY

In 1981, ANSI/IEEE Standard 519, IEEE Guide for Harmonic Control and Reactive Compensation of Static Power Converters, was published. It included maximum total-harmonic-voltage-distortion (THD_V) recommendations.

In the extreme, voltage distortion can cause flat-topping of power-system voltage waveforms (Figure 1), which can cause sensitive electronic processors to become confused and malfunction.

In 1992, ANSI/IEEE Standard 519 was revised. Renamed IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems, it now concentrates more on total harmonic current distortion (THD₁) than voltage distortion.

THD₁ can propagate through utility step-down/step-up transformers and make its way from one facility to another. For example, several years ago, a VFD manufacturer was creating high amounts of current distortion during its burn-in testing operation. The current distortion traveled through the utility transformers at the VFD manufacturer's plant to the utility feed at a neighboring printing plant, corrupting the logic circuits in the controls and direct-current (DC) drives running the printing plant's printing press and causing the printing-press registration to malfunction.

THD₁ results in additional heat in the distribution transformers typically provided by utilities, as well as the power-feeder cables of the equipment from which it originates. Basically, THD₁ is current that a utility has to generate and source to a facility, but that brings no revenue to the utility. While it is a real issue for utilities, THDI largely is a perceived problem from a facility manager's point of view.

ANSI/IEEE Standard 519-1992 addresses the system-issue nature of THD¹ by introducing total demand distortion (TDD), which can be calculated as follows:

where:

I_he = total harmonic current as measured by system

I_hc = total harmonic current contributed by VFDs

I_L = maximum demand-load current (fundamental frequency component) (15- or 30-min demand) at utility point of common coupling (PCC) as measured in system

I_C = fundamental frequency component contributed by vfds (included only if vfds are an addition to existing loads)

(All quantities are in amperes root mean square.)