Reducing harmonics from variable frequency HVAC drives

Pulse width modulation (PWM), variable frequency drives (VFDs) are considered the most energy-saving and efficient way to control motors in HVAC, fan, and pump systems. VFDs have proven their worth to facilities and plant managers, supported by bona fide energy savings and performance data.

However, the proliferation of drives has exacerbated a problem for some users. Harmonics, otherwise known as total harmonic distortion (THD), has always been an issue, but the inherent noise at the input terminal of a PWM diode bridge drive added to the problem.

The advent of PWM and insulated gate bi-polar transistors (IGBTs) in the late 1980s greatly improved the reliability and performance of VFDs, while at the same time reducing their size and cost.

A PWM drive converts ac power to a fixed dc voltage through the use of a full-wave diode bridge. This fixed dc voltage then feeds a PMW inverter that, in the past, had been SCRs, transistors, and gate turn-off thyristors (GTOs). IGBTs replaced these modulation devices because of their fast switching capabilities. The IGBT enhancement paved the way for the widespread use and popularity of PWM drives in many industrial and HVAC applications.

Harmonics can create many problems in a plant or facility. It can cause additional motor heating as well as higher RMS currents through connected transformers and feeder equipment. Sensitive equipment such as instrumentation, computers, and communications systems may fail to function correctly or get damaged in severe cases of voltage distortion. In addition to equipment breakdowns or malfunctions, harmonics can add costs in oversizing transformers to accommodate a perceived or false load requirement that is reflected back onto the power line.

To understand harmonics, one must first understand how a conventional PWM drive works and its power flow. Conventional PWM drives consist of a 6-pulse diode rectifier, dc link capacitor, IGBT inverter, and a processor-based controller (Fig. 2).

A diode rectifier is used to convert ac line voltage to a constant, fixed-level dc voltage. The dc link capacitor acts as a filter to smooth the dc link voltage and, as a storage device, to help keep the bus constant. The inverter is used to convert the dc link voltage to a variable-voltage, variable-torque, variable-frequency 3-phase output for controlling the speed and torque of an ac motor. This provides the overload capabilities necessary for dynamic motor performance.

The controller is used to supervise the operation of the inverter as well as to implement powerful vector control algorithms to obtain optimum dynamic performance from the induction motor.

When harmonic currents flow through the impedances of the power system, they cause corresponding voltage drops and introduce harmonics onto the voltage waveform. These changes cause the system voltage waveform to become distorted and, since this voltage is distributed to other users on the power system, it causes harmonic currents to flow through otherwise linear loads.

For example, if the system voltage has a 5^th harmonic component and it is applied to an induction motor, then some 5^th harmonic current will flow into the motor.

All periodic waveforms can be represented by a set of sinusoidal waveforms consisting of the fundamental frequency plus various other harmonic frequencies. The ac line harmonic currents, with a 6-pulse bridge, have characteristic frequencies at 6n±1 times the fundamental frequency, where n is an integer.

This means that if the fundamental frequency is 60 Hz then the harmonics present are 5x60, 7x60, 11x60, 13x60, and so on. The amplitude of the harmonic currents depends on the impedance of the ac power system, the size of the dc link impedence, and the load on the induction motor. The 5^th and 7^th harmonic currents with standard PWM drives are predominantly large (Fig. 3).

Building design specifications often require compliance with IEEE 519 standards, which limit the amount of harmonic distortion allowed on the power system. The recommended practices of IEEE 519 provide guidelines for the design of electrical systems that contain both linear and nonlinear loads.

It addresses the responsibility users have to not degrade the voltage of the utility serving other users by imposing excessive amounts of nonlinear currents back to the utility. It also addresses the responsibility of the utilities to provide users with power that is free from sags and distortions.

Recommended practices provide guideline limits on the amount of harmonic currents imposed on the utility at the point of common coupling as well as limits on the amount of voltage distortion the harmonic currents can produce.

The design of HVAC electrical systems using PWM drives is influenced by recommended practices and, in some cases, corrective measures that must be taken to comply with the guidelines. Until recently, conventional filtering devices such as ac line reactors and/or dc link chokes were added to help reduce the amount of 5^th and 7^th harmonic currents produced by PWM drives.

While these methods help reduce harmonics to acceptable levels, they by no means eliminate the problem, nor are they cost effective.

Typically, 3% or 5% line reactors reduce harmonic currents to the 35%–65% range; while dc link chokes reduce harmonic currents down to the 35%–50% range (Fig. 4). The drawbacks of these conventional methods for harmonic current reduction are extra hardware costs, space considerations, and increased heat losses. Any time additional components are added to a system, reliability and performance can be compromised.

Recent advances in VFDs with harmonics reduction technology have applications in HVAC for centrifugal fans and pumps with variable torque load characteristics. These drives have been designed for limited overload capabilities, limited ability to operate the motor above base speed, and provide a new method of reducing input line harmonic currents without the addition of extra components.

A typical HVAC drive system does not require advanced overload capability. The new drive design has used this fact to allow operation with significantly reduced values of dc link capacitors, typically 2% of a conventional PWM drive. This means the level of the dc link voltage is lower and has more ripple content than a conventional PWM drive, resulting in a longer conduction period for the diodes in the rectifier section.

The resulting line current is approximately equivalent to or less than a standard PWM drive equipped with additional ac line reactors or a dc link choke (Fig. 5). A control scheme was developed to compensate for the effects of the lower level, high ripple, dc link voltage to ensure smooth and quiet operation of the fan or pump motor.

Questions about variable frequency drives may be directed to Ed Smith at 847-941-6161. Article edited by Joseph L. Foszcz, Senior Editor, 630-288-8776, jfoszcz@reedbusiness.com.