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Six-Pulse Conversion and Harmonics

Dec 1, 2008 12:00 PM, By John DeDad, Senior Director, Editorial and EC&M Development

Devices using 6-pulse technology may cause motor overheating/failure and resonance problems if generated 5th and 7th harmonic currents are not eliminated

Most modern industrial facilities have widespread applications of nonlinear loads, many of which represent a significant portion of the total facility load. As a result, they can inject harmonic currents into the power system, causing harmonic distortion in voltage.

Compounding this problem is the fact that these nonlinear loads typically have a low power factor (PF), forcing many industrial facilities to use capacitor banks to improve PF to avoid utility penalty charges. Ironically, this solution can cause some unforeseen problems. For example, the PF correction potentially can magnify harmonic currents, possibly presenting resonance conditions within the facility. These conditions, in turn, can cause motor and transformer overheating and problematic operation of susceptible electronic equipment. Let's take a closer look at some of the harmonics problems and solutions found in typical industrial environments.

Adjustable-speed drives and harmonics

A typical nonlinear load is the adjustable-speed drive (ASD).

Figure 1 (click here to see Fig. 1) shows a basic 3-phase, 6-pulse, pulse-width modulation (PWM) drive, which is the most common type. All PWM drives contain the following main parts, with subtle differences in hardware and software components. The input section of the drive is the converter, and it contains six diodes (arranged in an electrical bridge). These diodes convert AC power to DC power. The next section, the DC bus, sees a fixed DC voltage and filters/smooths out the waveform. The diodes actually reconstruct the negative halves of the waveform onto the positive half. In a 460V unit, you would measure an average DC bus voltage of about 650V to 680V. You can calculate this as line voltage times 1.414.

The DC bus feeds the final section of the drive: the inverter. As the name implies, this section inverts the DC voltage back to AC. However, it does so in a variable voltage and frequency output by using various types of power devices, such as silicon-controlled rectifiers (SCRs) or insulated gate bipolar transistors (IGBTs).

It's the converter part of the ASD that dominates the interaction of the drive with its source system. Therefore, how it functions determines the extent of the harmonics it will introduce into the electrical distribution system to which it's connected.

Production of harmonics by line-commutated converters is related to the pulse number of the device (see Drive Configurations and Harmonic Orders on page 22). The harmonic spectrum of a 6-pulse drive consists of the 5^th harmonic (300 Hz) and 7^th harmonic (420 Hz) as the lowest predominant orders.

PF improvement vs. harmonic resonance

Problems involving harmonics often show up at capacitor banks first because these units experience high-voltage distortion during resonance, and the current flowing in these banks can be significantly large/rich in a specific harmonic.

One way of looking at this is shown in Fig. 2. (click here to see Fig. 2) Here, we see a PF improvement curve superimposed over a harmonic resonance curve. Note that the 5^th and 7^th order harmonics are shown interfering across a wide area of capacitor addition, specifically in the high PF area in which you would normally want to have their performance characteristic. As you can see, the interference is significant.

If we were able to totally remove those harmonics, the resonance curve would not exist except at the far left of the graph, beginning with the 11^th order. In other words, there would be no resonance interaction in the higher orders of PF.

For more detailed information, see the June 2003 EC&M issue article “Power Factor Correction and Harmonic Resonance: A Volatile Mix,” which can be accessed at www.ecmweb.com.

Harmonic impact on motors

According to the book “Electrical Power Systems Quality,” by Dugan, McGranaghan, Santoso, and Beaty (ISBN 0-07-138622-X), harmonics can significantly impact motors, basically through voltage distortion. This distortion at the motor's terminals translates into harmonic fluxes within the motor. These fluxes don't contribute much to motor torque, but they do rotate at a frequency different than the rotor synchronous frequency. This, in turn, induces high-frequency currents in the rotor.

The effect on motors is similar to that of negative-sequence currents at fundamental frequency: The additional fluxes do little more than induce additional losses. One indicator of harmonic voltage distortion is increased motor heating. Excessive motor heating problems usually begin when the voltage distortion reaches 8% to 10% and higher.

Another effect, according to the book “Power Quality,” by C. Sankaran (ISBN 9780849310409), is torsional oscillation due to the presence of harmonics. The 5^th harmonic is a negative sequence harmonic, and the resulting magnetic field revolves in a direction opposite to that of the fundamental field at a speed five times that of the fundamental field. The 7^th harmonic is a positive sequence harmonic, with a resulting magnetic field revolving in the same direction as the fundamental field at a speed of seven times the fundamental. The net effect is a magnetic field revolving at a speed of six times the speed of the rotor.

The resulting interaction between the magnetic fields and the rotor-induced currents produces torsional oscillations of the motor. If the frequency of the oscillation is the same as the natural frequency of the motor rotating members, severe damage to the motor can occur. Other indicators include decreased efficiency and high-pitched noises.

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