Application Note

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Troubleshooting

power harmonics

Basic troubleshooting using

multimeters and current clamps

A mystery is occurring in today’s office buildings and

manufacturing plants. Transformers supplying seemingly

average loads are overheating. Neutral conductors in

balanced circuits are overheating from excessive loads.

Circuit breakers are tripping for no apparent reason.

Yet the standard troubleshooting procedures show

everything to be normal. So what’s the problem?

In one word—harmonics.

New technology,

new challenges

Harmonics are the byproducts

of modern electronics. They

are especially prevalent wher-

ever there are large numbers of

personal computers, adjustable

speed drives, and other types of

equipment that draw current in

short pulses.

This equipment is designed

to draw current only during a

controlled portion of the incom-

ing voltage waveform. While

this dramatically improves

efficiency, it causes harmon-

ics in the load current. And that

causes overheated transformers

and neutrals, as well as tripped

circuit breakers.

If you were to listen to an ordi-

nary 60-cycle power line, you’d

hear a monotone hum. When

harmonics are present, you hear

a different tune, rich with high

Figure 1. Near sine wave.

Figure 2A. Distorted current waveform.

Figure 2B. Distorted voltage waveform.

notes. The problem is even more

evident when you look at the

waveform. A normal 60-cycle

power line voltage appears on

the oscilloscope as a near sine

wave (Figure 1). When harmon-

ics are present, the waveform

is distorted (Figure 2A and 2B).

These waves are described as

non-sinusoidal. The voltage and

current waveforms are no longer

simply related-hence the term

“non-linear.”

Getting to the root of the

problem

Finding the problem is relatively

easy once you know what to look

for and where to look. Harmonics

symptoms are usually anything

but subtle. This application note

provides some basic pointers on

how to find harmonics and some

suggestions of ways to address

the problems they create.

From the Fluke Digital Library @ www.fluke.com/library

Application Note

Summary of content (8 pages)

PAGE 1
Troubleshooting power harmonics Basic troubleshooting using multimeters and current clamps Application Note A mystery is occurring in today’s office buildings and manufacturing plants. Transformers supplying seemingly average loads are overheating. Neutral conductors in balanced circuits are overheating from excessive loads. Circuit breakers are tripping for no apparent reason. Yet the standard troubleshooting procedures show everything to be normal. So what’s the problem? In one word—harmonics.
PAGE 2
Sources of harmonics Defining the problem charge a large capacitor. After a few cycles, the capacitor is Harmonics are currents or voltcharged to the peak voltage of ages with frequencies that are the sine wave (e.g., 170 V for a integer multiples of the funda120 V ac line). The electronic mental power frequency. For equipment then draws current example, if the fundamental fre- from this high dc voltage to quency is 60 Hz, then the second power the rest of the circuit.
PAGE 3
Effects of harmonic currents Circuit breakers Common thermal-magnetic circuit breakers use a bi-metallic trip mechanism that responds to the heating effect of the circuit current. They are designed to respond to the true-rms value of the current waveform and will trip when the trip mechanism gets too hot. This type of breaker has a good chance of protecting against harmonic current overloads. A peak-sensing, electronic trip circuit breaker responds to the peak of current waveform.
PAGE 4
Generators Standby generators are subject to the same kind of overheating problems as transformers. Because they provide emergency backup for harmonic producing loads such as data processing equipment, they are often even more vulnerable. In addition to overheating, certain types of harmonics produce distortion at the zero crossing of the current waveform, which causes interference and instability for the generator’s control circuits.
PAGE 5
Troubleshooting tools To determine whether you have a harmonics problem you need to measure the true-rms value and the instantaneous peak value of the wave shape. For single applications, you need either a clamp meter like the Fluke 335, 336 or 337 or a multimeter like those in the Fluke 80, 170 and 180 Series that makes true-rms measurements. For three-phase applications, you’ll need a power quality analyzer like the Fluke 430 Series.
PAGE 6
Solving the problem The following are suggestions of ways to address some typical harmonics problems. Before taking any such measures you should call a power quality expert to analyze the problem and design a plan tailored to your specific situation. Derating transformers The Computer & Business Equipment Manufacturers Association has recommended a second method that involves several straightforward measurements that you can get with commonly available test equipment.
PAGE 7
Work safely The high voltage and current present in electrical power systems can cause serious injury or death by electrocution. Consequently, only trained, experienced electricians who have knowledge of electrical systems in general and the equipment under test should perform testing and modification of electrical systems. Fluke cannot anticipate all possible precautions that you must take when performing the measurements described here.
PAGE 8
The presence of harmonics was obvious by comparison of phase current and neutral current measurements. As Table 1 shows, the neutral current was substantially higher than any of the phase currents, even though the phase currents were relatively well balanced. The averageresponding meter consistently took readings approximately 20 percent low on all the phases. Its neutral current readings were only 2 percent low. The waveforms explain the discrepancy.