Get The Measure of Your Enemy

If you have harmonics, you should be able to measure it!

by Peter Kratochvil, President - Cos Phi Inc.
Electrical Business Magazine

 

So, you think you have a harmonics problem in your facility. How do you know for sure? First, you have to determine if you have harmonic-producing equipment such as: rectifiers, variable speed ac and dc drives, high efficiency lighting, arc equipment, induction heater, switch mode power supplies. Don't forget power factor correction capacitors which, while not in themselves harmonic-producing, do magnify the harmonics in your system.

Chances are your facility has one or more of these sources of harmonics. Therefore, you do probably have a harmonic problem to contend with. The next thing to know is how much of a problem this is likely to be in your operations. You will need to evaluate - which is to say, to quantify - the magnitude and the precise nature of the harmonics in your facility. Approximately what percentage of your total electrical load comprises equipment of this category? Is it 5%, 10%, 25%, or more? More than 20% will very likely cause you problems.

Now you have come to the stage of measuring or monitoring harmonics. The major questions that have to be answered first are;

• What level of Total Harmonic Distortion (THD) is present?
• Which order of harmonics (3rd, 5th, 7th, etc.) is prevalent
• What will you need to measure or monitor? Where, at which point of your network, should you measure?

The measuring and monitoring devices industry today places at your disposal a vast array of instruments that can be used in the search for a solution to your problem. There is a variety of instruments that measure harmonics, from very simple hand-held meters to very sophisticated analyzers. Each has a different purpose and can provide a different level of information.

In selecting measuring or monitoring equipment, you have to first determine exactly what is it that you want to do. Will a few instantaneous measurements resolve your problem? Or, will you need to monitor over a long period of time? To simply confirm your suspicion of harmonic-producing equipment a few instantaneous measurements with a relatively inexpensive instrument - e.g. a true RMS meter or hand held harmonic measuring instrument - will do the job. However, if you want to determine how a number of different harmonic sources interact with each other and exert an influence on your electrical distribution system as a whole, much more sophisticated monitoring equipment will be required.

The instantaneous measurements will reveal whether or not your equipment is in compliance with the IEEE 519 guidelines. These guidelines were drawn to establish the maximum level of harmonic distortion that could be tolerated without having an adverse effect on the electrical distribution system and sensitive electronic equipment. If the instantaneous measurements reveal levels of harmonics in excess of the guidelines the specific pieces of equipment that were checked by measurements may very well be the source of your problem.

The interaction of a variety of harmonic sources can only be determined by monitoring over a period of time; an hour, a day, a week, sometimes a month. The length of monitoring time depends not only on the variety of sources that are interacting, but also on the operating conditions which may change during the work cycle. The length of the work cycle itself will vary depending on the type of production. Therefore, the monitoring time will need to include at least one work cycle preferably two, to establish consistency. These types of monitoring analyzers are costly and the information obtained from them is often difficult to interpret. Sometimes additional software is required to analyze the monitoring results and to allow you to pinpoint the possible causes of the problem.

Most facilities should have measuring instruments to determine instantaneous harmonic readings. These instruments are relatively inexpensive and are a valuable tool. The purchase of a more sophisticated monitoring tool is, however, often not justifiable. This type of equipment is rather expensive and, unless there is a need to use it on a continuing basis, it may be more economical to rent it when required. The other facto to consider is that getting the most benefit out of such a monitoring is dependent on the availability of an experienced person to interpret the information dispensed by the monitoring device. Without the correct interpretation of the monitoring information, it is next to impossible to determine what corrective action has to be taken.

Whatever be the method used to measure and monitor, the results need to be documented and the corrective action needs to be specified. Once the corrective action has been implemented, a follow-up measurement should be done to ensure that the problem has indeed been resolved.

In many cases, a permanent type of monitoring device may have to be installed on the electrical distribution system in order to detect any recurrence of the problem. These devices usually have an alarm feature to warn you if and when harmonic problems reappear. Such permanent monitoring devices are capable of keeping track of many other electrical measurements as well; power factor, temperature, consumption, voltage and current characteristics. There are several of these monitoring devices on the market, most of them available at a reasonable cost. These devices can also be linked to a PC or used as stand-alone units.

In today's fast-changing electrical and electronic environment, no amount of information on your system's operating parameters can be considered excessive. The problem of poor power quality is not a matter of if, but when. The first step in its solution lies in measuring and monitoring, as your system grows and changes.

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