North Carolina Power & Light

Harmonics -- now that is a buzzword! How many times have we heard this word mentioned in a discussion on power and power distribution? Well, let’s talk about it a bit.

First, let me tell you that we could dedicate a year’s college course to this study. I also believe that most of you have a fair understanding on the subject. But simply put, harmonics is the name given to the multiples (1, 2, 3, 4, 5, 6, . . .) of the fundamental 60 Hz (cycle) frequency that powers our homes and businesses. The even frequencies -- 2, 4, 6, 8 -- are a result of DC voltages on the busses, and the odd frequencies -- 1, 3, 5, 7, 9 --are a result of AC voltages. Power electronics used in personal computers, electric motors, fluorescent lighting, laser printers, copiers, telephone switching gear, fax machines, and various test equipment provide the source. The terminology used to identify harmonic distortion is called Total Harmonic Distortion or THD. Voltage THD values of 0%-10% are normal within a facility, as are current THD values of 0%-5%. Voltage values of 15% and 30%-50% for current start presenting a problem.

Now, let’s talk about why we are concerned about harmonics. Harmonic frequencies do not perform any useful work. They circulate around getting canceled out or amplified, resulting in HEAT, STRESS, and MISOPERATION of equipment. The customer may not notice the heat, but will notice the intermittent booting of a computer, the increased utility bills, the lower capacity of the UPS due to increased neutral currents, and the inability of the UPS to synchronize with the utility feed and a backup generator. These problems are real and I have seen all of them personally with Exide Electronics’ customers.

The best way to work with harmonics is to minimize their effect. Techniques such as balancing of loads, isolation transformers, and filtering will all reduce the magnitude of the harmonics. The best way to know if the harmonics are causing a problem to your customer is to talk to your facility personnel. Ask the right questions. Are they having any of the problems identified above? Maybe you need a professional engineer to conduct a site survey of the facility.

No matter which direction you go, harmonics cannot be ignored. You must know what severity level of harmonics you have, before you decide whether you should do anything about it.

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