Power Factor Correction Catalog - GE Capacitors - #39

Harmonics are multiples of the fundamental frequency distortions found in electrical power, subjected to continuous disturbances. In a 60 Hz electrical system, 300 Hz is the 5th harmonic, 420 Hz is the 7th harmonic, and so on. These harmonics are created by the increased use of non-linear devices such as UPS systems, solid state variable speed motor drives, rectifiers, welders, arc furnaces, fluorescent ballasts, and personal computers. The source of these harmonics may be internal or external. Individual harmonic frequencies will vary in amplitude and phase angle, depending on the harmonic source. Variable speed drives are usually referred to by the number of rectifiers in the system. The most common are six (rectifiers) and twelve (rectifiers) pulse drives.

Harmonic Resonance occurs when the capacitor reactance and the system

reactance are equal. If this occurs, large harmonic currents will circulate between transformer and capacitor. These currents will result in greater voltage distortion. This provides a higher voltage across the capacitor and potentially harmful currents through all capacitor equipment. Harmonic resonance may occur at any frequency but the 5th, 7th, I Ith and I3th are the frequencies with which we are most concerned. If total bus load exceeds 15-20% of harmonic generation load, the potential for a resonance condition is high. Some indicators of resonance are overheating, frequent circuit breaker tripping, unexplained fuse operation, capacitor failure, electronic equipment malfunction, flickering lights and telephone interference.

Conquering Harmonic Resonance can be accomplished by: (1) adding or subtracting capacitance from the system to move the parallel resonance frequency to one that is not deleterious; (2) adding tuned harmonic suppression reactors in series with the capacitor to prevent resonance; (3) altering the size of non-linear devices. It is important that the tuned frequency, for the 5th harmonic, be at approximately the 4.7th harmonic to account for tolerance in manufacturing and to remove the largest offending portion of the 5th harmonic. Parallel resonance will occur around the 4th harmonic, at a much lower amplitude and in an area that does no harm to the system or capacitor. Tuning lower than 282 Hz is not efficient in removing large portions of the offending harmonic.

Considerations of how power factor correction capacitors affect a system are of utmost importance. In systems with more than 15-20% of harmonic loads, a harmonic survey should be performed to indicate potential problem areas. Readings taken over changing load conditions at potential capacitor locations are most useful in determining the types of systems best employed to accomplish the ultimate harmonic suppression, power factor improvement, KVA reduction and other goals.