Archive for the ‘Harmonics’ Category

Power Quality

May 2, 2009
Harmonics and electrical noise are potential problems when power electronics are utilized. As more AFD’s are put into use, utilities may force users to install harmonic filtering from entering their systems. IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems; IEEE Std. 519 – 1992; is written to attend to this issue. The standard has been written to limit the harmonic content introduced into the system by either the utilities or the customer.

(Note: The limits are, generally, 5% voltage distortion and 3% current distortion at the Point of Common Connection (PCC) or the point at which the utility power enters the customer plant.)
Harmonic content has attracted quite a bit of attention when discussing power quality and power electronics. Harmonics, created by the load, generally come from feedback into the line from electronic power supplies. Voltage and current harmonics tend to create alternate fields within motors and rotors, cause transformers to overheat, and interfere with other electronic systems. Odd harmonics of the fundamental frequency are generally found in power electronic systems.
In motor systems the following fundamentals of 60 Hz can be recognized:
Harm: 1st 3rd 5th 7th etc.
Rot: pos. zero neg. pos. etc.

Voltage and Current Harmonics

Positive harmonics rotate in the direction of the rotor. Other than the fundamental frequency, this type of harmonic causes heating within the stator. Negative rotating harmonics rotate against the rotor causing overheating of the rotor and reducing torque.
Zero rotating harmonics generally cause system neutrals to overheat. In the case of electronic drives, in general, the predominant harmonics are the 5th and 7th.