Power Quality - Harmonics

Linear vs. Non-Linear Power

Residential, commercial, and industrial power systems in direct use today in this country by personnel typically are 120 to 600VAC, 60Hz(cycles per second). AC(Alternating Current) systems operating at 60Hz will show a near-perfect sine wave on an oscilloscope which reverses its polarity twice per cycle, or 120 times per second. As long as this power is used in a linear fashion, that is, current flowing linearly with the voltage sine wave, the power is said to be clean, or linear. When this power is used by a non-linear device, such as a computer power supply, distortions appear. The electronic power supplies used in computers and similar devices take current at certain points of the sine wave, not evenly as a function of the voltage sine wave. This type of power usage causes harmonic distortions which are integer multiples of the fundamental frequency, 60Hz. In other words, the second harmonic is 120Hz, the third harmonic is 180Hz, etc.

Neutral Conductor Overheating due to Harmonics

In a 3-phase balanced linear system, the vector sum of the three phase currents and the neutral current will cancel out to zero. Current harmonics which are odd multiples of three times the fundamental, such as the 3^rd(180Hz), 9^th(540Hz), etc., are called triplens. Triplen harmonics do not cancel out, but rather sum in the neutral(return). This can cause very high currents and overheating in the neutral conductor and in connections associated with it.

Electronic Equipment Problems

Voltage harmonics can be produced by interactions between the current harmonics and the system impedance. Voltage harmonics can in turn affect the ability of the electronic power supplies mentioned above to absorb the power needed for correct functionality of their associated equipment.

Resonance

The inductive and capacitive components of a system can in some cases form a circuit which is resonant at a certain frequency. In some cases, this means that the circuit impedance is very low at that frequency when compared to what would be typical for that part of the spectrum. If the frequency of the harmonics in the system are close to the resonant frequency of the circuit, this low impedance will cause a high current flow. Other resonant circuit variations can cause high voltage transients. Either case can be very damaging to circuit components.

This damage can be likened mechanically to a drinking glass(electrical system), which, because of its structure, has a certain resonant frequency. The glass(system) can be shattered(damaged) by a audio tone(harmonic voltage/current) of the same resonant frequency.