AC Circuits and Power Systems in Practice

13Harmonics in Power Systems

Ideally the currents flowing throughout the power system should be sinusoidal, since the voltages exciting them are sinusoidal, or nearly so. However, when a customer’s load contains non‐linear (or distorting) elements then non‐linear currents will occur as a result. Such currents can be resolved into a component at the system frequency, henceforth referred to as the fundamental frequency (f₁), and harmonic components occurring at integral multiples of the fundamental (nf₁). Therefore we might expect 2nd, 3rd, 4th and 5th harmonic currents, for example, to appear as a result of distorting loads connected to the network; however, we will see that under most conditions even harmonics are generally absent.

Harmonics are not new; as long ago as 1916 the third harmonic, caused by the saturation of machines and power transformers, was noticeable. Improvements in transformer design reduced this problem substantially until the 1930s when the introduction of large rectifier equipment for rail transport again introduced significant harmonics into the power system. Interestingly, the main problem presented by harmonics was interference with the open wire telephony circuits of the day.

Prior to 1960 one of the main sources of harmonic distortion was television receivers with power supplied from a half wave rectifier. As a result each receiver introduced a small DC current into the LV network, as well as even and odd harmonics. The DC currents in the LV network ...