Harmonic Current.

If you are familiar with Fourier Transform theory, you know that any complex waveform can be expressed as the sum of sine waves of different frequencies and amplitudes. For example, the current through a thyristor used as a phase-angle control device would be zero through a half-cycle of the AC supply voltage until it receives the gate trigger. At that point, the current would jump to a value limited by the impedance of the circuit, and follow a sinusoid wave-form until the voltage returns to zero. If you express this current waveform as a Fourier sum, you will see that it includes components with frequencies that are multiples of the fundamental AC frequency. These are harmonic currents.

It is also possible to have currents that are harmonics of switch-mode power supply frequencies. That is why SMPS circuits should have line filtering components on their inputs; otherwise the harmonic currents can cause problems for other equipment on the power line, particularly motors, transformers and other low-frequency inductive devices.

current harmonics represent a distortion of the normal sine wave provided by the utility. When a product such as an SCR switched load or a switching power supply distorts the current, harmonics at multiples of the power line frequency are generated. Two significant consequences arise as a result of harmonic generation. First, because of finite impedances of power lines, voltage variations are generated that other equipment on the line must tolerate. Second, when generated in a three-phase system, harmonics may cause overheating of neutral lines.

Power line harmonics are generated when a load draws a non-linear current from a sinusoidal voltage. The harmonic component is an element of a Fourier series which can be used to define any periodic waveshape. The harmonic order or number is the integral number defined by the ratio of the frequency of the harmonic to the fundamental frequency (e.g., 150 Hz is the third harmonic of 50 Hz; n = 150/50). A second harmonic is therefore two times the fundamental frequency of the supply line volt current. If the supply voltage had been generated by an ideal source (zero impedance), the current distortion would have little effect on the supply voltage sine wave. However, because a power system has a finite impedance, the current distortion caused by a nonlinear load creates a corresponding voltage distortion in the supply lines. This voltage distortion can subsequently disrupt operation of other sensitive equipment connected to the same line. Voltage distortion can also cause motors operating on the line to overheat.

Because neutral lines are not fused or protected by circuit breakers, overheating of neutral conductors in a three-phase line can be a significant safety hazard. Such damaging occurrences are usually attributable to the use of single-phase loads attached to three-phase/single-phase wiring systems. Excessive neutral current is caused by the existence of "triplen" harmonics, which add in series in the neutral line. Triplen harmonics are those harmonics that are an integral multiple of three times the fundamental. In three-phase, four-wire systems, each of the three phases is separated in phase by multiples of 120°. Triplen harmonics have relative amplitudes that are also 120° out of phase with the fundamental. Therefore, these harmonics, when drawn from the single phase lines, can add together in phase to cause neutral currents that exceed the phase current. While the individual phase currents are less than the circuit breaker protection current, the neutral current caused by the triplen harmonics may exceed the current rating of the wire used.