Origin of harmonics

Harmonic currents

Equipment comprising power electronics circuits are typical non-linear loads and generate harmonic currents. Such loads are increasingly frequent in all industrial, commercial and residential installations and their percentage in overall electrical consumption is growing steadily.

Examples include:

• Industrial equipment (welding machines, arc and induction furnaces, battery chargers),
• Variable Speed Drives for AC or DC motors,
• Uninterruptible Power Supplies,
• Office equipment (PCs, printers, servers, etc.),
• Household appliances (TV sets, microwave ovens, fluorescent lighting, light dimmers).

Harmonic voltages

In order to understand the origin of harmonic voltages, let's consider the simplified diagram on Fig. M3.

Fig. M3 : Single-line diagram showing the impedance of the supply circuit for a non-linear load

The reactance of a conductor increases as a function of the frequency of the current flowing through the conductor. For each harmonic current (order h), there is therefore an impedance Z_h in the supply circuit.

The total system can be split into different circuits:

• One circuit representing the flow of current at the fundamental frequency,
• One circuit representing the flow of harmonic currents.

Fig. M4 : Split of circuit into fundamental and harmonic circuits

When the harmonic current of order h flows through impedance Z_h, it creates a harmonic voltage U_h, where U_h = Z_h x I_h (by Ohm's law).

The voltage at point B is therefore distorted. All devices supplied via point B receive a distorted voltage.

For a given harmonic current, the voltage distortion is proportional to the impedance in the distribution network.

Flow of harmonic currents in distribution networks

The non-linear loads can be considered to inject the harmonic currents upstream into the distribution network, towards the source. The harmonic currents generated by the different loads sum up at the busbar level creating the harmonic distortion.

Because of the different technologies of loads, harmonic currents of the same order are generally not in phase. This diversity effect results in a partial summation.

Fig. M5 : Flow of harmonic currents in a distribution network