Harmonic distortion and power quality indices in electric power systems

How to avoid harmonics?

Preventive solutions aim to avoid harmonics and their consequences. These technical solutions mainly focus on the reduction of the harmonic emission from non-linear loads. They usually concern modifications in the structure or technology of the loads.

The two most popular methods are the use of AC line reactors or DC link chokes with conventional converter systems and the use of multi-pulse converter systems.

AC line reactor

The use of AC line reactors leads to a significantly lower current distortion of converters. Basically a reactor is an inductor which forms a magnetic field around a coil with wires when current flows through it.

When energized, it is an electric magnet with the strength of the field being proportional to the amperage flowing and the number of turns. A simple loop of wire is an air core inductor; more loops give a higher inductance rating.

AC Line reactors have to be connected in series at the input of typically converter-based devices (VFDs) as they insert series inductive reactance into the circuit. The magnitude of harmonic distortion and the actual spectrum of harmonics depend on the effective impedance that the reactor represents in relation to the load.

For reasonable harmonic attenuation, a 4% impedance line reactor has proved to be state of the art. Figure 1 shows an example with an applied AC line reactor on a six-pulse rectifier.

Additionally, the harmonic mitigation capabilities of the reactor reduce proportionately as load current is reduced because the reactor’s effective percent impedance is reduced.

At full load, a 4% effective impedance reactor achieves harmonic distortion of 37% THDi, while, at 50% load it’s effective impedance is only 2.0% {0.5 x 4% = 2.0%}, and THDi will be around 53%.

DC link choke

DC Link Chokes are a cost effective way of filtering the DC bus voltage and current in a VSD. DC link chokes are installed between the input rectifier and bus capacitor to improve the DC bus waveform and the AC input waveform.

Figure 2 above shows an applied DC link choke in a six-pulse rectifier.

A DC link choke is simply an inductor in the ripple filter circuit, ahead of the DC bus capacitors. The added inductance limits the rate of change of line current relative to time (di/dt) into the capacitors.

This results in lower peak currents. A DC link choke is capable to reduce current distortion typically by 40% to 60%.