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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.

Harmonic distortion and power quality indices in electric power systems
Harmonic distortion and power quality indices in electric power systems

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.

Beside the attenuation of harmonics they also are able to absorb voltage transients which may otherwise cause a VFD to trip on overvoltage.

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.

AC line reactor applied
Figure 1 – AC line reactor applied

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.

Line reactors are causing a voltage drop which subsequently slightly increases system losses. With reasonable impedance values line reactors do not achieve current distortion levels much below 35% THD-I.

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.

DC link choke applied
Figure 2 – DC link choke applied

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%.

Title:Harmonic distortion and power quality indices in electric power systems – Alexander Kamenka; Schaffner Group
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Harmonic distortion and power quality indices in electric power systems
Harmonic distortion and power quality indices in electric power systems

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  1. Emeka Anukam
    Nov 18, 2019

    Excellent, widens my scope in the field and guide me on new challenges. Very knowledgeable, plz keep it rolling would like to be a member very soon.

  2. Vijay
    Nov 18, 2019

    Complete information about 22or 11kv circuit breaker

  3. Daniel gbenatey
    Oct 09, 2018


  4. Muhammad Bilal
    Oct 09, 2018

    Hello I am Electrical Engineer.
    I need your help in load calculation.
    also i need some examples of load calculation.
    i am waiting for your response.
    Best Regards.

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