ABB Guide to Harmonics with AC DrivesGuide to Harmonics
with AC Variable Frequency Drives

Contents

Other methods for harmonics reduction

Filtering is a method to reduce harmonics in an industrial plant when the harmonic distortion has been gradually increased or as a total solution in a new plant. There are two basic methods: passive and active filters.

Tuned single arm passive filter 

The principle of a tuned arm passive filter is shown in Figure 8.1. A tuned arm passive filter should be applied at the single lowest harmonic component where there is significant harmonic generation in the system. For systems that mostly supply an industrial load this would probably be the fifth harmonic. Above the tuned frequency the harmonics are absorbed but below that frequency they may be amplified.

Tuned single arm passive filter.

  • Detuned – Single tuning frequency
  • Above tuned frequency harmonics absorbed
  • Below tuned frequency harmonics may be amplified
  • Harmonic reduction limited by possible over compensation at the supply frequency and network itself

Figure 8.1 Tuned single arm passive filter.

This kind of filter consists of an inductor in series with a capacitor bank and the best location for the passive filter is close to the harmonic generating loads. This solution is not normally used for new installations.

Tuned multiple arm passive filter 

The principle of this filter is shown in Figure 8.2. This filter has several arms tuned to two or more of the harmonic components which should be the lowest significant harmonic frequencies in the system. The multiple filter has better harmonic absorption than the one arm system.

Tuned multiple arm passive filter.

  • Capacitive below tuned frequency/Inductive above
  • Better harmonic absorption
  • Design consideration to amplification harmonics by filter
  • Limited by KVAr and network

Figure 8.2 Tuned multiple arm passive filter.

The multiple arm passive filters are often used for large DC drive installations where a dedicated transformer is supplying the whole installation.

External active filter 

A passive tuned filter introduces new resonances that can cause additional harmonic problems. New power electronics technologies are resulting in products that can control harmonic distortion with active control. These active filters, see Figure 8.3, provide compensation for harmonic components on the utility system based on existing harmonic generation at any given moment in time.

External active filter principle diagram.

Figure 8.3 External active filter principle diagram.

The active filter compensates the harmonics generated by nonlinear loads by generating the same harmonic components in opposite phase as shown in Figure 8.4. External active filters are most suited to multiple small drives. They are relatively expensive compared to other methods.

External active filter waveforms and harmonics.

Figure 8.4 External active filter waveforms and harmonics.

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