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Passive Harmonic Filters

Tuned Filters

Most Passive Harmonic filters are "Tuned Filters" with a series resonant circuit in parallel with the input of the drive. There is almos always a line reactor on the supply side of the shunt trap and often a line reactor between the trap and the input of the VFD.

The trap must be tuned to the harmonics to be blocked. The most significant harmonics caused by a six pulse VFD, are the 5th and the 7th harmonics. To reduce these, either there must be two traps, one tuend to the 5th harmonic and one tuned to the seventh harmonic, or a lower Q trap tuned to the 6th harmonic so that it reduces the amplitude of both the 5th and the 7th harmonics.

Tuned filters tend to have a high capacitive current (high VARs, typically in the order of 50% of the filter rated current) and high losses (typically > 2%).
High VAR filters suffer from a voltage rise on the output of the filter at reduced load. This voltage rise can be high enough to cause the VFD to trip on over voltage, or even to damage the input of the VFD. Because of this, it is advisable to switch the capacitors out when the drive loading falls below 30% to avoid resonances on a high impedance supply, and excessive output voltage levels.
High VAR tuned trap filters are designed to provide a low impedance path to the "trap" harmonic frequencies. This low impedance path is also presented to the supply voltage harmonics (background harmonics) and can result in very high harmonic currents through the trap capacitors due to the imported voltage harmonics. Capacitors and reactors can be overloaded and can fail early due to these harmonic voltages.

Tuned filters are designed to filter one or two harmonics only. Higher order harmonics are reduced by virtue of the series line reactor but can still be significant.

Broadband Filters

Broad band filters are designed to remove all harmonics, not just the 5th and 7th.