Harmonic Mitigation
Passive Harmonic Filters. - Tuned Filters
Probably the most common form of harmonic mitigationused in industry today, is the tuned filter. Typically, these are available in two formats, 5% filters and 10% filters.
The tuned filter is usually designed to reduce the magnitude of one or two harmonics only. Tuned harmonic filters used with VFDs are tuned to either the 5th and seventh harmonics, or are tuend to the 6th, relying on the bandwidth to attenuate the 5th and 7th harmonics.
The VFD style tuned harmonic filters typically comprise an input reactor followed by two series tuned circuits in parallel with the load to "short out" the harmonics and an output reactor making four three phase reactors in each filter.
The capacitance required to tune to the 5th and seventh harmonics is typically in the order of : KVAR = rated KVA/2 so there is a high capcitive current. This requires that the capacitors are switched out of circuit when the VFD is off load.
The high capacitive loading causes major problems when using the tuned filters on generator sets as the AVR does not work stably into a capacitive load.
Hi VAR tuned filters are very sensitive to background harmonics on the supply. Supply voltage distortion increases the current through the capacitors and can lead to early capacitor failure and to very noisy and hot reactors.
Passive Harmonic Filters - Untuned Filters
Cleverly designed magnetics can provide harmonic cancellation in the same way that a delta to star transformers cancells the triplen harmonics. This would normally require a single reactor and small capacitors. An example of this is the Mirus Lineator AUHF series filters.
The AUHF series filters are not as badly affected by background harmonics and do not cause instability with Generator AVR systems.
Active Harmonic Filters
Active filters are essentially an inverter with the output coupled back into the supply. The active filter is designed to measure the harmonic current present in the load and to generate a negative harmonic to cancel the harmonics produced by the load.
At present, active filters are expensive but are reducing in cost and will become a much more viable technology with time.
Active filters are also able to provide displacement power factor correction and thus are a true power factor correction device, correcting both displacement power factor and distortion power factor. Being an active system, instability can be an issue with active filters, but generally this is not a problem.
Active Front End VFD
An active rectifier within the VFD can be designed to ensure sinusoidal current draw from the supply ensuring that the harmonic currents are low. This can be referred to as a "low harmonic drive". The active rectifier is made up of the same components as the output of a VFD plus a switching filter. The cost of the active front end drive is close to couble the cost of a standard VFD. In some cases, the active front end VFD is made up by using a reverse connected VFD with special software.
Twelve and Eighteen pulse VFDs
By using phase shifting transformers, it is possible to create a six phase or nine phase input to the VFD. When used with additional rectifiers within the VFD, the harmonics can be reduced.
The twelve pulse (six phase) solution is not sufficient to meet the harmonics regulations. In some cases the eighteen pulse (nine phase) input will meet the requirements but the requirement for a special phase shifting transformer and extra rectifiers are physically large and expensive.
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