Effective harmonic mitigation requires good analysis of harmonic power flows. Without knowing whether harmonics are being imported from the point of common coupling (pcc) or being exported by the installation, it is unwise to proceed with specification of harmonic filters or with their installation. Suitable instrumentation is available, such as the Hioki PQ3100 power analyser.
First consider some basics of mitigation techniques, as shown below:
Active filters (Ablerex Enersine APF)
The latter are a combination of active and passive filters and are sometimes used to suppress high order harmonics passively, thus allowing the active filter to have good dynamic range for lower order harmonics.
Chokes are a lower cost method. The impedance of a choke increases linearly with frequency which is helpful. On the other hand chokes do drop voltage to the load.
Passive filters are combinations of inductors and capacitors and are perforce tuned to specific harmonics.
Transformers have no capacity to stop harmonics from being exported, except for the third harmonic, and other triplens (6th, 9th, etc.) which can circulate in the delta wired primary.
Chokes and passive filters are suitable for suppressing harmonics in circuits with fixed loads. Passive filters can absorb harmonics without affecting load voltage. However the range of harmonics is limited, often to the 5th and 7th. Those harmonics are prevalent in 6-pulse drives so that a passive filter can be a practical solution. As drives generally are employed with varying loads, a form of passive filter, the broad band filter is used. As the name implies, it provides harmonic mitigation over a wide load range.
Where widely fluctuating loads occur, for example at the incomer on the main switchboard, active filters are the best solution. That does not mean that passive filters elsewhere in the installation are not required. The latter can limit energy loss in conductors servicing distant loads whereas an active filter at the main switchboard is only used in order to comply with maximum harmonic current distortion allowed at the point of common coupling (pcc). Harmonic current circulates towards the board up to the connection point of the active filter. Therefore it makes sense in an installation with a separate switchboard for a part of the plant, for example, mechanical services, to install a lower rating active filter.
Power factor correction
Power factor correction capacity is often available in active harmonic filters such as in the Ablerex Enersine APF series. Correction of displacement power factor improves the overall power factor which includes harmonic volt-amps (VA) as well. Displacement power factor correction, if power factors are poor and there is a large harmonic VA load, may require capacitor-bank correction such the PowerSave™ series in combination with active harmonic filtering. As an example, a power factor of 0.7 requires correction of kVAr equal to 71% of line current and therefore limits the harmonic load that can be compensated.
Active filters can be used in closed or open loop correction (refer fig 1). Both methods are used, however the closed loop with the monitoring CTs paced downstream from the filter is the preferred method as it insures line current (rather than load current, upstream from the filter) has minimum harmonic content.
Open loop configuration
Closed loop configuration
When combined with capacitive power factor correction, open loop connection (see fig 2) is preferred so as to minimise resonance destroying the capacitor bank. This is an open loop arrangement with the capacitor banks upstream from the filter.
Combination active harmonic filter and capacitor bank
Active filters are not voltage dependent, and can follow changes in power factor within milliseconds. Four-wire versions of active harmonic filters are also very good for phase balancing and therefore unloading neutrals.
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