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Active harmonic filters keep mains clean

A Mitsubishi Electric Automation Systems product story

Edited by the Engineeringtalk editorial team Sep 1, 2003

A new range of active harmonic filters aims to set new standards in fight against distortion and corruption of the mains electricity supply caused by back-harmonics from equipment.

Active harmonic filters newly launched by Mitsubishi Electric, Automation Systems will set new standards in fight against distortion and corruption of the mains electricity supply caused by back-harmonics from equipment such as UPSs, power factor correction units and variable speed drives.

The new filters continuously monitor the nonlinear current demanded by the load, and electronically generate an adaptive current waveform that matches the shape of the nonlinear portion on the load current.

This technique is referred to as active injection mode, or AIM, as is also used in the active noise suppression systems fitted to luxury cars' music systems.

Active filters are particularly dynamic and can react almost instantly to load changes, such as when a large drive comes online.

Harmonic filters have become commonplace in automation systems, including manufacturing automation, building and climate control, water and gas distribution etc, since legislation was enacted in the mid-1990s to reduce mains corruption and protect supply quality.

Currently the most common type of filter is the simple choke, but while attractive due to low cost, is only partially effective.

12-pulse bridge filters are far more effective, but prohibitively bulky and complicated.

Active filters achieve even better performance, cancelling out typically 95% of all corruption, yet take up far less space.

They are fitted centrally within a system, rather than one to each piece of equipment, so that are particularly useful for retrofit applications.

Mitsubishi's AIM filter technology has spun out of stealth developments for the US military and incorporates: harmonic mitigation; reactive current control; load side transient suppression; and load side surge suppression.

All this reduces the effect of supply sags and surges (flicker) so that there is virtually no 'signature' on the mains from the equipment used in conjunction with the filter.

The design is such that input current of the AIM filter is a true sinewave, irrespective of the distortion at the load, and nearly in phase with the input voltage, resulting in near-unity power factor (typically 0.98 lagging).

This is irrespective of the power factor of the connected load, as any reactive current required by the load is supplied by the filter as reactive power compensation.

Typically this is 37kVAr per 415V, 100A, is current limited and will function regardless of the magnitude of the harmonic current.

Mitsubishi's AIM filters respond to all non-fundamental current components, ie any components other than the steady-state fundamental frequency components.

The speed of response is less than 100us, and peak currents or spikes of up to three times the nominal current rating can be absorbed so that transient load currents cannot pass back into the mains, preventing the creation of voltage transients.

The filter is not affected by changes in system impedance and is inherently nonresonating and can be configured to accommodate individual or multiple nonlinear loads at the equipment, motor control centre, or switchboard level.

The AIM product range incorporates single- and three-phase version and two, three and four wire connection (four wire being for unbalanced loads with high neutral current returns).

Nominal line-to-line voltage capacity is 208-600V at 60Hz or 400-600V at 50Hz.

The harmonic current ratings are 25 to 400A, and units can be mounted in parallel to achieve ratings in excess of 1000A.

Typical reactive power ratings are 52-137kVAr/400V unit.

A comprehensive set of approvals include compliance with European EMC and LV directives, UL, cUL and CSA, Engineering Recommendation G5/4 (2001), EN61000-4-2/4-3/4-4/4-5, EN50081-2, EN50082-2 and IEEE519 (1992).

Mean time between failures is calculated at in excess of a decade.