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Active filters are not, as often stated, parallel connected, electronic harmonic filters which inject harmonic currents at the various frequencies and thereby cancel out those drawn by the non linear load(s).

The active filter is simply a very low impedance electronic source so for the non linear(s) loads obtain the harmonic currents from the filter which would otherwise be drawn from the source transformer or generator thus significantly reducing the harmonic voltage distortion (Uthd).

What is not often stated is that active filters ideally require 3% AC line (or DC bus reactance) in each VFD in order to meet the <5% Ithd (total harmonic current distortion) advertised. The lack such reactance in rectifier loads can result in significantly more harmonic current being drawn from the active filter then previously from transformers or generators. Depending in the type of non linear load, for example in VFDs, this additional current can be in the order of 150-200% more. Even with 3% AC line reactors installed the additional current is 25% more but this is taken into account in the designed current rating of the active filter.

Even if correctly dimensioned for drives with no additional reactance the active filter will be substantially larger and more expensive than would be required had suitable AC line reactors had been installed. The active filter performance will be significantly degraded with around 8-12% Ithd expected not the often advertised <5% Ithd.

Similar harmonic current issues are apparent when active filters are used with DC SCR drives. Due to inductive nature of the load (i.e. the DC motor) the additional current drawn when the active filter is connected is less compared to a VFD load (which is a capacitive load). In addition, however, if no suitable AC line (or ‘commutation’) reactors are employed in each rectifier load then the high frequency energy in the DC drive line voltage notches can destroy the active filter’s passive carrier frequency filter and render the active filter inoperative.

A combination of at least 3% AC line reactors and a fast response active filter can almost eliminate line notching, provide an excellent degree of harmonic mitigation and, if rated correctly, can provide power factor correction for systems with DC SCR drives. For information on AC line reactors for use with active filters or AC and DC drives generally up to 3000A, 690V click here.

However, active filters, when applied correctly, are an important tool in the harmonic mitigation toolbox. However, no form of harmonic mitigation is perfect. The secret of success is recognising the pros and cons of all types of mitigation and being able to apply the most appropriate form to any given application.

The staff at Harmonic Solutions Oil and Gas have been involved in applying active filters since 1997 and have applied active filters to a number of demanding applications including offshore DC SCR and VFD based drilling systems and offshore electrical submersible pumps (ESPs).

Harmonic Solutions Oil and Gas, offer a very advanced range of Comsys ADF active filters from 70A to 450A as discrete filters and up to 3000A as paralleled active filter mitigation. The P300 range (80A-450A) is available in voltages from 400V to 690V in both air and water cooled variants. 208V-400V for the low cost P100 range.

Depending on the application a range of hybrid passive/active filter are also available.


Contact us with your application details and let us assist you.