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

Active filters are very low impedance electronic sources which provide non linear loads the harmonic currents they require to function.  Without such devices the harmonic currents would be drawn from the generator or transfomer often resulting in excessive harmonic voltage distortion in the power system.

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) performance figure advertised. The lack of additional reactance in a rectifier load can result in significantly more harmonic current being drawn from the active filter then previously from the transformer or generator. This is due to the less than 1% source impedance for harmonics of the active filter.  Depending on the type of non linear load, for example in VFDs, this additional current can be in the order of 50-100% more for VFDs without additional reactance. Even with 3% AC line reactors installed the ‘additional current’ is around 25% more.  However, this additional harmonic current is usually designed into the current rating of the active filter.

Therefore, if correctly dimensioned for loads with no additional reactance the active filter will be substantially larger and significantly more expensive than required had suitable AC line (or DC bus) reactors been installed in the VFD. The active filter performance will be significantly degraded with around 8-12% Ithd expected and not the <5% Ithd often stated.  The lack of sufficient AC line (or DC bus) reactors in each drive is problematic if active filters are to be used effectively.

Similar issues are apparent when active filters are used with DC SCR drives. Due the to inductive nature of the load (i.e. the DC motor armature) the additional current drawn when the active filter is connected is less compared to a VFD load (which is a capacitive load). However, if no AC line (or ‘commutation’) reactors are employed then the high frequency energy in the DC drive line voltage notches, if near the active filter’s switching frequency, can potentially destroy the active filter’s passive carrier frequency suppression 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 also provide displacement power factor correction for systems with DC SCR drives.

Important tool…

Active filters however, when used 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 active (and passive) mitigation and being able to apply the most appropriate form of mitigation for any given application.

The staff at Harmonic Solutions Marine have been involved in applying active filters since 1996 and have applied active filters to a large number of demanding marine and offshore applications.

Advanced filters…

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

A complete range of accompanying AC line reactors to 3000A can also be supplied if required.


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