ACTIVE FILTER DESIGN AND SPECIFICATION FOR CONTROL OF HARMONICS IN INDUSTRIAL AND COMMERCIAL FACILITIES
Abstract
Active filters have become a viable alternativefor controlling harmonic levels in industrial andcommercial facilities. However, there are manydifferent filter configurations that can beemployed and there is no standard method forrating the active filters. This paper describesthe active filter operation characteristics anddevelops standard ratings that can be used forfiltering different types of nonlinear loads.Limitations of the active filters are alsodescribed.
1.0 Introduction
Applications of active filters have beendescribed in a number of previous publications[1-5]. The increasing use of power electronicsbasedloads (adjustable speed drives, switchmode power supplies, etc.) to improve systemefficiency and controllability is increasing theconcern for harmonic distortion levels in enduse facilities and on the overall power system.The application of passive tuned filters createsnew system resonances which are dependent onspecific system conditions. Also, passive filtersoften need to be significantly overrated toaccount for possible harmonic absorption fromthe power system.Passive filter ratings must be coordinated withreactive power requirements of the loads and itis often difficult to design the filters to avoidleading power factor operation for some loadconditions. Active filters have the advantage ofbeing able to compensate for harmonics withoutfundamental frequency reactive power concerns.This means that the rating of the active powercan be less than a conquerable passive filter forthe same nonlinear load and the active filter willnot introduce system resonances that can move aharmonic problem from one frequency toanother.The active filter concept uses power electronicsto produce harmonic components which cancelthe harmonic components from the nonlinearloads. These active filters are relatively new anda number of different topologies are beingproposed [6-10]. Within each topology, thereare issues of required component ratings andmethods of rating the overall filter for the loadsto be compensated. Development of a detailedmodel for the active filter using the Electro-Magnetic Transients Program (EMTP)facilitates the evaluation of these design andapplication considerations without extensivefield tests. This paper describes the results of anextensive investigation to evaluate specificdesign and application concerns.
2.0 Active Filter Configuration
The active filter uses power electronic switchingto generate harmonic currents that cancel theharmonic currents from a nonlinear load. Theactive filter configuration investigated in thispaper is based on a pulse-width modulated(PWM) voltage source inverter that interfaces tothe system through a system interface filter asshown in Figure 1. In this configuration, thefilter is connected in parallel with the load beingcompensated. Therefore, the configuration isoften referred to as an active parallel filter.Figure 1 illustrates the concept of the harmoniccurrent cancellation so that the current beingsupplied from the source is sinusoidal.


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