Adaptive Hysteresis Current Control of Shunt Active Filter for Three Phase Four-wire System

In a balanced three phase system, only harmonics of positive sequence(7 th , 13 th ,..etc) and negative sequence (5 th , 11 th ,…etc) are produced. However, the harmonic currents produced by single phase non-linear loads like switch-mode power supplies in computer which are connected between phase to neutral injects third order zero sequence harmonics (3 rd ,9 th ,15 th …etc) (Mehmet et al 2008, Roger C Dugan). These triplen harmonic currents unlike positive and negative sequence harmonic current do not cancel but add up arithmetically with the neutral current. This can result in heavy current in the neutral wire (Gruzs 1990). Under such condition, the neutral current may reach magnitude as high as 1.732 times the phase current. Because of this, the loads supplied by the three-phase four-wire distribution system may face severe power quality problems such as poor voltage regulation, high reactive power, load unbalancing and excessive neutral current. A suitable control strategy should be adopted to reduce the dominant harmonics below 5% and also to cancel the neutral current. This work proposes a fuzzy-adaptive hysteresis band current control based shunt active filter for this purpose. In this control strategy, the hysteresis bandwidth is calculated with the help of a fuzzy logic controller (FLC). The controller performance is analyzed under ideal and distorted voltage conditions.