A fault protection scheme for series active power filters

A protection scheme for series active power filters is presented and analyz ed in this paper. The proposed scheme protects series active power filters when short-circuit faults occur in the power distribution system. The princ ipal protection element is a varistor, which is connected in parallel to th e secondary of each current transformer. The current transformers used to c onnect in series the active power filter present a low-magnetic saturation characteristic increasing current ratio error when high currents circulate through the primary winding, thus generating lower secondary currents. In t his way, the power dissipated by the varistors is significantly reduced. Af ter a few cycles of short-circuit currents flowing through the varistor, th e gating signals applied to the active power filter switches are removed an d the pulsewidth-modulation (PWM) voltage-source inverter (VSI) is short ci rcuited through a couple of antiparallel thyristors.