A simple sag generator using SSRs

Power line voltage sags are among the most frequent and costly power quality problems. Most equipment must be designed such that they can tolerate voltage sags (within limits defined according to some standards). Furthermore, some equipment must continue proper operation even under extreme sag conditions (critical loads) and this property often may not be accommodated inside the device itself and sag compensating power conditioners have been developed for such purposes. While in practice voltage sags are not wanted, generating sags becomes necessary for the purpose of experimentally verifying the performances of the equipment (both the equipment under sag condition and the sag compensating power conditioner) under sag conditions. In this work, a simple and economical, yet highly performing sag generator is developed, its design discussed, and its feasibility demonstrated by experiments. The proposed SSR (solid state relay, a semiconductor power module of triac characteristics) and variable transformer (variac) based sag generator is built for three-phase 10kVA ratings and balanced/imbalanced voltage sags are demonstrated in the laboratory. The performance under resistive and inductive loads is evaluated and finally the utilization of the sag generator in the test of a series active filter based power quality conditioner is demonstrated. The proposed approach provides an effective solution for voltage sag generation.