Parallel processing UPS and unbalanced voltage sag compensation

Power quality problems such as voltage sag and unbalance are increasingly important to critical loads. A new type of custom power quality conditioner parallel processing UPS is introduced. Compared with the conventional two-stage conversion UPS systems, the parallel processing UPS system has several advantages such as compact structure, lower losses and improved power factor. Based on the topology and operation principle of the proposed device, a method to compensate voltage sag and unbalance is studied. In this method, the sequence components of supply voltage are extracted firstly by voltage space vector concept and weighted least-squares estimation (WLSE), then the negative sequence voltage is eliminated to zero and the positive sequence voltage is compensated to rate value at load side. The system and its control method are verified by simulation results.