A power quality transformer for feeding critical loads

In this paper a new single phase distribution transformer concept is proposed to improve power quality for critical loads. The secondary of the proposed transformer is composed of two windings, one of them equipped with a power electronic ac-ac converter. With the choice of proper turns ratio and the design of the PWM ac-ac converter, the proposed transformer has the following capabilities: (a) can compensate for 50% voltage sag and swells (b) can continuously shape the output voltage to be sinusoidal (low THD) even when the input voltage is highly distorted (c) can disconnect the load rapidly under fault conditions. The proposed approach does not employ any energy storage devices, such as large capacitors or inductors. The PWM ac-ac converter consists of four switching devices (IGBT) and is controlled with a four-step switching technique to achieve snubber-less operation. A design example is presented for a 480 V/120 V, 5 kVA transformer. Simulation results are discussed. Experimental results are included in the final paper.