Uninterrupted Transfer Switch

Growing sophistication in critical telecommunication and other modern computerized equipment requires uninterrupted power. This necessitates an uninterrupted transfer capability between normal and alternate sources of power, such as utility and engine-generator. Such a transfer is easily accomplished between two compatible sources (for example between two sources connected to the same bus feeder) by means of make-before-break techniques. However, when the two sources are not compatible (such as utility and engine-generator), make-before-break techniques are no longer practical. If the transfer is made when the sources are not synchronized, large circulating currents could result in the loss of one or both sources, and the resulting phase shift or hop could result in severe problems with the critical load. If a break-before-make transfer is attempted with electro-mechanical transfer switches, the resulting interruption in power of several cycles would result in loss of these sensitive loads. This paper examines the key design considerations, and describes a method developed to perform the required transfer using an Uninterrupted Transfer Switch. The Uninterrupted Transfer Switch (UTS) is a combination static-electro-mechanical device which will allow a load to be transferred without interruption from one source to another unsynchronized source. The transfer is controlled so that switching takes place only when the asynchronous sources approach each other in phase, resulting in an apparent uninterrupted transfer of power.