A New Control System for Automatic Parallel Operation of Load-Ratio-Control Transformers

The successful parallel operation of transformers equipped with automatic load-ratio control and line-drop compensation requires that stability of operation be insured. In addition, experience has shown that it is frequently highly desirable that other automatic features be included which will contribute toward operating flexibility, and which will permit switching transformers in and out of parallel operation without disturbing the voltage level at the load center or causing an excessive initial circulating current at the instant of paralleling. This paper discusses the conditions which make these added features desirable, and presents a new control system whereby they all may readily be secured. In this new system, load current and circulating current are made to flow in separate paths in the control circuit, and each produce their effects independently, thereby insuring stable operation and maintaining the desired loadcenter voltage level. This separation of the load and circulating current, in conjunction with a modification in the closing circuit of the paralleling circuit breaker, also makes possible the avoidance of excessive initial circulating current when the breaker is closed. The simplicity and effectiveness of this arrangement in providing a fully automatic means for meeting all the anticipated operating requirements recommends its use in many applications.