Analysis of energizing a large transformer from a limited capacity engine generator

In some industrial and utility installations, the emergency engine-generator provided for supplying essential loads is being considered for supplying loads in different configurations or for system restoration. In such installations, the necessary switching operations may require energizing a large transformer from the engine-generator. Energizing a large transformer draws a high magnetizing current from the generator. The high magnetizing current can last for several seconds and it is necessary to analyze its effect on the engine-generator. This paper describes simplified digital simulations performed using the Electromagnetic Transients Program (EMTP) to analyze the inrush current drawn by a 345 kV/13.8 kV/4.3 kV, 27 MVA/18 MVA/12 MVA base rated transformer when energized by a 8375 kVA, 4160 V diesel-generator at the Seabrook Nuclear Generating Station. The simulations also analyze the transient behavior of the diesel-generator during and following transformer energization. The analysis accounts for the transformer saturation characteristics, residual magnetism and the switching angle to predict the most severe conditions. The paper briefly describes the magnetizing inrush current phenomenon and presents the simulation results and the guidelines for modeling the synchronous generator and the saturable transformer.