Reducing the magnetizing inrush current by means of controlled energization and de-energization of large power transformers

Energizing unloaded transformers may result in magnetizing inrush current of high amplitude. These currents have many unfavorable effects including differential protection maloperation, deterioration of the insulation material and mechanical support structure of windings, voltage sag and other power quality issues on the HV, as well as on lower voltage terminal(s). If no point-on-wave controller is applied the energization of the transformer may occur at any time on the sinusoidal wave producing high inrush current peaks if the transformer core is driven into saturation. The point-on-wave control methodology presented in this paper has been elaborated to eliminate the inrush currents of 132/15 kV, Y/delta connected generator step-up transformers, which are switched very often in quick start, gas-turbine power stations. A new controller has been elaborated to operate the mechanically staggered, common drive type circuit breakers which are characterized by three-pole operated spring drive and fixed time delay between the operating poles. The paper proposes a new method to minimize the residual flux in the core by means of controlled de-energization. The new concept has been proved by several field tests and the new controllers have been put into service in several power stations in Hungary. (c) 2006 Elsevier B.V. All rights reserved.