Field application of a synchronous controller based on the measurement of residual fluxes for the energization of a step-up transformer

A random energization of unloaded transformers may cause large inrush currents and overvoltages; they lead to malfunction of protection relays and possibly to mechanical damages of the transformers windings. The paper describes the reduction of those stresses by the implementation of a controller system installed in a 400kV pumped storage power plant — Transformer, circuit breaker and controller system are from independent manufacturers. Firstly, the assessment of the circuit-breaker parameters (on site) is presented. Secondly, specific operating conditions of the 400kV circuit breaker have been performed, leading to non zero residual flux values. Thirdly, several energizations of the step-up transformer have been performed, leading to a substantial reduction of inrush currents, when the synchronous controller is active. The principles of such a synchronous controller are also addressed, and a comparison with EMTP simulations is presented.