Influence of system transients on the residual flux of three-phase transformers

The amount of switching operations of unloaded transformers increases with the developments of smart grids and integration of renewable energy sources. A random energization of an unload transformer can cause high amplitude and high frequency inrush currents. Controlled switching can eliminate the inrush currents effectively, the key to realize which is measuring residual flux in transformer core accurately. The residual flux, which is generally assumed as a stable value after a transformer de-energization, however can be influenced by system transients passing through a circuit breaker's grading capacitors. The influence of residual flux level also changes the controlled switching instant. The purpose of this paper is to research how system transients influence the residual flux in each phase of an unload three-phase transformer based on frequency response analysis. An improved method for residual flux estimation is proposed, and this provides a basis for the controlled switching.