Influence of system transients on the residual flux of an unloaded transformer

Developments of smart grids and integration of renewable energy sources will increase the amount of switching operations of unloaded transformer systems. A random energization of an unloaded power transformer can cause inrush currents and related voltage disturbances. State of the art controller systems control these phenomena taking into account the residual flux after a transformer de-energization, which is generally assumed as a stable and “locked in” value. However, the residual flux can be influenced by system transients passing through a circuit breaker´s grading capacitors. Furthermore, the frequency content of system transients can also change the transformer impedance at the instant while the system transients appear. These changes determine how extent system transients influence the residual flux and therefore change the optimal re-energization instant. The paper evaluates these phenomena based on frequency response analysis and proposes a filter for an improved approximation of the residual flux level.