Abstract
Controlled switching, taking into account the residual flux level within a transformer core, can effectively eliminate inrush currents. Many switching sequences assume the residual flux as a constant value, which can be obtained by a measurement after a transformer de-energization. However, in case of a transformer system that is switched by a circuit breaker equipped with a grading capacitor, the residual flux characteristic cannot be considered as constant. A source voltage will feed the deenergized transformer system through the grading capacitor, which will change the residual flux level and let oscillations appear. It follows that the optimal re-energization targets change and inrush currents might not be optimally minimized. Further, transient voltages based on line failures can influence the residual flux through a grading capacitor as well. At first, this paper evaluates the influence of a grading capacitor on the residual flux characteristic analytically. Further, measurements of two transformer systems at a varied de-energization instant and for different grading capacitors provide actual information for the development of future controller systems.
