Determination of Acceptable Closing Time Scatter and Residual Flux Measurement Uncertainty for Controlled Switching of Transformers

Theoretically, controlled switching taking into account the residual flux is able to fully eliminate inrush currents. Due to non-idealities in the field (closing time scatter, residual flux measurement uncertainty) that significantly affect the performance of controlled switching, inrush currents of 1.0 pu must be tolerated. To calculate the maximum acceptable tolerances of both nonidealities in this case, systematic energisation studies are carried out using a topological correct transformer model and the “Delayed Closing” strategy. The results show that there is a linear relationship between tolerable closing time scatter and maximum residual flux uncertainty. Consequently, the required accuracy in the determination of the residual flux can be calculated if the proposed analysis is carried out and the closing time scatter of the installed circuit breaker is known.