Switching large transformers on weak transmission networks — A real time case study

Transformer energisation is a common practice for the transmission of high voltage electrical power over long distances. Switching large power transformers arbitrarily on a system with high impedance or relatively low fault current levels drives the transformer core into saturation resulting in high inrush currents [1]. The magnitude of inrush currents can be very high especially when the transformer core has residual flux which adds to the core flux build-up after energisation [2]. This inrush current results in voltage drop at the transmission network and the level of voltage drop depends on the short-circuit MVA level at the source bus and the magnitude of the transient inrush current. This paper is based on a real time event that took place at Langage Power Station located in the south west peninsula of the UK in January 2010. Two high voltage generator step-up transformers were energised simultaneously resulting in high voltage drop at the transmission network thereby affecting the power quality of the system. A detailed MATLAB model has been developed to simulate the switching event. Simulations were carried out considering different system conditions to clearly demonstrate the impact of inrush current on voltage drop. The results have been produced for different scenarios considering various factors such as system impedance, residual flux and short circuit MVA level at the source network which would impact on the magnitudes of inrush and sympathetic inrush currents. Suitable mitigating measures were recommended and the advantages and disadvantages of each of the measures have also been discussed.