Optimised and pre-emptive strategies for on-load tap changing transformer control relays

The paper presents two strategies that enhance the voltage control of on-load tap changing transformers (OLTC). Simulation results demonstrate improvements in the voltage recovery times following a disturbance and that the schemes do not incur additional tap changes. These schemes also avoid the use of communications between automatic voltage control (AVC) relays associated with the network. The source drop compensation strategy is developed to determine the voltage condition of the upstream feeder to optimise the time grading between series operating OLTCs. This reduces the time delays usually required to eliminate reverse tap changes and hunting between units. The scheme improves voltage recovery times and hence the voltage quality. The duty of responsibility algorithm has been developed to consider the probable cause of the voltage deviation so that it either initiates an early tap change or retains the conventional time grading. This pre-emptive tap changing strategy is used to initiate the early tap change and then grade any successive tap changes with an upstream voltage control relay. It therefore enables an OLTC associated with the load change to make any necessary tap changes without the traditional delays, restoring the voltage deviation faster without compromising OLTC efficiency.