Electrical operational inteilocks for a fossil-fired power station auxiliary system

The paper reviews the derivation of interlock logic and the resultant interlock ciicuits for conventional double-busbar substations. Some of the assumptions made in deriving this logic are then shown to be invalid for highly complex circuits, complicated by the presence of generators and large induction motors, such as may be found in a power-station auxiliary system. A method of interlocking such a system so that its fault levels are always within the capacity of the circuit breakers is then proposed. This method, based on a semi-empirical approach to fault level computation, relies on a microprocessor to compute fault levels for all busbars on the system. Operation of a switch is then only permitted if no rated fault level would be exceeded as a result. The application of this technique to the Drax coal-fired power station is then given as an example and shown to be both sufficiently fast and accurate to be used as an online interlocking device.