High speed transmission line fault impedance calculation using a dedicated minicomputer

An algorithim for calculating apparent transmission line impedance to the point of a fault is presented as an approach to distance type protection via a dedicated digital computer. Phase voltages and currents are sampled asynchronously approximately 24 times per cycle and operated on to yield apparent resistances and reactances. Descrimination between phase to phase, phase to ground and two phase to ground faults is considered. The effects of disturbances such as transformer magnetizing inrush currents, assymetrical currents, sensor response, as well as results which can be expected when the power system frequency is higher or lower than nominal are discussed. The reach accuracy of the system is reported using simple statistical measures obtained from simulating the operation of the digital relay on an IBM 360/65 computer. These simulations verify the approach is feasible.