A Differential Line Protection Scheme for Power Systems Based on Composite Voltage and Current Measurements

Current Differential schemes for two terminal EHV transmission systems using digital techniques have the disadvantage that because they are totally current dependent, they are often insensitive to low levels of fault currents. A principal cause of this is due to the necessity to provide a bias signal proportional to through current to ensure security for external faults. This paper describes the basis of an alternative approach to Conventional Differential protection that involves deriving differential signals as functions of both voltages and currents at each end of a circuit. This approach obviates the need for relay bias, thus improving relay performance. The scheme is based on Master and Slave principles and utilises a standard Fibre Optic Link (FOL) as a means of communication between ends. Fig. 1 shows a block schematic of the complete protection scheme. The basic relay operating principle hinges upon deriving differential signals STk and SRk at the two ends T and R of the circuit and these are shown in terms of primary values as: