The fundamental characteristics of pilot-wire differential protection systems

In the design of pilot-wire protection systems the pilot circuit is usually considered to be either purely resistive or completely balanced and able to transmit information simultaneously from end to end. Characteristics are given for long practical circuits showing that these are invariably unsymmetrical owing to their heterogeneous composition, and it is shown that the change in pilot-circuit input admittance due to remote-end voltage variation gives a reliable indication of design parameters. A locus diagram is given for pilot-circuit admittance upon which the relay operating characteristics can be superimposed, giving complete design information for the protection scheme. By a change of axis this diagram can be redrawn in terms of power-system input currents to the relay system, showing the performance of the relays at both ends of the line. Consideration is given to the design of practical protection schemes using the longest pilot circuits, which are treated as completely unsymmetrical networks. The effect of the non-linearities in the voltage-limiting circuits is also considered. A design technique is illustrated which enables the protective scheme characteristics to be predicted and verified by simple tests. Practical limits and limitations inherent in the use of rented telephone-type circuits are discussed briefly, since these exert considerable influence on practical scheme design.