Transients in distance protection

The demand for high-speed operation in distance protection involves design consideration under three main headings, namely: the transient primary current and voltage at the relaying point; the modification of these signals due to secondary circuits; and the effect of the modified secondary signals on two input comparators currently or prospectively employed. These are considered in terms of simple formulation showing their effect on voltage transformers and current transducers (whether current transformers or linear couplers). Alternative means of eliminating, or equating, transient secondary current and voltage signals are considered. The use of secondary replica impedances is formulated, and the effect of the relaxation transient in the voltage-transformer secondary circuit and its relation to transient time delay are pointed out. General consideration is given to the effect of secondary transient signals, containing components of different time-constants determined, respectively, by the primary and the secondary circuits, and of unequal amplitude, on currently used comparators.