| It is an availability artifice to use series compensating capacitors (SC) toreduce the transfer reactance between the buses to which the line is connected, increase the maximum power that can he transmitted and improve the system stability, as well as to reduce the engineering cost of transmission system. Especially for long-distance transmission, it can considerably increase system stability margin and optimize the load distribution among lines. Protective relay is a very important component for the stable operation of transmission systems, it can disconnect the faulty lines timely, correctly and selectively, so as the system can return to normal operation quickly and in possibly maximum limit. But the series capacitor's switch-in and switch off will change the line impedance and influence the measurement and operation of the relays which performance are based on the line impedance characteristic.This paper studies the influences of series compensating capacitors on the protective relaying, the causes of these matters are analyzed and the ways to settle them are found out. The study mainly includes the following aspects: the characteristic of series compensated lines; the influences of SC on the line distance protection and on the distance relaying of power frequency variables; the influences of SC on the zero-sequence and negative-sequence relays; also the SC influences on the adjacent uncompensated lines. The measures to eliminate the aforementioned influences are also included. At the end of this paper, some simulations are carried out on Guangxi SOOkV system to illustrate the effects of series compensating capacitors, the engineering application of NARI RCS-902AS protective devices in TianShengQiao—PingGuo line with TCSC is used tofurther demonstrate this problem.
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