Vulnerability analysis of transmission lines based on the performance of Mho relays and series/shunt compensation systems

Protective relays have a remarkable impact on the security of the transmission systems after various disturbances. Undesirable relay operation will trip the lines and lead to cascading events. The difference between the apparent impedance seen by the distance relay and the relay characteristic, i.e., operation margin is a measure to identify the vulnerable relays. In this paper, the sensitivities of the operation margin to bus voltage and bus power is applied to quantify the vulnerability of transmission system. It is shown that shunt compensation has considerable effect on the bus voltage, especially during transients. The contribution of the shunt compensation by STATCOM is analyzed based on the sensitivities of the operation margin to the shunt susceptance. The control effect of STATCOM is investigated when the MHO relay has relative smaller operation margin. Since series compensation has direct influence on bus power, the influence of series compensation, i.e., TCSC on the operation margin is also investigated.