Power System Stability Using Decentralized Under Frequency and Voltage Load Shedding

Load shedding (LS) is the last emergency control action against voltage and frequency instability or even system blackout. In conventional Under Frequency Load Shedding (UFLS) relays, LS is carried out by curtailing a fixed amount of load when the frequency declines below a set of predefined thresholds and hence, may not be adaptive to disturbance scale. Besides, due to equality of the frequency throughout the entire power system, there is the possibility of coincidence operation of relays and perhaps over load shedding. Furthermore, the LS locations are already determined independent of disturbance location. In this paper, to aim adaptive performance of the relays, the amount of load shed in each stage is determined proportional to the measured Rate of Change of Frequency (ROCOF). In order to localize the LS scheme close to the disturbance place, this scheme utilizes the voltage drop information to shed the loads with higher voltage decay first. Therefore, this approach deals with coordination of voltage and frequency information instead of independent methods. Numerical simulations which are carried out in DigSilent PowerFactory software confirm the efficiency of proposed methodology to stabilize the power system after a severe contingency.