Power system stability using decentralized under frequency and voltage load shedding

Load shedding (LS) is the last emergency control action against voltage or frequency instability or even system blackout. Conventional LS relays, often rely on only frequency data and 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. Besides, the LS locations are already determined regardless of disturbance location and may not be adaptive to the event scale and place. This paper addresses the decentralized coordination of Under Frequency Load Shedding (UFLS) and Under Voltage Load Shedding (UVLS) schemes in the LS relays instead of independent methods. The independent and constant frequency and voltage stage thresholds are merged to achieve a united and new elliptical thresholds. Since in addition to frequency, the voltage drop information is also employed in the new method, the load curtailment is started from the vicinity of failure point and radially propagates in the network until not only the frequency collapse is completely prevented, but also the frequency settles down in the permissible range. 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.