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In the recent years, operation of power systems at lower stability margins has increased the importance of system protection methods that protect the system stability against various disturbances. Among these methods, the load shedding serves as an effective and last-resort tool to prevent system frequency/voltage instability. For major combinational disturbances, the active power deficit is usually accompanied by reactive power deficit. Therefore frequency stability and voltage stability of the system are jeopardised simultaneously. For such cases, the conventional non-adaptive underfrequency load-shedding scheme cannot offer adequate protection to prevent system collapse. In this study a new combinational load shedding method is proposed to increase the security of power system during large disturbances. The proposed method uses locally measured frequency and voltage signals and does not need any communication link. Performance of the conventional scheme and the proposed load-shedding method using different load shedding criterions have been simulated in two test systems: an actual large network and a standard test system. Obtained simulation results confirm that the proposed schemes provide considerable enhancement in the power system voltage stability margin and by using the proposed method various power system blackouts could be prevented.
Date of Publication: May 2011