Adaptive undervoltage load shedding relay design using Thevenin equivalent estimation

Power transmission systems of today are operating closer to their limit. In the event of a developing blackout, the disconnection of loads under controlled conditions can maintain system´s stability. Undervoltage load shedding (UVLS) is an economical solution to the voltage stability problem, and provides protection for unusual disturbances outside the planning and operating criteria. In this paper, system Thevenin equivalent estimation is firstly presented. Thevenin equivalent calculation based on varying system condition is analyzed and recursive least square estimation technique is applied to estimate Thevenin equivalent online. UVLS relay placement using modified maximum power transfer theorem is illustrated. Load shedding amount and load shedding time are determined by P-Q curve adaptively on-line. Finally, the proposed algorithm of the adaptive UVLS scheme is tested in New England 39 and Taipower systems using PSS/E combined with MATLAB to show its effectiveness.