Dynamic load-shedding for enhancement of power system stability for the Lesotho 132 kV transmission network

Abstract

The transient and dynamic stability are key elements that a system should satisfy to achieve stable system operation [1]. Fault type and location determine the impact that each fault puts on the system and whether power generators stay in synchronism. The Eskom-Mabote tie-line is key to satisfying Lesotho power system (LPS) peak load demand of 150 MW. Any faults on the tie-line that affect the power transfer capacity threaten the continuity of supply as they can result in system collapse. When clearing of faults is longer than the critical fault clearing time (CFCT), the generators lose synchronism and trip as a protective measure. The knock-on effect of system collapse contributes to Lesotho’s struggling techno-economic standing.The absence of a contingency to counteract the effects of the tie-line failure and so minimize the impact on the supply/demand mismatch, often results in frequency violations. Using DIgSILENT, this study investigates the impact of the tie-line failure during the peak load and proposes load curtailment measures by employing dynamic-load shedding to avoid complete system collapse. Secondly, the study outlines the solutions to overcome the existing supply/demand challenge during tie-line failure to minimize the recurrence of system collapse. Thirdly, the study investigates the causes of failure to resynchronize and the impact of live system load to satisfy the requisite resynchronizing requirements, viz. change in voltage, frequency, and load angle. The study also reflects on the impact of different amounts of live loads during restoration considering the hypothetical extreme cases.