Performance Investigation of New Combinational Load Shedding Schemes

Modern power systems are operating at ever-smaller capacity and stability margins due to limitations imposed by development and deregulation. In this situation, traditional entities involved in securing the power system, such as underfrequency load shedding scheme have become inadequate. Sometimes after initial frequency drop due to some abnormal events, system frequency is returned back to its acceptable values, however system eventually collapses due to severe voltage decline which results in voltage instability. In this paper two combinational load shedding methods are proposed. The purpose of these methods is to enhance the flexibility of under frequency relays and increase the security of power system during large disturbances by improving system voltage stability margins. The proposed methods use locally measured frequency and voltage signals to automatically select the best locations of the system for load shedding during under frequency conditions. The proposed load shedding methods are tested using dynamic model of an actual large network. Performance of these methods is compared with each other and also with the conventional method by several simulations in different operating conditions. Simulation results indicate that the proposed methods enhance voltage stability margins especially for weak points of the system.