Application of Combinational Adaptive Load Shedding Schemes to Improve Power System Voltage Stability - Part I: General Concept & the Algorithms

In this paper three combinational adaptive load shedding schemes are proposed to enhance the power system stability especially voltage stability margins of the system following severe events. Nowadays, the security margin of power systems against various instabilities is decreased due to the developments, deregulation and competitions in the power industry. In this situation, traditional system protection schemes can not offer adequate protection especially against combinational events. In some combinational disturbances, after initial frequency drop the conventional protection schemes returns back the system frequency to its permissible values; however, the system eventually collapses due to severe voltage declines which result in voltage instability. In some other disturbances, severe voltage declines cause troubles in appropriate operation of the under frequency load shedding relays. In this paper three adaptive combinational load shedding schemes are proposed to counteract such disturbances. The proposed schemes use locally measured frequency and voltage signals and do not need any communication link. In the proposed algorithms, during under frequency condition, load shedding is started from the locations which have higher voltage decay and for longer period of time. The speed, location and amount of load shedding are changed adaptively depending on the disturbance location, voltage status of the system, and the rate of frequency decline. In the second part of this paper using model of a real network, various simulation studies are performed and performance of the proposed schemes is investigated.