Implementation of Expert System for Power System Voltage Stability Improvement

This paper presents a concept of an expert system to monitor and improve the voltage stability condition in a power system based on the L-index of load busses. This index uses the load flow analysis information from the steady state voltages condition and is in the range of 0 which is indicated for no load condition to 1 which is indicated for voltage collapse. For improving the voltage stability index the control variable consists of switchable shunt VAR compensators (SVC), OLTC transformers and generator excitation. The control of voltage collapse is based on reducing the magnitude of L-indices of critical bus for a given system operating condition based on heuristic criteria using the sensitivities (change in L-index) of load busses to different reactive power control variables. These sensitivities are stored in the knowledge base of expert system in the form of facts called voltage stability control lists. The expert system identifies the critical load busses whose L-index values are above acceptable limits. The expert system selects the corresponding voltage stability control list and recommends control action at the most effective compensator. The proposed expert system technique has been tested on a standard test system. Results obtained for a modified IEEE-30 bus standard test system are presented for illustration purposes. 