A CLOSED-LOOP REACTIVE POWER CONTROLLER FOR POWER-SYSTEM VOLTAGE STABILITY

Any imbalance between reactive power supply and demand disturbs the no rmal operation of a power system. Power system contingencies sometimes cause excessive reactive power imbalance, even leading to total syste m breakdown. The problem is usually aggravated when the system is heav ily loaded. Generator overload due to excessive reactive power generat ion is one of the most common and serious causes of voltage instabilit y. Low voltage at load buses is another cause which is likely to resul t in voltage stability problems. By closely monitoring the reactive po wer generation and load voltages, it is possible to avoid such major d isturbances. A closed-loop controller can steer the system to a safer state by rescheduling the reactive power generation from generators an d SVC's through voltage setpoint controls in real time. A closed-loop controller which dynamically steers the system to a safer state on vio lation of specified operating conditions of reactive power generation and load bus voltages is designed. During normal operation, the contro ller minimizes active power transmission losses due to reactive power transfers. The principle of the controller is based on a multi-variabl e control concept: A closed-loop controller is synthesized to steer th e system dynamically to an acceptable state while satisfying Kuhn-Tuck er conditions of optimality in steady state. The proposed closed-loop reactive power controller which gives set point control signals to AVR s and SVCs in the secondary mode of control, can be viewed as a parall el to the AGC for active power/frequency control. The controller perfo rms this task on a real-time basis using SCADA measurements. The contr oller also issues load shedding commands in case it is necessary. The controller is tested under normal as well as contingency operation. Br oad derails of the closed-loop controller and its efficacy in avoiding system breakdown due to reactive power disturbance is demonstrated wi th examples. (C) 1997 Elsevier Science Ltd