A closed-loop reactive power controller for power system voltage stability

Abstract

Any imbalance between reactive power supply and demand disturbs the normal operation of a power system. Power system contingencies sometimes cause excessive reactive power imbalance, even leading to total system breakdown. The problem is usually aggravated when the system is heavily loaded. Generator overload due to excessive reactive power generation is one of the most common and serious causes of voltage instability. Low voltage at load buses is another cause which is likely to result in voltage stability problems. By closely monitoring the reactive power generation and load voltages, it is possible to avoid such major disturbances. A closed-loop controller can steer the system to a safer state by rescheduling the reactive power generation from generators and SVCs through voltage set point controls in real time. A closed-loop controller which dynamically steers the system to a safer state on violation of specified operating conditions of reactive power generation and load bus voltages is designed. During normal operation, the controller minimizes active power transmission losses due to reactive power transfers. The principle of the controller is based on a multi-variable control concept. A closed-loop controller is synthesized to steer the system dynamically to an acceptable state while satisfying Kuhn-Tucker conditions of optimality in steady state. The proposed closed-loop reactive power controller which gives set point control signals to AVRs and SVCs in the secondary mode of control, can be viewed as a parallel to the AGC for active power/frequency control. The controller performs this task on a real-time basis using SCADA measurements. The controller also issues load shedding commands in case it is necessary. The controller is tested under normal as well as contingency operation. Broad details of the closed-loop controller and its efficacy in avoiding system breakdown due to reactive power disturbance is demonstrated with examples.