Fast monitoring and optimal preventive control of voltage instability

Abstract

This paper discusses a new method for monitoring and controlling to prevent voltage instability in electrical power systems. Voltage instability phenomena or voltage collapse may occur at the critical loadflow point in a static sense. From the system operator's viewpoint, this critical point must be carefully monitored in a heavily loaded power system. In addition, the operators need to take an adequate control action when the current operating state approaches the critical point. These processes require very fast computation. Hence special attention is paid to the required computing time and on-line applicability. Monitoring methodology proposed here is based upon the multiple loadflow solutions and sensitivity analysis. The complex-valued multiple loadflow calculation method, which has already been developed by the authors, is free from the numerical instability in calculating the lower voltage solution. By using this algorithm, the operating personnel can monitor the lower voltage loadflow solution corresponding to the current operating condition. The critical point is calculated approximately using these two loadflow solutions, and the load forecast very quickly for on-line usage. The estimation error of the critical point becomes very small as the operating state becomes closer to the critical point. The existing preventive control against voltage collapse is based on the operator's experience, not on a theoretical basis. The preventive control proposed here gives the optimal operations of various kinds of control equipment in the sense that the total demand margin is enlarged most effectively. Control variables consist of voltage/active power setting values at generator terminals, switched capacitors, and tap ratios of OLTCs (on load tap chargers). The resultant control action satisfies all the constraints about bus voltages, power flows, etc. Its effectiveness is demonstrated by using several numerical examples.