Determining a Robust Lower Bound on Voltage Magnitudes to Enhance Voltage Stability

The bounds safeguarding the steady-state voltage stability of a power system must resist a wide variety of operating conditions. While the upper voltage magnitude bound is given by hardware restrictions, the lower bound must ensure that the loadability limit of the system is not exceeded. Capturing this limit for different load and generation patterns in a single voltage magnitude value is complex. In this work, the operating point which leads to the optimal lower voltage bound is first determined, and an iterative method for identifying this operating point is proposed. The method traces the transfer limit surface, which separates the feasible from the infeasible region of the parameter space, such that the minimum voltage magnitude at the maximum loading point increases. Both saddle-node bifurcations and limits caused by generators reaching their reactive power limits are taken into account. The method is validated on an IEEE 9-bus test case and then applied to the actual Swiss transmission grid with more than 300 buses.