The Modeling Requirements for Short-Term Voltage Stability Studies

Transmission planners are now very familiar with PV and VQ analysis. In fact, transfer limitations and new transmission facilities are now based on the problems found with this analysis. While these two forms of analysis are valuable, another form of analysis should also be performed to insure that critical problems are found. Transmission planners should analyze the same loadflow outages using their stability programs. What they will find in some of the cases are more critical problems than their PV and VQ loadflow analysis shows. Similar to generator stability analysis, short-term voltage stability analysis should be used to determine if the transmission system can survive three-phase fault on critical elements. To do this type of voltage stability analysis properly, the usual stability loadflow base case and stability modeling data will need to be modified. Modeling of the loads as constant current and constant impedance will need to be changed so that the impact of motors and other devices can be included. The modeling of the distribution step-down transformers and some distribution feeder impedances also need to be added. Lastly, the dynamic capability of the generators, especially over excitation limiters, needs to be accurately modeled. This paper will show how the above items can be modeled quite easily, and how a scan of a utility´s transmission system can be undertaken to determine if any short-term voltage stability problems exist