Voltage stability index for under voltage load sheeding scheme

Abstract

Power system is a nonlinear system that operates in a constantly changing environment. When subjected to a disturbance, the stability of the system depends on the initial operating condition as well as the nature of the disturbance. During fault occurrences, the system may experience a change in frequency or a change in voltage levels or a combination of both. These may range from localize instability event to a wide system collapse. With the introduction of protection relays, theoretically these faults may be isolated. Nevertheless, uncertainties in power system operation and cascading failures may lead to situations where existing security measures cannot cope. Hence, the last possible mitigating action in this case would be using Under Voltage Load Shedding (UVLS) schemes. UVLS has been successfully deployed in many systems throughout the world to protect local systems from voltage collapse. This is particularly true if the system conditions and the contingencies leading to voltage instabilities are of low probability, but would result in serious consequences. This is analogous to Under Frequency Load Shedding (UFLS), which has become a common utility practice to cater to extreme situations resulting in generation deficiency and underfrequency. However, the locations and how much load to shed requires expert knowledge and experience working with the power grid. Hence, the research proposed to utilize an established index called Fast Voltage Stability Index (FVSI) to act as a numerical verification of the shedding locations. The research work done shows that the FVSI index can be used and load shedding at these points does improve the stability of the system. However, it is worth noting that the analysis conducted is only based upon static analysis hence the dynamic behaviour of the system is not included.