Online voltage stability and load margin assessment using wide area measurements

A power system is continuously exposed to changes in load and operating conditions. This results in voltage variation which may lead to instability at times. Therefore, it is required to assess the voltage stability continuously for which a stability index is used. This paper uses phasor measurement data from selected buses to estimate the states of the system. Thevenin equivalent of the system is calculated and a voltage stability index obtained using ratio of change in load power to change in voltage magnitude. The index provides how close the system is towards instability, following load variation and/or change in topology. When the proposed index is equal to 1, the system reaches voltage collapse point. For any lower value of the index, the system is in stable region and when it approaches 1, it is vulnerable to collapse. Load margin assessment is performed when the system is voltage stressed and obtained for selected stressed buses using a least square technique. Load margins of buses around the stressed buses are also calculated which is useful in situational awareness of a system. Several case studies are performed on 39 bus New England test system and 233 bus North Eastern Indian grid by varying load and topology of the system. Different types of loads are also considered in the testing. The methods for stability index calculation and load margin assessment are found to be accurate. Comparative performance of the proposed technique with available method is also presented.