Diagnosis of Voltage Instability Using BSDC Index in the Presence of Voltage Dependent Loads

In this paper, an algorithm for diagnosis of voltage collapse point is proposed. The algorithm is based on load characteristics, voltage and current measurements. When all loads at a bus are voltage independent type, the voltage collapse point coincides with Maximum Loading Point. In maximum loading point, with voltage and current changes, apparent power delivered to the bus does not increase. In these conditions, BSDC index that is based on the apparent power difference at the bus equals 0 in the voltage collapse point. If load characteristics are not considered, the BSDC cannot detect the voltage collapse point. In this paper an improved version of the BSDC for diagnosis of the voltage collapse when a part of the load is voltage dependent, is proposed. This modification is based on dividing the load into voltage dependent and independent parts. The algorithm is simple enough to implement in a protective relay. The presented algorithm was tested on the IEEE 30 bus test system. The results show the problem of the old method and the correction made by the proposed method.