DISTRIBUTION-SYSTEM MODELING FOR VOLTAGE STABILITY STUDIES

This paper presents a method to include a radial distribution system, with cascaded tap-changers, in voltage stability studies. A rural dist ribution system, including the 50 kV, 20 kV, 10 kV and 0.4 kV levels, has been studied. A detailed model of the network down to the customer level, with all lines and transformers explicitly represented, has be en compared with a simple model consisting of an impedance in series w ith a modified tap-changer. The response to a step change in the trans mission system voltage, has been studied for the entire distribution s ystem as well as for the model derived. The series impedance of the mo del is based on the losses in the distribution system, and the equival ent tap-changer is derived from the behaviour of the real tap-changers which operate in series and in parallel. The detailed network has als o been compared with other methods of including the distribution netwo rk in voltage stability studies. From the detailed model of the networ k an equivalent transfer function, describing the ratio of the equival ent tap-changer from 130 kV down to 0.4 kV, is derived.