Distribution system modelling 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 distribution 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 been compared with a simple model consisting of an impedance in series with a modified tap-changer. The response to a step change in the transmission system voltage, has been studied for the entire distribution system as well as for the model derived. The series impedance of the model is based on the losses in the distribution system, and the equivalent tap-changer is derived from the behaviour of the real tap-changers which operate in series and in parallel. The detailed network has also been compared with other methods of including the distribution network in voltage stability studies. From the detailed model of the network an equivalent transfer function, describing the ratio of the equivalent tap-changer from 130 kV down to 0.4 kV, is derived.