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Top1. Introduction
Voltage collapse may occur in a power system due to lost in voltage stability in the system. Voltage collapse is the phenomenon of voltage instability that can appear in a transmission or distribution system operating under the heaviest loading conditions, in which the voltage decreases monotonically leading the system to be blackout. While in normal operating conditions, small loads increase causes a small voltage drop but if the entire network or a particular node is over a certain critical load level; further loads increase causes a fast decrease of the voltage which suddenly leads the system to the collapse. Therefore voltage stability analysis is important in order to identify critical nodes in a power system i.e. nodes which are closed to their voltage stability limits and thus enable certain measures to be taken by the control engineer in order to avoid any incidence of voltage collapse.
Voltage stability is a major concern in planning and assessment of security of large power systems in contingency situation, especially in developing countries because of non-uniform growth of load demand and lacuna in the reactive power management side (Van Cutsem, 1991). Radial distribution systems (Jasmon and Lee, 1991; Haque, 1995) having a low reactance to resistance ratio, which causes a high power loss. Hence, the radial distribution system is one of the power systems, which may suffer from voltage instability.
In the deregulated power market, electric utilities are now continuously searching new technologies to provide acceptable power quality and higher reliability to their valuable customers. Non-conventional generation is growing more rapidly around the world due to its small size, low cost and less environmental impact with high potentiality. Investment in distributed generation (DG) enhances economical, technical and environmental benefits.
Literature survey shows that a lot of work has been done on the voltage stability analysis of transmission systems (Ajjarapu and Lee, 1998), but little work has been done on the voltage stability analysis of radial distribution networks (Ranjan et al., 2003). Gubina and Strmchnik (1997) and Jasmon and Lee (1991) have studied the voltage stability analysis of radial networks but all of them have represented the entire network by a single line equivalent. The single line equivalent derived by these authors (Gubina and Strmchnik, 1997; Jasmon and Lee, 1991) is valid only at the operating point at which it is derived. It can be used for small load changes around this point. But due to highly nonlinear power flow equations of a simple radial system, the equivalent would be inadequate for assessing the voltage stability limit. Also their techniques (Gubina and Strmchnik, 1997; Jasmon and Lee, 1991) do not allow for the changing of the loading pattern of the various nodes which would greatly affect the collapse point.