A Quantitative Evaluation of Security Indices for Nigerian National Grid System

Abstract

Security of a power system is the degree of risk and ability to survive imminent disturbances (contingencies) without interruption of continuous service. Security indices are parametric variables used to represent the degree of operation or malfunction of power system before the system faces interruption of service or the element faces outage or malfunction. A concept opposite to security is vulnerability concept. An element or a system is vulnerable if contingencies lead to an interruption of service at a point or the entire element or system. Vulnerability index (VI) and Margin index (MI) are quantitative security indices that provide comprehensive information about the individual parts and the whole system. This paper presents a quantitative evaluation of security indices for the Nigerian national grid Mathematical models were formulated for the two prominent security indices. Twenty four generators, Twenty four buses and Twenty four branches were selected as case studies on the Nigerian national grid system while their impacts on the vulnerability and margin indices were stressed. The vulnerability indices increased as more generators were added while the margin indices also decreased proportionately as the number of generators increase. The average value for the vulnerability index was 0.0275 per generator while the average margin index was 0.8073 per generator. The vulnerability indices increased as more buses were added into the system while the margin index between 6 and 7 buses remained constant at 1.0 suggesting that the buses appeared to be at optimum even though, as the number of buses increased, the margin indices decreased. The average vulnerability and margin indices for the buses were 9.921 per bus and 14.0495 per bus respectively. The vulnerability indices for the branches increased with increase in branches while the margin indices decreased as more branches were included in the system. The average vulnerability and margin indices for the branches were 0.1906 and 0.4640 per branch respectively.