Protection system in a power system domain plays a vital role as watchmen to safe guard power system apparatus in the event of faults. The integrity and effectiveness of these watchmen is heavily dependent on the reliability of the components that makes up the system. One of the main components in a protection system is protective relays that provides decision making element for correct protection and fault clearing operations. Failure of the protection devices may reduce the reliability of the power system protection that will impact the overall performance of the power system. Hence it is imperative for power utilities, such as Tenaga Nasional Berhad (TNB) to assess the reliability of protective relays to assure it will perform its intended function without failure. This paper will discuss the application of reliability block diagram (RBD) to assess the reliability and availability of a typical 132kV transmission overhead line protection system using historical failure data of protective relays.

The integrity of high voltage transmission lines is extremely important to ensure the continuous electricity supply throughout the country. Faults, such as lightning strikes and flora and fauna encroachments to the transmission lines may cause disruption of electricity supply which will affect the overall power system and lead to a wide scale blackout. Therefore, fault clearing system is deployed to minimize the impact of the faults to the power system by disconnecting and isolating the affected transmission lines specifically. One of the main components of fault clearing system are the protective relays, which are vital in providing the decision making element for correct protection and fault clearing operations. As the electric utility company in Malaysia, Tenaga Nasional Berhad (TNB) is obliged to assess the reliability of the protective relays considering its importance to the fault clearing system. In this paper, a statistical method called Life Data Analysis using Weibull Distribution is applied to assess the reliability of the protective relays. Furthermore, the fault clearing system is modeled using Reliability Block Diagram to simulate the availability of the system and derive reliability indices which will assist TNB in managing the fault clearing system.