The use of Substation Communication Networks (SCN) in power distribution centres of thermal power plants for protection, control and monitoring is increasing as industry’s confidence grows in the application of the IEC-61850 based Substation Automation Systems (SAS). IEC-61508, which is the standard for functional safety, addresses Electrical/Electronic/Programmable Electronic (E/E/PE) safety based technologies in so far as safety lifecycle as well as reliability and verification of safety related systems are concerned. It follows therefore, that Substation Communication Networks (SCN), as well as associated substation equipment that forms part of the overall safety system should satisfy the requirements of functional safety standard IEC-61508. In this paper, Substation Automation System standard (SAS) IEC-61850, Substation Communication Networks (SCNs) as well as dependability evaluation methods are reviewed and discussed.

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The introduction of IEC-61850 digital-based Substation Automation System (SAS) eases implementation of elaborate schemes; however, its reliability and availability continue to be investigated for executing mission-critical applications. Independent repairable multi-channel systems with voting capability such as ‘one-out-of-two’ tripping schemes are often used for critical safety-related functions because the individual scheme channels hardly ever fail simultaneously. The system configuration enables the scheme to self-reconfigure when a link failure occurs in one of the channels, as well as being repairable with no interruption to the overall mission. This paper reviews the reliability and availability of evaluation methods to highlight their advantages and disadvantages. Structure-function modelling, as well as Markov process incorporating Systems Thinking and Mathematical Expectation are used to model the reliability of IEC-61850 based SCN to demonstrate the shortcomings of combinatorial methods in the evaluation of mission-critical systems where diagnostic capabilities of the systems and imperfect repairs should be considered. It is evident from the results of the study that system diagnostic capability and repair efficiency cannot be ignored for mission-critical applications for the reason that the simplifying assumptions of combinatorial analysis methods greatly over-state the reliability and availability performance of the system as observed from the results.

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