Reliability and Availability of Multi-Channel IEC-61850 Substation Communication Networks for Mission-Critical Applications

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.