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

Abstract

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.

Responsiveness of Multi-Channel IEC-61850 Substation Communication Network Reliability Performance to Changes in Repair Factors

“…Hence, high reliability is needed to enable execution of safety-related trips and commands as and when they are issued. In reliability studies, the use of the combinatorial methods to evaluate the reliability of IEC-61850 based SCN is preferred, owing to the ease of applying the methods [1], [2].…”

“…Nevertheless, the disadvantage of the methods is that repairs are assumed to be completed and correctly executed. Also, it is assumed that all system faults are known; whereas it is not the case in practice [2], [3]. In a thermal process plant, the control, monitoring and protection of auxiliary boiler systems is implemented through a dedicated Distributed Control System (DCS) and BPS, respectively.…”

“…As a Safety-Related System (SRS), BPS is required to meet the requirements of the IEC-61508, which is the standard for SRS. The standard requires that a system diagnostic coverage be considered in the evaluation of SRS reliability performance, where power distribution centre equipment form part of the final element of the SRS to isolate power supply to electrical machines [2], [4].…”

“…In [2], The reliability of IEC-61850 based SCN is modelled using Markov, where imperfect repairs and system diagnostic coverage are integrated into the model using Systems Thinking approach [2]. The Markov model approach enables the impact of both repair efficiency and system diagnostic coverage on the system reliability performance and dynamical behaviour to be investigated based on the concept of linear dynamical systems through the observation of the eigenvalues of the system [5].…”

“…The technique presented in this paper advances the eigenvalue analysis method, it uses sensitivity analysis to evaluate the responsiveness of the system reliability performance to incremental changes in repair efficiency and system diagnostic coverage [1], [2], [5], being cognizant that any factors of interest can be modelled based on the desired level of system abstraction. Hence, the sensitivity analysis approach enables the determination of the impact of imperfect repairs between any two system states; which in turn enables system optimisation at subsystem level compared to the eigenvalue method that only determines high-level behavioural dynamics based on the layout formation of the eigenvalues and the spectral gap of the second largest eigenvalue(s) [8].…”