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Reliability analysis of breaker arrangements in high voltage stations: A fault tree approach

State Enumeration technique for reliability evaluation is the process of selecting and
assessing the impact of all possible contingencies in a given system. It starts with first
order failure states to higher orders depending on the resolution requirement. In
general higher order failure states do not contribute significantly to the results and are
usually neglected from an engineering viewpoint.
Fault Tree Analysis (FTA) is a deductive analyzing technique that allows assessing
the reliability of complex systems. FTA has been used in the nuclear power,
aerospace and railway industries for evaluating the safety and reliability of complex
systems. In an FTA analysis, it is postulated that the system has failed in a certain way
(top event) and an attempt is made to find out what modes of subsystems behaviours
contribute to this failure.
In this thesis the reliability of high voltage breaker arrangements is studied using two
alternative techniques implemented in two different simulations tools, namely Subrel
and Riskspectrum. Dynamic state enumeration and fault tree analysis are used to
perform reliability studies of two breaker arrangements. The breaker-and-a-half and
double-breaker-double-bus arrangements are assessed and compared. The thesis
introduces a modeling technique for analyzing breaker arrangements via fault tree.
The modeling technique and corresponding results from the fault tree analysis are
validated against those obtained from Subrel. Uncertainty in reliability data is taken
into consideration by randomizing failure rate and time to repair in Riskspectrum. The
reliability performance of breaker arrangements is compared and discrepancies are
explained. It is concluded that the reliability of a breaker and half arrangement is in
principle equal to that of a double breaker double bus.