Reliability Assessment of Power Distribution Networks Incorporating Circuit Breaker Failure Modes

Power supply interruptions incur significant financial losses to utilities and their customers. Most of the interruptions arise from the failure of components in the power distribution system. Circuit breakers (CB) are primarily used to protect the system against failure of components and disturbances in the system. A CB can randomly fail in various failure modes, such as passive failure, active failure, and stuck conditions. An active failure or a stuck condition of a CB triggers the operation of circuit breakers in the backup protection zone and causes outages of a wider area of the distribution network. An active failure or a stuck condition of a CB in a faulty feeder will incite and extend unreliability events to several adjacent healthy feeders. Existing reliability evaluation techniques to incorporate active failure and stuck conditions can be applied to simple radial networks where the impacts of component failures can be easily identified. However, most urban utility distribution systems and large industrial/commercial power distribution networks are very complex with meshed configurations and a large number of components of various characteristics. Most existing reliability assessment software tools only consider first-order contingencies and may include active CB failure. Meshed distribution networks are designed with inherent redundancies to meet their high-reliability needs, and therefore, it is necessary to incorporate higher-order contingencies including active failure and stuck condition of CB in reliability assessment in order to determine investment options to continuously meet the reliability requirements of the customers. In this context, this thesis presents generalized analytical methodologies to incorporate active failure and stuck breaker condition into reliability studies of distribution systems which can be systematically implemented in a computer program. To consider the active failure of the breaker, this thesis presents a methodology to identify an active breaker incidence matrix that captures the active failure of the breaker leading to load point failures. The methodology concatenates this incidence matrix with the existing incidence matrix created from the paths between input power supplies and load point to form a modified incidence matrix. This modified incidence matrix is used to identify all orders of contingencies involving the active failure of the breaker. To consider the stuck condition of the breaker, the methodology identifies the set of stuck breakers whose backup breakers are the breakers in the path between supply and load point. If any of the breakers in the set utilize at least one breaker of all the paths as a backup breaker, the breaker is supposed to cause the failure of the load point due to stuck condition. This thesis also presents the application of the proposed methodologies to compare several reliability investment alternatives for reliability enhancement of a typical industrial and commercial distribution network provided in the IEEE Std 493-2007. The methodologies presented in this thesis offer the ability to conduct a range of alternative studies involving CB upgrades, protection system upgrade, maintenance scheduling upgrade, installation of backup supply and uninterrupted power supplies to make the reliability-based investment decision for power utilities and industrial and commercial customers