Power systems are in a phase of automation where intelligent components and sub-systems are employed to monitor, control and manage the grid. Quantifying the complex consequences on system reliability, from the integration of such automated and semi-automated equipment into the existing grid is important for maintenance optimisation and fault mitigation. This thesis identifies the  advanced approaches in power system reliability analysis with the potential to capture the complications and correlations in modern power grid after reviewing the traditional reliability evaluation methods.

A method for modelling the different modes of failures, possible in a substation and feeder architecture along with the probable false tripping scenarios was developed. An improved Reliability Block Diagram based approach was designed to count in the traditionally unaccounted failure cases affecting both the primary grid and the protectionand control equipment. The effect and corresponding trend of additional feeder lines in a radial distribution system on the net interruption rate experienced at load ends of feeders are derived and modelled. Such real-world substation architectures are analysed and the aforementioned trends are compared with those from the practical grid. Thus, the analysis was able to identify and measure the complex hidden failure probabilities due to both unwanted operation of breakers and functional failure of protection systems.

The measured probabilities were used to calculate the impact of protection and control equipment on system  reliability. The obtained results were verified by comparing it with the observations by energy researchers on ten years of protection system failure statistics. The application of the model and results, in optimal maintenance planning and power network optimisation are identified as the next step.

Power distribution networks are recognized as the constituent part of power systems with the highest concentration of failure events. Even though the faults in distribution networks have a local effect when compared to the generation and transmission sides, major contingency escalation events are being more frequently reported from this section. The various aspects regarding the reliability and performance of distribution networks are identified as an important topic. Integration of new technologies, automation and increased penetration of distributed generation is expected to make improving and even sustaining high reliability standards a complex task.

This thesis presents developed approaches to quantify and analyze the complex correlated failure probabilities of different failure modes in distribution networks. A theoretical simulation model that relates to real world data to measure false tripping probabilities is developed and tested. More simplified approaches that utilities can exercise with readily available data in fault registers are also established.  Optimal configurations that could improve system performance and respective investment costs are analyzed and savings in system reliability at the cost of grid investments are modelled. The optimization helps in prioritizing the most critical investments by considering the system impact of reconfigurations focusing on meeting customer demands and respecting transfer capacities of weak links. The value of existing networks and willingness of the grid owner in investing can be integrated into suggestive alterations to assist decision making in planning and maintenance allocation.

The thesis makes both system specific and generalizable observations from detailed data collection from power utilities. The observations and results have potential in aiding future research by giving important understanding of the reliability impacts of network structures and of control and protection equipment.

Eldistributionsnätet är den del av elnäten som leder till flest kundarbrott trots att felhändelserna oftast är mycket lokala jämfört med händelse på genererings och transmissions sidan. Alltså identifieras de olika aspekterna beträffande pålitlighet och prestanda för distributionssystem som viktiga ämnen. Integration av ny teknik, automatisering och ökad penetration av distribuerad produktion förväntas göra förbättringar och rent av upprätthållande av höga tillförlitlighetskrav till en komplex uppgift.

Denna avhandling presenterar metoder för att kvantifiera och analysera de komplexa och korrelerade sannolikheterna för olika fellägen i distributionsnätet. En teoretisk simuleringsmodell baserrad på verkliga data för att mäta sannolikheter för felaktiga brytarkommandon utvecklas och testas. Mer förenklade tillvägagångssätt där elnätsföretag kan använda lättillgängliga data i felregister presenteras också. Optimala konfigurationer som kan förbättra systemprestandan och investeringskostnader analyseras och minskningar i systemets tillförlitlighet genom minskade kostnader för nätinvesteringar modelleras. Optimeringen bidrar till att prioritera kritiska investeringar genom att påvisa systemets inverkan av omkonfigurationer. Optimeringen tär hänsyn till kundernas krav och att bevara överföringsförmågan hos svaga länkar. Värdet av befintliga nät och villigheten hos nätägaren att investera kan undersökas som förslag, till ändringar, som stöd detta för beslut om planering och underhåll.

Avhandlingen gör både systemspecifika och generaliserbara observationer från en detaljerad datainsamling från elnätägare. Observationerna och resultaten har potential att hjälpa framtida forskning genom att ge en viktig förståelse för tillförlitlighetseffekter från nätverksstrukturen och från kontroll- och skyddsutrustning.