The electric grid is important in modern society, so the outages should be few. The transmission grid should have a real high availability, since it transports electrical energy across the country. Then, it is difficult to plan outages for maintenance and installation. This thesis lists twenty surveys about failures of circuit breakers and disconnectors, with data about the cause and the lifetime. It also contains a survey of about forty condition-measuring methods for circuit breakers and disconnectors, mostly applicable to the electric contacts and the mechanical parts.

As there is no published method to monitor the electric contacts at voltages around 400 kV, a system with IR sensors has been installed at the nine contacts of six disconnectors. The purpose is to avoid outages for maintenance by estimating the condition of the contacts while the disconnectors are still connected to high voltage. The measured temperatures are collected by radio and regressed against the square of the current, since the best exponent for the current was found to be 2.0. The coefficient of determination $R^2$ is high, greater than 0.9. This gives for each contact a regression coefficient, which can serve as an estimate of the condition of the contact. The higher the coefficient is, the more heat is produced at the contact, which can damage the material. This enables a ranking of the different contacts and shows which the worst ones are. This coefficient can also give a dynamic rating of the disconnector.

Elnätet är viktigt i det moderna samhället, så antalet avbrott bör vara lågt. Särskilt stamnätet bör ha en särskilt hög tillgänglighet, då det transporterar elektrisk energi tvärsöver landet. Då blir det svårt att planera avbrott för underhåll och utbyggnad. Denna avhandling tar upp tjugo studier av fel i brytare och frånskiljare med data om felorsak och livslängd. Den har också en översikt av ett fyrtiotal olika metoder för tillståndsmätningar för brytare och frånskiljare, som huvudsakligen rör de elektriska kontakterna och de mekaniska delarna.

Eftersom det inte finns någon beskriven metod för att övervaka elektriska kontakter för spänningar på runt 400 kV, har ett system med IR sensorer installerats på de nio kontakterna på sex frånskiljare. Målet är att minska antalet avbrott för underhåll genom att skatta skicket hos kontakterna, när frånskiljarna fortfarande är kopplade till högspänning. De uppmätta temperaturerna tas emot genom radio och behandlas genom regression mot kvadraten av strömmen, då den bästa exponenten för strömmen visade sig vara 2,0. Förklaringsfaktorn $R^2$ är hög, över 0,9. För varje kontakt ger det en regressionskoefficient, som blir en skattning av kontaktens skick. Ju högre koefficienten är, desto mer värme utvecklas det i kontakten, vilket kan leda till skador på materialet. Kontakternas olika koefficienter ger en rankordning dem och därmed en möjlighet att prioritera vilken frånskiljare som ska underhållas först. Systemet kan också användas för att minska eller öka den tillåtna strömmen baserat på skicket.

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.

The electrical power grid is one of the most important infrastructures in the modernsociety. It supplies industrial and private customers with electricity and supportsother critical infrastructures such as the water supply. Thus, it is significant that the power grid is a reliable system. However, the power system experiences a hugetransition from classical production methods such as coal and nuclear power plantsto distributed renewable energy forms such as wind energy and photovoltaic. This change to a more distributed system challenges the existing power grid as well as the traditional business models of electric utilities. Thus, cost minimization to increase profitability and improvement of the power grid to increase customer satisfactionare in the focus. One approach to increase the reliability of the grid and decrease maintenance costs is a condition-based maintenance approach which requirescondition monitoring techniques.

This thesis introduces into failure rate modelling for individual power system components and develops a method to calculate individual failure rates based onthe average failure rate, failure statistics, and condition monitoring data. This approach includes the analysis of failure statistics to identify failure causes and failure locations which are population characteristics but can be utilized to describe the heterogeneity within the population. Thus, the thesis first introduces into the topic of failure analysis and heterogeneity in populations. Different factors are identified and categorized which describe the condition development of a component overtime. Then, the literature within failure rate estimation is reviewed to present the factors which are used within failure rate modelling and to outline the existingmethods which consider the individual. However, limitations are discussed which emphasize the demand for a new approach. Consequently, this thesis introduce intoa new approach for estimating the failure rate for individual components.