The dependency of continuous electricity supply is high in the Swedish society today, at the same time no one is willing to pay for a too costly or over dimensioned power distribution system. The owners of the distribution systems are the ones responsible for this balance act of maintaining a high quality of electricity delivery to customers at a reasonable cost. In this master thesis a limited part of the primary distribution system (10 kV), owned by Mälarenergi Elnät AB and located in the city centre of Västerås, was chosen for a deeper analysis of its reliability. Its ability to handle outages of system components (N-1 and N-2 contingency analysis) was investigated to find out potential weak spots and parts of the grid that showed signs of being overdimensioned. The results of the performed simulations showed that in 74 % of the studied N-1 contingencies cases, consisting of outages of a bus bar in a distribution substation, the system could handle this without causing outages in other parts of the grid. For the N-2 contingencies 61 % passed the test. It was also found that one of the two included high voltage substations could alone supply the investigated part of the grid. Some cables and transformers were found to be more or less unnecessary, whereas other components proved to be overloaded in several situations.

This master thesis is done within the Energy Systems Engineering program at Uppsala University and performed for Forsmarks Kraftgrupp, Vattenfall. The distribution grid for the outer area of the nuclear power plant at Forsmark is an old grid from the time during construction of the plant which later became permanent. Many changes have been made to the grid thus far and Forsmark’s need for an updated model to easily simulate upcoming changes to the system has increased. In this thesis, the distribution grid for the outer area of Forsmark is modeled and evaluated from the aspect of load and selectivity. The simulation software PowerFactory has been used to model and perform the necessary network calculations. The results from these calculations will serve as source material for a future system health report for the distribution grid. The results show that some components in the system are overloaded in some modes of operation. A few buses have a fault clearing time that is too long and the system is not selective. The problem with fault clearing time can be solved with new settings on a protection relay and swapping changing out a few fuses. The selectivity in the system would also be greatly improved by the new relay settings.