The aim of this M.Sc thesis is to analyse the present state of Vaasan Sähköverkko Oy´s medium voltage network and define the guidelines of development from the aspect of´reliability. In the present state examination the present state of the network is defined with network information system Xpower’s calculations of load flow, short-circuit and earth fault. In this thesis there is also analysed a situation where one substation at a time is out of use. It is examined if it is possible to replace a single substation via other substations.

According to results Vaasan Sähköverkko Oy’s medium voltage network fulfils mainly the set technical boundary conditions. Boundary conditions cover voltage reduction, thermal load capacity, and functionality of earth fault and over-current protection. From the result it can be noticed that four substation transformers have over 90 % load capacity rate. The development of the areas near these substations has to observed in the future. According the result of substations replacements there are problems with replacing five substations on the present maximum loads.

Based on the results 10 pole circuit breakers were added to the Vaasan Sähköverkko Oy’s medium voltage network and potential guidelines for the development of the network were presented.