The transformer is one of the most common and most important electric machines for power companies. Cost of investment and the economic worth of a transformer are counted in millions of SEK. Because of that, proper maintenance actions that may lead to longer technical life time should be the aim in all situations. The aim with the report was to perform transformer diagnosis by dissolved gas analysis on Umeå Energi AB´s power transformers and a risk assessment. The report should also contribute to longer life time for the transformers. Oil samples were taken from each transformer two times. The first sample in summer 2013 and the second in April 2014. The gas concentrations in the samples was measured by an portable gas analyzing monitor. The samples were analyzed regarding to gas concentrations and the gas rate of change. The analyze was based on scientific methods like the total amount of dissolved combustible gases, Duvals triangle and Rogers ratios. The risk assessment was made as an internal comparison between Umeå Energi´s transformers. The results of the study showed large gas concentrations in several transformers in both times of sampling. The high concentrations appeared in old as well as in newer transformers. Therefore can the presence of gases not only be explained by natural aging, and different fault types can be suspected in some of the transformers. Duval´s triangle and Rogers ratios indicated various fault types. One of the transformers was immediately taken out of service after the first sample because of high level of a separate gas and the discussion about what actions to take was ongoing. A number of other transformers were taken into further consideration. Conclusions of the report were that transformer diagnosis is a very complex science where it is important to collect as much adequate information as possible to be able to make a correct analyze for every single transformer and to take proper actions. To be able to secure fault free operation of the power grid and to minimize the risk of wasting economic and material resources, and from a personal safety point of view, the power transformers must be maintained and tested regularly and be based on scientific principles. Faults in power transformers can lead to large impact on the environment and large effect on society if electricity is not available.

Svenska kraftnät is the Swedish transmission system operator, responsible for the national electrical grid with power lines of 220 kV and 400 kV, and the electrical substations of the grid. In this master thesis an old substation located in a sensitive environmental area is inspected and its environmental risks defined and assessed. From this “worst case scenario” measures are recommended, that should be applicable on other existing substations, and for new constructed substations. This thesis is limited to a more detailed analysis on how oil spill can be handled. The insulating oil is used in different machines and equipment on the substation to avoid electrical charges. The inspected substation has a special feature; a pipe system that connects wells on the substation area, water pipes from the substation house, and transformer pits to an oil-water separation house. The oil pits are made of concrete and placed under machines with large amounts of oil to capture the oil in case of a breakdown or leakage. In the oil-water separation house oil is separated from water and the clean water is sent to a lake in the vicinity. The most severe environmental risks found are the filter in the oil-water separation house and the risk of oil leakage from the open water-cooling system of the synchronous generator. In addition, there is no way to catch oil spill from current and voltage transformers and it is not known if the pipe system that transports water and oil spills is completely sealed. Finally, there is no permanent installation to catch spills from tank filling areas and Svenska kraftnät does rarely follow up the environmental work of the entrepreneurs. Svenska kraftnät is recommended to use the products and services from the companies Qlean Scandinavia AB and TTL Miljöteknik AB to attend to the noted environmental risks. Qlean Scandinavia AB offers an environmental friendly cleaning procedure with ultraclean water and sealing of concrete, which can be used for the transformer pits and oil-water separation house. They also sell profile planks as a fire-extinguishing layer for transformer pits. These are considered better and safer than the conventional stones used. The price is almost the same, however the maintenance costs are lower for the profile planks, since they are easier to clean and do not require heavy transports to landfill. They also require less space than the stones, therefore a smaller transformer pit can be built, and cost savings can be made on concrete. TTL Miljöteknik AB has products based on environmental friendly oil solidifying polymers placed in different textiles and filters. They let water and other substances pass, but react with hydrocarbons, like oil, forming a tight rubberlike mass. These installations are new to Europe, but more than 12 000 installations have been made, mainly in the United States and Australia. For the inspected substation in this thesis it is recommended to manage oil spill at each machine and to clean the pipe system. An HFF-filter should be installed in cleaned and sealed transformer pits. The filter lets water from precipitation pass, but binds oil, thanks to the solidifying polymers. The clean water is further transported through the pipe system to the oil-water separation house and from there water is pumped to the lake. Wells must be sealed and the pipes from the substation house led to a septic tank. Agent X, a type of filter mat, should be placed on the substation area to catch oil spills from current and voltage transformers, since previous breakdowns show that oil can be spread over a large area. For plants without a pipe system the water will be released outside the transformer pits. For new constructions it is suggested that transformer pits are built with a polyvinyl mat in the bottom and a filter on the sides, which will capture the oil. A concrete fundament still needs to be built to support the machine; therefore the price will almost be the same as for the conventional transformer pit in concrete, although less concrete is needed. However, with this technique the transformer pit will not require any maintenance.