Vakuumbrytare i småskaliga vattenkraftstationer
This master thesis has been performed at Vattenfall Power Consultant in Gothenburg and Trollhättan for Vattenfall Vattenkraft. In this report vacuum circuit breaker in small-scale hydropower stations with the rated power 4 MVA and below has been studied. Models of the stations are modeled and analyzed in PSCAD/EMTDC to study the high voltage transients that are created after current chopping with vacuum circuit breaker. Vacuum circuit breaker may chop the current before it reaches zero value, which means that high voltage transients can be created. The equipment of the stations can be destroyed by high voltage transients.The purpose of this thesis is to examine the use of vacuum circuit breaker in small-scale hydropower stations and the risk for high voltage transients from current chopping.According to the results there is a risk of high voltage transients after current chopping with small synchronous generators in stations with the operating voltages 0.4 kV and 3.6 kV. When the vacuum circuit breaker is placed on the high voltage side of the transformer the risk increases of high voltages transients and reignitions due to the low capacitance of the transformer.The conclusions is that high voltage transients occurs when the vacuum circuit breaker chops the current at a high chopping level in stations where the synchronous generator rated power is low, high rated voltage and a small capacitance. The risks of high voltage transients in small-scale hydropower stations can be reduced with surge arrester, RC-circuits or protective capacitors.The report is written in Swedish.