Cold start of a 5.5 MVA offshore transformer

SLIM® transformers are compact liquid-immersed transformers according to IEC 60076-14 and customised for typical applications such as on- and off-shore wind turbines. State of the art SLIM® wind turbine generator transformers (WTGT) have to operate in wind farms which are often located in remote locations with harsh conditions and sometime very low temperatures. After a few days of no wind the transformer can be cooled down to -30 °C or even -40 °C so these conditions need to be tested in advance. To ensure the reliability of CG Power System Belgium’s WTGT’s and the possibility to start in cold conditions, several tests were conducted in OWI-Lab’s large climatic test chamber. OWI-Lab’s test facility is the first public test centre in Europe that deals with extreme climatic tests of heavy machinery applications up to 150 tonnes with a special focus on wind turbine components. CG wants to prove that when WTGT’s have to operate in cold conditions, the internal cooling is still working properly. Due to higher viscosity at low temperature of the used cooling liquids, the natural convection cooling of the internal windings may be limited. According to the properties of the cooling liquid that is used inside the WTGT, it remains ‘liquid’ above -45 °C (pour point), but due to the high viscosity, the natural convection may be limited and there is a possibility that the initial losses generated inside the transformers’ windings cannot be evacuated fast enough. To verify if the natural convection starts, a full load cold start test was conducted at -30 °C to prove that the natural cooling of the internal windings starts immediately. During the cold start test the internal pressure and several temperatures such as the top oil were measured. Also a storage test was done at -40 ºC to check if the transformer can resist this ambient temperature. This storage test was conducted to prove that no leaks or other visual issues occurred on the tank and gaskets