Simulation of shaft vibrations due to interaction between turbine-generator train and power electronic converters at the Visund oil platform

The Norwegian company Norsk Hydro AS has frequently experienced severe vibration problems within the power generator trains at some of their oil platforms in the North Sea. Initially, these problems were believed to have a pure mechanical nature. The generator shaft was therefore modified to increase its inherent damping, but this only partly solved the problem. The focus was eventually directed to possible sources for the problems within the electrical power grid, especially two 7 MW variable speed drives (VSD) that serve as compressor drives. These drives are designed as current source converters (CSC), which feed synchronous motors. In order to clarify whether the vibration problems were caused by interaction between the VSDs and the generator train it was decided to carry out a detailed analysis using the simulation tool PSCAD/EMTDC. The simulated system includes a fully detailed model of the two 7 MW compressor drives and the turbine-generator train, as well as other relevant parts of the electrical system. The simulation results show that the VSDs can cause excessive vibration levels by two different phenomena: interharmonics and negative damping. It is verified that the redesigned shaft gives a lower vibration level, and that the resulting vibration level becomes reduced in the presence of passive electrical loads. The results and the simulation model are subject to further analysis in order to find permanent corrective actions to the problems, e.g. by means of dedicated power electronic devices.