Åsgard subsea gas compression - Technology qualification testing with high-speed VSD and very long step-out cable

The power system including Variable Speed Drives, transformers and long subsea power cables is one of the main building blocks in the development of subsea gas compression. This paper presents results from full scale technology qualification testing of a power system onshore where the Variable Speed Drive is controlling a 188 Hz, 7.8 MW gas compressor motor over a simulated distance of 47 km. The extensive testing shows that it is possible to control the compressor/motor speed from start-up throughout the whole load envelope without need for motor voltage and current feedback. By use of estimator-based control it is also possible to provide motor overcurrent protection. The paper shows how resonances in the long cable system can be handled by use of optimization of the generated voltage waveform instead of using Variable Speed Drive sine filter. The successful technology qualification shows applicability for subsea compressors with a rated frequency of up to 200 Hz, at least 8 MW shaft power and a step-out distance of at least 50 km.