Review of maturing multi-megawatt power electronic converter technologies and reliability modeling in the light of subsea applications

• Maturing technologies on multi MW power electronic converters are discussed.
• Reliability modeling is done on a 6 MW standard VSI power electronic converter.
• Modeling extended with redundancies and considered subsea ambient conditions.
• Identified that a suitably configured subsea VSI system has MTBF of about 20 years.
• Subsea pressure compensated CSI with active rectifier has MTBF of about 25 years.

The increasing demand for establishing deep water enhanced oil recovery systems for subsea boosting, power generation, transmission from remote subsea marine current turbine farms, efficient power transmission and operation of subsea dredgers and mining machines requires subsea based power conversion, as the subsea pumps, compressors and turbines need to be operated at varying power and voltage levels. Even though techno-economic studies indicate the absolute need for locating the multi megawatt power electronic converters in subsea, the recorded failure-in-time data of multi megawatt capacity industrial standard power converters are not encouraging. Reliability is the key factor, as subsea breakdowns lead to huge production losses, and system retrieval for repair and maintenance is very costly and time consuming. This paper reviews the present technical maturity of a typical industry standard 6 MW capacity 6.6 kV 3-level voltage source inverter based power electronic frequency converter, and describes a reliability model in the light of their use in subsea environments. Reliability modeling is done for different possible configurations, with Mean Time Between Failure as the driver. The results presented can be directly used by the system designers to select a suitable configuration based on the Mean Time Between Failure requirements. The results obtained provide confidence that a suitably configured multi megawatt power converter, when used in an oil filled enclosure in a deep water environment, could have an MTBF of more than twenty years.