Subsea processing has been increasingly accepted by the offshore oil and gas industry as a solution to boost the production and reduce cost. Accordingly, the subsea power demand is growing in order to support the various processing loads including pumps and compressors. Depending on the application, the power rating of a field ranges from tens of kW to tens of MW. A reliable subsea electrical power system which is suitable for subsea deployment is desired.
The hostile and remote subsea environment has imposed significant technical challenges on the electrical system. Due to the extremely limited accessibility, the electrical system needs to be very reliable and require minimum regular maintenance in its lifetime. The system should be able to detect and isolate local faults without shutting down the whole system in order to maximize the availability and avoid unplanned stops. The electrical system should also be as compact as possible and modularized, so that they can be installed and retrieved without heavy vessels and lifts which have very limited availability.
A novel modular stacked DC (MSDC) development has been developed to address the above mentioned challenges. The proposed system consists of a number of power converter building blocks connected in series on both the on-shore station and the seabed stations. The dc-link current is controlled to be constant, and the dc-link voltage will vary according to the loading condition. This architecture renders simplified power distribution, fault-tolerant capability and feasibility for field extension.
This paper will focus on fault protection of the MSDC system. A fast bypass circuit is proposed to isolate the faulty equipment in tens of micro-second without affecting the operation of the rest of the system. The concept has been verified by both simulation and experiments. The paper will also cover the cooling concept for the power electronics in the subsea power converter. A phase change approach has been investigated and tested in the lab. Latest testing results will be shown in the paper.
