Control of an LCC HVDC system for connecting large offshore wind farms with special consideration of grid fault

This paper describes the control and operation of an HVDC system comprising a line-commuted converter (LCC) HVDC and a STATCOM for connecting offshore wind farms based on DFIGs. During fault on the main grid, fast communications have previously been relied upon to make the wind farm aware of the condition and reduce its power output. Here, an alternative method is examined which enables automatic power balancing during fault. This is achieved through frequency modulation on the offshore network via the STATCOM. Several methods of fault detection using frequency threshold, rate of change of frequency (ROCOF) and rate of change of AC voltage (ROCOVac) are used to indicate when the wind farm power output should be reduced to achieve power balancing, and are compared with results using direct communications. PSCAD/EMTDC simulations show the effectiveness of the proposed control, which allows for faster fault identification. As a result the STATCOM DC over-voltage can be significantly reduced, requiring small DC capacitor, and tripping of the wind farm can be avoided.