1 Control of Multiterminal HVDC Transmission for Offshore Wind Energy

Abstract — With increased interest for environmental friendly renewable energy, offshore wind power has been attracting the attention of researchers and energy developing companies as a promising source of future electricity. Considering the long distance from onshore involved with such offshore windfarm sites, it will be necessary to find a robust grid integration solution for transmitting the power generated to points of power consumption onshore. In the case of the North Sea, the presence of offshore loads at oil & gas platforms can make it even more attractive to consider offshore multiterminal HVDC interconnecting the windfarms, the oil &gas platforms and onshore national grid. This paper discusses proposed control strategy for such a system. Voltage source converter (VSC) was selected for its suitability for MTDC system and for its flexibility in control. An equivalent circuit of the VSC in synchronous d-q reference frame has been established and decoupled control of active and reactive power was developed. A four terminal MTDC using VSCs was modeled and simulated in EMTDC/PSCAD software. A modified mix of voltage-margin-method and DC-voltaged-droop control has been used for reliable operation of the HVDC system avoiding the need for communication. Simulation results show that the proposed MTDC control system was able to maintain the DC voltage with in desired range during load switchings, step changes in power demand and was able to secure power to passive loads during loss of a DC voltage regulating converter terminal without the use of communication between terminals