Grid Integration of Large Offshore Wind Farm and Oil Gas Installations Using HVDC Technologies
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- Institutt for elkraftteknikk 
This thesis investigates the modeling and control of High Voltage Direct Current (HVDC) for connection of offshore large variable speed wind farm and oil & gas installations to main grid. Both Line Commutated Converter (LCC) and Voltage Sourced Converter (VSC) based HVDC systems are discussed.First, a LCC HVDC transmission system scheme for grid integration of an offshore wind farm and an oil & gas installation was studied. After the derivation of system mathematical model, the operation principle and control strategies of proposed system are described and validated by a PSCAD/EMTDC based model. Additionally, a STATCOM was proposed to provide commutation voltage and reactive power to the system. Furthermore, an effective fault ride through method is give and demonstrated during the on shore grid fault. The reactive power support by wind farm during fault condition can not only reduce the STATCOM size but also improve the system stability. The simulation results demonstrated the robust system performance during both the normal operation and the various grid fault conditions.Second, a VSC HVDC transmission system is further investigated for its ability to integrate the offshore wind farm and oil & gas installations to shore. An analysis and new fault mitigation methods of the VSC HVDC transmission system are presented. A fixed frequency controller is described for the voltage and power regulation. On the fault occurrence in the onshore AC grid, the proposed DC link voltage controller initiates fast output power drop of the wind farm, in order to avoid the overvoltage in the DC transmission link. On the faults occurrence at the oil & gas installation, the proposed control employs a voltage-dependent limiter, to limit the maximum amount of active power through the offshore VSC. Simulation results in PSCAD / EMTDC show the satisfied performance of the proposed control strategy for VSC transmission system connecting an offshore wind farm and oil & gas installations to the grid.Finally, a comparison based on the simulation study shows that the VSC HVDC solution has better operation performance and fault ride through capabilities compared with LCC HVDC solution.Keywords: Line Commutate Converter (LCC), Voltage Source Converter (VSC), Offshore Wind Farm, Oil & Gas Installation, Grid Integration, STATCOM.