Control Design Strategy to Enhance the Fault-Ride-Through Capability of VSC-HVDC Transmission System Interconnecting Offshore Wind Power Plant

Most of the wind power plants currently installed at North Sea are near to shore connected with the onshore grid through AC cable. AC transmission technology is proven and reliable, but there will be a need to switch from AC to DC transmission since large number of offshore power plants at longer distance to shore are expected to be installed by the end of 2020. Furthermore, to enhance the integration of European energy market, offshore power plants are proposed to be interconnected with each other through HVDC transmission system. This has developed a new concept of establishing the mesh DC network [1]. High voltage direct current (HVDC) transmission system based on voltage source converter (VSC) become imperative to cater the offshore wind power plant integration challenges. IGBT based converter offer distinguish feature compare to thyristor based converter such as ease of power reversal, and black start operation; that are crucial for the integration of offshore power plants. One of the important aspect in realizing the integrated DC offshore network is to establish it in fragments and must be able to operate in isolation, in case of fault. Thus in first phase, point-to-point connection from offshore substation to onshore station is expected to be developed using HVDC transmission system. In [2] dynamic model of VSC and its operational behavior has been analyzed for point-to-point configuration. The fundamental operation of the offshore converter is to provide the reference frequency to the network and maintain the voltage level at the reference bus. LCL-filters are preferable over L-filters as reference bus would not remain floating and same switching harmonics damping can be achieved using smaller inductance [3], [4]. One of the problem with the LCL-filters is the resonance effect that causes instability in the network which is usually overcome through active damping method [5], [6].