Grid connection of offshore wind farms through multi-terminal high voltage direct current networks

This thesis investigated the capability of multi-terminal high voltage direct current (HVDC) networks based on voltage source converter (VSC) technology to transfer power generated from offshore wind farms to onshore grids and interconnect the grids of different countries. Variable speed wind turbines and other low-carbon generators or loads that are connected through inverters do not inherently contribute to the inertia of AC grids. A coordinated control scheme for frequency support from multi-terminal VSC-HVDC (MTDC) scheme was designed to transfer additional power to AC grids from the kinetic energy stored in the wind turbine rotating mass and the active power transferred from other AC systems. The wind turbine inertia response limited the rate of change of AC grid frequency and the active power transferred from the other AC system reduced the frequency deviation. The wind turbines recovered back to their original speed after their inertia response and transferred a recovery power to the AC grid. An alternative coordinated control scheme with a frequency versus active power droop controller was designed for frequency support from MTDC schemes, in order to transfer the recovery power of wind turbines to other AC systems. This prevented a further drop of frequency on the AC grid. The effectiveness of the alternative coordinated control scheme was verified using the PSCAD simulation tool and demonstrated using an experimental test rig. A scaling method was demonstrated for a multi-terminal DC test rig to represent the equivalent steady state operation of different VSC-HVDC systems. The method uses a virtual resistance to extend the equivalent DC cable resistance of the test rig through the action of an additional DC voltage versus DC current droop controller. Three different VSC-HDC systems were modelled using the PSCAD simulation tool and demonstrated on the DC test rig with virtual resistance, showing good agreement