Research On Control Strategy Of Offshore Wind Farm Integration Via VSC-HVDC

Recently, Doubly Fed Induction Generator (DFIG) is applied into the offshore wind power generation system popularly. Moreover, VSC-HVDC is a typical way of connecting large-scale offshore wind power to main grid over a long distance. This thesis focuses on the control strategies on offshore wind power intergration via VSC-HVDC transmission, including:(1) Build a simulation platform that is offshore wind power integration via VSC-HVDC transmission on DIgSILENT/Powerfactory. As for the wind turbine based on DFIG, it includes the model of DFIG, convertors in DFIG and pitch angle control system. As for the VSC-HVDC, the Wind Farm VSC (WFVSC) mainly controls the magnitude and the frequency of the offshore AC grid; the Grid Side VSC (GSVSC) controls the DC voltage and the reactive power exchanged with onshore AC grid.(2) Decreasing inertia by the high level penetration of the wind energy makes the power system an "inertia-less" system that has less rotating kinetic energy for abrupt changes in load and generation. A new coordinated control strategy which uses electrical energy stored in DC capacitors and kinetic energy in DFIG rotors to emulate the inertia of a synchronous generator is proposed. DC link capacitors release or absorb energy by the droop DC voltage control of the GSVSC. No remote communication devices are needed since the artificial coupling of the frequency of the two-side AC systems is obtained through the variable frequency control of the WFVSC. The DFIG rotor speed alters if the reference power changes by the power controller, which is in response to the WFVSC frequency. As a result, the DFIGs are utilized to keep the frequency stable. Within the permissible DC voltage variations, the proposed control can provide a wide range of inertia time constants, which improves the overall stability of the system.(3) Analyze the DC capacitors and the current limtation value of GSVSC control on the fault DC voltage. The fault ride through control strategy for the offshore wind farm integration via VSC-HVDC is proposed. When GSVSC side undertakes an AC fault, GSVSC will convert from the DC voltage control strategy to the current limitation strategy. Then, WFVSC will perceive the high fault DC voltage. Then this high fault DC voltage will convert to a high offshore AC frequency via WFVSC. Finally, DFIG will perceive this increasing AC frequency of the WFVDC, then reduce its active power automatically using two methods:the frequency error control and the user-defined frequency-power curve. These two methods are compared on fault ride through capacibility of offshore wind farm integration via VSC-HVDC.