Wind Power Integration Using Multi-terminal HVDC Technology

With the development of the wind power technology, the scale of the offshore and onshore wind farms is increasing. In China, the huge onshore wind farm bases in10GW scale, for example, Jiuquan wind power center in Gansu province and Hami in Xinjiang province, have been planned to establish among the seven planned wind power bases. They are characterized by large-scale, long-distance and high concentration. In Europe, the Supergrid plans to interconnect the offshore wind farm and the national grid. Large scale wind farms being constructed or under construction are far away from the load center. The wind power can't be consumed by the local load and needs to be transmitted to the remote load center. Due to stochastic and variable feature of the wind power, large-scale wind power development is bound to result in challenges in terms of the integration into the power network. How to integrate and transmit such bulk wind power to load centers is really a challenge and is not fully studied yet.Firstly, the disseration reviews the technology features of the AC and DC transmission for wind power integration. It indicates that the high voltage DC (HVDC) transmission is suitable for the long-distance and large-scale wind farm integration. The present HVDC technologies include the Line-Commutated Converter-HVDC (LCC-HVDC), Voltage Source Converter-HVDC (VSC-HVDC) as well as the multi-terminal HVDC (MTDC). The research status concerned with those technologies is investigated. The structure of the whole disseration is listed as follows:In the second part, the operational principle of the doubly fed induction generator (DFIG) comprising the wind farm is investigated. The de-couple control of the active and reactive power is achieved by the outer power control and the inner current control. The research on the wind power integration technology is based on the DFIG wind farm. Later, the performances of the LCC-HVDC and VSC-HVDC technologies under the faulted condition are compared to explain the features of the two technologies. The low voltage ride-through capability of the large-scale wind farm integration using LCC-HVDC and VSC-HVDC are verified.respectively.The third part focuses on the LCC-MTDC system research for DFIG based wind farm grid integration in the context of the Northwest grid. The control strategy for the LCC-MTDC operation is then proposed, and the comparison in control strategy with and without coordinated operation is also carried out and verified by PSCAD/EMTDC simulation. Various operation scenarios such as ac fault on rectifier and inverter sides are simulated to investigate the system performance during disturbances. Results show that the proposed LCC-MTDC configuration and its control strategy are effective and the LCC-MTDC system is well controlled over the whole operating range.The fourth part proposes the LCC-MTDC combining with the static synchronous compensator (STATCOM) in the case of no AC system or weak AC system existing at the point of common coupling. The control strategies design is approached from two aspects. One is aimed to the ac system on the rectifier side. To coordinate the wind farm, STATCOM and rectifier, the inverse system method is introduced to design the controller in view of the nonlinear characteristic of STATCOM. Meanwhile, the control strategy for the rectifier is proposed to balance the active power in the subsystem and ensure the dc link voltage of STATCOM constant. On the other hand, the single-point voltage control strategy suitable for LCC-MTDC is conceived to guarantee the stable operation of the system by considering the control characteristics of both rectifiers and inverters Simulations carried out in PSCAD/EMTDC have proven that the proposed control strategies possess favorable control performance and fast response speed.The last part explores the feasibility of using HVDC transmission technology, especially hybrid multi-terminal HVDC (MTDC), as one of the preferable solutions to solve the grid interconnection issue of wind generation. The paper mainly focuses on the application of the hybrid MTDC to integrate wind farms into the electric power grid. A five-terminal hybrid MTDC model system including a large capacity wind farm is set up in PSCAD/EMTDC, in which the corresponding control strategy is designed. The operational characteristis of the hybrid system is studied and the proposed control strategy is verified through simulation under various conditions, including the wind speed variation and faults on ac side and dc side.