Research On Technology Of Grid Integration And Power Transmission For Large Capacity Of Offshore Wind Turbines

With the crisis of the world serious energy shortage, renewable energy development has been paid more attention, including wind power with its cost and technical advantagesdeveloping most rapidly. With the continuous development of wind power technology, offshore wind power with its unique environment-friendly and abundant resources has gainedmore and more attention, investment on construction of offshore wind farms is increasing. However, due to a short time of the development of offshore wind power, the accumulation of past experience is not enough, especially the offshore wind power collection and transmission technology still need the systematic study, the wind turbines connection topology is lack of demonstration and the control strategy of power transmission also needs to optimize. Therefore, the research of offshore wind power collection and transmission technology has important theoretical significance and application value.This paper is based on VSC-HVDC technology, combined with the characteristics of offshore wind power DC transmission technology, integrating the structural advantages of several DC transmission topologies, and then proposes a DC parallel-series topology transmission system for offshore wind farm. This structure has the advantage of DC seriesboost, which reducing the required offshore platform construction of transformer and converter station, reducingthe losses during commutation and transforming, alsodecreasing the complexity of the system, construction costs and while conducive to system maintenance. This structure to some extent solves the problems of system fluctuations and branch circle current caused by series system and parallel system failure. Therefore, this structure is the main object of study in the paper, the operating andcontrol characteristics of the wind turbines, the offshore wind farms and the converters in the DC transmission system were studied and analyzed.The research work is carried out mainly from three aspects on the principle analysis, mathematical modeling and simulation.According to the characteristics of DC parallel-series topology structure, the paper gives thestructures of Double-Fed Induction Generator (DFIG) and Wind Turbine with Permanent Magnet Synchronous Generator (PMSG) in this topology. DFIG uses the distributed converter which connected independently to each turbine; but PMSG simplifies the structure, and reduces the system complexity and commutation loss. According to the working principle and characteristics of the wind power generation system, conduct the principles and modeling analysis separately through the DFIG and PMSG turbines, and design the control algorithm of the wind turbine in the DC parallel-series structure. The start-up control of the DFIG is relatively complex in DC parallel-series structure. In this paper, in order to decrease the control fluctuation caused by switching-over of the direct power control algorithm in the steady-state, it proposes the direct virtual power control algorithm. This algorithm is used before the DFIG starts, and aims to control the distributed converter that connects to the DFIG maintaining stable AC voltage output flow, and verifies the effectiveness of the proposed algorithm through the software simulation and simplified physical circuit experiments. Therefore, the effectiveness of the wind turbine control algorithm is validated by doing the software simulation.Base on the effective wind turbine control, through analysis on the steady-state characteristics and operating characteristics of wind farms, also combiningthe operational requirements of the DC parallel-series structure, this paperproposes the hierarchical control module of wind farms. Then conduct the system analysis and control methods design for the wind farms startup and exit. According to thecharacteristics of the system wind turbine control and cluster control, design a wind farm low voltage ride through method, which can effectively control wind farms operationwhen the grid failure happens. By simplifying the wind turbine model, constructing a simulation model of the wind farm, and conducting the simulation test to verify the wind farm control algorithm.On the basis of analyzing the operating characteristics of the DC parallel-seriesstructure, design the control algorithm of the grid-connection converter. In this paper, it uses a dual closed loop direct current control, the outer loop uses a traditional PI control algorithm, and the current inner loop control uses a differential flatness control algorithm, this algorithm can solve the conflict situation caused by the time constant differences in different control loop, simplify the system control and make the system more robust at the same time. Finally, build thesimplifiedmodel of wind farms, verify the validity of the grid inverter control algorithm through the simulation results.