Networking Mode Of Offshore DC Wind Farm And Its Operation And Control

With the fast development of offshore wind farms, the cost-effective method of electric power collection and transmission for large-scale offshore wind farms is becoming one of interesting topics. Currently, for offshore wind farms, the AC internal collection grid is used to delivery electric power, and AC output voltages are lifted by bulky transformers. The main drawback is the high cost of the offshore platform which needs to install the bulky transformer and static var compenstor(SVC). High voltage direct current(HVDC) technology is proved to be a promising solution for delivering large-scale wind power with long transmission distance, due to its reduced power losses and very low reactive power requirements. However, the power transformer still cannot be eliminated from the offshore platform infrastructure. To better integrate the HVDC technology to the future large-scale wind farms, advanced DC-DC power conversion concepts can be extended to the wind farm design, by which different schemes of DC wind farm are thus proposed. In the future DC wind farm, large-size power transformers can be replaced by power converters; and AC cables can be replaced by DC cables with lower losses and less materials.The DC wind farm(DCWF) can be composed of DC wind turbines(DCWT) radical-connected or series-parallel-connected. For the radical scheme, the control method is relatively simple, but a DC-DC converter station of high-capacity is necessary to lift the DC voltage. This is because the DC output voltage of individual DCWT is not high enough for HVDC transmission. The offshore platform is still challenged by the high-capacity DC-DC converter with high efficiency. On the contrary, the series-parallel scheme can directly step-up the DC voltage to HVDC transmission level by series-connecting a number of DCWTs. Since both the DC-DC converter station and offshore platform are eliminated, the capital cost is thus reduced. However, the coupling between series-connected DCWTs will bring troubles to the control system design.This thesis takes offshore DC wind farm parallel-connected and series-connected as research objectives; mainly focuses on networking mode, modeling method, operation and control of the wind farm; and carries out the research from the point view of theory analysis, modeling construction and simulation validation. The main contributions are as follows.(1) Networking mode of offshore DC wind farm and its scientific assessment methodThe recommended networking modes of offshore DC wind farm are given with regards to wind farm scale, voltage level and the correspondent monopole-to-bipole transition method as well. In the absence of key equipment of DC wind farm, based on the concept of "incremental method", the technique-economics assessment from the loss, cost and reliability are put forward for internal grid topology of large-size offshore DC wind farm. Combined with the actual calculation case from AC wind farm example, the related indicators are compared and analyzed.(2) Large time-scale dynamic modeling theory of offshore DC wind farm with consideration of random wind speedGeneralized dynamic mathematic model of DC wind turbine are established, including the aerodynamic model, drive train model, generator model, AC-DC rectifier model and DC-DC converter model. The control model of blade control, power control and DC-link voltage control are given. Based on converter average model, dynamic model of DC wind turbine with electromechanical transients is proposed for wind farm studies and all control functions can be implemented.(3) Decoupling control scheme of series-connected DC wind farm with multi random input-single output and strong couplingBased on the operation and control study for parallel-connected DC wind farm, the steady state characteristics, dynamic characteristics and DC wind turbine operation boundary are analyzed for series-connected DC wind farm. The control strategy of series-connected DC wind turbine with voltage limitation is proposed according to the description of control characteristics. While there is wind power variance among series-connected DC wind turbines, reasonable layout of wind turbines and interconnection of internal grid can smooth the wind power variance above minutes time scale, and variable rotor speed control and additional ESS control can smooth the wind power variance below minutes time scale. While there are series-connected DC wind turbines cutting out, a certain number of wind turbines cutting out allowable can be realized by the internal grid design. Moreover, the transmission voltage control of onshore converter station can reduce the coupling among series-connected DC wind turbines and increase the fault-tolerance capability.