Filter Design And Coordination Control For Wind Farms To Supply The Passive Loads Through MMC-HVDC

Using modular multilevel converter based HVDC(MMC-HVDC) technology to transmit the bulk wind power to passive load makes great importance to the specialized applications, such as supplying the marine drilling platform. At present, there exist some problems considering the wind power supply to passive loads through MMC-HVDC. Firstly, the wind turbines adopt the back to back converters to realize the power grid integration. But converters using PWM modulation and MMC with low voltage level will bring the harmonic distortion to the PCC voltage. Secondly, conventional controller of MMC-HVDC system can't balance the wind power output and passive loads without communication systems. To solve these two problems, it is necessary to study the filter design and the coordination control for wind farms to supply the passive loads through MMC-HVDC.In this paper, the structure and parameters of the MMC-HVDC system using wind power to supply the passive loads are firstly introduced, and then the mathematic models of DFIG and MMC are also introduced in detail. To improve the quality of PCC voltage, the systematic design method of LCL filter applied in the back-to-back converter of DFIG at different constraints is proposed. Aimed at solving the problem of LCL grounding and taking advantages of the leakage inductance at rotor side, the LCL filter at rotor side are further improved. The DFIG using the proposed LCL design method is simulated in PSCAD/EMTDC. The results demonstrate the validity of the proposed method.To solve the problem that system can't start up with the steady-state control strategy, a start-up strategy for the whole system is studied in this paper. Diesel generator with small capacity is used to build the DC voltage of the DC lines and AC voltage at wind farm side. The start strategy is validated in PSCAD/EMTDC. To solve the problem that communication system is needed to balance the power between the wind farm and the passive load, this paper proposes a coordination control strategy based on the conventional double-loop control. Local DC voltage is applied to eliminate the power difference without the need of communication system. Coordination controller is designed and simulated in PSCAD/EMTDC. The results demonstrate the validity of the proposed coordination control strategy.Finally, several cases are simulated under different operating conditions. The allowable range of the power factor of the load and wind power fluctuation for the coordinated controller is obtained.