Study On Control Strategy Of Hybrid Multi-terminal HVDC System Applicable To Wind Power Integration

High voltage direct current transmission(HVDC)has many advantages,such as high power capability,low line cost and good control performance.It has a broad application prospect in the field of wind power integration.Line commutated converter based high voltage direct current(LCC-HVDC)system has the advantages of large capacity,high voltage level,low loss of commutation valve and mature technology.Voltage source converter based high voltage direct current(VSC-HVDC)has many advantages such as independent control of active power and reactive power,ability to provide dynamic reactive power support for AC grid.Due to its advantages,it is a potential solution to the wind power grid-connection.Hybrid multi-terminal high voltage direct current(Hybrid-MTDC)system combining the advantages of LCC-HVDC and VSC-HVDC is formed with VSC and LCC,which is developed for wind power integration.Firstly,the structure of Hybrid-MTDC system is introduced.The wind turbine model,the operation principle of doubly fed induction generator(DFIG),the basic structure and mathematical model of VSC and LCC converter station are described,and the basic control strategies of VSC and LCC converter station are analyzed.Then the control strategy of Hybrid-MTDC system for wind power grid connection is proposed.To overcome the problems such as complex control structure and difficulties to tuning multiple PI parameters of direct current control,an optimal model predictive control strategy is proposed.The control strategy adopts the fast vector selection method to obtain the optimal voltage vector and the suboptimal voltage vector.On the basis of considering both optimal and suboptimal voltage vectors,the optimal voltage vector for two control cycles is obtained,and the corresponding switching signals are applied to VSC converter.The proposed model predictive control strategy has high control precision,excellent steady state performance,dynamic performance and fault recovery performance,which can provide stable ac voltage for wind farm.In order to solve the problems of large DC voltage deviation and unreasonable power distribution in traditional droop control,a DC voltage adaptive droop control strategy based on DC voltage-current characteristics is proposed.On the basis of adjusting the droop coefficient of the converter station,according to the direction of DC current variation in converter station,the power margin of converter station is obtained to introduce the influence factor of the droop coefficient.Based on the principle that the larger the power margin of converter station is,the smaller the droop coefficient is,the adaptive droop coefficient is defined.When changes in the hybrid multi-terminal high voltage direct current system power flow occur,the droop coefficient of the converter station can be adjusted by the influence factor of the droop coefficient,so that the converter station can participate in power regulati on according to its own power regulating ability.Consequently,the proposed control strategy can reasonably allocate the active power and minimize the deviation of the DC voltage.Besides,the stability analysis of the DC voltage adaptive droop control strategy is also carried out.Finally,a four-terminal hybrid HVDC transmission system is built based on PSCAD/EMTDC to verify the feasibility and effectiveness of the proposed control strategies.