Research On Control Technology Of The DC Series-connected Direct-drive Permanent Magnet Wind Farm Based On VSC-HVDC

The DC series-connected direct-drive permanent magnet wind farm based on VSC-HVDC is a practical and effective solution to solve the difficulties in the construction of large offshore wind farm,as it could effectively reduce the amount of the required electrical equipment and improve the global stability of the offshore wind farm.Considering the feature that the DC side voltage is highly coupled in the DC series-connected direct-drive permanent magnet wind farm based on VSC-HVDC,this dissertation mainly studies the control technology on how to improve the stability and reliability of the DC series-connected direct-drive permanent magnet wind farm under various working conditions.The main contents and innovative points of the research are listed as following.The working principle of the DC series-connected direct-drive permanent magnet wind farm based on VSC-HVDC is described in detail in this dissertation,and the mathematical model is established to analyze the dynamic operation process of the wind farm and especially illustrates the coupling relationship among the rotor speed,powerand the DC side voltage of the generators in the wind farm.According to the working conditions,the control scheme for the DC series-connected topology is proposed.A novel variable step hill-climbing MPPT algorithm is proposed to realize the coordinated control of the DC series wind turbines which is able to find the best power point in the safety operating range,and achieve the goal of optimal wind power capture.Meanwhile,a novel control scheme based on current loop feed-forward is adopted to improve the response speed for the speed tracking and to reduce the steady-state error.Based on the coordinated control of the wind farm,the battery storage based technology for the offshore wind farm is put forward to further solve the problem of wind curtailment caused by the severe coupling of the DC side voltage of the DC wind turbine.The proposed battery storage based technology may help to decrease the coupling degree of DC side voltage,optimize the stored energy in each storage unit of the DC wind turbine in the DC series group,and reduce the capacity of energy storage unit.Furthermore,the topology and control stradegy of the bidirectional DC/DC converter that applied to the energy storage unit of the DC series-connected offshore wind farm is analyzed in detail.The simulation shows that the coordinated control of the DC series-connected wind farm can be effectively realized by the proposed control method.The MMC convertor is used as the convertor station of the DC series-connected direct-drive permanent magnet wind farm based on VSC-HVDC,the basic operating principle of the MMC convertor station is analyzed.The carrier phase-shifting pulse width modulation(PWM)and the nearest level modulation are introduced to form the routine control strategy of the MMC convertor station.Based on the deep study of the control strategy of MMC convertor station,the concept of the virtual inertia is further analyzed.According to the feature that the capacity of distributed energy storage capacitor adopted in MMC convertor station is higher than traditional 2-level lumped capacitor,a novel virtual inertia control method based on MMC-HVDC is proposed.The energy stored in sub-module capacitor of MMC and the kinetic energy stored in DC wind turbines of the DC series-connected wind farm is fully used.The power outputted to grid from DC transmission system could adjust automatically in according to the change of AC system frequency in the case of cutting down the need of extra energy storage devices.The dynamic estimation of the system inertia is realized,and the fluctuation of system frequency under fault state is effectively restrained.The condition of bridge arm circulation and capacitor voltage fluctuation in sub-module in unbalanced network is also analyzed.In the case of voltage unbalance from the grid side,the 2nd harmonic will appear in the DC side voltage,and the capacitor voltage fluctuation in sub-module will increase.To solve these problems,the method of adding the zero sequence circulation and 2nd negative sequence circulation is adopted to stabilize the DC voltage and the capacitor voltage of sub-module,which may effectively improve the stability of the MMC convertor.Finally the validity of control method is proved by abundant simulation.The dynamic performance of grid-side converter with voltage drops is analyzed,and the low voltage ride-through mechanism of the DC series-connected direct-driven permanent magnet offshore wind farm is expounded.A feasible plan which is suitable for the low voltage ride-through of DC series-connected offshore wind farm is proposed.The application of Crowbar circuit in low voltage ride-through of the DC series-connected VSC-HVDC offshore wind farm is illustrated in detail.It is found that the unbalanced energy produced under voltage drops can be eliminated and the low voltage ride-through can then be realized by adding Crowbar circuit in the DC side of DC wind turbine.A new low voltage topology of the DC series-connected offshore wind farm combin ed with energy storage unit is employed,in which the unbalanced energy can be stored in the energy storage unit to realize low voltage ride-through.The method of energy storage in rotor impellers is also discussed,when voltage drops,the outer loop controller of the torque current in generator side converter controls the DC bus voltage,and when the output voltage of generator is unsaturated,the excitation current loop controls excitation current to be 0.When the output voltage of generator is saturated,the stator voltage field-weakening control will be adopted to make DC bus voltage stable.In this scheme,the rising speed of the wind turbine can be adjusted by the coordinate pitch control of the DC series-connected wind turbine set,no extra equipment is needed,and the low voltage ride-through ability of the entire wind farm is enhanced by storing maximum wind energy in fault crossing period.Finally the simulation shows that these three methods can realize the low voltage ride-through of the DC series-connected new type topology offshore wind farm based on VSC-HVDC.According to the characteristics of the DC series-connect direct-drivepermanent magnet wind farm based on VSC-HVDC,a new type DC series-connected topology is proposed.In the new proposed topology,the DC wind turbine is connected with a half-bridge circuit by a capacitor,and the lower bridge arm of the half-bridge circuit is connected in series to compose the high voltage DC transmission circuit.The working principle of the proposed topology is analyzed and the mathematical model is proposed.The sub-module capacitor and sensor circuit inductive is studied,this topology realizes the decoupling control of the DC wind turbine in the DC series-connected wind farm.The bypass unit can be cut rapidly when the fault occurs,so,the fault tolerance of the system can be improved.In addition,the control strategy for voltage-sharing of the single DC wind turbine in the DC side,coordination of the output power and capacitor voltage of sub-module is proposed.Finally the average duty ratio of the sub-module and an optimal scheme of the average capacitor voltage value are studied.The simulation shows that the control strategy can realize stable operation in the DC series-connected new type topology offshore wind farm based on VSC-HVDC.