Actively Commutated Current Source Converter And Its Application In Offshore Wind Power

High voltage direct current(HVDC)transmission technology is widely used in long-distance and large-capacity transmission system due to its long transmission distance,large transmission capacity and strong control ability.At present,line commutated converter(LCC)and voltage source converter(VSC)are widely used in HVDC transmission system.Actively commutated current source converter(CSC)is a new type of converter with application prospects.It has some advantages of VSC and LCC.For example,it does not need large-area reactive power compensation and filter field on the AC side,nor does it need a large number of energy storage capacitors on the DC side.It has small size,light weight,high power density,no commutation failure,and can also supply power to passive systems.Therefore,it provides a new scheme for the HVDC transmission system.China is rich in offshore wind energy resources,and large-scale development of offshore wind power is one of the important measures to achieve carbon peak in 2030 and carbon neutrality in 2060.The existing HVDC transmission system for offshore wind power mainly adopts modular multilevel converter(MMC),which has problems such as low power density,high construction cost,and difficulty in DC fault ride-through.Applying CSC to HVDC transmission system for offshore wind power can provide a feasible solution to the above problems.Focusing on CSC and its application in HVDC transmission and offshore wind power,this paper studies CSC operation mechanism,steady-state control strategy,DC fault characteristics and recovery strategy,and offshore wind power grid-connected system.(1)Exploring the operation mechanism and new modulation method of CSC.The widely used modulation of CSC is pulse width modulation(PWM).Under the existing modulation method,the switching frequency of CSC is high,which makes its DC-side harmonics high,switching loss high,and voltage equalizing of series switching devices difficult.To solve these problems,a fundamental frequency modulation(FFM)method is proposed.The FFM-CSC has lower switching frequency,less voltage fluctuation on the DC side,smaller AC-side LC filter and DC-side smoothing reactor,less number of series switching devices and lower difficulty in voltage equalizing,so it is more suitable for HVDC transmission system.Further,based on the mathematical model of the FFM-CSC,considering its reactive power and harmonic characteristics,the main parameters design method of FFM-CSC is proposed.According to the results of the parameters design,the technical and economic comparison of CSC with MMC and LCC is carried out.The total cost of CSC is between these of LCC and MMC.The volume of CSC is 81.11%lower than that of MMC,and the weight is 60.66%lower than that of MMC.(2)Studying the steady-state control strategy of the CSC-HVDC transmission system.The FFM-CSC can realize independent control of DC voltage or DC current,but it is difficult to control the active power and reactive power of the converter at the same time because there is only a single degree of control freedom.To solve this problem,the cooperative power decoupling control strategy for the two-terminal CSCs is proposed.The main station and the sub-station participate in the control of the active and reactive power of the main station at the same time.The effectiveness of the control strategy is verified by the simulation platform and the physical experiment platform.The power operation range of the cooperative power decoupling control strategy is analyzed.The results show that the the power of CSC can operate in four quadrants,and the operation range is wide.In addition,in order to avoid the cooperative control of converter stations on both sides,a cascaded CSC and its unit power factor control strategy without relying on the communication of converter stations on both sides are proposed.Two 12-pulse FFM-CSCs are cascaded,and the firing angles of the two are controlled differently.The independent unit power factor control of single-end FFM-CSC can be achieved.The simulation results verify the effectiveness of the scheme.(3)Analyzing the DC fault characteristics and recovery strategy of the CSC-HVDC transmission system.The DC fault of the CSC-HVDC transmission system will endanger the safe operation of the electrical equipment,interrupt the power transmission,and the fault clearing and recovery after the fault is particularly important to ensure equipment safety and improve power supply reliability.Based on the mathematical model of the CSC,the DC fault characteristics of CSC-HVDC transmission system are analyzed in stages.The suppression methods of AC overvoltage and oscillation during DC fault and the control strategy for system restart and recovery after fault are proposed.The simulation results show that the proposed f’ault suppression method can reduce the AC overvoltage after the fault and suppress the non-fundamental frequency oscillation after the fault,and the proposed recovery strategy can effectively and quickly realize the recovery of the HVDC transmission system.(4)Researching the DC transmission and grid connection system based on CSC for offshore wind power.The CSC does not need large energy storage capacity and has black start capability,which provides a feasible scheme for the lightweight of the offshore platform.Compared with MMC,CSC makes the offshore platform smaller.lighter and more economical;Compared with the diode rectifier unit(DRU).CSC has stronger control ability and black start ability.To solve the problem that the offshore CSC has only a single control freedom degree and can not establish the amplitude and frequency of the offshore AC voltage at the same time,the cooperative control strategy between the offshore converter station and the onshore converter station relying on remote communication is proposed,and the cooperative control strategy between the wind turbines and the offshore CSC without relying on remote communication is also proposed.The black-start control strategy of offshore wind farms is proposed and the corresponding control sequence is designed.The simulation results show that the proposed black-start control strategy can establish stable AC voltage of the offshore wind farms and achieve stable black-start.The proposed steady-state control strategy can achieve constant AC voltage amplitude and frequency control of the wind farms,and can adapt to continuous multiple fluctuations of wind power.(5)Studying the hybrid cascaded DC transmission and grid connection system based on CSC for offshore wind power.In order to further reduce costs,a hybrid DC transmission system based on cascaded CSC and diode rectifier for offshore wind power is proposed.The steady-state control strategy and black-start control strategy of the system are designed,and the simulation results verify the effectiveness.The economic comparison between the proposed scheme and the existing scheme shows that the hybrid cascaded scheme of CSC and DRU has lower cost and converter loss.Compared with the scheme based on MMC,the cost of this scheme is reduced by 28.75%and the converter loss is reduced by 84.3%;Compared with the scheme based on CSC,the cost of this scheme is reduced by 10.9%and the converter loss is reduced by 68.6%.