Research On Hybrid Modular Multilevel High-Voltage Converter And Its Low-Frequency Operation Technology

Modular multilevel converter(MMC)has a promising application prospect in the field of high-voltage variable-speed drive applications due to its modularity,easy-to-expand structure and ideal sinusoidal output waveforms characteristics.Despite the salient features of MMC,one of the major obstacles to its widespread adoption in variable-speed drives is that,if no special measures were taken,the capacitance value of sub-modules(SMs)for a given maximum voltage ripple has to be proportional to output current amplitude and inversely proportional to output frequency.This means MMC may face extremely serious capacitor voltage ripple at low-frequency operation with high torque.A commonly used solution is the high frequency components injection,which injects high frequency circulating current in each MMC arm while superposes the same frequency common-mode voltage at the ac side of the MMC.However,the injected common-mode voltage is harmful to the bearings and insulation of the motor.To solve these problems,this paper proposes a low-frequency operation scheme of a hybrid modular multilevel converter topology(H-MMC),and introduces its topology and control methods.On this basis,for low-frequency operation,this thesis optimizes H-MMC from three aspects: reducing capacitance,reducing power loss and expanding four-quadrant applications,aiming to provide a complete H-MMC solution for high-voltage variable-speed drive applications.The specific content of this thesis is as follows:This thesis proposes to connect the thyristor as a DC switch in series on the MMC DC bus to form a H-MMC topology,which can suppress SM capacitor voltage ripple by reducing the DC voltage.Compared with the traditional capacitor voltage ripple suppression method like high-frequency components injection,H-MMC avoids the common mode voltage from acting on the motor and will not cause the motor insulation and shaft current problems.To ensure the thyristor can be softly turned on and safely turned off,a new control method is proposed by analyzing the thyristor switching process.When using this proposed method,H-MMC can also tolerate the failure of thyristor’s turning-off without shutting down the system,improving the reliability effectively.The capacitor voltage ripple analysis is also conducted,which theoretical proves that H-MMC has good capacitance voltage ripple suppression characteristics when operating at low frequencies.A 7.5kW experimental H-MMC motor drive platform is built,verifying the effectiveness of the proposed topology and control method.However,the capacitor voltage ripple of the HMMC at very low speed is still higher,which also calls for a larger capacitance.To solve this problem,an operation method of the H-MMC decreasing the capacitor voltage is proposed.By reducing the DC component of the SM capacitor voltage,a larger capacitor voltage ripple can be tolerated without exceeding the allowed voltage limit.Moreover,the selection method of the DC component of capacitor voltage within full-frequency range is studied.In addition,in order to avoid the over-modulation of the SM when decreasing the capacitor voltage,the limit range of the capacitor voltage ripple is analyzed,and dimension of SM capacitance is proposed based on this.Finally,the effectiveness of the proposed method is verified by simulation and experimental platform.Aiming at the loss problem caused by high switching frequency and high current stress during H-MMC low frequency operation,this thesis adopts Si CMOSFET devices in some SMs.The Si C-MOSFET sub-modules(Si C-SMs)are operated at high switching frequencies while the traditional Si-IGBT sub-modules(Si-SMs)are operated at low switching frequencies.By taking advantage of Si C devices such as high switching frequency and low switching loss,the efficiency of H-MMC is improved.In order to ensure the balance of Si C-SM capacitor voltage,a balancing control method is proposed.Simulation and experimental results verify the effectiveness of the proposed method.For high-voltage and high-power four-quadrant variable-speed drive applications,H-MMC can be expanded to a back-to-back structure.However,the circulating current control of the motor-and grid-side MMC will cause the conflict of DC current control.In this regard,this thesis analyzes the working mode of both motor-and grid-side MMC.Then equivalent circuit of back-to-back H-MMC DC-side loop circuit is established.Based on this circuit,the mechanism of the conflict is revealed.Then a circulating current decoupling control is proposed to handle this issue,which includes the design of the circulating current references to realize the input decoupling of the circulating current controller,and the use of digital control to realize the output decoupling of the circulating current controller.On this basis,this paper designs a control method of the back-to-back H-MMC.The effectiveness of the proposed topology and related control methods are verified by simulation and experiment respectively.