| Based on the modular cascaded structure,modular multilevel converter(MMC)not only has strong scalability and avoids the difficulty of voltage balancing in switches connected in series,but also can achieve high-quality output waveforms with low switching frequency and achieve high reliability under high risk of failure through mutual backup of submodules(SMs).With these advantages,MMC has a broad development prospect in the field of medium-/high-voltage and large power.Especially in the high-voltage DC(HVDC)transmission system,MMC is being rapidly applied and becomes an important support for building a new electric power system based on new energy sources.With the continuous advancement of related research,a few common problems closely related to the multi-SM structure of MMC have feasible solutions,but they are still far from perfect for practical applications.Around these issues,the internal and external characteristics of MMC under different working conditions are cooperatively optimized in this paper by fully exploiting the potential of limited hardware resources and utilizing sharing redundancy among arms and selectivity of switching transitions within arms in MMC.The specific research contents include:(1)As the final link of generating SM switching pulses,modulation in MMC is often combined with voltage balancing control,which plays a crucial role in the actual control effect.However,due to the contradictory relationship between high modulation accuracy,low capacitor voltage difference and low switching frequency,it is difficult to balance these control objectives.Thus,optimal allocation of switching-pulse edge is proposed in this paper.Based on this,the nearest level pulsewidth modulation(NL-PWM)and carrier phase-shifted pulsewidth modulation(CPS-PWM)methods are improved,in which switching transitions necessary for modulation are utilized to decrease capacitor voltage differences to the greatest extent.As a result,the conflicts between multiple targets are reduced,thus improving the output harmonic characteristics and voltage balancing effect of MMC with low switching frequency.In addition,a parallel sorting method suitable for voltage balancing control is provided in this paper,which can significantly reduce the time overhead of determining the priority of SM insertion/bypass,thereby reducing the total control delay of MMC and improving control performance.(2)Due to the long closing time of the mechanical bypass switch,the open-circuit fault of the lower switch in SMs may cause serious overvoltage,which may further develop into a chain fault,even forcing MMC to shut down.Therefore,an overvoltage suppression method based on arm current reshaping is proposed in this paper.After locating the faulty SM and delivering the corresponding isolating signal,the arm current before fault isolation will be reshaped according to the pre-estimated peak capacitor voltage and expected voltage,so as to limit the degree of overvoltage to a safe range? Appropriate common-mode voltage will be injected when necessary to maintain the normal DC-port and AC-port voltages.As the prerequisite for overvoltage suppression,accurate localization for faulty SMs is also important.Considering the high error rate of fault detection based on arm voltage/current characteristics when the number of SMs in an arm is large,this paper also proposes a parallel fault diagnosis method and determines the selection principle of relevant thresholds based on error analysis.(3)Due to the limited number of hardware redundant SMs,if multiple SMs fail at the same time or over a period of time,the remaining SMs may be insufficient to generate the expected arm voltage,leading to severe distortion of the current waveforms.To solve this problem,a fault-tolerant control method based on bidirectional pulse level compensation is proposed in this paper.On one hand,a quasi-rate forward compensation is carried out in the section of arm voltage missing to maintain the DC-side and AC-side port voltages of MMC consistent with normal operation? On the other hand,reverse compensation shall be carried out in other appropriate sections to offset the arm energy imbalance caused by forward compensation to the greatest extent,thus suppressing the generation of fundamentalfrequency circulating current from the root cause.Compared with full-time common-mode voltage injection,the phased level compensation method not only further improves the fault tolerance margin of SMs,but also minimizes the harmonics of circulating currents during fault-tolerant operation,which has both high reliability and high economy.(4)The bus communication delay of massive SM-related data in MMC is long,which not only leads to the deterioration of control performance,but also may cause high-frequency oscillation of power system.Aiming at the optimization of communication delay,this paper proposes a precise voltage estimation and state monitoring method for SMs under voltage grouping measurement.During the capacitor pre-charging stage,the measured values of capacitor voltages can be obtained with the proposed self-checking method,thus determining whether each part is working normally? During the period of transmitting power normally,accurate estimated values of SM capacitor voltages can be obtained with the improved predictor-corrector algorithm and the proposed capacitance self-updating method?In the case of SM open-circuit failure,fault occurence and fault isolation can be detected in one sampling cycle,and accurate fault localization can be achieved within several sampling cycles based on the measured values of group voltages under orderly switching,almost achieving seamless operation.Compared to the scheme of minimizing the number of sensors,the proposed scheme effectively avoids the adverse effects on control accuracy and reliability caused by reduction of measurement data,and thus it has stronger practicality.Corresponding to the above research contents,each chapter of this paper contains extensive simulation and experimental results to prove the correctness and effectiveness of the achievements in this paper. |
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