Research On Key Technologies For Modular Multilevel Converters

As a preferred solution for high power energy conversion, multilevel converters are attracting increasing research interests worldwide. Compared with diode-clamped, flying-capacitor, and cascadedH-bridge multilevel converters, the modular multilevel converter (MMC)achieves the highest level of modularization, and therefore has been considered themost promising multilevel converter schemes in the future. This paper has carried out some research on the operating principle, working characteristic and key technology of MMC, which are organized as follows:Building up the steady-state model as well as dynamic model is essential to the high-quality control of MMC. After analyzing the operating principle, switching states and input-output energy regulation of MMC, the ideal switching model and average switching model of a three-phase MMC are established. Moreover, to eliminate the influence on input and output of MMC introduced by the coupling between upper and lower arms, a decoupled low-frequency model is also developed. To describe such a multivariable system, a multi-loop control system is proposed. As for internal voltage balance, the analysis and control design of one phase leg can be easily expanded to other legs since each phase are independently operated. Afteranalyzing the characteristic of energy stored inphase leg, an energy based control structure of MMC is proposed, which consists of total energy control and differential energy control. The circulating current control and ac output control can be easily emedded into the structure. Simulation and experimental results validate the proposed mathematical model and control structure. Simulation models in MATLAB/SIMULINK as well as reduced-scall prototype based on DSP+FPGA platform are established to verify the above model and control systems.The voltage balance among all SMs in one arm is crutial to the normal operation of MMC. At present, realizing SM voltage balance in modulation is a preferred altenative, since this utilizes the freedom of switching state. However, existing voltage balancing methods more or less have some drawbacks, such as lack of stability, need of long-time-excuted sorting algorithm, increased switching frequency, etc. To overcome these drawbacks, this paper proposes two balancing methods. Firstly, combined with altenative pulse distribution PWM, a pulse width compensation based balancing method is proposed. This method is easy to implement, leaving out complex sorting algorithm and will not introduce extra switching states. Secondly, modification is made on traditional sorting method, so that it is suitable for large submodule (SM)number, low switching frequency, high-power applications. Without sorting algorithm, it greatly saves computation burden and achieves good balancing effect as well.Currently, the researches of MMC are mostly focused on constant-frequency application.However, when operating at low frequency, there exists huge voltage fluctuation in SM capacitors, which greatly limits its application in high-voltage high-power ac drives. Mostly adopted method to restrict this low-frequency voltage ripple is simultaneously injecting high-frequency (relative to AC output frequency) common-mode voltage as well as injecting the same frequency circulating currents. There are two main shortcomings existed in this method:the waveform of circulating current is complex and the amplitude of arm current is large. In order to track the complex injected circulating current reference, a Proportional-Integraltogether withQuasi-Resonantcontrol scheme is firstly presented in this paper. Morever, to reduce the arm current stress of proposed control scheme, the concept of "promoting common-mode votage utilization" is proposed. Arm currents are decreased by modifying the waveform of injected common-mode voltage. At last, the full range operaion of MMC-based ac drive is realized by changing capacitor voltage reasonably. Simulation and experimental results indicate that proposed control scheme can realize the accurate control of circulating currents, effectively restrain voltage ripple and reduce arm currents. With this method, HB-SM based MMC can be applied to most ac drives applications where start-up and low-frequency torque are low.FB-SM based MMC is firstly applied to ac drives here. Though many efforts have been made on control, HB-SM based MMCs are still not applicable to large start-up and low-frequency torque applications, such as traction drives and electrical ship propulsion. To radically overcome this technical problem, this paper applies FB-SM based MMCs to ac drives. Although the number of power devices is doubled, the capacitor voltage ripple is significantly reduced and device stress is greatly relieved. Hence the economic effect is obvious. Some researches are focused on FB-SM’s application to ac drives. Compared with HB-SM MMC, although the number of power devices is doubled, the problems in low-frequency operation of MMC can be totally solved. Since the capacitor voltage ripple is greatly reduced, the stress of devices is significantly aliviated. Thereby the economic effect is obvious.