| Modular multilevel converter(MMC) is a new topology applied in VSC-HVDC in recent years.Compared with traditional VSC-HVDC which adopts two-level or three-level converter. MMC not only has its advantages, but also, because of modular design, MMC can adjust the size of output level according to the number of tandem submodules. In the condition of high level, MMC can greatly reduced the switching frequency and output harmonic without ac filter.In all, MMC topology is suitable for VSC-HVDC.This paper analyzes the topological structure and principle of operation of MMC in detail. The cause of capacitor voltage unbalance of submodules is analyzed from the perspective of bridge arm energy balance. By defining the average switch state to analyze capacitor current of submodules and the theory of bridge arm voltage fluctuation.the cause and mathematical expression of second harmonic frequency circulation are obrained.This paper design nearest level modulation which combines ranking method to realize capacitor voltage balancing control of submodules.By analyzing the the Fourier spectrum, the size of circulation and the composition of bridge arm current are verified. According to the essence of circulation, this paper designs voltage balancing control strategy with three-phase brige arm equipartition of energy component. which realizes voltage balancing control and circulating current suppressing.By simulating single-phase ground fault in ac side.the result improves that three-phase bridge arm equipartition of energy control has stronger anti-interference ability than single-thase bridge arm equipartition of energy control.In the condition of adopting NLM and CPS-PWM modulation strategy, respectively, this paper compares the influence of the level number of MMC and switching frequence to the harmonic content of output voltage.The mathematical models of MMC in abc coordinate system and dq coordinate system respectively are built.Both the inside loop and an outside loop controllers of direct-current control are designed in dq coordinate system.Two-terminal with21level of MMC-HVDC simulation model is built in PSCAD and the simulation results demonstrate the good decoupling and fast dynamical response characteristic.According to the characteristic of mathematical model of MMC-HVDC and the advantage of deadbeat in digital control.this paper discretizes the mathematical model and designs the deadbeat decoupling control strategy in abc coordinate system.From the simulation results.not only the good decoupling and fast dynamical response characteristics are demonstrated.but also.in consequence of the reduction of PI control link and coordinate transformation frequency, the computation load and complexity of controller is reduced so that it can be easily designed and realized in digital control.Based on the connection type and control principles of multi-terminal MMC-HVDC, the advantage and disadvantage of master-slave control and master-slave combined DC voltage error control are analyzed.A three-terminal MMC-HVDC system model is established with PSCAD and the two kinds of control strategy mentioned are simulated.The simulation results indicate that master-slave combined DC voltage error control can realize mode-swtich of convertor station without high-speed communication.The flexibility and reliability of MMC-HVDC are improved, so the application range will be wider. |
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