Research On Control And Fault Operation Strategy Of The Modular Multilevel Converter

The modular multilevel converter(MMC)has the advantages of modularity,scalability,and low voltage stress of power components,and it has been widely applied to medium/high-voltage high-power applications,especially for reactive power compensation.This thesis focuses on the half-bridge submodle(SM)based MMC,and targets to guarantee the high-performance control and consistent operation under fault conditions.It mainly researches on decoupled control,largesignal stability control,fault diagnosis,and fault-tolerant operation.The main work includes:Firstly,considering the high THD value of output currents and high circulating currents ripple caused by the coupling effect between the ac and circulating current control paths,this paper proposes a decoupled control scheme between the ac and circulating current control paths.In the ac control path,based on the 2N+1 output voltage level modulation method,the differences in inserted SM numbers between the lower and upper arms are calculated according to the ac output voltage reference.In the circulating current control path,based on the fuzzy controller,the sum of inserted SM numbers in the upper and lower arms are calculated according to the voltage references.Based on the sum and differences of inserted SM numbers,the inserted SM number in each arm can be calculated according to the circulating current tracking error.Based on the above results,the inserted SM number in each arm can be calculated,and the decoupled control can be realized between the ac and circulating current control paths.Verification results show that the proposed method can reduce the total harmonic distortion(THD)of output currents and lower the amplitude of the circulating current ripple.Secondly,considering the issues of issues of complicated parameter design and not guaranteed large-signal stability in the conventional cascaded linear control structure of MMC,a Lyapunov function-based dual-layer back-stepping control method is proposed.The first layer takes the stored total energy in SM capacitors and the output reactive current as the control targets.The state variables are selected,and their Lyapunov functions are established.The control laws of the control variables are designed so that the tracking error converges to zero asymptotically.The second layer takes the circulating current as the control target and state variable.The Lyapunov function is established,and the control laws are designed so that the tracking error converges to zero asymptotically.By the proposed method,the largesignal stability of the proposed method can be guaranteed theoretically.Simulation and experimental results verify that the proposed method operates normally under variable parameters and unbalanced grid voltage conditions and has fast dynamics under current reference step conditions.Next,considering the large computation burden of fault detection and localization algorithm caused by the large number of power switches,a fault diagnosis method of SM open-circuit switch fault is proposed based on predictions of state variables.Based on the MMC model,the output currents and circulating currents can be predicted,and their errors with real values are calculated.By comparing the error values and their threshold values,the fault can be detected and localized in the faulty arm.Considering the fact that the capacitor voltage in the faulty SM continuously increases,the SM with the highest capacitor voltage is selected in the faulty arm.According to its switching status and capacitor voltage increment,the fault can be confirmed in this SM,and the faulty SM can be localized.Simulation and experimental results verify that the proposed method can realize fast fault detection and localization,and the system operation safety can be guaranteed.Finally,considering the reduced range of output voltages under multiple SM fault conditions,a fault-tolerant operation approach is proposed based on the dynamic reference regulation of SM capacitor voltages.This approach increases the capacitor voltages in both the faulty arm and healthy arms,thus enlarging the output voltage ranges in the faulty and healthy phases.Furthermore,a lower amplitude of the zero-sequence voltage is injected to adjust the three-phase modulation waves,and the overmodulation can be avoided.The proposed method uses the three-phase healthy SMs to share the missing power from the faulty SMs,and the fault-tolerant operation ability can be improved under multiple SM fault conditions.Simulation and experimental results indicate that the proposed method can guarantee the consistent operation of MMC under multiple SM faults.