Research On The Control And Fault Diagnosis Technology Of Modular Multilevel Converter

As one of the key technologies of the flexible high voltage direct current transmission,the modular multilevel converter(MMC)has many advantages,such as modularization of power unit,low harmonics of output waveforms,low voltage of the power devices,and common DC bus,etc.It has attracted much attention and the research on it has great practical significance.This paper intensively studies the technologies of the modulation strategy,circulating current injection control,fault diagnosis and fault tolerant control on the basis of summarizing recent research of MMC at home and abroad.The modulation strategy is one of the essence control technologies of MMC.An optimized modulation strategy can reduce the IGBT switching frequency and improve the reliability of the system.The modulation method based on submodule capacitor voltage sorting can avoid complex close-loop controllers,but the IGBT switching frequency is quite high with the traditional strategy.In this paper,an optimized pulse width modulation method based on the capacitor voltage sorting is proposed by improving the carrier generation scheme and the inserted submodules selecting method.It can reduce the IGBT switching frequency effectively,and the simulation and experimental results are presented to verify it.Besides,a hybrid MMC topology with an improved nearest level modulation method is proposed in this paper.It can generate an output voltage of 2N+1 level,and the total level number of the inserted submodules is a constant,thus there is no pulse voltage across the arm inductors.The validity of the developed method is verified by the simulation and experimental results on a MMC-based inverter.Power passive components,such as submodule capacitors and arm inductors,are important for MMC to operate steadily.This paper studies the control technology to reduce the submodule capacitor voltage ripple while considering the power losses.The essence is the control of the circulating current,since it has a great impact on the capacitor voltage ripples,the power losses and the rating of power devices.Existing analysis of circulating current has only considered the capacitor voltage and the modulation signal.In this paper,the effect of semiconductor devices on the circulating current is further presented.In order to realize the different circulating current injection targets under different conditions,a unified on-line calculation scheme by using the instantaneous information of MMC is proposed to generate the circulating current injection references.It avoids detecting the amplitude and phase of the output current or using an extensive lookup table.Besides,a mathematical analysis model is built and it reveal the impact on the capacitor voltage ripple and the RMS value of arm current with different injection references,thus guide the determination of the injection ratio.The effectiveness of proposed method is verified with several circulating current control targets based on a three-phase MMC prototype with 6 SMs per arm.Besides,this paper proposes an arm inductor fault detection method by identifying the arm inductance variation online.The inductor characteristics and the impacts on the MMC are firstly introduced with an inductor fault in a phase.Based on these,the relation between the circulating current and the inductance variation of two arm inductors is analyzed and the arm inductor fault detection method is developed.When a fault occurs in one of the arm inductors,changes of the inductance can be obtained by the discrete Fourier transform of the compensation in the circulating current suppression controller,thus the fault condition can be determined.Finally,experimental results based on a three-phase MMC prototype with 6 submodules per arm are presented to verify the proposed method.Reliability is one of the substantial issues in MMC since many switching devices are used and each of these devices is a potential failure point.It is essential to detect the fault occurrence,locate the faulty submodule,isolate the fault and apply the fault tolerant control within a short time.In this paper,a strategy including fault detection,localization,redundancy and recovery is proposed to ensure continuous operation of MMC under IGBT open-circuit faults conditions.The features of MMC and SMs with three types of failures are studied respectively.Based on these,the fault detection method is proposed by using a simple hardware circuit,and the strategy to locate the faulty SM and identify the fault type is further developed by detecting the submodule capactior voltages and the capacitor cumulative discharge voltages in software.After this,the fault redundancy and the proposed fault recovery method are applied to eliminate the fault and then exit the failure state.Therefore,the ability of the submodule fault ride-through can be realized and it do not need complex calculations and complicated hardware circuit.Finally,the proposed method is verified by the simulation and experimental results on a MMC-based inverter.Besides,this paper studies the fault tolerant control method when the redundant submodules are exhausted or the MMC does not equip the redundant submodules.The previous strategy is firstly introduced and the performance is analyzed.Based on it,an improved fault tolerant control method by injecting a zero-sequence ac harmonic into the modulation signals is further proposed.The circulating current characteristics with the developed method are analyzed in detail,and the simulation and experimental results with two fault-tolerant methods are presented and compared to verify the proposed method.