Research On Fault Ride-through Technology Of Hybrid Modular Multilevel Converter

Modular multilevel converter(MMC)based high-voltage direct-current(HVDC)has become an important technology for larger-capacity,longer-distance,and more efficient power transmission in China.The fault ride-through capability of MMC is directly related to the reliable operation of the transmission system and the development prospect of flexible DC transmission.With the construction cost,operation efficiency and fault ride-through capability of MMC taken into consideration,the hybrid MMC composed of half half-bridge submodule(HBSM)and half full-bridge submodule(FBSM)has a broad application prospect in the flexible direct current transmission technology.When hybrid MMC dc-side short-circuit fault occurs,it will not only cause a large DC fault current,but also penetrate to the AC side of the hybrid MMC,which will threaten the security and stability of the AC grid.Similarly,the AC side short-circuit fault hazard will spread to MMC and its DC side,which will threaten the safe operation of MMC and DC grid.MMC internal submodule(SM)fault will both affect the normal operation of AC and DC side at the same time.However,there is no systematic theoretical research on the comprehensive fault hazard penetration of hybrid MMC as well as its handling techniques.Due to the deficiency of theoretical research,the fault handling technology is immature,which will cause cascading failures and threaten the security and stability of the transmission system once the fault penetration influence is beyond the expectation.Therefore,this thesis carries out researches on the fault-tolerant reconstruction methodologies of the DC side,AC side and internal submodules of the hybrid MMC.The main research contents are as follows:(1)When DC voltage sags occur,if the hybrid MMC is directly locked to clear the DC fault current,the active power transmission capability and the fault ride-through capacity of the hybrid MMC are sacrificed,which will reduce the utilization of the hybrid MMC.For the dc voltage sags,a power adaptive compensation method is proposed to ensure the continuous and stable operation of the hybrid MMC,that is,according to the specific DC voltage drop situation,four operating intervals are divided to correspondingly adjust the power transmission mode of the hybrid MMC.With the proposed method,the safe and stable operation of the hybrid MMC can be guaranteed,and meanwhile the maximization of active power transmission can be realized,which will suppress the influence of the dc voltage sags on the ac-side instantaneous power outage.Furthermore,for the instantaneous high power outage and AC-grid frequency oscillation problem caused by the most serious DC-side short-circuit fault,a flexible compensation method based on power-flow port reconstruction is proposed to clear DC fault current and fully excavate the internal capacitor energy storage of the hybrid MMC to attenuate the transient change of AC-side active power and realize the flexible compensation of AC-side active power.Moreover,the generalized design and the mathematical quantification analysis of the proposed flexible compensation method is carried out,whose results show that the proposed flexible compensation method can be applicable to other MMC with different capacity ratings.The proposed flexible compensation method enables the hybrid MMC to act as both a DC circuit breaker and an energy storage converter in addition to power converter.Compared with the conventional schemes,the proposed method can not only eliminate the DC fault current,but also effectively suppress the penetration impact of DC-side fault hazards on the AC-side point of common coupling(PCC),and gain a certain time for the power grid dispatching to deal with fault events,which will enhance the stability of the AC grid.(2)Different from AC grid-side short-circuit faults,the valve-side single-phase grounding fault and two-phase short-circuit grounding fault will cause valve-side nonfault-phase overvoltage and produce fundamental-frequency zero-sequence voltage and current,which will further penetrate to DC side through the hybrid of MMC and distributed capacitance to ground of the DC transmission lines.As a result,the fundamental-frequency oscillation of the DC bus voltage to ground and DC current will be caused,which will lead to DC-line insulation breakdown and overcurrent damage of MMC internal devices.Even though the hybrid MMC is shut down after the fault,the submodule(SM)capacitors and power semiconductor switches in the nonfault-phase bridge arms will be damaged by overvoltage.Therefore,by taking full use of the inherent voltage boundary of the hybrid MMC,a port voltage adaptive matching method based on the modulation benchmark reconstruction is proposed to realize the AC valveside short-circuit fault ride-through,which can effectively eliminate the fundamental-frequency zero-sequence current at the AC valve side and the fundamental-frequency oscillation of the DC bus voltage to ground and DC current.Furthermore,by analyzing the fault-tolerant operation characteristics of the hybrid MMC,the proposed port voltage adaptive matching method can guarantee that:(a)In the case of single-phase grounding fault at the AC valve side,the short-circuit fault current in the entire AC and DC transmission system can be effectively cleared,and the hybrid MMC can continuously provide reactive power support to the AC grid,in which the capacitive reactive power output range is larger than that of the inductive reactive power;(b)In the case of two-phase short-circuit grounding fault at the AC valve side,the fundamental-frequency oscillation of the DC side is effectively eliminated before the breaking of the AC circuit breaker on the grid side,which can avoid the overvoltage damage of SM capacitors and power switches in the nonfault-phase bridge arms caused by the direct shutting down of the hybrid MMC.Compared with the conventional schemes,the proposed method makes up for the vacant mechanism of the fault-tolerant operation of AC valve-side overvoltage without increasing the hardware cost,and eliminates the security threat of AC valve-side shortcircuit faults to DC grid and hybrid MMC.(3)For the open-circuit faults of the internal SM switches of hybrid MMC,the existing fault diagnosis methods take a long time to diagnose or only consider a single fault diagnosis or a specific modulation strategy,and some of them are complicated in calculation and require high processing resources.Therefore,a novel fault diagnosis method based on the operating characteristics of the SM with maximum capacitor voltage is proposed in this thesis.By judging the deviation between the actual capacitor voltage variation and the theoretical variation of the SM with maximum capacitor voltage,the multi-SM faults can be diagnosed quickly,which is prepared for the fault-tolerant control.The proposed fault diagnosis method is not only applicable to the nearest level modulation,but also to carrier phase shift modulation,carrier disposition modulation and model predictive control combined with SM capacitor voltage balance algorithm.Due to the limited number of hardware redundant SMs,once the number of faulty SMs exceeds the redundancy,the operating performance of hybrid MMC will be deteriorated seriously.Therefore,this thesis further proposes a SM fault-tolerant reconstruction method based on the inherent SMs sharing.According to the phase alternating characteristics of arm voltages,the idle SMs in the other arms are utilized to compensate for the missing level of the faulty arm,which can maintain the DC-side and AC-side port voltages of the hybrid MMC consistent with normal operation.After employing the proposed method,the hybrid MMC can still operate normally without fault disturbances.Compared with the inherent modulation redundancy in MMC,the proposed fault-tolerant reconstruction method can achieve SM fault ride-through without hardware redundancy,and the redundancy rate and the maximum number of fault-tolerant SMs are expanded by two times,which greatly improves the economy and operating reliability of the hybrid MMC.This thesis fully exploits the inherent characteristics of the hybrid MMC to realize the fault-tolerant reconstruction operation of the hybrid MMC under the faults of DC side,AC side and internal submodule,which effectively restrains the fault hazard and its penetration effects.The proposed methods guarantee the security and stability of AC grid,DC grid and hybrid MMC itself,which makes hybrid MMC play a more significant role in multi-terminal interconnected HVDC transmission applications.