Modular multilevel converters(MMCs)have been one of the most broadly used multilevel converter topologies in industrial applications,particularly in medium-voltage motor drives and high-voltage dc power conversion sys...Modular multilevel converters(MMCs)have been one of the most broadly used multilevel converter topologies in industrial applications,particularly in medium-voltage motor drives and high-voltage dc power conversion systems.However,due to the utilization of large amount of semiconductor devices,the reliability of MMCs becomes one of the severe challenges constraining their further development and applications.In this paper,common electrical faults of the MMC have been summarized and analyzed,including open-circuit switching faults,short-circuit switching faults,dc-bus short-circuit faults,and single line-to-ground faults on the ac side.A thorough and comprehensive review of the existing online fault diagnostic methods has been conducted.In addition,fault-tolerant operation strategies for such various fault scenarios in MMCs have been presented.All the fault diagnosis and fault-tolerant operation strategies are comparatively evaluated,which aims to provide a state-of-the-art reference on the MMC reliability for future research and industrial applications.展开更多
The modular multilevel converter(MMC)has become a promising topology for widespread power converter applications.However,an evident circulating current flowing between the phases will increase system losses and compli...The modular multilevel converter(MMC)has become a promising topology for widespread power converter applications.However,an evident circulating current flowing between the phases will increase system losses and complicate the heatsink design.This paper proposes a novel hybrid model predictive control method for MMCs.This method utilizes an indirect structure MPC and a sorting algorithm to implement current tracking and capacitor voltages balancing,considerably resulting in reduced calculation burden.In addition,different from the conventional MPC solutions,we add a simple proportional-integral(PI)controller to suppress circulating current through modifying the submodule(SM)inserted number,which is parallel to the MPC loop.This hybrid control solution combines both advantages of MPC and linear control,evidently resulting in improved performance of circulating current.Finally,the MATLAB/Simulink results of an 11-level MMC system verify the effectiveness of the proposed solution.展开更多
Modular Multilevel Converters(MMC)have emerged as a key technology for medium-to high-voltage power conversion applications such as HVDC transmission,large-scale renewable integration,and flexible AC transmission syst...Modular Multilevel Converters(MMC)have emerged as a key technology for medium-to high-voltage power conversion applications such as HVDC transmission,large-scale renewable integration,and flexible AC transmission systems.Despite their superior performance and modularity,MMCs present complex control challenges due to their multi-level structure,numerous submodules,and circulating currents.This paper comprehensively reviews advanced control strategies developed for MMCs,including fundamental current and voltage balancing controls,sophisticated switching algorithms,and multifunctional approaches addressing harmonic suppression,power factor correction,and fault tolerance.Emphasis is placed on modern methods such as model predictive control,adaptive and robust controls that accommodate system nonlinearities and parameter uncertainties.Implementation aspects,including digital control hardware and the impact of control delays,are also discussed.The paper highlights recent advancements that improve system stability,efficiency,and reliability,thus facilitating MMC deployment in evolving power grids with high renewable penetration and stringent grid code requirements.展开更多
This paper develops a detailed equivalent model for modular multilevel converters with partially-integrated battery energy storage.The proposed model gains computational efficiency in two ways.Firstly,it markedly redu...This paper develops a detailed equivalent model for modular multilevel converters with partially-integrated battery energy storage.The proposed model gains computational efficiency in two ways.Firstly,it markedly reduces the large number of nodes in the conventional switching model of the converter,thereby shrinking the size of its admittance matrix.Secondly,it avoids computationally expensive re-triangularization of the admittance matrix during the normal operation of the converter and restricts it only to the rare occasions of converter blocking.Mathematical derivation of the model is carried out using differential equations of the converter.The computational efficiency and accuracy of the proposed model are confirmed by comparison of the results from its implementation in the PSCAD/EMTDC simulator against conventional detailed switching models and measurements from a single-phase scaleddown laboratory setup.This paper also shows a case study wherein a converter with partially-integrated batteries is included in the CIGRE B4-5 benchmark system.展开更多
In the present scenario,modular multilevel converters(MMCs)are considered to be one of the most promising and effective topologies in the family of high-power converters because of their modular design and good scalab...In the present scenario,modular multilevel converters(MMCs)are considered to be one of the most promising and effective topologies in the family of high-power converters because of their modular design and good scalability;MMCs are extensively used in high-voltage and high-power applications.Based on their unique advantages,MMCs have attracted increasing attention from academic circles over the past years.Several studies have focused on different aspects of MMCs,including submodule topologies,modeling schemes,modulation strategies,control schemes for voltage balancing and circulating currents,fault diagnoses,and fault-tolerant control strategies.To summarize the current research status of MMCs,all the aforementioned research issues with representative research approaches,results and characteristics are systematically overviewed.In the final section,the current research status of MMCs and their future trends are emphasized.展开更多
Power loss management is one of the most significant challenges for reliability improvement of modular multilevel converters(MMCs).In the MMC,the bottom switch/diode in each submodule(SM)normally takes the maximum pow...Power loss management is one of the most significant challenges for reliability improvement of modular multilevel converters(MMCs).In the MMC,the bottom switch/diode in each submodule(SM)normally takes the maximum power loss.In this paper,a power loss optimization control(PLOC)for MMCs is proposed,where the maximum power losses in the bottom switch/diode of each SM can be effectively reduced through injecting optimum second-order harmonic current into the circulating current of MMCs,and accordingly the reliability of MMCs can be improved by the proposed PLOC.Simulation results with PSCAD software and experimental results with a 1 kW MMC platform are provided to confirm the validity of the proposed PLOC for MMCs.展开更多
Dead time is necessary for the coupled power switches to prevent shoot-through,especially in the modular multilevel converters(MMCs)with a large number of power switches.This paper proposes a dead-time effect suppress...Dead time is necessary for the coupled power switches to prevent shoot-through,especially in the modular multilevel converters(MMCs)with a large number of power switches.This paper proposes a dead-time effect suppression strategy for MMCs with nearest level modulation.The operational principles of MMCs are first analyzed.According to the operational features of MMCs,the method that removes a switching signal from the coupled switches and the reduced switching frequency voltage balancing algorithms(RSFVBAs)are mixed in the proposed method.In the intervals that are furthest away from the zerocrossing points(ZCP)of arm currents,the single switching signal method can completely eliminate the dead-time effect(DTE).Alternatively,the DTE is suppressed by the RSFVBA in intervals that are close to the ZCP.By the combination of the two methods,the dependence of the DTE suppression method on currents is reduced and the influences of ZCP are also released without degrading the normal operation performance of MMCs.Moreover,the output performance of MMCs is improved and the voltage stress on the arm inductor dramatically decreases.Finally,the validation of the method is verified by the simulation results with the professional tool Matlab/Simulink.展开更多
To understand the operation principle of the modular multilevel converter(MMC)deeply,it is necessary to study the harmonic characteristics of the MMC theoretically.Besides,the analytical harmonic formulas of the MMC a...To understand the operation principle of the modular multilevel converter(MMC)deeply,it is necessary to study the harmonic characteristics of the MMC theoretically.Besides,the analytical harmonic formulas of the MMC are useful in designing the main circuit,reducing the losses and improving the waveform quality.Based on the average switching function and the Fourier series harmonic analysis,this paper deduces the analytical expressions for such electrical quantities as the arm voltage,the arm current,the capacitor voltage,the capacitor current and the circulating current of the MMC.Finally,a digital model of a 21-level MMC-HVDC system is realized in PSCAD/EMTDC.The results of the analytical expressions coincide with the simulation results,which verify the effectiveness and feasibility of the proposed analytical expressions.展开更多
Grid-forming control(GFC)is promising for power electronics based power systems with high renewable energy penetration.Naturally,the impedance modeling for GFC is necessary and has gained significant attention recentl...Grid-forming control(GFC)is promising for power electronics based power systems with high renewable energy penetration.Naturally,the impedance modeling for GFC is necessary and has gained significant attention recently.However,most of the impedance analyses for GFC are based on a twolevel converter(TLC)rather than a modular multilevel converter(MMC).MMC differs from TLC with respect to its dominant multi-frequency response.It is necessary to analyze the impedance of GFC-based MMC owing to its superiority in highvoltage direct current(HVDC)transmission to interlink two weak AC systems with high renewable energy penetration.As the main contribution,this paper presents the AC-and DC-side impedance analyses for the GFC-based MMC with both power and DC voltage control using the harmonic transfer function(HTF),and compares the impedances of GFC-based MMC and TLC.It is inferred that although the impedance is mainly influenced within 200 Hz,the instability still could occur owing to negative resistance triggered by relatively larger parameters.The difference in AC-side impedance with power and DC voltage control is not apparent with proper parameters,while the DC-side impedance differs significantly.The generalized Nyquist criterion is necessary for AC-side stability owing to the relatively large coupling terms under GFC.Moreover,the coupling between AC-and DC-side impedances is noneligible,especially considering the DC-side resonance around the system resonant peak.The effects of parameters,system strength,and virtual impedance on the impedance shaping are analyzed and verified through simulations.展开更多
The modular multilevel converters(MMCs) are popularly used in high-voltage direct current(HVDC) transmission systems. However, for the direct modulation based MMC, its complex internal dynamics and the interaction wit...The modular multilevel converters(MMCs) are popularly used in high-voltage direct current(HVDC) transmission systems. However, for the direct modulation based MMC, its complex internal dynamics and the interaction with the grid impedance would induce the frequency coupling effect, which may lead to instability issues, especially in the case of weak grid. To effectively suppress the sub-and super-synchronous oscillations, this paper proposes a linear active disturbance rejection control(LADRC) based MMC control strategy. The LADRC mainly consists of the linear extended state observer(LESO) and the linear state error feedback(LSEF). And it is a potential method to enhance the system stability margin, attributing to its high anti-interference capability and good tracking performance. Thereupon, the system small-signal impedance model considering frequency coupling is established. And the effect of the introduction of the LADRC on the system stability is further investigated using the Nyquist criterion. Particularly, the influences of key control parameters on the stability are discussed in detail. Meanwhile, the impact of LADRC on the transient performance is explored through closed-loop zero poles. Finally, the correctness of the theoretical analysis and the effectiveness of the proposed control strategy are verified via electromagnetic simulations.展开更多
The high-voltage direct current(HVDC)grid has been recognized as an effective solution for renewable energy integration.Currently,two main development trends for HVDC grids are being studied:a DC breaker based HVDC gr...The high-voltage direct current(HVDC)grid has been recognized as an effective solution for renewable energy integration.Currently,two main development trends for HVDC grids are being studied:a DC breaker based HVDC grid and fault-blocking converter based HVDC grid.Although the former has a perfect performance for fault clearance,its development is still highly constrained by the cost and maturity of DC breakers.The latter can extinguish DC faults by the fault-blocking converters.Without using DC breakers,there is no bottleneck in its technical feasibility.Nevertheless,in fault scenarios,such types of HVDC grids will be blocked at length for air-deionization,which is its main drawback.The aim of this paper is to minimize its power interruption time,by optimizing protection coordination strategies.To cover the most complex cases,the overhead line applications,in which the reclosure actions are required to be implemented,are considered.In this paper,the protection requirements of HVDC grids are first discussed,then the benefits of fault-blocking modular multilevel converters(MMCs)and their fault features are analyzed.Based on this,a control function is designed to reduce the air-deionization time.To minimize the influence of the DC faults,a separation methodology for restarting the system is proposed.The effectiveness of the proposed protection coordination schemes is validated by PSCAD/EMTDC simulations.展开更多
Due to the large number of submodules(SMs),and modular multilevel converters(MMCs)in high-voltage applications,they are usually regulated by the nearest level modulation(NLM).Moreover,the large number of SMs causes a ...Due to the large number of submodules(SMs),and modular multilevel converters(MMCs)in high-voltage applications,they are usually regulated by the nearest level modulation(NLM).Moreover,the large number of SMs causes a challenge for the fault diagnosis strategy(FDS).This paper proposes a currentless FDS for MMC with NLM.In FDS,the voltage sensor is relocated to measure the output voltage of the SM.To acquire the capacitor voltage and avoid increasing extra sensors,a capacitor voltage calculation method is proposed.Based on the measurement of output voltages,the faults can be detected and the number of different-type switch open-circuit faults can be confirmed from the numerous SMs in an arm,which narrows the scope of fault localization.Then,the faulty SMs and faulty switches in these SMs are further located without arm current according to the sorting of capacitor voltages in the voltage balancing algorithm.The FDS is independent of the arm current,which can reduce the communication cost in the hierarchical control system of MMC.Furthermore,the proposed FDS not only simplifies the identification of switch open-circuit faults by confirming the scope of faults,but also detects and locates multiple different-type faults in an arm.The effectiveness of the proposed strategy is verified by the simulation results.展开更多
The capacitor is one of the most important compo-nents in a modular multilevel converter(MMC).Due to the chemical process and the aging effect,the capacitor is subject to deterioration over time which is usually manif...The capacitor is one of the most important compo-nents in a modular multilevel converter(MMC).Due to the chemical process and the aging effect,the capacitor is subject to deterioration over time which is usually manifested by a drop in capacitance.To identify the abnormal capacitors and enhance the reliability of MMCs,an improved submodule(SM)capacitor condition monitoring method is proposed in this pa-per.The proposed method estimates the capacitance during each control cycle based on the switching states of SMs,offer-ing advantages such as high accuracy and no adverse influence on the operation of MMCs.Firstly,the aging differences of ca-pacitors in different SMs per arm of MMC are analyzed.Then,the capacitances of SMs that switch on the state are calculated based on the relationship between the capacitor voltage and cur-rent during each control cycle.A data processing algorithm is proposed to improve the accuracy of capacitance estimation.Fi-nally,the simulation and the real-time control hardware-in-the loop test results based on real-time digital simulator(RTDS)show the effectiveness of the proposed method.展开更多
Modular multilevel resonant converter is an promising candidate for high voltage applications since it has advantageous features,such as high efficiency,high voltage capability and easy fault-tolerant operation.Howeve...Modular multilevel resonant converter is an promising candidate for high voltage applications since it has advantageous features,such as high efficiency,high voltage capability and easy fault-tolerant operation.However,the inequality of arm inductance in practice will lead to imbalance between the upper and lower arm voltages,which will induce large ripples in the circulating current and a dc bias on the voltage generated by modular circuits.To compensate for the voltage imbalance,effects of arm duty cycle changes on arm voltages are discussed.An arm voltage balancing control method is proposed:adjust arm duty cycle according to arm voltage deviation in every switching cycle.Simulation and experimental results are presented to validate the theoretical analysis and the proposed control method.展开更多
Solid state transformer(SST)can provide more advanced functionalities compared with conventional transformer,and has great potential in smart grid application.Recently,the SST with medium frequency(MF)isolation link a...Solid state transformer(SST)can provide more advanced functionalities compared with conventional transformer,and has great potential in smart grid application.Recently,the SST with medium frequency(MF)isolation link and magnetic integration feature has been proposed,which can reduce the system volume and thus increase the power density.However,the magnetic integration also introduces strong coupling between the line frequency(LF)and MF variables,which poses a great challenge on modeling and control issues.This paper proposes a modeling and control method for an SST with magnetic integration and mixed-frequency modulation.A mathematical model based on dual d-q references is deduced,and then a cascaded control system is designed according to the model.Parameters of the controller for the variables at one frequency are properly designed to avoid disturbance from the variables at the other frequency.The simulation and experimental results show good decoupling effect and satisfactory dynamics performance of the proposed control system.展开更多
To facilitate rapid analysis of the oscillation stability mechanism in modular multilevel converter-based high voltage direct current(MMC-HVDC)systems and streamline the simulation process for determining MMC impedanc...To facilitate rapid analysis of the oscillation stability mechanism in modular multilevel converter-based high voltage direct current(MMC-HVDC)systems and streamline the simulation process for determining MMC impedance characteristics,a simplified mathematical simulation model for MMC closed-loop impedance is developed using the harmonic state space method.This model considers various control strategies and includes both AC-side and DC-side impedance models.By applying a Nyquist criterion-based impedance analysis method,the stability mechanisms on the AC and DC sides of the MMC are examined.In addition,a data-driven oscillation stability analysis method is also proposed,leveraging a global sensitivity algorithm based on fast model results to identify key parameters influencing MMC oscillation stability.Based on sensitivity analysis results,a parameter adjustment strategy for oscillation suppression is proposed.The simulation results from the MATLAB/Simulinkbased MMC model validate the effectiveness of the proposed method.展开更多
A modular multilevel converter(MMC)integrated with split battery cells(BIMMCs)is proposed for the battery management system(BMS)and motor drive system.In order to reduce the switching losses,the state of charge(SOC)ba...A modular multilevel converter(MMC)integrated with split battery cells(BIMMCs)is proposed for the battery management system(BMS)and motor drive system.In order to reduce the switching losses,the state of charge(SOC)balancing strategy with a reduced switching-frequency(RSF)is proposed in this paper.The proposed RSF algorithm not only reduces the switching losses,but also features good balancing performance both in the unbalanced and balanced initial states.The results are verified by extensive simulations in MATLAB/Simulink surroundings.展开更多
This paper proposes a continuous control set model predictive control(CCS-MPC)algorithm of a modular multilevel matrix converter(M3C)for low-frequency AC transmission(LFAC),via which the offshore wind farm(OWF)is inte...This paper proposes a continuous control set model predictive control(CCS-MPC)algorithm of a modular multilevel matrix converter(M3C)for low-frequency AC transmission(LFAC),via which the offshore wind farm(OWF)is integrated.The M3C is operated with a 16.7 Hz frequency at the OWF side and a 50 Hz frequency at the onshore grid side.The balance of the capacitor voltages and the regulation of circulating currents in the M3C are performed using the proposed CCSMPC algorithm,which is based on the online solution of a cost function with constraints.Simulation and experimental work(with a 5 kW M3C prototype)are provided,showing the performance of the LFAC system to operate with symmetrical and asymmetrical voltage dips,active and reactive power steps,and optimal limitation of currents and voltages using constraints.Unlike previous publications,the predictive control system in this paper allows seamless operation under balanced and unbalanced conditions,for instance,during asymmetrical voltage dips.展开更多
This paper presents a novel inter-cluster direct current(DC)capacitor voltage balancing control scheme for the single-star configured modular multilevel cascaded converter(MMCC)-based static synchronous compensator(ST...This paper presents a novel inter-cluster direct current(DC)capacitor voltage balancing control scheme for the single-star configured modular multilevel cascaded converter(MMCC)-based static synchronous compensator(STATCOM)under unbalanced grid voltage.The negative-sequence component of grid voltage at the point of common connection(PCC)causes unbalanced active power flow in the phase limbs of converter.This leads to the imbalance of DC voltages of the sub-module capacitors across the MMCC phases,and consequently,the malfunction of converter.The proposed solution is to inject both negative-sequence current(NSC)and zero-sequence voltage(ZSV)into the phase limbs of MMCC.A quantification factor Qf is used to achieve the sharing of inter-cluster active pow-er between the NSC and ZSV injection methods.Accurate determination of the quantification factor has been presented.In addition to maintaining the DC voltages of sub-module capacitor across the MMCC phases balanced,it also prevents the overcurrent and overvoltage of converter by injecting NSC and ZSV with the right proportion.The control scheme is validated on a 3.54 kV 1.2 MVA power system using MMCC-based STATCOM with 3-level bridge cells as sub-modules.The results show that the proposed scheme provides superior effectiveness in eliminating the voltage imbalance of DC capacitor in the phase limb while maintaining low voltage and current ratings.展开更多
Simultaneously imposed challenges of highvoltage insulation,high dv/dt,highswitching frequency,fast protection,and thermal management associated with the adoption of 10 kV SiC MOSFET,often pose nearly insurmountable b...Simultaneously imposed challenges of highvoltage insulation,high dv/dt,highswitching frequency,fast protection,and thermal management associated with the adoption of 10 kV SiC MOSFET,often pose nearly insurmountable barriers to potential users,undoubtedly hindering their penetration in mediumvoltage(MV)power conversion.Key novel technologies such as enhanced gatedriver,auxiliary power supply network,PCB planar dcbus,and highdensity inductor are presented,enabling the SiCbased designs in modular MV converters,overcoming aforementioned challenges.However,purely substituting SiC design instead of Sibased ones in modular MV converters,would expectedly yield only limited gains.Therefore,to further elevate SiCbased designs,novel highbandwidth control strategies such as switchingcycle control(SCC)and integrated capacitorblocked transistor(ICBT),as well as highperformance/highbandwidth communication network are developed.All these technologies combined,overcome barriers posed by stateoftheart Si designs and unlock system level benefits such as very high power density,highefficiency,fast dynamic response,unrestricted line frequency operation,and improved power quality,all demonstrated throughout this paper.展开更多
文摘Modular multilevel converters(MMCs)have been one of the most broadly used multilevel converter topologies in industrial applications,particularly in medium-voltage motor drives and high-voltage dc power conversion systems.However,due to the utilization of large amount of semiconductor devices,the reliability of MMCs becomes one of the severe challenges constraining their further development and applications.In this paper,common electrical faults of the MMC have been summarized and analyzed,including open-circuit switching faults,short-circuit switching faults,dc-bus short-circuit faults,and single line-to-ground faults on the ac side.A thorough and comprehensive review of the existing online fault diagnostic methods has been conducted.In addition,fault-tolerant operation strategies for such various fault scenarios in MMCs have been presented.All the fault diagnosis and fault-tolerant operation strategies are comparatively evaluated,which aims to provide a state-of-the-art reference on the MMC reliability for future research and industrial applications.
基金This work was partially supported by the National Natural Science Foundation of China(11847104)General Program of National Natural Science Foundation of China(51977124)+2 种基金Shandong Natural Science Foundation(ZR2019QEE001)Natural Science Foundation of Jiangsu Province(BK20190204)National Distinguished Expert(Youth Talent)Program of China(31390089963058)。
文摘The modular multilevel converter(MMC)has become a promising topology for widespread power converter applications.However,an evident circulating current flowing between the phases will increase system losses and complicate the heatsink design.This paper proposes a novel hybrid model predictive control method for MMCs.This method utilizes an indirect structure MPC and a sorting algorithm to implement current tracking and capacitor voltages balancing,considerably resulting in reduced calculation burden.In addition,different from the conventional MPC solutions,we add a simple proportional-integral(PI)controller to suppress circulating current through modifying the submodule(SM)inserted number,which is parallel to the MPC loop.This hybrid control solution combines both advantages of MPC and linear control,evidently resulting in improved performance of circulating current.Finally,the MATLAB/Simulink results of an 11-level MMC system verify the effectiveness of the proposed solution.
文摘Modular Multilevel Converters(MMC)have emerged as a key technology for medium-to high-voltage power conversion applications such as HVDC transmission,large-scale renewable integration,and flexible AC transmission systems.Despite their superior performance and modularity,MMCs present complex control challenges due to their multi-level structure,numerous submodules,and circulating currents.This paper comprehensively reviews advanced control strategies developed for MMCs,including fundamental current and voltage balancing controls,sophisticated switching algorithms,and multifunctional approaches addressing harmonic suppression,power factor correction,and fault tolerance.Emphasis is placed on modern methods such as model predictive control,adaptive and robust controls that accommodate system nonlinearities and parameter uncertainties.Implementation aspects,including digital control hardware and the impact of control delays,are also discussed.The paper highlights recent advancements that improve system stability,efficiency,and reliability,thus facilitating MMC deployment in evolving power grids with high renewable penetration and stringent grid code requirements.
基金supported in part by the Natural Sciences and Engineering Research Council(NSERC)of Canada,MITACS Accelerate,Manitoba Hydro,and by the University of Manitoba。
文摘This paper develops a detailed equivalent model for modular multilevel converters with partially-integrated battery energy storage.The proposed model gains computational efficiency in two ways.Firstly,it markedly reduces the large number of nodes in the conventional switching model of the converter,thereby shrinking the size of its admittance matrix.Secondly,it avoids computationally expensive re-triangularization of the admittance matrix during the normal operation of the converter and restricts it only to the rare occasions of converter blocking.Mathematical derivation of the model is carried out using differential equations of the converter.The computational efficiency and accuracy of the proposed model are confirmed by comparison of the results from its implementation in the PSCAD/EMTDC simulator against conventional detailed switching models and measurements from a single-phase scaleddown laboratory setup.This paper also shows a case study wherein a converter with partially-integrated batteries is included in the CIGRE B4-5 benchmark system.
基金Supported by the Science and Technology Program of State Grid Corporation of China(5100-201999330A-0-0-00)。
文摘In the present scenario,modular multilevel converters(MMCs)are considered to be one of the most promising and effective topologies in the family of high-power converters because of their modular design and good scalability;MMCs are extensively used in high-voltage and high-power applications.Based on their unique advantages,MMCs have attracted increasing attention from academic circles over the past years.Several studies have focused on different aspects of MMCs,including submodule topologies,modeling schemes,modulation strategies,control schemes for voltage balancing and circulating currents,fault diagnoses,and fault-tolerant control strategies.To summarize the current research status of MMCs,all the aforementioned research issues with representative research approaches,results and characteristics are systematically overviewed.In the final section,the current research status of MMCs and their future trends are emphasized.
基金supported in part by the National Natural Science Foundation of China under Grant No.61873062in part by the Natural Science Foundation of Jiangsu Province under Grant No.BK20180395in part by the Six Talent Peaks Project of Jiangsu Province under Grant No.GDZB-002.
文摘Power loss management is one of the most significant challenges for reliability improvement of modular multilevel converters(MMCs).In the MMC,the bottom switch/diode in each submodule(SM)normally takes the maximum power loss.In this paper,a power loss optimization control(PLOC)for MMCs is proposed,where the maximum power losses in the bottom switch/diode of each SM can be effectively reduced through injecting optimum second-order harmonic current into the circulating current of MMCs,and accordingly the reliability of MMCs can be improved by the proposed PLOC.Simulation results with PSCAD software and experimental results with a 1 kW MMC platform are provided to confirm the validity of the proposed PLOC for MMCs.
基金supported by the State Key Laboratory of Advanced Power Transmission Technology(GEIRI-SKL-2020-011)。
文摘Dead time is necessary for the coupled power switches to prevent shoot-through,especially in the modular multilevel converters(MMCs)with a large number of power switches.This paper proposes a dead-time effect suppression strategy for MMCs with nearest level modulation.The operational principles of MMCs are first analyzed.According to the operational features of MMCs,the method that removes a switching signal from the coupled switches and the reduced switching frequency voltage balancing algorithms(RSFVBAs)are mixed in the proposed method.In the intervals that are furthest away from the zerocrossing points(ZCP)of arm currents,the single switching signal method can completely eliminate the dead-time effect(DTE).Alternatively,the DTE is suppressed by the RSFVBA in intervals that are close to the ZCP.By the combination of the two methods,the dependence of the DTE suppression method on currents is reduced and the influences of ZCP are also released without degrading the normal operation performance of MMCs.Moreover,the output performance of MMCs is improved and the voltage stress on the arm inductor dramatically decreases.Finally,the validation of the method is verified by the simulation results with the professional tool Matlab/Simulink.
基金supported by the National High Technology Research and Development Program of China("863" Project)(Grant No.2012AA050205)
文摘To understand the operation principle of the modular multilevel converter(MMC)deeply,it is necessary to study the harmonic characteristics of the MMC theoretically.Besides,the analytical harmonic formulas of the MMC are useful in designing the main circuit,reducing the losses and improving the waveform quality.Based on the average switching function and the Fourier series harmonic analysis,this paper deduces the analytical expressions for such electrical quantities as the arm voltage,the arm current,the capacitor voltage,the capacitor current and the circulating current of the MMC.Finally,a digital model of a 21-level MMC-HVDC system is realized in PSCAD/EMTDC.The results of the analytical expressions coincide with the simulation results,which verify the effectiveness and feasibility of the proposed analytical expressions.
基金supported by the State Grid Corporation Science and Technology Project(No.5100-202158335A-0-0-00).
文摘Grid-forming control(GFC)is promising for power electronics based power systems with high renewable energy penetration.Naturally,the impedance modeling for GFC is necessary and has gained significant attention recently.However,most of the impedance analyses for GFC are based on a twolevel converter(TLC)rather than a modular multilevel converter(MMC).MMC differs from TLC with respect to its dominant multi-frequency response.It is necessary to analyze the impedance of GFC-based MMC owing to its superiority in highvoltage direct current(HVDC)transmission to interlink two weak AC systems with high renewable energy penetration.As the main contribution,this paper presents the AC-and DC-side impedance analyses for the GFC-based MMC with both power and DC voltage control using the harmonic transfer function(HTF),and compares the impedances of GFC-based MMC and TLC.It is inferred that although the impedance is mainly influenced within 200 Hz,the instability still could occur owing to negative resistance triggered by relatively larger parameters.The difference in AC-side impedance with power and DC voltage control is not apparent with proper parameters,while the DC-side impedance differs significantly.The generalized Nyquist criterion is necessary for AC-side stability owing to the relatively large coupling terms under GFC.Moreover,the coupling between AC-and DC-side impedances is noneligible,especially considering the DC-side resonance around the system resonant peak.The effects of parameters,system strength,and virtual impedance on the impedance shaping are analyzed and verified through simulations.
基金supported in part by the National Natural Science Foundation of China (No.52077222)in part by the Natural Science Foundation of Shandong Province (No.ZR2020ME202)。
文摘The modular multilevel converters(MMCs) are popularly used in high-voltage direct current(HVDC) transmission systems. However, for the direct modulation based MMC, its complex internal dynamics and the interaction with the grid impedance would induce the frequency coupling effect, which may lead to instability issues, especially in the case of weak grid. To effectively suppress the sub-and super-synchronous oscillations, this paper proposes a linear active disturbance rejection control(LADRC) based MMC control strategy. The LADRC mainly consists of the linear extended state observer(LESO) and the linear state error feedback(LSEF). And it is a potential method to enhance the system stability margin, attributing to its high anti-interference capability and good tracking performance. Thereupon, the system small-signal impedance model considering frequency coupling is established. And the effect of the introduction of the LADRC on the system stability is further investigated using the Nyquist criterion. Particularly, the influences of key control parameters on the stability are discussed in detail. Meanwhile, the impact of LADRC on the transient performance is explored through closed-loop zero poles. Finally, the correctness of the theoretical analysis and the effectiveness of the proposed control strategy are verified via electromagnetic simulations.
文摘The high-voltage direct current(HVDC)grid has been recognized as an effective solution for renewable energy integration.Currently,two main development trends for HVDC grids are being studied:a DC breaker based HVDC grid and fault-blocking converter based HVDC grid.Although the former has a perfect performance for fault clearance,its development is still highly constrained by the cost and maturity of DC breakers.The latter can extinguish DC faults by the fault-blocking converters.Without using DC breakers,there is no bottleneck in its technical feasibility.Nevertheless,in fault scenarios,such types of HVDC grids will be blocked at length for air-deionization,which is its main drawback.The aim of this paper is to minimize its power interruption time,by optimizing protection coordination strategies.To cover the most complex cases,the overhead line applications,in which the reclosure actions are required to be implemented,are considered.In this paper,the protection requirements of HVDC grids are first discussed,then the benefits of fault-blocking modular multilevel converters(MMCs)and their fault features are analyzed.Based on this,a control function is designed to reduce the air-deionization time.To minimize the influence of the DC faults,a separation methodology for restarting the system is proposed.The effectiveness of the proposed protection coordination schemes is validated by PSCAD/EMTDC simulations.
基金supported by the State Key Laboratory of Advanced Power Transmission Technology(GEIRI-SKL-2020-011)。
文摘Due to the large number of submodules(SMs),and modular multilevel converters(MMCs)in high-voltage applications,they are usually regulated by the nearest level modulation(NLM).Moreover,the large number of SMs causes a challenge for the fault diagnosis strategy(FDS).This paper proposes a currentless FDS for MMC with NLM.In FDS,the voltage sensor is relocated to measure the output voltage of the SM.To acquire the capacitor voltage and avoid increasing extra sensors,a capacitor voltage calculation method is proposed.Based on the measurement of output voltages,the faults can be detected and the number of different-type switch open-circuit faults can be confirmed from the numerous SMs in an arm,which narrows the scope of fault localization.Then,the faulty SMs and faulty switches in these SMs are further located without arm current according to the sorting of capacitor voltages in the voltage balancing algorithm.The FDS is independent of the arm current,which can reduce the communication cost in the hierarchical control system of MMC.Furthermore,the proposed FDS not only simplifies the identification of switch open-circuit faults by confirming the scope of faults,but also detects and locates multiple different-type faults in an arm.The effectiveness of the proposed strategy is verified by the simulation results.
基金This work was supported in part by the Joint Funds of the National Natural Science Foundation of China(No.U22B6006).
文摘The capacitor is one of the most important compo-nents in a modular multilevel converter(MMC).Due to the chemical process and the aging effect,the capacitor is subject to deterioration over time which is usually manifested by a drop in capacitance.To identify the abnormal capacitors and enhance the reliability of MMCs,an improved submodule(SM)capacitor condition monitoring method is proposed in this pa-per.The proposed method estimates the capacitance during each control cycle based on the switching states of SMs,offer-ing advantages such as high accuracy and no adverse influence on the operation of MMCs.Firstly,the aging differences of ca-pacitors in different SMs per arm of MMC are analyzed.Then,the capacitances of SMs that switch on the state are calculated based on the relationship between the capacitor voltage and cur-rent during each control cycle.A data processing algorithm is proposed to improve the accuracy of capacitance estimation.Fi-nally,the simulation and the real-time control hardware-in-the loop test results based on real-time digital simulator(RTDS)show the effectiveness of the proposed method.
基金the National Key Research and Development Program of China(No.2016YFB0100603)National Natural Science Foundation of China(No.51877193)。
文摘Modular multilevel resonant converter is an promising candidate for high voltage applications since it has advantageous features,such as high efficiency,high voltage capability and easy fault-tolerant operation.However,the inequality of arm inductance in practice will lead to imbalance between the upper and lower arm voltages,which will induce large ripples in the circulating current and a dc bias on the voltage generated by modular circuits.To compensate for the voltage imbalance,effects of arm duty cycle changes on arm voltages are discussed.An arm voltage balancing control method is proposed:adjust arm duty cycle according to arm voltage deviation in every switching cycle.Simulation and experimental results are presented to validate the theoretical analysis and the proposed control method.
基金the National Natural Science Foundation of China under Grant 51777085.
文摘Solid state transformer(SST)can provide more advanced functionalities compared with conventional transformer,and has great potential in smart grid application.Recently,the SST with medium frequency(MF)isolation link and magnetic integration feature has been proposed,which can reduce the system volume and thus increase the power density.However,the magnetic integration also introduces strong coupling between the line frequency(LF)and MF variables,which poses a great challenge on modeling and control issues.This paper proposes a modeling and control method for an SST with magnetic integration and mixed-frequency modulation.A mathematical model based on dual d-q references is deduced,and then a cascaded control system is designed according to the model.Parameters of the controller for the variables at one frequency are properly designed to avoid disturbance from the variables at the other frequency.The simulation and experimental results show good decoupling effect and satisfactory dynamics performance of the proposed control system.
基金National Natural Science Foundation of China(52307127)State Key Laboratory of Power System Operation and Control(SKLD23KZ07)。
文摘To facilitate rapid analysis of the oscillation stability mechanism in modular multilevel converter-based high voltage direct current(MMC-HVDC)systems and streamline the simulation process for determining MMC impedance characteristics,a simplified mathematical simulation model for MMC closed-loop impedance is developed using the harmonic state space method.This model considers various control strategies and includes both AC-side and DC-side impedance models.By applying a Nyquist criterion-based impedance analysis method,the stability mechanisms on the AC and DC sides of the MMC are examined.In addition,a data-driven oscillation stability analysis method is also proposed,leveraging a global sensitivity algorithm based on fast model results to identify key parameters influencing MMC oscillation stability.Based on sensitivity analysis results,a parameter adjustment strategy for oscillation suppression is proposed.The simulation results from the MATLAB/Simulinkbased MMC model validate the effectiveness of the proposed method.
文摘A modular multilevel converter(MMC)integrated with split battery cells(BIMMCs)is proposed for the battery management system(BMS)and motor drive system.In order to reduce the switching losses,the state of charge(SOC)balancing strategy with a reduced switching-frequency(RSF)is proposed in this paper.The proposed RSF algorithm not only reduces the switching losses,but also features good balancing performance both in the unbalanced and balanced initial states.The results are verified by extensive simulations in MATLAB/Simulink surroundings.
基金supported by ANID BECAS/DOCTORADO NACIONAL 21230608supported by the Projects Fondecyt Nr.1221392,Anillo grant ATE230035,and Basal project FB0008(AC3E)supported by Fondecyt Nr.1230596 and Fondequip EQM200234.
文摘This paper proposes a continuous control set model predictive control(CCS-MPC)algorithm of a modular multilevel matrix converter(M3C)for low-frequency AC transmission(LFAC),via which the offshore wind farm(OWF)is integrated.The M3C is operated with a 16.7 Hz frequency at the OWF side and a 50 Hz frequency at the onshore grid side.The balance of the capacitor voltages and the regulation of circulating currents in the M3C are performed using the proposed CCSMPC algorithm,which is based on the online solution of a cost function with constraints.Simulation and experimental work(with a 5 kW M3C prototype)are provided,showing the performance of the LFAC system to operate with symmetrical and asymmetrical voltage dips,active and reactive power steps,and optimal limitation of currents and voltages using constraints.Unlike previous publications,the predictive control system in this paper allows seamless operation under balanced and unbalanced conditions,for instance,during asymmetrical voltage dips.
文摘This paper presents a novel inter-cluster direct current(DC)capacitor voltage balancing control scheme for the single-star configured modular multilevel cascaded converter(MMCC)-based static synchronous compensator(STATCOM)under unbalanced grid voltage.The negative-sequence component of grid voltage at the point of common connection(PCC)causes unbalanced active power flow in the phase limbs of converter.This leads to the imbalance of DC voltages of the sub-module capacitors across the MMCC phases,and consequently,the malfunction of converter.The proposed solution is to inject both negative-sequence current(NSC)and zero-sequence voltage(ZSV)into the phase limbs of MMCC.A quantification factor Qf is used to achieve the sharing of inter-cluster active pow-er between the NSC and ZSV injection methods.Accurate determination of the quantification factor has been presented.In addition to maintaining the DC voltages of sub-module capacitor across the MMCC phases balanced,it also prevents the overcurrent and overvoltage of converter by injecting NSC and ZSV with the right proportion.The control scheme is validated on a 3.54 kV 1.2 MVA power system using MMCC-based STATCOM with 3-level bridge cells as sub-modules.The results show that the proposed scheme provides superior effectiveness in eliminating the voltage imbalance of DC capacitor in the phase limb while maintaining low voltage and current ratings.
基金conducted under ARPA-e from DOE with the award number DE-AR0000892.
文摘Simultaneously imposed challenges of highvoltage insulation,high dv/dt,highswitching frequency,fast protection,and thermal management associated with the adoption of 10 kV SiC MOSFET,often pose nearly insurmountable barriers to potential users,undoubtedly hindering their penetration in mediumvoltage(MV)power conversion.Key novel technologies such as enhanced gatedriver,auxiliary power supply network,PCB planar dcbus,and highdensity inductor are presented,enabling the SiCbased designs in modular MV converters,overcoming aforementioned challenges.However,purely substituting SiC design instead of Sibased ones in modular MV converters,would expectedly yield only limited gains.Therefore,to further elevate SiCbased designs,novel highbandwidth control strategies such as switchingcycle control(SCC)and integrated capacitorblocked transistor(ICBT),as well as highperformance/highbandwidth communication network are developed.All these technologies combined,overcome barriers posed by stateoftheart Si designs and unlock system level benefits such as very high power density,highefficiency,fast dynamic response,unrestricted line frequency operation,and improved power quality,all demonstrated throughout this paper.
