In wind power transmission via modular multilevel converter based high voltage direct current(MMCHVDC)systems,under traditional control strategies,MMC-HVDCcannot provide inertia support to the receiving-end grid(REG)d...In wind power transmission via modular multilevel converter based high voltage direct current(MMCHVDC)systems,under traditional control strategies,MMC-HVDCcannot provide inertia support to the receiving-end grid(REG)during disturbances.Moreover,due to the frequency decoupling between the two ends of the MMCHVDC,the sending-end wind farm(SEWF)cannot obtain the frequency variation information of the REG to provide inertia response.Therefore,this paper proposes a novel coordinated source-network-storage inertia control strategy based on wind power transmission via MMC-HVDC system.First,the grid-side MMC station(GS-MMC)maps the frequency variations of the REG to direct current(DC)voltage variations through the frequency mapping control,and uses submodule capacitor energy to provide inertial power.Then,the wind farm-side MMC station(WF-MMC)restores the DC voltage variations to frequency variations through the frequency restoration control and power loss compensation,providing real-time frequency information for the wind farm.Finally,based on real-time frequency information,thewind farmutilizes the rotor kinetic energy and energy storage to provide fast and lasting power support through the wind-storage coordinated inertia control strategy.Meanwhile,when the wind turbines withdraw from the inertia response phase,the energy storage can increase the power output to compensate for the power deficit,preventing secondary frequency drops.Furthermore,this paper uses small-signal analysis to determine the appropriate values for the key parameters of the proposed control strategy.A simulation model of the wind power transmission via MMCHVDC system is built in MATLAB/Simulink environment to validate and evaluate the proposed method.The results show that the proposed coordinated control strategy can effectively improve the system inertia level and avoid the secondary frequency drop under the load sudden increase condition.展开更多
To enhance power flow regulation in scenarios involving large-scale renewable energy transmission via high-voltage direct current(HVDC)links and multi-infeed DC systems in load-center regions,this paper proposes a hyb...To enhance power flow regulation in scenarios involving large-scale renewable energy transmission via high-voltage direct current(HVDC)links and multi-infeed DC systems in load-center regions,this paper proposes a hybrid modular multilevel converter–capacitor-commutated line-commutated converter(MMC-CLCC)HVDC transmission system and its corresponding control strategy.First,the system topology is constructed,and a submodule configuration method for the MMC—combining full-bridge submodules(FBSMs)and half-bridge submodules(HBSMs)—is proposed to enable direct power flow reversal.Second,a hierarchical control strategy is introduced,includingMMCvoltage control,CLCC current control,and a coordinationmechanism,along with the derivation of the hybrid system’s power flow reversal characteristics.Third,leveraging the CLCC’s fast current regulation and theMMC’s negative voltage control capability,a coordinated power flow reversal control strategy is developed.Finally,an 800 kV MMC-CLCC hybrid HVDC system is modeled in PSCAD/EMTDC to validate the power flow reversal performance under a high proportion of full-bridge submodule configuration.Results demonstrate that the proposed control strategy enables rapid(1-s transition)and smooth switching of bidirectional power flow without modifying the structure of primary equipment:the transient fluctuation ofDC voltage from the rated value(UdcN)to themaximumreverse voltage(-kUdcN)is less than 5%;the DC current strictly follows the preset characteristic curve with a deviation of≤3%;the active power reverses continuously,and the system maintains stable operation throughout the reversal process.展开更多
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
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
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
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
Submodule capacitor aging poses significant challenges to the safe operation of modular multilevel converter(MMC)systems.Traditional detection methods rely predominantly on offline tests,lacking real-time evaluation c...Submodule capacitor aging poses significant challenges to the safe operation of modular multilevel converter(MMC)systems.Traditional detection methods rely predominantly on offline tests,lacking real-time evaluation capabilities.Moreover,existing online approaches require additional sampling channels,thereby increasing system complexity and costs.To address these issues,this paper proposes an online evaluation method for submodule capacitor aging based on CapAgingNet.Initially,an MMC system simulation platform is developed to examine the effects of submodule capacitor aging on system operational characteristics and to create a dataset of submodule capacitor switching states.Subsequently,the CapAgingNet model is introduced,incorporating key technical modules to enhance performance:the Deep Stem module,which extracts larger receptive fields through multiple convolution layers and mitigates the impact of data sparsity in capacitor aging on feature extraction;the efficient channel attention(ECA)module,utilizing onedimensional convolution for dynamic weighting to adjust the importance of each channel,thereby enhancing the ability of the model to process high-dimensional features in capacitor aging data;and the multiscale feature fusion(MSF)module,which integrates capacitor aging information across different scales by combining fine-grained and coarse-grained features,thus improving the capacity of the model to capture high-frequency variation characteristics.The experimental results reveal that the CapAgingNet model achieves a TOP-1 accuracy of 95.32%and a macro-averaged F1 score of 95.49%on the test set,thereby providing effective technical support for online monitoring of submodule capacitor aging.展开更多
As key equipment in medium voltage DC(MVDC)systems,modular multilevel AC/DC and DC/DC converters(MM-AC/DC,MM-DC/DC)have drawn marvelous attractions.However,research on DC fault ride-through focuses on MM-AC/DC,and the...As key equipment in medium voltage DC(MVDC)systems,modular multilevel AC/DC and DC/DC converters(MM-AC/DC,MM-DC/DC)have drawn marvelous attractions.However,research on DC fault ride-through focuses on MM-AC/DC,and the fault current elimination for MM-DC/DC remains a research gap,which limits the wide application of the MVDC system.To fulfil this research gap,the contribution of this paper is revealing the fault current characteristics of MM-DC/DC based on half-bridge and full-bridge submodules(HBSM and FBSM)and proposing a novel MM-DC/DC based on hybrid HBSM and thyristor-diode module(TDM).By integrating TDM in the upper bridge arm of one phase and the down bridge arm of the other phase in MM-DC/DC,the MM-DC/DC achieves self-elimination of fault currents.The basic concept is using the energy at the healthy side to modulate a reverse voltage source(RVS)at the faulty side of MM-DC/DC,forcing fault current through TDM pass across zero.TDM can extinguish the resulting fault current.The parameter design and control strategy of the novel MM-DC/DC are discussed.Simulation is carried out for verification,and the results show that fault current can be eliminated within several milliseconds without causing excessive operating losses and costs.展开更多
A hybrid of line commutated converters(LCCs)and modular multi-level converters(MMCs)can provide the advantages of both the technologies.However,the commutation failure still exists if the LCC operates as an inverter i...A hybrid of line commutated converters(LCCs)and modular multi-level converters(MMCs)can provide the advantages of both the technologies.However,the commutation failure still exists if the LCC operates as an inverter in a hybrid LCC/MMC system.In this paper,the system behavior during a commutation failure is investigated.Both halfbridge and full-bridge MMCs are considered.Control strategies are examined through simulations conducted in PSCAD/EMTDC.Additionally,commutation failure protection strategies for multi-terminal hybrid LCC/MMC systems with AC and DC circuit breakers are studied.This paper can contribute to the protection design of future hybrid LCC/MMC systems against commutation failures.展开更多
This paper proposes a hybrid submodule modular multilevel converter(MMC)topology which is suitable for multi terminal direct current(MTDC)transmission systems.Each arm of the proposed MMC topology consists of a half-b...This paper proposes a hybrid submodule modular multilevel converter(MMC)topology which is suitable for multi terminal direct current(MTDC)transmission systems.Each arm of the proposed MMC topology consists of a half-bridge submodule(HBSM)branch and two parallel full-bridge submodule(FBSM)branches.Comparing with the conventional MTDC transmission system,the proposed topology can selectively block the DC fault current and isolate the corresponding fault line without expensive DC circuit breakers(DCCBs).Thus,the influence range of the DC fault can be reduced and the reliability of the power supply can be improved as well.The corresponding modulation and voltage balancing strategies are developed for the proposed hybrid MMC topology.The feasibility of the proposed topology and control strategy is verified in the MATLAB/Simulink simulation.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
Recently,high-frequency oscillation of themodularmultilevel converter(MMC)based high-voltage direct current(HVDC)projects has attracted great attentions.In order to analyze the small-signal stability,this paper uses t...Recently,high-frequency oscillation of themodularmultilevel converter(MMC)based high-voltage direct current(HVDC)projects has attracted great attentions.In order to analyze the small-signal stability,this paper uses the harmonic state-space(HSS)method to establish a detailed frequency domain impedance model of the AC-side of the HVDC transmission system,which considers the internal dynamic characteristics.In addition,the suggested model is also used to assess the system’s high-frequency oscillationmechanism,and the effects of the MMC current inner loop control,feedforward voltage links,and control delay on the high-frequency impedance characteristics and the effect of higher harmonic components.Finally,three oscillation suppression schemes are analyzed for the oscillation problems occurring in actual engineering,and a simplified impedance model considering only the highfrequency impedance characteristics is established to compare the suppression effect with the detailed impedance model to prove its reliability.展开更多
In the medium voltage direct current(MVDC)transmission system,a small number of MMC sub modules will reduce the power quality.In this paper,based on the research background of Photovoltaic Medium VoltageDirect Current...In the medium voltage direct current(MVDC)transmission system,a small number of MMC sub modules will reduce the power quality.In this paper,based on the research background of Photovoltaic Medium VoltageDirect Current(PV-MVDC)system,aHybrid Modulation Strategy based on theDecoupledDouble SynchronousReference Frame(DDSRF)control strategy is proposed.The dual armcomplementary hybrid modulation combining nearest-level-SPWM(NL-SPWM)can keep the number of SMs in the ON state constant.Then,the corresponding voltage sharing control algorithm of sub module(SM)is introduced.Through theoretical calculation,the modulation strategy can be found to stabilize the DC voltage and reduce the harmonic content.A32-level MMC systemhas been developed to verity that the proposed hybrid modulation strategy and its SM voltage sharing algorithm have the advantages of restraining circulating current and maintaining capacitor voltage balance.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
Modular multilevel converters(MMC)have been widely applied in high voltage direct current(HVDC)transmission systems,while complex dynamic characteristics pose challenges for stable operation under large-signal disturb...Modular multilevel converters(MMC)have been widely applied in high voltage direct current(HVDC)transmission systems,while complex dynamic characteristics pose challenges for stable operation under large-signal disturbance.In this paper,large-signal stability of MMC is investigated under grid voltage and power disturbances based on mixed potential function(MPF).First,an MMC simplified model based on average value model(AVM)is introduced.Then,the physical mechanism of large-signal instability phenomenon is revealed for MMC.Based on the simplified model,large-signal stability of MMC is analyzed by applying MPF.Furthermore,to diminish the conservatism of MPF,a large-signal stability criterion is defined by introducing a conservative correction factor.The influences of system parameters on large-signal stability of MMC are presented by the region of asymptotic stability(RAS).Finally,PLECS simulation case studies are carried out to verify the proposed stability criterion under grid voltage and power disturbances.展开更多
Innovative dynamic models for the DC modular multilevel converter(DC-MMC)in rotating dq frame are presented in this paper,which are specifically designed to enhance converter design and stability analysis.Open-loop an...Innovative dynamic models for the DC modular multilevel converter(DC-MMC)in rotating dq frame are presented in this paper,which are specifically designed to enhance converter design and stability analysis.Open-loop and closed-loop models are developed using three dq frames,providing a detailed examination of the impact of 2^(nd)and 3^(rd)harmonic components on the model accuracy.A novel contribution of this paper is the integration of a 2^(nd)harmonic current suppression controller(SHCSC)within the closed-loop model,offering new insights into its effects on system stability.The DC-MMC model is further extended by coupling it with high-voltage direct current(HVDC)cables on each side,forming an interconnected system model that accurately represents a more authentic scenario for future DC grids.The proposed model is rigorously validated against PSCAD benchmark model,confirming their precision and reliability.The interconnected system model is then utilized to analyze the influence of cable length on system stability,demonstrating practical applications.The closed-loop model is subsequently employed for stability assessment of the interconnected system,showcasing its applicability in real-world scenarios.Additionally,a damping controller is designed using participation factor and residue approaches,offering a refined approach to oscillation damping and stability optimization.The effectiveness of the controller is evaluated through eigenvalue analysis,supported by simulation results,underscoring its potential for enhancing system stability.展开更多
The fractional frequency transmission system is an emerging technology for long-distance wind power integration,and the modular multilevel matrix converter(M3C)is the keen equipment.Since the M3C directly connects two...The fractional frequency transmission system is an emerging technology for long-distance wind power integration,and the modular multilevel matrix converter(M3C)is the keen equipment.Since the M3C directly connects two ac grids with different frequencies,the external and internal harmonics have complex coupling relationships with a unique dual-fundamental-frequency spectrum,which has not been properly investigated due to a lack of an effective method.To address this issue,a novel harmonic state-space method is proposed to achieve comprehensive modelling of the harmonic dynamics of the M3C.Based on the principle of two-dimensional Fourier transform,the decomposition of the dual-fundamental-frequency harmonics is realized,and the multiplicative coupling between time-domain variables is modelled through double-layer convolution on the frequency domain.Besides,the general expression of the proposed method is provided,which highlights a modularized matrix with easy scalability to meet different truncation requirements.Then,the HSS model of M3C considering the close-loop control is established,based on which a panoramic harmonic coupling relationship between the system-and the low-frequency side is concluded.Finally,the M3C model and harmonic coupling relationship are validated by simulation tests conducted in MATLAB/Simulink environment.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
基金funded by State Grid Corporation of China Central Branch Technology Project(52140024000C).
文摘In wind power transmission via modular multilevel converter based high voltage direct current(MMCHVDC)systems,under traditional control strategies,MMC-HVDCcannot provide inertia support to the receiving-end grid(REG)during disturbances.Moreover,due to the frequency decoupling between the two ends of the MMCHVDC,the sending-end wind farm(SEWF)cannot obtain the frequency variation information of the REG to provide inertia response.Therefore,this paper proposes a novel coordinated source-network-storage inertia control strategy based on wind power transmission via MMC-HVDC system.First,the grid-side MMC station(GS-MMC)maps the frequency variations of the REG to direct current(DC)voltage variations through the frequency mapping control,and uses submodule capacitor energy to provide inertial power.Then,the wind farm-side MMC station(WF-MMC)restores the DC voltage variations to frequency variations through the frequency restoration control and power loss compensation,providing real-time frequency information for the wind farm.Finally,based on real-time frequency information,thewind farmutilizes the rotor kinetic energy and energy storage to provide fast and lasting power support through the wind-storage coordinated inertia control strategy.Meanwhile,when the wind turbines withdraw from the inertia response phase,the energy storage can increase the power output to compensate for the power deficit,preventing secondary frequency drops.Furthermore,this paper uses small-signal analysis to determine the appropriate values for the key parameters of the proposed control strategy.A simulation model of the wind power transmission via MMCHVDC system is built in MATLAB/Simulink environment to validate and evaluate the proposed method.The results show that the proposed coordinated control strategy can effectively improve the system inertia level and avoid the secondary frequency drop under the load sudden increase condition.
基金supported by Science and Technology Project of the headquarters of the State Grid Corporation of China(No.5500-202324492A-3-2-ZN).
文摘To enhance power flow regulation in scenarios involving large-scale renewable energy transmission via high-voltage direct current(HVDC)links and multi-infeed DC systems in load-center regions,this paper proposes a hybrid modular multilevel converter–capacitor-commutated line-commutated converter(MMC-CLCC)HVDC transmission system and its corresponding control strategy.First,the system topology is constructed,and a submodule configuration method for the MMC—combining full-bridge submodules(FBSMs)and half-bridge submodules(HBSMs)—is proposed to enable direct power flow reversal.Second,a hierarchical control strategy is introduced,includingMMCvoltage control,CLCC current control,and a coordinationmechanism,along with the derivation of the hybrid system’s power flow reversal characteristics.Third,leveraging the CLCC’s fast current regulation and theMMC’s negative voltage control capability,a coordinated power flow reversal control strategy is developed.Finally,an 800 kV MMC-CLCC hybrid HVDC system is modeled in PSCAD/EMTDC to validate the power flow reversal performance under a high proportion of full-bridge submodule configuration.Results demonstrate that the proposed control strategy enables rapid(1-s transition)and smooth switching of bidirectional power flow without modifying the structure of primary equipment:the transient fluctuation ofDC voltage from the rated value(UdcN)to themaximumreverse voltage(-kUdcN)is less than 5%;the DC current strictly follows the preset characteristic curve with a deviation of≤3%;the active power reverses continuously,and the system maintains stable operation throughout the reversal process.
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.
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 China Yangtze Power Co.,Ltd.(No.ZSF2502001).
文摘Submodule capacitor aging poses significant challenges to the safe operation of modular multilevel converter(MMC)systems.Traditional detection methods rely predominantly on offline tests,lacking real-time evaluation capabilities.Moreover,existing online approaches require additional sampling channels,thereby increasing system complexity and costs.To address these issues,this paper proposes an online evaluation method for submodule capacitor aging based on CapAgingNet.Initially,an MMC system simulation platform is developed to examine the effects of submodule capacitor aging on system operational characteristics and to create a dataset of submodule capacitor switching states.Subsequently,the CapAgingNet model is introduced,incorporating key technical modules to enhance performance:the Deep Stem module,which extracts larger receptive fields through multiple convolution layers and mitigates the impact of data sparsity in capacitor aging on feature extraction;the efficient channel attention(ECA)module,utilizing onedimensional convolution for dynamic weighting to adjust the importance of each channel,thereby enhancing the ability of the model to process high-dimensional features in capacitor aging data;and the multiscale feature fusion(MSF)module,which integrates capacitor aging information across different scales by combining fine-grained and coarse-grained features,thus improving the capacity of the model to capture high-frequency variation characteristics.The experimental results reveal that the CapAgingNet model achieves a TOP-1 accuracy of 95.32%and a macro-averaged F1 score of 95.49%on the test set,thereby providing effective technical support for online monitoring of submodule capacitor aging.
基金supported by Science and Technology Project of SGCC(5108-202218280A-2-370-XG).
文摘As key equipment in medium voltage DC(MVDC)systems,modular multilevel AC/DC and DC/DC converters(MM-AC/DC,MM-DC/DC)have drawn marvelous attractions.However,research on DC fault ride-through focuses on MM-AC/DC,and the fault current elimination for MM-DC/DC remains a research gap,which limits the wide application of the MVDC system.To fulfil this research gap,the contribution of this paper is revealing the fault current characteristics of MM-DC/DC based on half-bridge and full-bridge submodules(HBSM and FBSM)and proposing a novel MM-DC/DC based on hybrid HBSM and thyristor-diode module(TDM).By integrating TDM in the upper bridge arm of one phase and the down bridge arm of the other phase in MM-DC/DC,the MM-DC/DC achieves self-elimination of fault currents.The basic concept is using the energy at the healthy side to modulate a reverse voltage source(RVS)at the faulty side of MM-DC/DC,forcing fault current through TDM pass across zero.TDM can extinguish the resulting fault current.The parameter design and control strategy of the novel MM-DC/DC are discussed.Simulation is carried out for verification,and the results show that fault current can be eliminated within several milliseconds without causing excessive operating losses and costs.
基金supported by the Science and Technology Project of the State Grid Corporation of China,HVDC Systems/Grids for Transnational Interconnections(Project number:SGTYHT/16-JS-198).
文摘A hybrid of line commutated converters(LCCs)and modular multi-level converters(MMCs)can provide the advantages of both the technologies.However,the commutation failure still exists if the LCC operates as an inverter in a hybrid LCC/MMC system.In this paper,the system behavior during a commutation failure is investigated.Both halfbridge and full-bridge MMCs are considered.Control strategies are examined through simulations conducted in PSCAD/EMTDC.Additionally,commutation failure protection strategies for multi-terminal hybrid LCC/MMC systems with AC and DC circuit breakers are studied.This paper can contribute to the protection design of future hybrid LCC/MMC systems against commutation failures.
基金This work was supported in part by the National Natural Science Foundation of China under Grant 51637007in part by the Science and Technology Projects of State Grid Corporation of China under project SG-TYHT/16-JS-198.
文摘This paper proposes a hybrid submodule modular multilevel converter(MMC)topology which is suitable for multi terminal direct current(MTDC)transmission systems.Each arm of the proposed MMC topology consists of a half-bridge submodule(HBSM)branch and two parallel full-bridge submodule(FBSM)branches.Comparing with the conventional MTDC transmission system,the proposed topology can selectively block the DC fault current and isolate the corresponding fault line without expensive DC circuit breakers(DCCBs).Thus,the influence range of the DC fault can be reduced and the reliability of the power supply can be improved as well.The corresponding modulation and voltage balancing strategies are developed for the proposed hybrid MMC topology.The feasibility of the proposed topology and control strategy is verified in the MATLAB/Simulink simulation.
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.
基金supported by Research on the Oscillation Mechanism and Suppression Strategy of Yu-E MMC-HVDC Equipment and System(2021Yudian Technology 33#).
文摘Recently,high-frequency oscillation of themodularmultilevel converter(MMC)based high-voltage direct current(HVDC)projects has attracted great attentions.In order to analyze the small-signal stability,this paper uses the harmonic state-space(HSS)method to establish a detailed frequency domain impedance model of the AC-side of the HVDC transmission system,which considers the internal dynamic characteristics.In addition,the suggested model is also used to assess the system’s high-frequency oscillationmechanism,and the effects of the MMC current inner loop control,feedforward voltage links,and control delay on the high-frequency impedance characteristics and the effect of higher harmonic components.Finally,three oscillation suppression schemes are analyzed for the oscillation problems occurring in actual engineering,and a simplified impedance model considering only the highfrequency impedance characteristics is established to compare the suppression effect with the detailed impedance model to prove its reliability.
基金This work was supported by the National Key Research and Development Project of China(2018YFB0905803).
文摘In the medium voltage direct current(MVDC)transmission system,a small number of MMC sub modules will reduce the power quality.In this paper,based on the research background of Photovoltaic Medium VoltageDirect Current(PV-MVDC)system,aHybrid Modulation Strategy based on theDecoupledDouble SynchronousReference Frame(DDSRF)control strategy is proposed.The dual armcomplementary hybrid modulation combining nearest-level-SPWM(NL-SPWM)can keep the number of SMs in the ON state constant.Then,the corresponding voltage sharing control algorithm of sub module(SM)is introduced.Through theoretical calculation,the modulation strategy can be found to stabilize the DC voltage and reduce the harmonic content.A32-level MMC systemhas been developed to verity that the proposed hybrid modulation strategy and its SM voltage sharing algorithm have the advantages of restraining circulating current and maintaining capacitor voltage balance.
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 National Natural Science Foundation of China under Grant 51807056in part by the Major Special Project of Hunan Province under Grant 2020GK1010in part by the Science and Technology Innovation Program of Hunan Province under Grant 2020RC4015.
文摘Modular multilevel converters(MMC)have been widely applied in high voltage direct current(HVDC)transmission systems,while complex dynamic characteristics pose challenges for stable operation under large-signal disturbance.In this paper,large-signal stability of MMC is investigated under grid voltage and power disturbances based on mixed potential function(MPF).First,an MMC simplified model based on average value model(AVM)is introduced.Then,the physical mechanism of large-signal instability phenomenon is revealed for MMC.Based on the simplified model,large-signal stability of MMC is analyzed by applying MPF.Furthermore,to diminish the conservatism of MPF,a large-signal stability criterion is defined by introducing a conservative correction factor.The influences of system parameters on large-signal stability of MMC are presented by the region of asymptotic stability(RAS).Finally,PLECS simulation case studies are carried out to verify the proposed stability criterion under grid voltage and power disturbances.
基金supported by Petrofac through the Petrofac Ph.D.Scholarship.
文摘Innovative dynamic models for the DC modular multilevel converter(DC-MMC)in rotating dq frame are presented in this paper,which are specifically designed to enhance converter design and stability analysis.Open-loop and closed-loop models are developed using three dq frames,providing a detailed examination of the impact of 2^(nd)and 3^(rd)harmonic components on the model accuracy.A novel contribution of this paper is the integration of a 2^(nd)harmonic current suppression controller(SHCSC)within the closed-loop model,offering new insights into its effects on system stability.The DC-MMC model is further extended by coupling it with high-voltage direct current(HVDC)cables on each side,forming an interconnected system model that accurately represents a more authentic scenario for future DC grids.The proposed model is rigorously validated against PSCAD benchmark model,confirming their precision and reliability.The interconnected system model is then utilized to analyze the influence of cable length on system stability,demonstrating practical applications.The closed-loop model is subsequently employed for stability assessment of the interconnected system,showcasing its applicability in real-world scenarios.Additionally,a damping controller is designed using participation factor and residue approaches,offering a refined approach to oscillation damping and stability optimization.The effectiveness of the controller is evaluated through eigenvalue analysis,supported by simulation results,underscoring its potential for enhancing system stability.
基金supported by the National Natural Science Foundation of China(52207103)in part by Basic and Appiled Basic Research Foundational of Guangdong Province(2020A1515111117).
文摘The fractional frequency transmission system is an emerging technology for long-distance wind power integration,and the modular multilevel matrix converter(M3C)is the keen equipment.Since the M3C directly connects two ac grids with different frequencies,the external and internal harmonics have complex coupling relationships with a unique dual-fundamental-frequency spectrum,which has not been properly investigated due to a lack of an effective method.To address this issue,a novel harmonic state-space method is proposed to achieve comprehensive modelling of the harmonic dynamics of the M3C.Based on the principle of two-dimensional Fourier transform,the decomposition of the dual-fundamental-frequency harmonics is realized,and the multiplicative coupling between time-domain variables is modelled through double-layer convolution on the frequency domain.Besides,the general expression of the proposed method is provided,which highlights a modularized matrix with easy scalability to meet different truncation requirements.Then,the HSS model of M3C considering the close-loop control is established,based on which a panoramic harmonic coupling relationship between the system-and the low-frequency side is concluded.Finally,the M3C model and harmonic coupling relationship are validated by simulation tests conducted in MATLAB/Simulink environment.
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.
