On-Line Fault Diagnosis and Fault-Tolerant Operation of Modular Multilevel Converters –A Comprehensive Review

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.

Hybrid Predictive Control with Simple Linear Control Based Circulating Current Suppression for Modular Multilevel Converters

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.

Modeling and Control of the Modular Multilevel Converter (MMC) based Solid State Transformer (SST) with Magnetic Integration

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.

Synchronization analysis on cascaded multilevel converters with distributed control

Cascaded multilevel converters built with integrated modules have many advantages such as increased power density,flexible distributed control,multi-functionality,increased reliability and short design cycles.However,the system performance will be affected due to the synchronization errors among each integrated modules.This paper analyzes the impact of the three kinds of synchronization errors on the whole system performance,as well as detailed synchronization implementation.Some valuable conclusions are derived from the theoretical analysis,simulations and experimental results.

Reduced Switching-Frequency State of Charge Balancing Strategy for Battery Integrated Modular Multilevel Converter

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.

DC Traction Power Supply System Based on Modular Multilevel Converter Suitable for Energy Feeding and De-icing

A novel DC traction power supply system suitable for energy feeding and de-icing is proposed in this paper for an urban rail transit catenary on the basis of the full bridge submodule (FBSM) modular multilevel converter (MMC). The FBSM-MMC is a novel type of voltage source converter (VSC) and can directly control the output DC voltage and conduct bipolar currents, thus flexibly controlling the power flow of the urban rail transit catenary. The proposed topology can overcome the inherent disadvantages of the output voltage drop in the diode rectifier units, increase the power supply distance and reduce the number of traction substations. The flexible DC technology can coordinate multiple FBSM-MMCs in a wide area and jointly complete the bidirectional control of catenary power flow during the operation of the electric locomotive, so as to realize the local consumption and optimal utilization of the recovered braking energy of the train. In addition, the FBSM-MMCs can also adjust the output current when the locomotive is out of service to prevent the catenary from icing in winter. The working modes of the proposed topology are illustrated in detail and the control strategy is specially designed for normal locomotive operations and catenary de-icing. Simulation cases conducted by PSCAD/EMTDC validate the proposed topology and its control strategy.

Mixed Dead-time Effect Suppression Strategy for Modular Multilevel Converters

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.

Currentless Multiple Switch Open-Circuit Faults Diagnosis for Modular Multilevel Converters with Nearest Level Modulation in HVDC Systems

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.

A DC Current Flow Controller for Meshed Modular Multilevel Converter Multiterminal HVDC Grids

This paper proposes the design of a novel DC current flow controller(CFC)and evaluates the control performance of balancing and regulating the DC branch currents using the DC CFC in a meshed multi-terminal HVDC(MTDC)grid.The DC CFC consists of two identical full bridge DC-DC converters with the capacitors of the two converters being connected in parallel.The scalability of the DC CFC is easily achievable due to the identical bridge converter topology;the cost of this DC CFC is also relatively low due to its simple physical structure and low voltage ratings.The control performance of the DC CFC is tested on a meshed 3-terminal(3-T)HVDC grid,which is based on modular multilevel converters(MMC).The DC branch current control in the meshed MTDC grid is achieved using the proposed control strategy of the DC CFC,and is verified through case studies on the real-time digital simulator(RTDS).

DC fault analysis for modular multilevel converter-based system

DC fault protection is the key technique for the development of the DC distribution and transmission system. This paper analyzes the transient characteristics of DC faults in a modular multilevel converter(MMC) based DC system combining with the numerical method. Meanwhile,lots of simulation tests based on MATLAB/Simulink are carried out to verify the correctness of the theoretical analysis. Finally, the technological difficulties of and requirements for the protection and isolation are discussed to provide the theoretical foundation for the design of dc fault protection strategy.

Modeling,Control,and Protection of Modular Multilevel Converter-based Multi-terminal HVDC Systems:A Review

Multi-terminal direct current(MTDC)grids provide the possibility of meshed interconnections between regional power systems and various renewable energy resources to boost supply reliability and economy.The modular multilevel converter(MMC)has become the basic building block for MTDC and DC grids due to its salient features,i.e.,modularity and scalability.Therefore,the MMC-based MTDC systems should be pervasively embedded into the present power system to improve system performance.However,several technical challenges hamper their practical applications and deployment,including modeling,control,and protection of the MMC-MTDC grids.This paper presents a comprehensive investigation and reference in modeling,control,and protection of the MMC-MTDC grids.A general overview of state-of-the-art modeling techniques of the MMC along with their performance in simulation analysis for MTDC applications is provided.A review of control strategies of the MMC-MTDC grids which provide AC system support is presented.State-of-the art protection techniques of the MMCMTDC systems are also investigated.Finally,the associated research challenges and trends are highlighted.

Theoretical analysis of the harmonic characteristics of modular multilevel converters

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.

Harmonic influence analysis of unified power flow controller based on modular multilevel converter

The unified power flow controller(UPFC)based on modular multilevel converter(MMC) is the most creative flexible ac transmission system(FACTS) device. In theory, the output voltage of the series MMC in MMCUPFC can be regulated from 0 to the rated value. However,there would be relatively large harmonics in the output voltage if the voltage modulation ratio is small. In order to analyze the influence of MMC-UPFC on the harmonics of the power grid, the theoretical calculation method and spectra of the output voltage harmonics of MMC are presented. Subsequently, the calculation formulas of the harmonics in the power grid with UPFC are proposed. Based on it, the influence of UPFC on the grid voltage harmonics is evaluated, when MMC-UPFC is operated with different submodular numbers and voltage modular ratios. Eventually, the proposed analysis method is validated using digital simulation. The study results would provide guideline for the design and operation of MMC-UPFC project.

Overview on Submodule Topologies,Modeling,Modulation,Control Schemes,Fault Diagnosis,and Tolerant Control Strategies of Modular Multilevel Converters

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.

Operation, Analysis and Experiments of DC Transformers Based on Modular Multilevel Converters for HVDC Applications

This paper studies the operation,analysis and experiments of multilevel high frequency link transformers(MHFLT)based on modular multilevel converters(MMC)for high voltage DC applications.The multilevel dual active phase shift is proposed to operate the MHFLT using a high switching frequency,which brings about many advantages and makes the operation quite different from that of the traditional DC transformer(DCT)based on a dual active bridge and the fundamental frequency MMC widely used in flexible HVDC transmission.Specifically,MHFLT is suitable for high voltage levels,which is due to its good switching characterization,ability to cut itself off from an external fault,and it can also achieve redundancy operations when a sub-module fault occurs.In this paper,the operation,modulation method,multilevel high frequency link voltage,current,and power characterization,high frequency commutation,and switching characterization are analyzed comprehensively;the pulse rotation and distributed delay control,and fault handling strategies of MHFLT are then proposed.Finally,a prototype is built,and the experimental results verify the correctness and effectiveness of the proposed solution.

Impedance Analysis of Grid Forming Control Based Modular Multilevel Converters

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.

Reverse-blocking modular multilevel converter for battery energy storage systems

Energy storage systems with multilevel converters play an important role in modern electric power systems with large-scale renewable energy integration.This paper proposes a reverse-blocking modular multilevel converter for a battery energy storage system(RB-MMCBESS). Besides integrating distributed low-voltage batteries to medium or high voltage grids, with the inherited advantages of traditional MMCs, the RB-MMC-BESS also provides improved DC fault handling capability. This paper analyzes such a new converter configuration and its operating principles. Control algorithms are developed for AC side power control and the balancing of battery state of charge. The blocking mechanism to manage a DC pole-topole fault analyzed in depth. Comprehensive simulation results validate both the feasibility of the RB-MMC-BESS topology and the effectiveness of the control and fault handling strategies.

Neutral-point-clamped hybrid multilevel converter with DC fault blocking capability for medium-voltage DC transmission

This paper proposes a novel hybrid multilevel converter with DC fault-blocking capability, i.e., the neutral-point clamped hybrid multilevel converter(NHMC).By employing two types of unipolar full-bridge submodules along with director switches, which are composed of seriesconnected insulated-gate bipolar transistors, the NHMC combines the features and advantages of the neutral-point clamped converter and the modular multilevel converter.The basic topology, operating principles, modulation scheme, and energy-balancing scheme of the NHMC are presented. The DC fault-blocking capability of the NHMC is investigated. The number of power electronic devices used by the NHMC is calculated and compared with other multilevel converters, showing that the proposed NHMC can be an economical and feasible option for medium-voltage DC transmission with overhead lines. Simulation results demonstrate the features and operating scheme of the proposed NHMC.

Harmonic Circulating Current Injection Based Power Loss Optimization Control of Bottom Switch/Diodes for Modular Multilevel Converters

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.

Optimized Protection Strategies for HVDC Grid with Fault-blocking Modular Multilevel Converters for Overhead Line Applications

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.