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.

FPGA-Based Real-Time Simulation of Modular Multilevel Converter HVDC Systems

AC-HVDC-AC energy conversion systems using MMC （modular multilevel converters） are becoming popular to integrate distributed energy systems to the main grid. Such multilevel converters pose a serious problems for HIL （hardware in the loop） simulators required for control, protection design and testing due to the large number of cells that must be simulated individually using very small time steps. This paper demonstrates the advantages of using a very small time step to simulate a MMC topology. The MMC is implemented on FPGA （fiel-programmable gate array） to simulate fast transient with a time step of 250 ns. The AC network and HVDC bus is simulated on the PC, with a slower time step of 10 μs to 20 μs. The simulator architecture and the components simulated on the FPGA and on the PC will be discussed, as well as the method allowing the interconnection of this slow and fast system.

Data-Driven Fault Detection of Multiple Open-Circuit Faults for MMC Systems Based on Long Short-Term Memory Networks

This paper presents a long short-term memory(LSTM)-based fault detection method to detect the multiple open-circuit switch faults of modular multilevel converter(MMC)systems with full-bridge sub-modules(FB-SMs).Eighteen sensor signals of grid voltages,grid currents and capacitance voltages of MMC for single and multi-switch faults are collected as sampling data.The output signal characteristics of four types of single switch faults of FB-SM,as well as double switch faults in the same and different phases of MMC,are analyzed under the conditions of load variations and control command changes.A multi-layer LSTM network is devised to deeply extract the fault characteristics of MMC under different faults and operation conditions,and a Softmax layer detects the fault types.Simulation results have confirmed that the proposed LSTM-based method has better detection performance compared with three other methods:K-nearest neighbor(KNN),naive bayes(NB)and recurrent neural network(RNN).In addition,it is highly robust to model uncertainties and Gaussian noise.The validity of the proposed method is further demonstrated by experiment studies conducted on a hardware-in-the-loop(HIL)testing platform.

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.

Thyristor-inserted MMC Sub-module Topology with DC Fault Blocking Capability

Modular multilevel converter(MMC)is increasingly being applied to high voltage direct current(HVDC)systems.However,dc short circuit situations restrain the application of a conventional half bridge MMC system.In this paper,a new sub-module topology with inserted thyristor can help the MMC system clear a dc side fault.Working states and devices voltage stress of the proposed topology are analyzed and the conduction loss comparison between the proposed topology with several existing topologies with dc fault blocking capability is carried out.Results show the proposed topology is superior to other topologies in terms of conduction loss while using the same voltage rating devices.Besides,compared with traditional half bridge topology,only thyristors and diodes are added in the proposed topology.Therefore,cost of the proposed topology can be lower than conventional hybrid MMC sub-modules.At last,the fault blocking capability of the proposed topology is verified in the simulation.

Design of a 10 kV SiC MOSFET-based high-density, high-efficiency, modular medium-voltage power converter

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.

Control and protection strategy for MMC MTDC system under converter-side AC fault during converter blocking failure

This paper investigates a control and protection strategy for a four-terminal modular multilevel converter(MMC)based high-voltage direct current(HVDC)system under a converter-side AC fault.Based on the system operating condition,a control and protection strategy against the fault with normal blocking of the converter is proposed.In practical,applications encountering such a fault,the MMC at the fault side may experience different conditions of blocking failure.The blocking failures may occur on:①the whole converter;②one converter arm;③one sub-module(SM)/several SMs of one converter arm;④other conditions.The phenomenon of the multi-terminal HVDC(MTDC)system following the fault is analyzed under the first three conditions with real-time simulations using the real-time digital simulator(RTDS).Based on the impact of different conditions on the MTDC system,the necessity of utilizing special control and protection is discussed.A special control and protection strategy is proposed for emergency conditions,and its effectiveness is verified by real-time simulation results.

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.

Mechanism Analysis of Additional Damping Control Strategies for the High-frequency Resonance of MMC Connected to AC Grid

High-frequency resonance can occur when a modular multilevel converter(MMC)is inserted into an AC grid.Additional damping control is a relatively low-cost resonance suppression strategy compared to passive damping strategies.This paper analyzes the influences of a feed-forward voltage filter and feedback current filter for the inner controller for the high-frequency impedance characteristics of the MMC based on a model.Moreover,the mechanism,influencing factors,and limitations of the existing strategy including an additional lowpass filter in the voltage feed-forward stage are investigated.Secondly,a resonance suppression strategy for the inclusion of additional cascaded notch filters in the voltage feed-forward stage is proposed,and its parameter design method and applicable scenarios are analyzed.In addition,this paper analyzes the effects of the inclusion of an additional control in other stages for the inner controller of the MMC.Finally,the correctness of the theoretical analysis and the proposed strategy is verified based on the simulation of an actual project on PSCAD/EMTDC.

Three-way Subsynchronous Torsional Interactions Between LCC HVDC,MMC HVDC and a Thermal Generator

This paper performs a study on three-way subsynchronous torsional interactions(SSTI)between a hybrid dual-infeed high-voltage direct current(HVDC)system and a nuclear generator.The test case is based on the French IFA2000 line commutated converter(LCC)HVDC(2 GW)and the new Eleclink modular multilevel converter(MMC)HVDC(1 GW)interacting with the Gravelines generator(1 GW).The analysis is performed by the means of the eigenvalue stability assessment on an analytical model,while the accuracy of the conclusions is verified using the detailed non-linear electromegnetic transient program(EMTP)model.The study shows that the dual-infeed system may introduce higher risk of the SSTI compared with the point-to-point HVDC systems.It shows that MMC operating as static synchronous compensator(STATCOM)may further reduce the torsional damping at 6.3 Hz mode.This conclusion may be unexpected since it is known fact from literature that STATCOM has a beneficial impact on the transient performance of LCC.Further studies show that in a sequential HVDC loading,it may be beneficial to load the MMC HVDC first.Also,the risk of the SSTI may be minimized by changing HVDC controller gains,in particular,by increasing phaselocked-loop(PLL)gains on the LCC rectifier.

Application of MMC with Embedded Energy Storage for Overvoltage Suppression and Fault Ride-through Improvement in Series LCCMMC Hybrid HVDC System

The series line-commutated converter(LCC)and modular multilevel converter(MMC)hybrid high-voltage direct current(HVDC)system provides a more economical and flexible alternative for ultra-HVDC(UHVDC)transmission,which is the so-called Baihetan-Jiangsu HVDC(BJ-HVDC)project of China.In one LCC and two MMCs(1+2)operation mode,the sub-module(SM)capacitors suffer the most rigorous overvoltage induced by three-phase-to-ground fault at grid-side MMC and valve-side single-phase-to-ground fault in internal MMC.In order to suppress such huge overvoltage,this paper demonstrates a novel alternative by employing the MMC-based embedded battery energy storage system(MMC-BESS).Firstly,the inducements of SM overvoltage are analyzed.Then,coordinated with MMC-BESS,new fault ride-through(FRT)strategies are proposed to suppress the overvoltage and improve the FRT capability.Finally,several simulation scenarios are carried out on PSCAD/EMTDC.The overvoltage suppression is verified against auxiliary device used in the BJ-HVDC project in a monopolar BJ-HVDC system.Further,the proposed FRT strategies are validated in the southern Jiangsu power grid of China based on the planning data in the summer of 2025.Simulation results show that the MMC-BESS and proposed FRT strategies could effectively suppress the overvoltage and improve the FRT capability.

Joint Limiting Control Strategy Based on Virtual Impedance Shaping for Suppressing DC Fault Current and Arm Current in MMC-HVDC Systems

This paper proposes a joint limiting control strategy for suppressing DC fault current and arm current in modular multilevel converter-based high-voltage direct current(MMC-HVDC) systems, which includes two target-oriented current limiting controls. To limit the DC fault current in the early fault stage, an equivalent modular multilevel converter(MMC) impedance is obtained, and its high-frequency part is reshaped by introducing virtual impedance, which is realized by adjusting the inserted submodules adaptively. Following the analysis of MMC control characteristics, the arm current limiting strategy is investigated, with results showing that the inner-loop control has significant effects on arm current and that a simple low-pass filter can reduce the arm current in the fault period. Finally, by combining the virtual impedance shaping and innerloop control, the fault currents of DC lines and MMC arms can be suppressed simultaneously, which can not only alleviate the interrupting pressure of the DC circuit breaker, but also prevent the MMC from being blocked by the arm overcurrent. Theoretical analysis conclusions and the proposed strategy are verified offline by a digital time-domain simulation on Power Systems Computer Aided Design/Electromagnetic Transients including DC platform, and experiment on a real-time digital simulator platform.

Modular Multilevel Converter with Thyristor DC-link Switch for Full-torque Variable-speed Drives

The modular multilevel converter(MMC)is a promising topology for medium-voltage drive applications due to its high-quality output waveforms,low device switching frequency and voltage rating.However,the large cell capacitor voltage ripple is a severe challenge faced by MMC at low motor speeds.Recently,a hybrid MMC(HMMC)topology is proven to be a competitive solution because of its lower cell capacitor voltage ripple and not demonstrating a common-mode voltage(CMV)problem compared with other methods.However,the DC-link switch with IGBT limits HMMC from being applied in highvoltage applications.This paper uses a thyristor instead of IGBT as the DC-link switch.To ensure the thyristor can be softly turned on and safely turned off,a new control scheme is proposed.When using this proposed scheme,HMMC can also tolerate the failure of the thyristor being turned-off without shutting down the system,effectively improving its reliability.The cell capacitor voltage ripple analysis is presented considering the effects of the thyristor switching process.In addition,a decoupled energy balancing control is utilized to suppress the fluctuation of the DC current.Experimental results obtained from a 380 V/7.5 kW downscaled prototype validate the effectiveness of starting up a motor from the standby mode to rated speed applying full-torque.

Stability Comparison Between Grid-forming and Grid-following Based Wind Farms Integrated MMC-HVDC

Grid-forming(GFM)control based high-voltage DC(HVDC)systems and renewable energy sources(RESs)provide support for enhancing the stability of power systems.However,the interaction and coordination of frequency support between the GFM-based modular multilevel converter based HVDC(MMC-HVDC)and grid-following(GFL)based RESs or GFM-based RESs have not been fully investigated,which are examined in this study.First,the detailed AC-and DC-side impedances of GFM-based MMC-HVDC are analyzed.The impedance characteristics of GFL-and GFM-based wind turbines are next analyzed.Then,the influences of GFL-and GFM-based wind farms(WFs)on the DC-and AC-side stabilities of WF-integrated MMC-HVDC systems are compared and evaluated.The results show that the GFM-based wind turbine performs better than the GFL-based wind turbine.Accordingly,to support a receiving-end AC system,the corresponding frequency supporting strategies are proposed based on the GFM control for WF-integrated MMC-HVDC systems.The GFM-based WF outperforms the GFL-based WF in terms of stability and response time.Simulations in PSCAD/EMTDC demonstrate the DC-and AC-side stability issues and seamless grid support from the RESs,i.e.,WFs,to the receiving-end AC system.

Linear Active Disturbance Rejection Control and Stability Analysis for Modular Multilevel Converters Under Weak Grid

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.

FPGA-based Digital Implementation of Flexible Power Control for Three-phase to Single-phase MMC-based Advanced Co-phase Traction Power Supply System

A three-phase to single-phase modular multilevel converter based advanced co-phase traction power supply(MMC-ACTPS) system is an effective structure to address the concerns of phase splitting and poor power quality of the conventional electrified railway. Due to the large number of MMCACTPS system modules, I/O resources and computing speed have high requirements on processors. Moreover, the module capacitor balance is challenging because the sorting time is too long when the traditional sorting algorithm for voltage balance is used. To solve the above issues, a digital implementation scheme of flexible power control strategy for three-phase to single-phase MMC-ACTPS system based on field programmable gate array(FPGA), which has sufficient I/O resources, has been proposed. Due to the parallel execution characteristics of the FPGA, the execution time of the controller and the modulator can be greatly reduced compared with a digital signal processor(DSP) + FPGA or DSpace. In addition, an improved sorting algorithm is proposed to reduce the sorting time and the implementation steps are analyzed. Finally, simulation and experimental results are presented to demonstrate the effectiveness and correctness of the proposed control strategy.

Phasor Analytical Model of Non-isolated DC/DC Converter Based on Modular Multilevel Converter for DC Transmission Grids

Non-isolated DC/DC converter based on modular multilevel converter(MMC)technology is expected to play an important role in future DC transmission grids.This paper presents a phasor analytical model for this new family of converters which is suitable for a range of studies like DC grid power flow or DC/DC parametric design.The 30th-order phasor model is derived in 3 coordinate frames:zero sequence(DC),fundamental frequency(dq),and double frequency(d2q2).The second-harmonic current suppression control is included as an option.Additionally,an estimation of the required control signals is presented,and a closed-loop model is developed which facilitates direct calculation of all variables and fast parametric studies.The accuracy of the proposed models is verified against a detailed PSCAD model for a wide range of parameters.The studies illustrate the importance of the second-harmonic components on the model accuracy.Finally,the impact of the converter parameters on the performance is studied,and a basic eigenvalue stability analysis is given.

R&D and application of voltage sourced converter based high voltage direct current engineering technology in China

As a new generation of direct current(DC)transmission technology,voltage sourced converter(VSC)based high voltage direct current(HVDC)has been widely developed and applied all over the world.China has also carried out a deep technical research and engineering application in this area,and at present,it has been stepped into a fast growing period.This paper gives a general review over China’s VSC based HVDC in terms of engineering technology,application and future development.It comprehensively analyzes the technical difficulties and future development orientation on the aspects of the main configurations of VSC based HVDC system,topological structures of converters,control and protection technologies,flexible DC cables,converter valve tests,etc.It introduces the applicable fields and current status of China’s VSC based HVDC projects,and analyzes the application trends of VSC based HVDC projects both in China and all over the world according to the development characteristics and demands of future power grids.

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.

Dynamics and Small Signal Stability Analysis of PMSG-based Wind Farm with an MMC-HVDC System

The permanent magnet synchronous generator (PMSG)-based wind farm with a modular multilevel converter (MMC) based HVDC system exhibits various oscillations and can experience dynamic instability due to the interactions between different controllers of the wind farm and MMC stations, which have not been properly examined in the existing literatures. This paper presents a dynamic modeling approach for small signal stability analysis of PMSG-based wind farms with a MMC- HVDC system. The small signal model of the study system is validated by the comprehensive electromagnetic transient (EMT) simulations in PSCAD/EMTDC. Then the eigenvalue approach and participation factors analysis are utilized to comprehensively evaluate the impact of different controllers, system’s parameters and the circulating current suppressing controller (CCSC) on the small signal stability of the entire system. From eigenvalue analysis, it is revealed that as the output active power of the wind farm increases within the rated range, the overall system will exhibit a sub-synchronous oscillation (SSO) instability mode, an extremely weak damping mode, and a low frequency oscillation instability mode. From participation factors analysis, it is observed that the SSO mode and weak damping mode are primarily related to the internal dynamics of the MMC, which can be suppressed or improved by CCSC. It is determined that the low frequency oscillation mode is primarily caused by the interactions between the phase locked loop (PLL) control of the wind farm and the voltage and frequency (V-F) control of the MMC station. The analysis also depicts that the larger proportional gain value of the V-F control of the MMC station and smaller PLL bandwidth of the wind farm can enhance the small signal stability of the entire system.