Modular Multilevel Converter System Research Articles

Analysis of Single-Phase-to-Ground Faults at the Valve-Side of HB-MMCs in HVDC Systems

Although the probability of occurrence of station internal ac grounding faults in modular multilevel converter (MMC)-based high-voltage direct-current systems is low, they may lead to severe consequences that should be considered when designing protection systems. This paper analyzes the characteristics of valve-side single-phase-to-ground (SPG) faults in three configurations of MMC systems. Fault responses for symmetrical monopole MMCs are first studied. Upper arm overvoltages and ac-side nonzero-crossing currents arising from SPG faults in asymmetrical and bipolar configurations are then investigated. DC grounding using an LR parallel circuit is employed to create current zero-crossings, which will enable the operation of grid-side ac circuit breakers. The theoretical analysis is verified through simulations performed in PSCAD/EMTDC, with simulation results and the theoretical analysis showing a good agreement. The studies in this paper will be valuable for the design of protection systems for station internal ac grounding faults.

On Exploiting Active Redundancy of a Modular Multilevel Converter to Balance Reliability and Operational Flexibility

This paper presents a practical strategy for utilizing the submodule (SM) redundancy of a modular multilevel converter (MMC) for its fault tolerance. This strategy provides a systematic framework for balancing the tradeoff between two conventional methods for using the active redundancy and, thus, achieves operational flexibility. One of the existing methods improves SM reliability owing to less voltage stress on the SM components by employing all of the SMs to form the ac or dc voltages (voltage-sharing mode). The other avoids transients by keeping the average SM voltage constant at the cost of slightly increased stress on the SM components (fixed-level mode), which, however, can be controlled to provide the grid-adaptive operation by reserving the energy of the SMs not in service. We, thus, develop a new redundancy management scheme by integrating these two methods and exploiting their technical benefits to meet the PQ requirements and MMC control performance. This research provides a theoretical basis and a technical guide to determining the number of SMs, which can further increase the voltage steps as per the MMC and grid conditions. This paper also connects the remaining PQ capability of the MMC at a particular operating point with the SM redundancy concept by defining a potential redundancy, especially useful when the physical redundancy is exhausted. The theoretical findings and efficacy of the proposed strategy are validated through PSCAD/EMTDC time-domain simulations followed by experiments using a nine-level single-phase MMC system.

Reliability evaluation of MMC system considering working conditions

The reliability is a key issue of the high-voltage high-power modular multilevel converter (MMC) system due to the usage of fragile insulated gate bipolar transistor (IGBT) modules. In this study, it is proposed that the reliability of the MMC system can be modelled and calculated considering practical working conditions. Firstly, based on the MMC power level, the power loss and the junction temperature are analysed. Then considering the aging process of IGBT, the failure rate is calculated with temperature coefficient. Finally, the MMC system failure rate and its relationships with some key parameters are found.

Simplified Thermal Modeling for IGBT Modules With Periodic Power Loss Profiles in Modular Multilevel Converters

One of the future challenges in modular multilevel converters (MMCs) is how to size key components with compromised costs and design margins, while fulfilling specific reliability targets. It demands better thermal modeling compared to the state-of-the-arts in terms of both accuracy and simplicity. Different from two-level power converters, MMCs have inherent dc-bias in arm currents and the power device conduction time is affected by operational parameters. A time-wise thermal modeling for the power devices in MMCs is, therefore, an iteration process and time-consuming. This paper, thus, proposes a simple analytical thermal modeling method, which adopts equivalent periodic power loss profiles. More importantly, time-domain simulations are not required in the proposed method. Benchmarking of the proposed methods with the prior-art solutions is performed in terms of parameter sensitivity and model accuracy with a case study on a 30-MW MMC system. Experiments are carried out on a specifically designed scaled-down system to verify the electrothermal aspects.

Unipolar Double-Star Submodule for Modular Multilevel Converter With DC Fault Blocking Capability

Adopting overhead lines is an effective way to reduce the cost of flexible high voltage direct current (HVDC) systems. However, the dc short-circuit fault is easily occurred and the development of the overhead lines based HVDC system is hampered. This paper proposes a unipolar double-star submodule (UDSSM) with the dc fault blocking capability. Under the dc fault condition, the UDSSM based modular multilevel converter (MMC) could block the fault current by switching all power switching devices off to protect the MMC system. In each proposed submodule, two capacitors work under almost the same condition and could be in series or parallel connection to output different voltage levels. Then two voltage sensors of the two capacitors are redundant each other, which improves the reliability of the MMC system. Additionally, voltage balancing methods for the capacitors under both normal operation and sensor fault conditions of the MMC submodule are developed in this paper. The effectiveness of the proposed submodule and the capacitor voltage balancing strategy are validated by the simulation and experiment results.

A New Topology of Modular Multilevel Converter With Voltage Self-Balancing Ability

The modular multilevel converter (MMC) has been used in the field of HVDC transmission due to its many advantages, but its voltage imbalance problem of sub-module capacitors affects the stable operation of the system. There are two kinds of methods for solving the capacitor voltage imbalance of the MMC sub-module. The one is voltage balancing algorithm and the other one is improved topology. However, the complexity of the voltage balancing algorithm increases sharply when the number of voltage levels is high, which is not suitable for the MMC system with a high number of voltage levels. In this paper, a new topology with bidirectional voltage self-balancing capability is proposed. This topology adds the voltage balancing circuit between sub-modules on the conventional MMC topology, realizing voltage self-balancing without voltage balancing algorithm, and having a better effect of inhibition phase-to-phase circulation. The bidirectional voltage balancing capability, the voltage balancing effect and the circulation inhibition effect of the bidirectional voltage self-balancing topology are verified by MATLAB/Simulink simulation and experiment.

A Discrete-Time Low-Frequency-Ratio Nearest Level Modulation Strategy for Modular Multilevel Converters With Small Number of Power Modules

There has been increased usage of modular multilevel converters (MMCs) in the medium-voltage traction drives for increased efficiency and reduced switching losses. One of the disadvantages of utilizing the conventional nearest level modulation (NLM) scheme is the poor performance of output voltage distortion and dynamic response. The control design of low-frequency ratio for the MMC system with a small number of power modules should, therefore, be considered due to the industrial operation requirements. This paper focuses on the voltage state switching mechanism of a discrete-time low-frequency-ratio NLM strategy. A five-segment voltage state allocation guideline based on voltage state smooth transition is analyzed in the discrete-time domain. The proposed strategy features a linearly synthesized output voltage by means of simple arithmetic calculations to determine the switching sequences and the duration times. The overall strategy benefits from reduced dv/dt in the rising/falling edges of the output voltage and the unnecessary switching actions are highly constrained. The simulation analysis and the experimental results validate the proposed modulation strategy and establish its effectiveness.

Measurement and Analysis of Radiated Disturbance Characteristics of ±320 kV Modular Multilevel Converter System

The radiated electromagnetic disturbance was evaluated by measuring the electric field intensity in a bandwidth of 10 kHz-2.75 GHz near the valve hall, direct-current (DC) area, reactor room, and transformers in a ±320-kV modular multilevel converter-based high-voltage, direct-current (MMC-HVDC) converter station. In addition, the distribution of electric field intensity was analyzed in various regions in the converter station. According to the law of electric fields attenuation with distance, the radiated electric field intensities at the position 200 m away from the converter station was calculated by using the electric fields measured in the converter station. The results show that the levels of radiated disturbance inside and outside the converter station meet the standards' requirements. The amplitude spectra of the electric field intensity in a traditional HVDC system and MMC-HVDC system were compared. Finally, the mechanism for radiated electromagnetic interference of the MMC-HVDC converter system was analyzed from the aspects of disturbance source characteristics, wall shielding and radiated structure.

Analysis and Compensation of the System Time Delay in an MMC System

The system time delay (STD) is typically neglected in the design of a modular multilevel converter system. However, the STD can cause different types of deteriorations, including a decrease in system stability, a weakening of the steady-state performance, and a deceleration of the transient response when the load changes. This study first provides the generation mechanism and classification of the system time delay. Then, a linear model is developed to form the basis of the analysis for the system time delay. Subsequently, an analytical method is used to provide an accurate description of the relation between the STD and corresponding deteriorations. Based on the analysis, the optimal design methods of the system controller parameters are proposed to effectively mitigate these deteriorations. Finally, the deteriorations caused by the STD and efficiency of the optimal design methods are demonstrated via experiments.

A unified MMC reliability evaluation based on physics-of-failure and SM lifetime correlation

The reliability of a modular multilevel converter (MMC) is determined both by the component lifetime distribution and the MMC structure. In this paper, a unified submodule (SM) component lifetime is estimated based on the physics- of- failure, and the overall reliability of the MMC system is evaluated considering the SM lifetime correlation. First, the component lifetime estimation is performed in three sub-steps. (a) The current stress of each component, i.e., the insulated gate bipolar transistor modules (IGBTs) and capacitors, is computed according to the working principle of the MMC. (b) The core temperatures of the components are calculated using thermal equivalent networks. For the IGBTs, the thermal cycles are extracted to compute the cumulative damage. (c) The lifetime of each device is estimated in a static state using the lifetime model and damage model. Second, the lifetime Weibull distribution of each component is obtained using the Monte Carlo method. This distribution links the component lifetime estimation and MMC reliability analysis. Finally, considering the SM lifetime correlation, the overall reliability of an MMC system with redundancy is evaluated using a Copula function. The reasons for the impact of SM lifetime correlation on MMC reliability are revealed in a case study.

Modulated Model Predictive Control of Modular Multilevel Converters in VSC-HVDC Systems

This paper proposes a new modulated model predictive control method for the control of modular multilevel converters (MMCs) in voltage source converter-high voltage dc systems. The proposed method retains the advantages of the conventional finite control set-model predictive control methods by programing the nonlinear properties of the MMC into the design calculations while minimizing the ac line and circulating current ripples and steady-state error by generating modulated switching signals with a fixed switching frequency. In this paper, first, the predictive modeling of the MMC is provided. Next, the proposed control method is described. Then, the application of the proposed method to an MMC system is detailed. Finally, experimental results from an MMC system connected to a three-phase grid are provided to validate the theoretical outcomes.

Accurate modeling and simulation analysis of modular multilevel converter system

Modular multi-level converter (MMC) mathematical analysis, including the main circuit parameter design, the main circuit electrical state, controller design and parameter tuning, and so on. From the external characteristics, MMC can be considered that the equivalence is reasonable and effective to some extent; but from the internal characteristics, the kind of equivalent will ignore the MMC some internal system characteristics, so the way modeling cannot reflect the constraints of voltage and current within the system. Therefore, it is necessary to carry on the system modeling research to the MMC converter to reveal the constraint relation of the internal electrical parameters of the MMC converter, and provide the theoretical basis for the system analysis and the complete set design.

Hybrid Modular Multilevel Converter With Redistributed Power to Reduce Submodule Capacitor Voltage Fluctuation

This paper introduces a hybrid modular multilevel converter (MMC) with the full-bridge submodules (FBSMs) inserted in the ac side. It is intended for redistributing arm powers of the MMC system to reduce the submodule (SM) capacitor voltage fluctuation by injecting high-frequency powers in upper and lower arms without disturbing the output voltage. New power distribution of the hybrid MMC system can be realized by controlling the SMs and FBSMs working under the high-frequency powers. The proposed FBSMs in the hybrid MMC system also perform as the harmonic power generator to eliminate the common-mode voltages in the ac side. Since the FBSMs locate in the ac side of the hybrid MMC system, the traditional MMC control method can be directly applied in arm control of the hybrid MMC system. Meanwhile, relationship between the arm powers and SM capacitor voltage has been analyzed in detail to reduce the SM capacitor voltage ripples by redistributing the frequency spectrum of arm powers. Simulation and experimental results have also verified the effectiveness of the proposed method.

Distributed Control for a Modular Multilevel Converter

Conventional centralized control strategies may reduce the flexibility and expandability of a modular multilevel converter (MMC) system. To tackle this issue, this paper proposes a distributed control architecture that is capable of assigning certain control tasks to distributed local controllers and improves the modularity of an MMC system. A central controller dealing with the output current regulation based on sensed arm currents is adopted. The control of MMC internal dynamics and the pulsewidth modulation (PWM) generation are distributed into local controllers. Unlike the conventional MMC control that needs all submodule capacitor voltages for capacitor voltage averaging, the proposed capacitor voltage control only relies on local submodule voltage measurement. Consequently, communication-intensive capacitor voltage transmission in each control cycle is not required and the communication burden of the control system can be significantly reduced. The control loops and possible control conflicts among submodules are presented and considered for system stability analysis. The effectiveness of the proposed distributed control architecture and capacitor voltage control for an MMC are confirmed by the start-up, steady state, and transient experimental results.

Large-Scale Nonlinear Device-Level Power Electronic Circuit Simulation on Massively Parallel Graphics Processing Architectures

Device-level power electronic circuit simulation is so cumbersome that engineers are forced to make model simplification or reduce circuit size to obtain a reasonable execution time for repeated simulation runs. This paper proposes a massive-thread parallel simulation of large-scale power electronic circuits employing device-level modeling on the graphics processors (GPUs) to obtain higher data throughput and lower execution times. Parallel massive-thread modules are proposed for the nonlinear physics-based insulated gate bipolar transistor and power diode components. The nonlinear solution algorithm comprised of Newton–Raphson iterations and partial LU decomposition is fully parallelized on the GPU. Furthermore, the commonly used behavioral model with reduced computational complexity is also employed to represent the switches. The developed simulation codes are used to run large-scale test cases of the modular multilevel converter (MMC) system. The accuracy and efficiency of the GPU-based parallel simulation are compared with sequential CPU-based codes and the SaberRD program to show the advantages of the parallelized simulation; execution time speedups of 15 times and 70 times are reported for the MMC system using the nonlinear physics-based modeling and behavior-based modeling, respectively.

Gradient-Based Energy Balancing and Current Control for Alternate Arm Converters

The alternate arm converter (AAC) is an emerging fault-tolerant multilevel converter topology from the same family of multilevel converters as the modular multilevel converter (MMC). Due to the alternate operation of the converter arms, energy balancing in the AAC is not continuous, but restricted to small time intervals. This paper develops a gradient-based current control and energy-balancing method for the AAC. The proposed strategy enforces the dynamic limits on the redundant submodules (SMs) during the overlap period and allocates effectively the maximum available number of redundant SMs to control the circulating current. The choice of the gradient as the circulating current control parameter improves the energy regulation capability of the AAC, and the enforcement of dynamic limitations avoids distortions of the output voltage. Results from an AAC-based HVDC converter model derived from the CIGRE benchmark MMC system demonstrate that the proposed strategy delivers improved energy control and balancing with good harmonic performance compared to existing current control methods for the AAC while also maintaining zero-current switching of the director switches of the AAC arms.

The Fault Detection, Localization, and Tolerant Operation of Modular Multilevel Converters with an Insulated Gate Bipolar Transistor (IGBT) Open Circuit Fault

Reliability is one of the critical issues for a modular multilevel converter (MMC) since it consists of a large number of series-connected power electronics submodules (SMs). In this paper, a complete control strategy including fault detection, localization, and tolerant operation is proposed for the MMC under an insulated gate bipolar transistor (IGBT) open circuit fault. According to the output characteristics of the SM with the open-circuit fault of IGBT, a fault detection method based on the circulating current and output current observation is used. In order to further precisely locate the position of the faulty SM, a fault localization method based on the SM capacitor voltage observation is developed. After the faulty SM is isolated, the continuous operation of the converter is ensured by adopting the fault-tolerant strategy based on the use of redundant modules. To verify the proposed fault detection, fault localization, and fault-tolerant operation strategies, a 900 kVA MMC system under the conditions of an IGBT open circuit is developed in the Matlab/Simulink platform. The capabilities of rapid detection, precise positioning, and fault-tolerant operation of the investigated detection and control algorithms are also demonstrated.

Small-signal modeling of MMC based DC grid and analysis of the impact of DC reactors on the small-signal stability

This paper proposed a generic modeling framework of the modular multilevel converter (MMC) based DC grid for small-signal stability studies. A modularized modeling method is employed to form the overall state-space model and small-signal model with considering the detailed dynamics of MMC internal electrical quantities, MMC control patterns and the DC network. The validity and accuracy of the state-space model and small-signal model for a five-terminal MMC system are verified versus the electromagnetic transient model in PSCAD/EMTDC. Based on the small-signal model, the impact of the DC reactors on the small-signal stability of the tested system is investigated through root locus analysis. Key DC reactors that have a dominant effect on the stability are identified. Critical values of the DC reactors that preserve stability are recognized. Damping control is designed and installed according to the participation factor analysis. The time-domain simulation results show excellent agreement on the oscillating frequency calculated by eigenvalue studies and verify the validity of the proposed damping control.

Active Power Decoupling for Submodules of a Modular Multilevel Converter

The modular multilevel converter (MMC) is receiving wide acceptance in both high- and medium-voltage (MV) applications. However, due to the low (fundamental and second-order) frequency ripple powers in the submodule (SM) capacitors, large capacitance is required to smooth the SM voltage. The SM capacitors account for a large portion of volume and weight in the MMC system. Present methods (e.g., circulating current control, power channels linking upper and lower arms) cannot eliminate the fundamental and second-order ripple powers simultaneously. This paper investigates the feasibility of an active power decoupling technique for solving this issue. By adding a buck-type active power filter (APF) circuit (which contains another energy-storage capacitor), the low-frequency ripple powers can be transferred to the APF capacitor. This significantly reduces the SM voltage ripple and therefore the total capacitance of the SM. To enhance voltage ripple suppression, APF capacitor voltage reference is modified in a closed-loop manner, and a proportional-integral plus repetitive controller is proposed. Simulations and experimental results prove the validity of the method. A comparison with the traditional MMC shows that it can significantly reduce system volume and improve power density. The method is best suited for MV applications where power density is given a high priority.

Circulating Current Suppression in Modular Multilevel Converters With Even-Harmonic Repetitive Control

Due to voltage mismatch between phase legs and the dc bus in modular multilevel converters (MMCs), the differential current in MMCs is inherently subjected to circulating even-order harmonics. Repetitive control based active harmonic suppression methods can be adopted to eliminate such harmonics. Nevertheless, conventional repetitive controllers have a relatively slow dynamic response, because all the sampled errors in the past one cycle have to be stored, which causes a response delay for one fundamental period. This paper proposes an improved repetitive control scheme that exclusively copes with even-order harmonics based on the circulating current characteristics of MMC systems. The design details of the even harmonic repetitive control scheme according to the harmonics characteristics are provided. The proposed even-harmonic repetitive control scheme requires halved data memory to store error samplings and the delay introduced by the repetitive controller is also reduced. According to the frequency domain analysis, the even-harmonic repetitive control features faster convergence rate, greater low-frequency gains, higher crossover frequency, and higher tolerance against system frequency deviation, while possessing the same even-order harmonics suppression capability and stability as conventional ones. Simulation and experimental results are presented to show the steady-state harmonics suppression capability, dynamic response, and disturbance tolerance of the proposed even-harmonic repetitive control scheme.