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This paper proposes an adaptive model of modular multilevel converter (MMC) for electromagnetic transient (EMT) simulations. The model is applicable to MMCs with arbitrary numbers of half-bridge and full-bridge submodules. The proposed design includes average value model, arm equivalent model, and detailed equivalent model. It allows smoothly transitioning from one model to another during time-domain simulations depending on the desired accuracy and execution time constraints. Modifications required in conventional MMC models to achieve smooth transitions are presented in the paper. Time-domain initialization methods are developed for each constituting model, including initialization of the appropriate control system blocks. Validity and effectiveness of the proposed adaptive model is demonstrated using EMT simulations of 401-level MMC-HVDC system. [ABSTRACT FROM AUTHOR]

The missing capability of DC fault current limitation is an inherent disadvantage in the typical half-bridge modular multilevel converter (MMC), which restricts its application in long-distance high voltage direct current (HVDC) systems. The full-bridge submodule (FBSM) and clamped double submodule (CDSM) topologies allow for fault current self-clearing, but also suffer from additional hardware cost and power losses. In this paper, an improved submodule topology utilizing the new reverse-blocking insulated gate bipolar transistor (RB-IGBT) is proposed to improve system economy without compromising its performance. The operating principles and DC fault characteristics of the proposed topology are analyzed in details, including comparison with various topologies in terms of cost, losses, fault current clearing speed and thermal. Compared with the traditional CDSM topology, the proposed RBSM can clear DC faults faster with reduced cost of switching devices. A DC fault protection strategy for MMC-HVDC system is also presented. Simulation results under normal operating, permanent and transient fault conditions are carried out to verify the effectiveness of the proposed topology. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Transient voltage and current stresses are essential for the design and protection of multi-terminal DC (MTDC) transmission systems, especially under the short-term dynamics after severe grid faults. In this paper, we propose a discrete-time model for the modular multilevel converter based MTDC (MMC-MTDC) system considering the DC-side disturbances in the system. Based on the piecewise linear modeling, the state variables such as the transient voltage on the sub-module (SM), arm current, etc., can be calculated analytically for the critical short term (within 10 ms) after the severe DC fault while the system protection is yet to be activated. Parameter sensitivity analysis is also performed to give a better system design guideline. The estimation results are validated by detailed electromagnetic (EMT) circuit simulations and the scaled-down experimental laboratory prototype. [ABSTRACT FROM AUTHOR]

The voltage ratings for dc transmission and distribution are not currently standardized because cable technology is advancing rapidly and therefore a low step-ratio dc–dc converter at multi-megawatt scale is a crucial technology for combining point-to-point dc links into multi-terminal networks. This paper presents a modular multilevel dc–dc converter configuration with only a single stack of half-bridge sub-modules (SM) and a resonant tank. The power processed through the stack is a small fraction of the throughput power if the step ratio is small ($1 - {\boldsymbol{v}_{\boldsymbol{L}}}/{\boldsymbol{v}_{\boldsymbol{H}}}$). Detailed analysis shows the required semiconductor ratings are much smaller compared to the front-to-front MMC and modular converters derived from standard dc–dc topologies. Further, the requirement for SM capacitive energy storage is also significantly less than those converters. The theoretical analysis and the operating principles have been verified by full-scale simulation examples and experiments with a down-scaled prototype. The SMs are operated with an adjustable phase shift to create a high frequency excitation across the resonant tank for further volume reduction. The SM capacitors self-balance without the need for switching pattern adjustments and the resonant operation provides soft switching for all power switches to increase efficiency. [ABSTRACT FROM AUTHOR]

This paper presents an accurate steady-state initialization method for electromagnetic transient (EMT) models of modular multilevel converters (MMCs). The proposed method uses steady-state arm voltages and currents to initialize internal electrical variables and control systems of the converter, including capacitor voltage and gating signals of each sub-module. In addition to typical voltage source converter upper level control, MMC specific controls are also considered, including circulating current control, dc ripple control, and capacitor voltage balancing algorithm. EMT simulations on 101-level and 401-level MMC-HVDC systems confirm the accuracy of the proposed method. [ABSTRACT FROM AUTHOR]

The dc tap or dc transformer will play an important role in interfacing different voltages of dc links in dc grids. This paper presents an isolated resonant mode modular converter (RMMC) with flexible modulation and assorted configurations to satisfy a wide variety of interface requirements for medium-voltage dc (MVdc) networks. The transformerless RMMC, as introduced in the literature, implemented a restricted modulation scheme leading to a very limited range of step ratio and the diode rectifier resulted in unidirectional power flow. Both of these limitations are removed in this proposal and galvanic isolation has also been added. Moreover, this new RMMC approach can serve as a building block for variety of configurations. Two such derived topologies are given, which inherently balance the voltage and current between different constituent circuits and realize the high power rating conversion for very low or very high step-ratio application. The theoretical analysis is validated by a set of full-scale simulations and a downscaled experimental prototype. The results illustrate that this isolated RMMC and its derivatives have promising features for dc taps or dc transformers in MVdc applications. [ABSTRACT FROM PUBLISHER]

The large number of switching elements in the modular multilevel converter (MMC) is a challenging problem when modeling the MMC-HVDC systems for the computation of electromagnetic transients. The modeling complexity increases even further when a multiterminal (MT) MMC-HVDC system is used to integrate offshore wind farms (OWFs) with power-electronics-based wind energy converters, such as doubly fed induction generators (DFIGs). This paper compares modeling accuracy and computational performances for various combinations of MMC and OWF models. Onshore and offshore ac fault simulations are performed for an OWF system composed of DFIG-type wind turbines and connected to a practical ac grid through an MT MMC-HVDC system. The OWF system model includes the detailed representation of the offshore collector grid and the associated overcurrent protection. The offshore MMC controls include an offshore fault current limiter and fast OWF power generation reduction-based fault-ride-through function. [ABSTRACT FROM PUBLISHER]

This paper proposes an optimal design method of modular multilevel converter (MMC) levels to approach the reasonable compromise between accuracy and computational efficiency. The entire procedure is benchmarked by a Thévenin equivalent 451-level MMC model. First, the calculation method of the critical maximum simulation time step is proposed to faithfully represent each voltage level of the staircase waveforms. Second, the previously developed improved analytic hierarchy process is adopted to integrate the relative errors of all the main electrical parameters into one comprehensive relative error at each MMC level. Third, the steady state, ac, and dc transient state relative error results as well as the central-processing unit times of all models are normalized and curve fitted based on the benchmark model. Finally, the optimal MMC level is obtained by the plotted curves and the given subjective weights considering the accuracy and efficiency. And the effectiveness and robustness of the proposed method are validated by various simulation scenarios on PSCAD/EMTDC. [ABSTRACT FROM PUBLISHER]

The modular multilevel converter (MMC) is a newly introduced switch-mode converter topology with the potential for high-voltage direct current (HVDC) transmission applications. This paper focuses on the dynamic performance of an MMC-based, back-to-back HVDC system. A phase-disposition (PD) sinusoidal pulsewidth modulation (SPWM) strategy, including a voltage balancing method, for the operation of an MMC is presented in this paper. Based on the proposed PD-SPWM switching strategy, a mathematical model for the MMC-HVDC system, under both balanced and unbalanced grid operation modes, is developed. Dynamic performance of the MMC-based back-to-back HVDC converter system, based on time-domain simulation studies in the PSCAD/EMTDC environment, is then evaluated. The reported time-domain simulation results show that based on the adopted PD-SPWM switching strategy, the MMC-HVDC station can respond satisfactorily to the system dynamics and control commands under balanced and unbalanced conditions while maintaining voltage balance of the dc capacitors. [ABSTRACT FROM PUBLISHER]

The modular multilevel converter (MMC) is a potential candidate for medium/high-power applications, specifically for high-voltage direct current (HVDC) transmission systems. One of the technical challenges associated with the control of an MMC is to carry out the submodule (SM) capacitor voltages balancing task without imposing any unnecessary switching transition among the SMs. This paper develops a general framework for the capacitor voltage balancing of an MMC. Based on the developed framework and with the objective of reducing the unnecessary switching transitions among the SMs, three capacitor voltage-balancing strategies are proposed and investigated: 1) a slow-rate balancing strategy based on the conventional sorting method; 2) a hybrid balancing strategy which combines a predictive method with the conventional sorting method; and 3) a fundamental-frequency balancing strategy. Performance of the proposed strategies for a point-to-point 21-level MMC-based HVDC system is evaluated based on time-domain simulation studies in the PSCAD/EMTDC software environment. The reported study results demonstrate the capability and tradeoffs of the proposed strategies to reduce the number of switching transitions and, consequently, the MMC losses, under various conditions. [ABSTRACT FROM AUTHOR]

This paper presents design and control methods for fault-tolerant operations with redundant converter modules, one of the most prominent features in modular multilevel converter (MMC) topology. In fully implementing MMC functionalities, a nearest level control is applied as a low-switching modulation method. A dual sorting algorithm is newly proposed for effectively reducing the switching commutations of each power module as well as for voltage balancing control. Built upon these primary MMC topological and control features, its redundant operation is comprehensively investigated for fail-safe energy transfer. In particular, a novel spare process is proposed to handle an emergency situation when the number of faulty power modules exceeds the module redundancy. Since topological redundancy may cause the switching commutations of power modules in an arm to be unevenly distributed, a practical and effective mitigation measure is incorporated to keep the energy balance while avoiding the undesired switching stresses. Rigorous simulation studies for MMC and its application for high-voltage direct current are performed to demonstrate the validity and effectiveness of the proposed spare process under normal and emergency conditions. [ABSTRACT FROM AUTHOR]

Voltage-source-converter (VSC) technologies present a bright opportunity in a variety of fields within the power system industry. New modular multilevel converters (MMCs) are expected to supersede two- and three-level VSC-based technologies for HVDC applications due to their recognized advantages in terms of scalability, performance, and efficiency. The computational burden introduced by detailed modeling of MMC-HVDC systems in electromagnetic-transients (EMT)-type programs complicates the study of transients especially when these systems are integrated into a large network. This paper presents a novel average-value model (AVM) for efficient and accurate representation of a detailed MMC–HVDC system. It also develops a detailed 401-level MMC-HVDC model for validating the AVM and studies the performance of both models when integrated into a large 400-kV transmission system in Europe. The results show that the AVM is significantly more efficient while maintaining its accuracy for the dynamic response of the overall system. [ABSTRACT FROM AUTHOR]