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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]

This paper introduces the control of the parallel operation of two voltage source converter (VSC)-HVdc links interconnecting an offshore wind farm. The aim of the study is to propose and validate a control system that allows the parallel operation of two VSC-HVdc links by controlling the currents injected by the VSC converters. The currents set points are established by a voltage controller in order to maintain constant voltage and frequency in the capacitor of the output filter and therefore within the offshore wind farm (OWF). It is demonstrated that the decoupled control of the d-q component of the voltage at the capacitor allows achieving the direct control of voltage and frequency, respectively. The voltage and frequency control is implemented by orienting the capacitor voltage toward a synchronous axis that is generated within the controller and therefore is not subjected to any grid disturbance. Both converters collaborate therefore in maintaining constant voltage and frequency, achieving in this way the parallel operation of the converters. The validation of this approach is demonstrated by simulation where the OWF and the VSC-HVdc rectifier have been modeled. Simulation results demonstrate that the proposed control system allows the parallelization of the converters while maintaining constant voltage and frequency within the OWF, even during transient faults. [ABSTRACT FROM AUTHOR]

A modular multilevel converter control system, based on converter energy storage, is proposed in this paper for two different control modes: active power and dc voltage. The proposed control system decouples the submodule (SM) capacitor voltages from the dc bus voltage. One of the practical applications is the management of active redundant SMs. A practical HVDC system with 401-level MMCs, including 10% redundancy in MMC SMs, is used for validating and demonstrating the advantages of the proposed control system. This paper also presents a novel capacitor voltage balancing control based on \max–\min functions. It is used to drastically reduce the number of switchings for each SM and enhances computational efficiency. [ABSTRACT FROM AUTHOR]

The analysis of circulating current in modular multilevel converter (MMC) has aroused extensive research interests recently. Most studies on the derivation of harmonic components of circulating current neglect the harmonics in the line currents, but the harmonics evidently distort the line currents in many MMC applications where a circulating path for the dominant third-order harmonic current exists. So far, little research has been done on the model of the harmonics in line currents of MMC and neglecting them inevitably causes large error in the previous analytical expressions of circulating current. To solve these problems, this paper proposes a novel accurate steady-state mathematical model of harmonic components existing both in arm and line currents, and presents the detailed relations between the inner and outer harmonic characteristics of MMC. Thus, the derived instantaneous voltages and currents both in arm and line from the proposed models are far more accurate than the present research. What's more, each existing higher order harmonic is precisely calculated for the first time, filling up the blank of the conventional studies, and providing a better reference for the understanding, designing, and controlling of the MMC engineering applications. The correctness and high accuracy of the proposed models are verified by simulation results in PSCAD/EMTDC. [ABSTRACT FROM AUTHOR]

When investigating voltage and current stresses in critical main circuit components during faults of the converter, a detailed equivalent model capable of representing the balancing control strategies of the capacitor voltages on the submodule level, along with blocking and delocking, is always necessary. Among previously proposed equivalent models of the modular multilevel converter (MMC), only submodule averaged models (SAMs) can capture interested inner dynamics inside each arm. However, the simulation accuracy of SAMs is not always satisfactory, especially when the time step is larger than 10 $\mu$s. In order to further improve the simulation accuracy with guaranteed simulation efficiency, the shifted frequency modeling of the half- and full-bridge hybrid MMC is proposed in this paper. Therein, each submodule is represented by Thévenin equivalents derived by submodule dynamic phasors. The arm of the MMC is represented by Norton equivalents to guarantee the efficiency, considering both normal and dc-blocking conditions. The effectiveness of the proposed model in terms of accuracy and efficiency is validated by simulating an MMC-based HVdc transmission. [ABSTRACT FROM AUTHOR]

This paper presents a transformerless high-voltage dc–dc converter based on cascaded sub-modules. It is intended for interconnecting high-voltage or medium-voltage dc grids. The proposed dc–dc converter consists of two phase legs, each of which consists of an upper arm and a lower arm with their middle nodes crossly connected through a branch of active sub-modules. By properly controlling the output voltage of the cross-connected branch, a trapezoidal ac current is induced to interact with the ac voltage components in the upper and lower arm for rebalancing the power amongst converter arms. The features of modular design, single-stage power conversion, and transformerless structure make the proposed dc–dc converter gain the outstanding merits of wide voltage ratio range, high system efficiency, and light converter weight. A control scheme is also elaborated for guaranteeing the normal operation of the proposed dc–dc converter. A 100-kV 100-MW simulation model performed in MATLAB/Simulink verifies the feasibility of the proposed dc–dc converter. Experimental results obtained from a 100-V 1-kW laboratory setup also confirm the validation of the proposal. [ABSTRACT FROM PUBLISHER]

Plug-in electric vehicle (PEV) charging equipment incorporates protection that ensures that grid disturbances do not damage the charger or vehicle. When the grid voltage sags below 80% of nominal, undervoltage protection is likely to disconnect the charging load from the grid. Most PEV charging will occur overnight, when non-PEV load is at a minimum. This paper argues that PEV voltage-sag response, when synchronized across large numbers of PEVs, could result in the loss of a significant proportion of the total load. It is shown that this load loss can lead to unacceptably high voltages once the initiating event has been cleared. This paper explores the nature of this voltage-rise phenomenon. Analysis tools are developed to assist in determining PEV loading conditions that demarcate acceptable postdisturbance voltage response from unacceptable outcomes. Two examples, based on standard distribution test systems, are used to illustrate PEV-induced overvoltage behavior, and demonstrate applications of the analysis tools. [ABSTRACT FROM AUTHOR]

A new class of modular multilevel dc/dc converters has recently emerged that eliminates the traditional full-rated intermediate ac stage inherent to two-stage dc/dc converter structures, that is, two cascaded dc/ac modular multilevel converters (MMCs). This paper introduces the first averaged dynamic state-space model for this new class of single-stage dc/dc converter, called the DC-MMC, which captures all input and output terminal dynamics as well as internal capacitor dynamics. Based on the developed analytical model, a dynamic phasor model is derived to accommodate the multifrequency components that exist at steady state. The phasor model enables: 1) solving for the full steady-state solution of the DC-MMC under arbitrary loading and 2) evaluation of DC-MMC open-loop stability via eigenvalue analysis. These analyses are not possible with existing DC-MMC models. Simulations of a comprehensive switched model verify the accuracy of the presented analytical models, which represent valuable study tools for the analysis of emerging modular multilevel dc/dc converters containing multiple frequency components. [ABSTRACT FROM PUBLISHER]

The modular multilevel converter (MMC) has become one of the most promising topologies for high-voltage applications. Although the MMC has a modular and redundant structure, which can significantly enhance its reliability and fault tolerance, its fault handling capability can be further improved by adopting a postfault strategy. In this paper, a postfault strategy based on the neutral-shift method is proposed, which, subsequent to bypassing the faulty submodule(s), aims at improving the maximum attainable line-to-line voltages of the MMC. Based on a newly developed internal variable analysis method, the impact of the neutral-shift strategy on the operation of the MMC is analyzed. In conjunction with the proposed postfault strategy, a modified circulating current suppression (CCS) method is used to mitigate the double-frequency component of the circulating currents. Performance and effectiveness of the proposed neutral-shift strategy with and without the CCS method is evaluated based on time-domain simulation studies in the PSCAD/EMTDC software environment. [ABSTRACT FROM PUBLISHER]

The number of semiconductor switches in a modular multilevel converter (MMC) for HVDC transmission is typically two orders of magnitudes larger than that in a two or three level voltage-sourced converter (VSC). The large number of devices creates a computational challenge for electromagnetic transient simulation programs, as it can significantly increase the simulation time. The paper presents a method based on partitioning the system's admittance matrix and deriving an efficient time-varying Thévenin's equivalent for the converter part. The proposed method does not make use of approximate interfaced models, and mathematically, is exactly equivalent to modelling the entire network (converter and external system) as one large network. It is shown to drastically reduce the computational time without sacrificing any accuracy. The paper also presents control algorithms and other modelling aspects. The efficacy of the proposed method is demonstrated by simulating a point-to-point VSC-MMC-based HVDC transmission system. [ABSTRACT FROM AUTHOR]

The modular multilevel converter (MMC) is one of the most potential converter topologies for high-power/voltage systems, specifically for high-voltage direct current (HVDC). One of the main technical challenges of an MMC is to eliminate/minimize the circulating currents of converter arms while the capacitor voltages are maintained balanced. This paper proposes a model predictive control (MPC) strategy that takes the advantage of a cost function minimization technique to eliminate the circulating currents and carry out the voltage balancing task of an MMC-based back-to-back HVDC system. A discrete-time mathematical model of the system is derived and a predictive model corresponding to the discrete-time model is developed. The predictive model is used to select the best switching states of each MMC unit based on evaluation and minimization a defined cost function associated with the control objectives of MMC units and the overall HVDC system. The proposed predictive control strategy: 1) enables control of real and reactive power of the HVDC system; 2) achieves capacitor voltage balancing of the MMC units; and 3) mitigates the circulating currents of the MMC units. Performance of the proposed MPC-based strategy for a five-level back-to-back MMC-HVDC is evaluated based on time-domain simulation studies in the PSCAD/EMTDC software environment. The reported study results demonstrate a satisfactory response of the MMC-HVDC station operating based on the proposed MPC strategy, under various conditions. [ABSTRACT FROM AUTHOR]