A Steady-State Analysis Method for a Modular Multilevel Converter
TL;DR: In this article, a steady-state analysis method for an MMC-based VSC-HVDC system is proposed to find a circular interaction among the electrical quantities in a MMC and a key equation can be established to solve the unknown circulating current.
Abstract: Modular multilevel converters (MMC) are considered a top converter alternative for voltage-source converter (VSC) high-voltage, direct current (HVDC) applications. Main circuit design and converter performance evaluation are always important issues to consider before installing a VSC-HVDC system. Investigation into a steady-state analysis method for an MMC-based VSC-HVDC system is necessary. This paper finds a circular interaction among the electrical quantities in an MMC. Through this circular interaction, a key equation can be established to solve the unknown circulating current. A new steady-state model is developed to simply and accurately describe the explicit analytical expressions for various voltage and current quantities in an MMC. The accuracy of the expressions is improved by the consideration of the circulating current when deriving all the analytical expressions. The model's simplicity is demonstrated by having only one key equation to solve. Based on the analytical expressions for the arm voltages, the equivalent circuits for MMC are proposed to improve the current understanding of the operation of MMC. The feasibility and accuracy of the proposed method are verified by comparing its results with the simulation and experimental results.
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TL;DR: An overview of the state of the art and recent developments enabling higher intelligence in future smart grids is provided and the integration of renewable sources and storage systems into the power grids is analyzed.
Abstract: Renewable energy sources are one key enabler to decrease greenhouse gas emissions and to cope with the anthropogenic climate change. Their intermittent behavior and limited storage capabilities present a new challenge to power system operators to maintain power quality and reliability. Additional technical complexity arises from the large number of small distributed generation units and their allocation within the power system. Market liberalization and changing regulatory framework lead to additional organizational complexity. As a result, the design and operation of the future electric energy system have to be redefined. Sophisticated information and communication architectures, automation concepts, and control approaches are necessary in order to manage the higher complexity of so-called smart grids. This paper provides an overview of the state of the art and recent developments enabling higher intelligence in future smart grids. The integration of renewable sources and storage systems into the power grids is analyzed. Energy management and demand response methods and important automation paradigms and domain standards are also reviewed.
463 citations
TL;DR: The development of MMC circuit topologies and their mathematical models over the years are presented and the evolution and technical challenges of the classical and model predictive control methods are discussed.
Abstract: Modular multilevel converter (MMC) is one of the most promising topologies for medium to high-voltage high-power applications. The main features of MMC are modularity, voltage and power scalability, fault tolerant and transformer-less operation, and high-quality output waveforms. Over the past few years, several research studies are conducted to address the technical challenges associated with the operation and control of the MMC. This paper presents the development of MMC circuit topologies and their mathematical models over the years. Also, the evolution and technical challenges of the classical and model predictive control methods are discussed. Finally, the MMC applications and their future trends are presented.
404 citations
Cites methods from "A Steady-State Analysis Method for ..."
...These models are used in the design of the control schemes and the converter stability analysis [35], [36]....
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TL;DR: In this paper, a hybrid modular multilevel converter (MMC) which combines full-bridge submodules (FBSM) and half-branch sub-modules (HBSM), which has the same dc fault blocking capability but uses fewer power devices hence has lower power losses, is presented.
Abstract: This paper presents a hybrid modular multilevel converter (MMC), which combines full-bridge submodules (FBSM) and half-bridge submodules (HBSM). Compared with the FBSM-based MMC, the proposed topology has the same dc fault blocking capability but uses fewer power devices hence has lower power losses. To increase power transmission capability of the proposed hybrid MMC, negative voltage states of the FBSMs are adopted to extend the output voltage range. The optimal ratio of FBSMs and HBSMs, and the number of FBSMs generating a negative voltage state are calculated to ensure successful dc fault blocking and capacitor voltage balancing. Equivalent circuits of each arm consisting of two individual voltage sources are proposed and two-stage selecting and sorting algorithms for ensuring capacitor voltage balancing are developed. Comparative studies for different circuit configurations show excellent performance balance for the proposed hybrid MMC, when considering dc fault blocking capability, power losses, and device utilization. Experimental results during normal operation and dc fault conditions demonstrate feasibility and validity the proposed hybrid MMC.
361 citations
Cites background from "A Steady-State Analysis Method for ..."
...Voltage ripple and capacitance of the SM capacitors are a crucial factor for the operation, size and cost of MMC systems [18]–[23]....
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...A detailed analysis on the impact of the circulating current on capacitor voltage ripple is provided in [23]....
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TL;DR: In this paper, a modular multilevel converter structure under different operating modes is analyzed and control algorithms are developed for the balancing of the battery state of charges and the respective gain limitations are established.
Abstract: Multilevel converters and battery energy storage systems are key components in present and future medium voltage networks, where an important integration of renewable energy sources takes place. The modular multilevel converter offers the capability of embedding such energy storage elements in a split manner, given the existence of several submodules operating at significantly lower voltages. This paper analyzes such a converter structure under different operating modes. In order to eliminate the low-frequency components of the submodule output currents, the latter are interfaced to the batteries by means of nonisolated dc/dc converters. Control algorithms are developed for the balancing of the battery state of charges and the respective gain limitations are established. Unbalanced grid conditions are also taken into account through the theory of symmetrical components and solutions are proposed. Finally, the development of a down-scaled prototype is described and experimental results are presented.
319 citations
Cites background from "A Steady-State Analysis Method for ..."
...A wide range of applications have been considered including HVDC systems [7]–[10], medium-voltage electric drives [11], [12], as well as direct and indirect grid interfaces [13], [14]....
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TL;DR: In this paper, a quasi two-level operating mode of the modular multilevel converter is proposed, where the converter generates a square wave with controllable dv/dt by employing the cell voltages to create transient intermediate voltage levels.
Abstract: DC fault protection is one challenge impeding the development of multi-terminal DC grids. The absence of manufacturing and operational standards has led to many point-to-point HVDC links built at different voltage levels, which creates another challenge. Therefore, the issues of voltage matching and DC fault isolation are undergoing extensive research and are addressed in this paper. A quasi two-level operating mode of the modular multilevel converter is proposed, where the converter generates a square wave with controllable dv/dt by employing the cell voltages to create transient intermediate voltage levels. Cell capacitance requirements diminish and the footprint of the converter is reduced. The common-mode DC component in the arm currents is not present in the proposed operating mode. The converter is proposed as the core of a DC to DC transformer where two converters operating in the proposed mode are coupled by an AC transformer for voltage matching and galvanic isolation. The proposed DC transformer is shown to be suitable for high-voltage high-power applications due to the low switching frequency, high efficiency, modularity, and reliability. The DC transformer facilitates DC voltage regulation and near instant isolation of DC faults within its protection zone. Analysis and simulations confirm these capabilities in a system-oriented approach.
311 citations
References
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23 Jun 2003
TL;DR: In this article, a new multilevel converter topology suitable for very high voltage applications, especially network interties in power generation and transmission, is presented, and a suitable structure of the converter-control is proposed.
Abstract: This paper presents a new multilevel converter topology suitable for very high voltage applications, especially network interties in power generation and transmission. The fundamental concept and the applied control scheme is introduced. Simulation results of a 36 MW-network intertie illustrate the efficient operating characteristics. A suitable structure of the converter-control is proposed.
2,806 citations
"A Steady-State Analysis Method for ..." refers background in this paper
...To maintain the dc voltage as a constant [4]–[6], [14], the relationship between the switching functions of the upper arm and that of the lower arm is usually controlled as...
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...Modular design, low switching frequency, high efficiency, and excellent output voltage waveforms are the advantages offered by MMC [6]–[15]....
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TL;DR: An overview of the recent advances in the area of voltage-source converter (VSC) HVdc technology is provided in this paper, where a list of VSC-based HVDC installations worldwide is included.
Abstract: The ever increasing progress of high-voltage high-power fully controlled semiconductor technology continues to have a significant impact on the development of advanced power electronic apparatus used to support optimized operations and efficient management of electrical grids, which, in many cases, are fully or partially deregulated networks. Developments advance both the HVDC power transmission and the flexible ac transmission system technologies. In this paper, an overview of the recent advances in the area of voltage-source converter (VSC) HVdc technology is provided. Selected key multilevel converter topologies are presented. Control and modeling methods are discussed. A list of VSC-based HVdc installations worldwide is included. It is confirmed that the continuous development of power electronics presents cost-effective opportunities for the utilities to exploit, and HVdc remains a key technology. In particular, VSC-HVdc can address not only conventional network issues such as bulk power transmission, asynchronous network interconnections, back-to-back ac system linking, and voltage/stability support to mention a few, but also niche markets such as the integration of large-scale renewable energy sources with the grid and most recently large onshore/offshore wind farms.
2,023 citations
TL;DR: In this article, two types of pulsewidth-modulated modular multilevel converters (PWM-MMCs) with focus on their circuit configurations and voltage balancing control are investigated.
Abstract: A modular multilevel converter (MMC) is one of the next-generation multilevel converters intended for high- or medium-voltage power conversion without transformers. The MMC is based on cascade connection of multiple bidirectional chopper-cells per leg, thus requiring voltage-balancing control of the multiple floating DC capacitors. However, no paper has made an explicit discussion on voltage-balancing control with theoretical and experimental verifications. This paper deals with two types of pulsewidth-modulated modular multilevel converters (PWM- MMCs) with focus on their circuit configurations and voltage-balancing control. Combination of averaging and balancing controls enables the PWM-MMCs to achieve voltage balancing without any external circuit. The viability of the PWM-MMCs, as well as the effectiveness of the voltage-balancing control, is confirmed by simulation and experiment.
1,506 citations
TL;DR: In this paper, a modified phase-shifted carrier-based pulsewidth-modulation (PSC-PWM) scheme for modular multilevel converters (MMC) is presented.
Abstract: This paper describes a modified phase-shifted carrier-based pulsewidth-modulation (PSC-PWM) scheme for modular multilevel converters (MMC). In order to reduce the average device switching frequency, a reduced switching-frequency (RSF) voltage balancing algorithm is developed. This paper also proposes a circulating current suppressing controller (CCSC) to minimize the inner circulating current in an MMC. Based on the double line-frequency, negative-sequence rotational frame, the three-phase alternative circulating currents are decomposed into two dc components and are minimized by a pair of proportional integral controllers. Simulation results based on a detailed PSCAD/EMTDC model prove the effectiveness of the modified PSC-PWM method and the RSF voltage-balancing algorithm. The proposed CCSC not only eliminates the inner circulating current but also improves the quality of the converter ac output voltage. A simple loss evaluation demonstrates that the RSF voltage-balancing algorithm and the CCSC reduce the converter power losses.
1,183 citations
TL;DR: In this paper, the authors proposed a phase-disposition (PD) sinusoidal pulsewidth modulation (SPWM) switching strategy for the operation of an MMC-HVDC system.
Abstract: 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.
1,104 citations
"A Steady-State Analysis Method for ..." refers background in this paper
...H IGH-VOLTAGE, direct current (HVDC) transmission using voltage-source converters (VSCs) provides a means of connecting distant renewable sources to a large ac network [1]–[5]....
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