Circulating Current Suppression in Modular Multilevel Converters With Even-Harmonic Repetitive Control
TL;DR: In this article, the authors proposed an improved repetitive control scheme that exclusively copes with even-order harmonics based on the circulating current characteristics of MMC systems, which requires halved data memory to store error samplings and reduces the delay introduced by the repetitive controller.
Abstract: 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.
Citations
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TL;DR: In this article, a distributed control architecture that is capable of assigning certain control tasks to distributed local controllers and improves the modularity of an MMC system is proposed, where a central controller dealing with the output current regulation based on sensed arm currents is adopted.
Abstract: 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.
108 citations
Cites background from "Circulating Current Suppression in ..."
...operated with highly centralized control strategies for multiple control objectives such as output voltage or current regulation, submodule capacitor voltage averaging and balancing, and circulating ripple current suppression or injection [4]–[7]....
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...In conventional capacitor voltage averaging strategies [7]–[9], the average...
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TL;DR: This paper quantitatively exploits the MMC synthetic inertia in a microgrid with flexible renewable penetration levels by proposing an MMC inertia coefficient concept and showing that, with proper system parameters and control loops, the line frequency deviation and the rate of change of frequency can be significantly reduced.
Abstract: High penetration of renewable energies through fast-response power converters results in a considerable displacement of conventional synchronous generators and losing of system inertia for frequency control. Modular multilevel converters (MMCs) can be employed serving as an interface between the large-scale renewable generation and power grids. In a microgrid with high shares of renewables integrating through MMCs, submodule (SM) capacitors can be used as energy storage to provide a degree of synthetic inertia for system frequency support. This paper quantitatively exploits the MMC synthetic inertia in a microgrid with flexible renewable penetration levels. An MMC inertia coefficient concept is proposed. It is mainly affected by the penetration ratio, the SM capacitance, and the modulation index of an MMC, with the system operation constraints. According to the analysis, a substantial portion of system inertia can be provided by properly designed MMCs. Detailed MMC frequency control loops are presented. The capacitor average voltage is proportionally linked to the frequency deviation, in order to flexibly adjust the capacitor energy during frequency events. The MMC output active power is deliberately and simultaneously regulated according to the capacitor energy change rate. By controlling the MMC capacitor voltage, dc side power, and output active power, an amount of energy can be released or absorbed by SM capacitors to improve the system frequency response during the frequency event transients, while the renewable generation is scarcely influenced. The proposed concept is experimentally verified and the results show that, with proper system parameters and control loops, the line frequency deviation and the rate of change of frequency can be significantly reduced. Good agreements of the system frequency characteristics have been achieved between the theoretical calculation and experimental results.
50 citations
Cites background from "Circulating Current Suppression in ..."
...If the circulating current ripples are perfectly suppressed as discussed in [18] and [19], and the three phase legs are identical,...
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TL;DR: A novel MMC-based advanced co-phase traction power supply system is proposed in this paper to solve power quality issues and eliminate the neutral sections in the traditional tractionPower supply system.
Abstract: Due to the attractive advantages such as modularity, scalability and excellent power quality of modular multilevel converter (MMC), converters based on MMC could be a promising alternative solution for traction transformer. A novel MMC-based advanced co-phase traction power supply system is proposed in this paper to solve power quality issues and eliminate the neutral sections in the traditional traction power supply system. And in the proposed system, a DC power transmission system is designed, which provides convenient access for distributed energies benefiting the utilization of the natural resources such as solar energy and wind energy along railways. In order to ensure the normal operation of the proposed system, nearest-level modulation considering voltage balancing is designed for MMCs. The mathematic model three-phase MMC-based rectifier is derived in detail. Based on the mathematic model, dual current-loop control is designed for the rectifier. Besides, the parallel operating traction substations suffer circulating current issue. A droop control combining with double closed-loop control is designed to deal with the problem. The correctness and feasibility of the system and its modulation and control strategies is verified through simulation and a small-scale experiment.
44 citations
Cites methods from "Circulating Current Suppression in ..."
...A circulating current suppression method based on even-harmonics repetitive control scheme is presented in [22]....
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TL;DR: In this article, the effect of the level-increased nearest level modulation (NLM) to the circulating current of the modular multilevel converter (MMC) is analyzed. And a circulating harmonic currents suppression method is proposed by applying deadbeat control for the MMC with level-increasing NLM.
Abstract: This article first analyzes the effect of the level-increased nearest level modulation (NLM) to the circulating current of the modular multilevel converter (MMC). The total inserted submodule (SM) number in one phase-leg of the MMC is variable using the level-increased NLM, which can significantly increase the peak-to-peak value of the circulating harmonic currents. Then, a circulating harmonic currents suppression method is proposed by applying deadbeat control for the MMC with level-increased NLM. Compared with the existing direct circulating current control approaches for the MMC, the proposed method presents specific extension and adjustment principle of the total inserted SM number. Therefore, the proposed method can improve the dynamic control performance and handle large circulating harmonic currents at steady state. And it can well coordinate the modulation and circulating harmonic currents suppression stages to avoid disturbing the ac-side output voltage while regulating the circulating current. The effectiveness of the proposed method is evaluated by a single-phase industrial-level simulated MMC system and a laboratory experimental platform.
35 citations
Cites background from "Circulating Current Suppression in ..."
...Indirect control approaches based on proportional integral (PI) controller [18], [19], nonideal proportional resonant (PR) controller [20], and repetitive controller [21], [22], are done by introducing a common mode component in the arm voltage references at the modulation stage [23]....
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TL;DR: A new control architecture comprising of a spatial comb filter and a spatial repetitive controller is proposed to effectively attenuate the even-order harmonics in the circulating current independent of the operating ac line frequency of the MMC.
Abstract: Circulating current control is one of the critical issues in modular multilevel converters (MMCs). When a frequency-droop-regulated MMC is used to integrate an offshore wind farm into an high-voltage dc transmission system, the variation in its operating ac line frequency induces a change in circulating current harmonic frequencies. Hence, the conventional circulating current controllers, such as proportional-resonant or repetitive controller, which are tuned to a specific frequency, fail to alleviate the circulating current harmonics. In this paper, a new control architecture comprising of a spatial comb filter (SCF) and a spatial repetitive controller (SRC) is proposed to effectively attenuate the even-order harmonics in the circulating current independent of the operating ac line frequency of the MMC. The proposed controller incorporates the phase sampling technique to achieve the dynamic change in the sampling frequency, which is the key to strong periodic disturbance rejection capability of the SCF and SRC even under variable frequency operation. A system level simulation model has been developed on PLECS simulation software to demonstrate the performance of the proposed control architecture. In addition, a scaled-down laboratory prototype of a 1-kW, 400-V, five-level single-phase MMC is developed and the experimental results are presented to substantiate the performance of the proposed control scheme.
31 citations
Cites background from "Circulating Current Suppression in ..."
...A repetitive controller (RC) has high periodic disturbance rejection capability and a single repetitive controller can suppress multiple harmonics [18]....
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References
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TL;DR: This paper first presents a brief overview of well-established multilevel converters strongly oriented to their current state in industrial applications to then center the discussion on the new converters that have made their way into the industry.
Abstract: Multilevel converters have been under research and development for more than three decades and have found successful industrial application. However, this is still a technology under development, and many new contributions and new commercial topologies have been reported in the last few years. The aim of this paper is to group and review these recent contributions, in order to establish the current state of the art and trends of the technology, to provide readers with a comprehensive and insightful review of where multilevel converter technology stands and is heading. This paper first presents a brief overview of well-established multilevel converters strongly oriented to their current state in industrial applications to then center the discussion on the new converters that have made their way into the industry. In addition, new promising topologies are discussed. Recent advances made in modulation and control of multilevel converters are also addressed. A great part of this paper is devoted to show nontraditional applications powered by multilevel converters and how multilevel converters are becoming an enabling technology in many industrial sectors. Finally, some future trends and challenges in the further development of this technology are discussed to motivate future contributions that address open problems and explore new possibilities.
3,415 citations
"Circulating Current Suppression in ..." refers background in this paper
...MODULAR multilevel converter (MMC) is one of the most attractive topologies in recent years for medium or high voltage industrial applications [1], such as high-voltage dc transmission [2], [3], medium voltage variable speed mo-...
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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
"Circulating Current Suppression in ..." refers background in this paper
...The basic structure and operation of an MMC have been extensively explained in the literature [11], [28] and will not be discussed in this paper....
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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
"Circulating Current Suppression in ..." refers background or methods in this paper
...Moreover, individual capacitor voltage balancing control [11] shown in Fig....
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...As the equivalent switching frequency of the MMC is 6fc = 12 kHz [11], the sampling frequency is designed to be fs = 12 kHz and the control system is synchronized with it, having a period of Ts = 1/fs ....
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...A simple proportional-integral (PI) controller based feedback control of the dc loop current is presented in [11], where obvious lowfrequency ac components can still be found in the differential current....
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...Such low-order harmonics not only increase the current stress of semiconductors and introduce more power losses in phase legs, but also, in turn, bring disturbances on the submodule (SM) capacitor voltage, which consequently deteriorate the performance of the MMC system [7], [9]–[11]....
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..., a dc reference idiff x dc∗ = mxIoxcos(φox)/4 for power balance, a current reference idiff x va ∗ for voltage averaging control [11] to maintain the SM capacitor voltages at the same level, and a fundamental frequency current reference idiff x vd∗ for the differential voltage control [6] between the upper and lower arms....
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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
"Circulating Current Suppression in ..." refers background in this paper
...According to (4) and (5), idiff x can be controlled by legitimately adjusting the voltages uux and ulx [8]....
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...A wellknown problem within an MMC system is that the differential current in phase legs may be distorted by even-order harmonics [6]–[8], and this is because of the inherent mismatch between the inserted voltage of each phase leg and the dc bus voltage....
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...Feedback controls in the d-q or rotating frames were proposed in [8], [18], and [19] for second-order harmonic suppression of MMC systems....
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TL;DR: In this article, the authors describe the control and operating performance of a modular multilevel PWM inverter for a transformerless medium-voltage motor drive, where the inverter is prominent in the modular arm structure consisting of a cascaded stack of multiple bidirectional choppercells.
Abstract: This paper describes the control and operating performance of a modular multilevel PWM inverter for a transformerless medium-voltage motor drive. The inverter is prominent in the modular arm structure consisting of a cascaded stack of multiple bidirectional chopper-cells. The dominant ac-voltage fluctuation with the same frequency as the motor (inverter) frequency occurs across the dc capacitor of each chopper-cell. The magnitude of the voltage fluctuation is inversely proportional to the motor frequency. This paper achieves theoretical analysis on the voltage fluctuation, leading to system design. A downscaled model rated at 400 V and 15 kW is designed and built up to confirm the validity and effectiveness of the nine-level (17-level in line-to-line) PWM inverter for a medium-voltage motor drive.
653 citations
"Circulating Current Suppression in ..." refers background in this paper
...tor drives [4], and static synchronous compensators [5]....
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...that is, a dc current Idc/3, assuming the dc source current Idc is evenly split into the three phases that ensures the power balance between the dc input and ac output [4], and a circulating current icirx that is dominated by even-order harmonics [7], [10]....
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