Operation Of Modular Multilevel Converter Research Articles

A capacitance reduction method of hybrid modular multilevel converter based on multi‐harmonic voltage injection

AbstractIn a modular multilevel converter (MMC) system, the sub‐module (SM) capacitors account for a large proportion of cost, volume and weight. Increasing modulation index (m) is an effective method to reducing the capacitance. Traditional MMC based on half‐bridge SM (HBSM) cannot boosting m while the hybrid MMC composed of HBSMs and full‐bridge sub‐modules (FBSM) can increase the m. The negative voltage is required when hybrid MMC works at the boosted m, which is only provided by the FBSMs because the HBSMs cannot output negative voltage. Therefore, appropriate control strategies are needed to achieve the normal operation of hybrid MMC under boosted m. This paper presents a control method suitable for hybrid MMC with high m based on multi‐harmonic voltage injection. The m is increased to 1.41 with the proposed multi‐harmonic voltage injection. Compared with the HBSM‐MMC, the proposed method reduces the total capacitance by more than 32%. Boosting m also reduces the RMS and peak value of the arm current by 18%. A comprehensive comparison was presented to demonstrate the cost and effect. Simulation and experiment verified the proposed method.

Local outlier factor based condition monitoring for sub-module capacitors in modular multilevel converters

Modular multilevel converters (MMC) as a key technology for future power grids is rapidly developing. The failure rate of sub-module (SM) capacitors is only lower than that of power devices in MMC, so accurate evaluation of the reliability of capacitors is crucial for the safe operation of MMC. Both the increase of equivalent series resistance (ESR) and decrease of equivalent series capacitance (ESC) are features for capacitor deteriorating. ESC or ESR of the capacitor reaching to limited values indicates that the end of its lifespan and it must be replaced. Most of contemporary researches only focus on ESC or ESR. This article considering the characteristics of both ESC and ESR, proposes a method for monitoring the status of SM capacitors based on machine learning algorithm: local outlier factor (LOF). By utilizing the existing SM switching function and capacitor voltage signals within a fundamental frequency cycle in the control system of MMC, the proposed strategy can not only monitor the faulty capacitors, but also can sort the degrees of abnormal states of capacitors, and the strategy is applicable to MMC with single and multiple types of SM topologies. The proposed method is verified by the hybrid MMC model on Matlab/Simulink.

A Decision Tree Based Ultra-high-speed Protection Scheme for Meshed MMC-MTDC Grids with Hybrid Lines

The reliable operation of modular multi-level converter based multi-terminal high voltage direct current (MMC-MTDC) grids requires high-speed and selective DC line protection. A single-ended protection technique is proposed for MTDC grids with offshore wind farms and hybrid lines that involve overhead lines and submarine cables in series. Positive and negative poles’ currents at one end of each line section are analyzed using discrete wavelet analysis (DWA). To classify fault type and identify fault zone, a fine decision tree is supplied with an energy index and the envelope slop of detail 1 coefficient (D1) obtained from DWA. Only 0.2 ms following the fault inception are needed for calculating the energy index and the envelope slope to perform relay functions. The approach was tested on a three-terminal two-poles ±\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\pm$$\\end{document} 400 kV MMC-MTDC model. The simulation results validate the effectiveness of the suggested protection technique under various fault scenarios, even with up to 200 Ω fault resistance. The proposed technique is not only able to detect the faulty line, but also it identifies overhead line faults and submarine cable faults for hybrid type lines. Moreover, the proposed technique is not affected by wind power injection changes, AC faults, or data noise. The simulated model was produced in the MATLAB/Simulink platform.

Circulating current suppression and natural voltage balancing using phase-shifted modulation for modular multilevel converter

The challenge of achieving a balanced capacitor voltage is one of the factors affecting the efficient operation of modular multilevel converters (MMC). This paper investigates this challenge through a proposed method that utilizes a high carrier frequency phase-shifted pulse width modulation (PS-PWM) scheme. This method aims to achieve natural balancing without the need for any additional control mechanisms. Moreover, the number of output voltage levels is affected by the phase shift between the carriers of the upper and lower arms. When there is no phase shift, N<sup>+1</sup> discrete levels are achieved, but when there is a phase shift, the number of discrete levels increases to 2<sup>N+1</sup>. The proportional-resonant (PR) controller and moving average filter (MAF) are employed to decrease the capacitor voltage ripples by suppressing the fourth and second harmonics in the circulating currents. The MMC inverter structure is modeled and simulated in the PLECS and MATLAB/Simulink environments to evaluate the impact of this control scheme on the converter’s performance.

Experimental Assessment of a Decentralized Control Strategy for a Back-to-Back Modular Multilevel Converter Operating in Low-Frequency AC Transmission

The Modular Multilevel Converter (MMC) has been widely used in high-power applications owing to its inherent advantages, including scalability, modularity, high-power density, and fault tolerance. MMCs have recently been used in Low-Frequency Alternating Current (LFAC) transmission, particularly in the integration of offshore wind power with onshore grids. However, LFAC applications produce significant voltage oscillations in floating capacitor voltages within the MMC. Early research efforts have successfully established and validated decoupled control strategies for LFAC-based MMC systems. However, validations are usually based on simulations or small-scale prototypes equipped with limited power cells. Consequently, this paper presents a decentralized voltage control strategy based on Nearest Level Control for an MMC-based LFAC system. Experimental results obtained with a 120-cell MMC prototype are presented to validate the effectiveness and operation of the MMC in LFAC applications.

A Dual-Ring Control Structure for Enhancing the Fault-Tolerance Ability Under Communication Interruption in MMC

The control structure of a modular multilevel converter (MMC) consists of a central controller and numerous local controllers with complex communication links. However, the communication interruption may lead to MMC operation failure or frequent bypassing of submodules (SMs). Although the fault-tolerance can be improved by adding extra communication link for each local controller in centralized star control structure, it will greatly increase the cost considering the large number of SMs. In this article, a novel dual-ring control structure based on consensus algorithm is proposed to enhance the fault-tolerance ability of MMC to the communication interruption. Compared with the star control structure, control tasks in central controller, such as capacitor voltage control and circulating current control, can be reasonably allocated to all local controllers. Each local controller only needs to communicate with adjacent local controllers to obtain the average value of specified information. Two types of communication interruptions, occurring between central controller and local controller or among local controllers, are analyzed respectively with the corresponding redundancy design implemented. Simulated and experimental results show that proposed control structure greatly enhance the fault-tolerance ability to communication interruption as well as effectively reduces the computational burden of central controller compared with star control structure.

Two‐stage deadbeat‐based predictive torque control strategy for modular multilevel converter‐fed three‐phase induction motors

AbstractRecently, Modular Multilevel Converters (MMC) are considered to be extensively used in medium‐voltage AC drives A two‐stage deadbeat‐based strategy is developed by the authors to make the implementation of Predictive Torque Control (PTC) technique for an induction motor that is fed by a three‐phase MMC possible . Despite various benefits of PTC strategy, it has not been widely employed for multilevel topologies due to the huge number of switching states of these converters, and the complex internal dynamics of topologies such as MMC. The proposed control scheme results in significant reduction of the computational burden, using an initial reference voltage that is determined at the first stage of the strategy by the deadbeat technique. Next, the predictive evaluation process is conducted at the second stage to simultaneously control the stator flux, electromagnetic torque and circulating current. The number of admissible vectors that should be evaluated in the cost function is effectively limited by only considering some adjacent vectors of the reference voltage in the first stage. Moreover, the circulating current control capability, which is necessary for effective operation of MMC, is also considered in the developed scheme. Finally, the satisfactory performance of the proposed strategy is validated through various experimental tests.

Phase Delay Compensation and Average Capacitor Voltage Based Currentless Voltage Balancing Methods for Modular Multilevel Converters

Voltage balancing algorithms (VBAs) are vital for the reliable operation of modular multilevel converters (MMCs). The existing VBAs depend on arm currents to adjust the capacitor voltages. However, the repetitive transmissions of arm currents in the hierarchical control system of MMCs increase communication burdens and failure risks. In this article, the performance of an existing currentless VBA is improved and a new currentless VBA is proposed. Firstly, the phase delay compensation is added to improve the currentless VBA with the filter. Then, in the proposed VBA, the polarity of an arm current is reflected by the minimum capacitor voltage. To eliminate the disturbance of measurement noises, the average of the minimum capacitor voltage is further utilized in the proposed VBA method. Compared with the existing currentless VBAs, both the improved and proposed methods can provide more similar performances as the conventional VBA with the sampling arm current. Meanwhile, the improved and proposed methods are both immune to the measurement noises and robust to several types of disturbances in MMC. Finally, the feasibilities of the two methods are verified by the simulation studies with the professional tool Matlab/Simulink.

A Space Vector Based Zero Common-Mode Voltage Modulation Method for a Modular Multilevel Converter

A modular multilevel converter (MMC) can generate different common-mode voltage (CMV) values due to the high-frequency changing of the switching state under various modulation strategies. The high-frequency dv/dt will produce common-mode current in the equivalent common-mode loop to the ground, which will affect the insulation and shorten the life of the equipment. To eliminate the effect of common-mode voltage on MMC operation, a common-mode voltage elimination strategy (0CMV-SVPWM) is proposed for five-level MMC space vector pulse width modulation (SVPWM) by using the vector that does not generate common-mode voltage as the reference vector in this paper. The proposed strategy is studied and analyzed by the rapid prototype development experimental system based on RT-LAB to verify the feasibility and effectiveness of the proposed strategy.

Inverse Application of Artificial Intelligence for the Control of Power Converters

This article proposes a novel application method, inverse application of artificial intelligence (IAAI) for the control of power electronic converter systems. The proposed method can give the desired control coefficients/references in a simple way because, compared to conventional methods, IAAI only relies on a data-driven process with no need for an optimization process or substantial derivations. Noting that the IAAI approach uses artificial intelligence to provide feasible coefficients/references for the power converter control, rather than building a new controller. After illustrating the IAAI concept, a conventional application method of artificial neural network is discussed, an optimization-based design. Then, a two-source-converter microgrid case is studied to choose the best droop coefficients via the optimization-based approach. After that, the proposed IAAI method is employed for the same microgrid case to quickly find good droop coefficients. Furthermore, the IAAI method is applied to a modular multilevel converter (MMC) case, extending the MMC operation region under unbalanced grid faults. In the MMC case, both simulation and experimental online tests validate the operation, feasibility, and practicality of IAAI.

Circulating current suppression in low frequency operation of modular multi-level converters

Circulating current suppression in low frequency operation of modular multi-level converters

Circulating Current Suppression in Low Frequency Operation of Modular Multilevel Converters

Circulating Current Suppression in Low Frequency Operation of Modular Multilevel Converters

A Hierarchic Capacitor Condition Monitoring Strategy for High-Voltage Modular Multilevel Converters

The capacitor condition monitoring is an important issue for the reliable operation of modular multilevel converters (MMCs) in high-voltage applications. The numerous capacitors make the monitoring for the capacitor condition complex and computation-consuming. This paper proposes a hierarchic capacitor condition monitoring strategy to assure the reliability of high-voltage MMCs. In the strategy, the switching times of the switching signals are counted to detect the condition of the capacitor in each submodule (SM). Then, the subsequent capacitance calculation method is only conducted to the abnormal capacitors indicated by the above condition detection algorithm. Meanwhile, the variations of the measured and reference capacitor voltages are employed to calculate the capacitance in the proposed method. The condition detection algorithm shrinks the scope where the abnormal capacitors locate, which avoids calculating the total capacitances in MMCs. In the proposed capacitance calculation method, the complex operations are further saved. Both the two measures reduce the computation burden effectively. The implementation of the whole monitoring strategy is simple and fit for high-voltage MMCs. Moreover, the strategy has no adverse influence on the normal operation of MMCs. The effectiveness of the proposed strategy is verified by the simulation studies with the professional tool Matlab/Simulink.

Analysis of Capacitor Voltage Unbalance in Hybrid MMC and Its Novel Operation With Reduced Submodule Capacitance

The hybrid modular multilevel converter (MMC) consists of both half-bridge submodules (HBSMs) and full-bridge submodules (FBSMs) in the same arm. The negative insertion of FBSMs allows it to generate higher ac output voltages for a given dc-link voltage than a traditional MMC with only positive insertion. However, the negative voltage in an arm forces HBSMs to bypass as they can only generate positive voltage. The higher the negative voltage, the larger is the duration for which HBSMs are bypassed. This results in an unequal distribution of voltage among the submodules, which further leads to unbalance in their capacitor voltages beyond a certain operating point. This article analyses voltage distribution among the submodules to evaluate the limiting operating point of hybrid MMC beyond which capacitor voltages cannot be balanced. Additionally, a novel operation of hybrid MMC is also proposed, where the arm voltage is generated as a sum of fixed negative voltage and a varying positive voltage, such that the average capacitor voltage increases, resulting in a significant reduction of submodule capacitance required to transfer the same power with the same arm energy as that in the conventional operation. The proposed scheme is verified with detailed simulation as well as experimental results.

Implementation of a Modular Distributed Fault-Tolerant Controller for MMC Applications

Centralized control algorithm limits the hardware flexibility of a modular multilevel converter (MMC). Therefore, distributed control structure has recently started to be seen in the industry application. Even though distributed controller reduces a single point of failure risk compared to the centralized controller, the failure risk of the entire control systems increases due to the number of local controllers. However, the distributed controller can be programmed in such a way as to replace the faulty local controller and sustain the MMC operation. In this paper, the distributed modular fault-tolerant controller is implemented in a laboratory-scale MMC prototype. The controller is built to control four SMs per phase for the proof-of-concept. Therefore, the MMC prototype is also built by two SMs per arm. The controller capability is validated with experimental and the Opal-RT result-time simulator results in a control-hardware-in-loop (CHIL) environment.

Improved CPS-PWM Approach for Over-Modulation Operations of Hybrid Modular Multilevel Converter

The over-modulation capability of the hybrid modular multilevel converter (MMC), mixed by half-bridge and full-bridge submodules (SMs), has acquired widespread attention recently, which can significantly increase the design freedom of the hybrid MMC. When the hybrid MMC is applied in the medium-voltage occasions, an improved carrier phase shift pulsewidth modulation (CPS-PWM) approach is put forward in this article for its over-modulation operation under the nominal and variable dc conditions. It is featured with high flexibility, adjustability, and simple implementation. A new standard voltage is introduced to normalize the modulation wave, by which there is always a sufficient margin between the peaks of the normalized modulation wave and carrier waves under any modulation indexes. Also, it can perform well when the nominal dc voltage of the hybrid MMC is not the integer multiple of SM voltage. Moreover, the carrier wave arrangements are decoupled with the number of SMs, dc voltage, and ac voltage, so the variable dc operation of the hybrid MMC can be achieved without rearranging the carrier waves. Finally, the simulations and experiments are carried out to confirm the effectiveness of the proposed CPS-PWM approach for over-modulation operations under the steady-state and variable dc conditions.

Unified Strategy for Fault-Tolerant Operation of MMC with Multiple SMs Failure Based on SMs Grouping Management

Modular multilevel converter (MMC) is distinguished by its modularity. In order to improve its reliability and avoid unscheduled maintenance, it requires the MMC to continue operating even though some of its submodules (SMs) have failed. In this paper, a grouping management method of SMs in MMC is first presented where every six SMs are conceptually grouped into a virtual subunit (SU), and on this basis, a simplified space vector modulation (SVM) implementation is developed. Then, the fault-tolerant solutions are proposed by investigating the post-fault operation of the MMC based on the virtual SU concept. It is analyzed that the SU failures can be categorized into three basic types, according to the phase where the failed SMs are located, and also multiple SM failures can be decomposed into one or multiple basic fault types. In this way, the fault-tolerant solutions are unified regardless of the number of faulty SMs. Further, the sorting capacitor voltage control is combined into the proposed methods to balance all of the SM capacitor voltages. With the proposed fault-tolerant method, the amplitude and THD of the line voltages are almost unchanged under fault conditions when compared with normal operations. The simulation and experiment verify the propositions.

An Efficient Sorting Algorithm for Capacitor Voltage Balance of Modular Multilevel Converter With Space Vector Pulsewidth Modulation

Thisarticle presents an efficient analogue sorting algorithm for balancing the submodule (SM) capacitor voltages of modular multilevel converter (MMC). The proposed analogue sorting algorithm offers the advantage of fast convergence rate without any need of recursive loops for the implementation on embedded devices. It can be easily implemented with combinational logic operations on field programmable gate array (FPGA) and provides less hardware and computational overhead. The functionality and performance of the proposed analogue sorting algorithm is evaluated with the simulation model of three phase five-level MMC in MATLAB/Simulink environment. The real time implementation of the proposed sorting algorithm with the SM capacitor voltage balancing strategy is implemented on Altera/Cylone - I (EP1C12Q240C8N) FPGA. A five-level continuous space vector pulsewidth modulation (CSVPWM) is realized on a PIC microcontroller (PIC18F452). A down-scaled model of single-phase five-level MMC is designed and constructed to investigate the reliable and stable operation of MMC with the proposed analogue sorting algorithm and SVPWM method. Simulation and experimental results are presented for validation.

Power Loss Reduction Control for Modular Multilevel Converters Based on Resistor Controllable Submodule

A bleeding resistor is usually adopted to be parallel with the capacitor in the submodule (SM) to avoid static voltage imbalance caused by auxiliary power supply during start-up process of modular multilevel converters (MMCs). However, the bleeding resistor causes unnecessary power loss during normal operation. In this article, a resistor controllable SM based on a normally- <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on</small> silicon carbide (SiC) junction field-effect transistor is proposed to reduce the power loss on bleeding resistor at normal operation of MMCs. The proposed control can not only ensure the static voltage balance during start-up of MMCs, but can also completely eliminate the power loss on bleeding resistor during normal operation of MMCs for high efficiency. In addition, the proposed control can shorten the SM capacitor discharging time at shut-down process. Experimental studies are conducted and the results confirm the effectiveness of the proposed control.

Suppression of third‐order harmonic current in transformerless MMC

The modular multilevel converter (MMC) based high voltage DC (HVDC) system can be effectively used for bulk power transmission. The MMC topology for the voltage source converter (VSC) has several advantages. In this work, the transformerless operation of MMC is explored. The internal dynamic of MMC can induce a third-order zero-sequence harmonic current. Its effect on the system is analysed, including the adverse impacts on energy requirement per arm and power transfer capability. The internal dynamic equations of the transformerless MMC configuration are derived, and two different controllers: the proportional-resonant (PR) controller and proportional-integral (PI) controller, were applied to suppress the unwanted third-order current. The performance analysis of these controllers is presented and the results indicate that the controllers efficiently suppress the third-order harmonic current. Moreover, the electromagnetic transient (EMT) models of MMC under different configurations have been developed on the real-time digital simulator (RTDS) platform. An analysis of internal variables of the MMC is also included to ensure that the controller does not adversely affect the system. Lastly, the state-space model is developed, and the stability is analysed.