A Modified Modular Multilevel Converter With Reduced Capacitor Voltage Fluctuation

The modular multilevel converter (MMC) has unique topological characteristics and has gained significant popularity in medium-voltage applications. However, during MMC operation, circulating currents inevitably arise, exacerbating arm current distortion, causing additional losses in the converter system, and thereby increasing system costs. This paper primarily addresses the circulating current issue in traditional half-bridge MMCs by introducing a control strategy combining model predictive control and proportional resonance controllers. First, a value function is established using a discrete prediction model of the output current, followed by a rolling optimization that combines a capacitor voltage sorting strategy to determine the duty cycles of each submodule in the arm. Secondly, a proportional resonance controller (PR) is designed to eliminate the second- and fourth-order harmonic components in the circulating current. Finally, the output of the resonance controller is used to correct the duty cycles, which are then compared with the PWM triangular carrier to generate more switching actions that suppress the circulating current. The effectiveness of the strategy is experimentally verified. The results show that the proposed method yields better output characteristics, smaller capacitor voltage fluctuations, and significantly suppresses harmonic components in the arm currents.

Modular multilevel converters (MMCs) have become one of the most foremost power converter topology for medium and high power applications in recent years. In this paper, single-phase traditional and hybrid MMC are designed, simulated and compared under various voltage levels. Output voltage and current waveform are analyzed in terms of total harmonic distortion by changing modulation index at operated 7-level, 19-1evel, 31-level and 43-level traditional and hybrid converter systems. Both types of MMCs are investigated under different conditions. At the end of the study, simulation results are deeply presented with different cases and evaluated to show the performances of the designed circuit topologies.

Modular multilevel converter (MMC) has been widely used in high-voltage direct current (HVDC) transmission system due to its advantages of modularity, low switching frequency and excellent output voltage waveforms. Proper operation of MMC is based on capacitor voltage balancing. Conventional MMC capacitor voltage balancing control needs a large number of voltage sensors and sorting algorithm which increases computing burden on the processor. In this paper, a capacitor voltage self-balancing MMC with DC fault ride-through capability is proposed. Capacitor voltage balancing control of this proposed MMC is significantly simple due to its self-balancing auxiliary circuit. Sorting algorithm is not needed, each phase leg only needs one voltage sensor for top SM. So control complexity and computing burden are reduced greatly. Furthermore, DC fault ride-through capability is also reserved. Simulation results validates capacitor voltage of this new MMC topology can be well balanced and DC fault ride-through capability is effective.

The second-harmonic injection strategy for modular multilevel converters (MMCs) can effectively suppress the capacitor voltage fluctuation in steady states. However, the capacitor voltage fluctuation under grid-side single-line-to-ground (SLG) faults will be more severe. Moreover, there is no research on the mitigation of capacitor voltage fluctuation under grid-side SLG faults for the hybrid MMC with mixed half-bridge and full-bridge sub-modules. To address this issue for hybrid MMCs, an optimal second-harmonic current injection scheme for mitigating the capacitor voltage fluctuation in the hybrid MMC under grid-side SLG faults is proposed in this paper. The mathematical models of arm power fluctuations of the hybrid MMC are firstly developed under both steady states and grid-side SLG faults. Then, the principle and optimal strategy of the proposed capacitor voltage fluctuation strategy are presented. The capacitor voltage fluctuation under grid-side SLG faults can be reduced by suppressing the three-phase arm power fluctuations. A hybrid MMC model with the proposed scheme has been established in PSCAD/EMTDC to verify its effectiveness and feasibility. Simulation results show that the average voltage fluctuation reduction rate can reach 55.77% with the proposed strategy.

SummaryThe Modular Multilevel Converter (MMC) has gained the widespread application in the DC power transmission, owing to the advantages such as modularity and low output current harmonic content. However, its adoption in variable‐speed pumped storage systems remains limited due to the significant voltage fluctuations in its sub‐module capacitors during the low‐speed operation. In this paper, a capacitor voltage ripple suppression method employing bidirectional‐switching channel is proposed. It aims to improve the driving performance of MMC when the variable‐speed pumped storage motor operates at low speed. The proposed method creates bidirectional‐switching channels between the sub‐modules of the upper and lower bridge arms and moves the sub‐module capacitors that originally fixed to the upper and lower bridge arms into the channels. By controlling the bidirectional‐switching channel, the same capacitor can be put into the sub‐modules of different bridge arms. At the same time, a threshold switching control method is proposed to reduce the switching loss. Without introducing the common mode voltage, the capacitor voltage fluctuation can be substantially suppressed when the pumped storage motor operates at low speed. Experimental and simulation results verify the effectiveness of the proposed method.

Modular Multilevel Converter (MMC) bridge arm circulating current increases the MMC bridge arm current distortion and bridge arm current stress, whereas it also increases converter losses and capacitor voltage fluctuations. In this paper, based on the average value of the switching function of MMC, the mathematical relationship between the second harmonic component of the Bridge Arm Common-Mode Voltage (BACMV) and the Circulating Current Suppressing Controller (CCSC) is derived in the d <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> q <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> rotating coordinate system. It proposed an MMC's CCSC based on BACMV. The proposed controller consists of two PI controllers, namely the outer controller and the inner controller. The inner controller suppresses the common-mode voltage, and the outer controller eliminates the second harmonic component of the bridge arm common-mode voltage. The MMC-HVDC simulation model is established, and PI-CCSC, PR-CCSC, and BACMV-CCSC are used for comparative study. The results show that the overshoot of PRCCSC is the largest and that of BACMV-CCSC is the smallest in response transient process. After entering the steady-state, the fluctuation of BACMV-CCSC is the smallest, whereas that of PI-CCSC and PR-CCSC is larger. The harmonic component of the secondary current of BACMV-CCSC is 0.08% and THD content is 0.39%, which is significantly lower than the other two suppression controllers. When the load changes abruptly, compared with the dynamic simulation, BACMV-CCSC has a smaller overshoot in the transient response process and has less disturbance after reaching the steady-state, which improves the steady-state performance of the system and the circulating current suppression controller effect, and provides a new idea about MMC circulating current suppression controller.

The modular multilevel converter (MMC) is one of the most promising topologies for high- and medium-voltage applications. However, the conventional MMC relies on large arm inductance and complicated feedback control to maintain submodule (SM) capacitor voltages balanced, which requires a large number of voltage sensors and complicated arm current control loops. In contrast to the conventional perspective, the MMC can be regarded as a derivative from the general multilevel topology based on switched-capacitor (SC) structure. The SC structure endows excellent properties to the MMC, such as minimum or no arm inductor and automatic SM capacitor voltage balancing capability. This MMC with minimum or no arm inductor is named as SC-MMC. Since the arm inductances are very small, the SM capacitor voltages can be balanced automatically. Small stray/parasitic inductance is enough to limit inrush current during SM's switching instants to the rated current level. To unleash the automatic SM capacitor voltage balancing capability of the SC-MMC, a Y-matrix modulation (YMM) has been proposed, which does not require closed loop control or any sensor, has no limitations on the number of SMs, and works for both half-bridge and full-bridge SMs. An unresolved issue of the emerging YMM is that, in the published literature, the existence of the Y matrix with full rank has not been proved theoretically beyond the observation for the SC-MMC when the number of SMs is large. In this paper, an analytical proof of the existence of the full rank Y matrix is provided, which implies the automatic voltage balance of SMs for the MMC with arbitrary number of SMs in each arm. Simulation results using the YMM on a 21-level SC-MMC with minimum arm inductance are provided to verify the voltage balance theory/observation of the SC-MMC using the YMM.

Accurate and efficient evaluation of losses in modular multilevel converters (MMC) is of great significance for the design and reliable operation of high‐voltage direct current transmission systems. In this paper, an efficient online loss evaluation method for MMC is proposed. First, a switching frequency surface is constructed based on the determined modulation and capacitor voltage balancing strategy, which enables the estimation of the switching frequency under any steady‐state operating condition. Then, an arm average value model (AAVM) is employed to estimate the upper bounds of the switching frequency and associated switching losses. Moreover, combined with the switching frequency surface, an approximate and efficient estimation of the actual switching losses is achieved, which ensures a balance between computational accuracy and efficiency. In addition, the thermal network model is employed to represent the temperature‐dependent characteristics of losses. Considering the impact of losses on simulation results, the energy absorbed by a controlled voltage source in series with the arm is used to characterize the losses. Subsequently, a switching loss injection method based on a decay function is proposed, which mitigates voltage spikes and enhances the effectiveness of loss injection. Finally, an AAVM considering loss injection is developed in PSCAD to verify the computational efficiency and accuracy of the proposed method.

This paper discusses a novel isolated three-phase AC-DC converter using Modular Multi-level Converter (MMC) topology for a medium voltage application. The MMC at a primary stage of an isolated AC-DC converter can directly convert a three-phase AC into a high-frequency AC voltage of 1 kHz. In this paper, the features of the proposed circuit are discussed according to the voltage ratings of the switching devices. Moreover, the total harmonic distortion (THD) of the input current, the output voltage, the number of cells and the capacitance of the DC capacitor are evaluated on each switching devices in the proposed system. Finally, the proposed system performance with SiC MOSFET of 3.3 kV is demonstrated by the simulation. As a result, the proposed converter achieves the low THD of the input current waveform which is approximately 2.64%.

Summary form only given. 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.

This paper proposes a modular multilevel converter (MMC) topology, designed particularly for load balancing in 50 Hz railway applications. The asymmetrical character of the traction load requires higher device rating for a Double-star or Delta-connected MMC compared to a symmetrical static compensator. This is due to the symmetrizing components to rebalance the capacitor voltages in the submodules. In this paper a reduced chain-link MMC topology, consisting of a two phase-leg converter, is proposed in order to avoid overrating the semiconductors in the main current path. In addition, the main circuit is equipped with dc-dc dual half-bridge (DHB) converters that connect the submodule capacitors of the two legs. The DHBs are operating as “Power Channels,” enabling energy exchange to restore the power imbalance among the capacitors. Simulation results show that the proposed converter is capable of achieving load balancing under any operating conditions, while the balance of the capacitor voltage is ensured.

This paper presents a new, simple and generalized approach to calculate flying capacitor currents for an n-level flying capacitor converter without the use of look-up tables. The performance of the proposed method has been studied for a three-, four- and five-level flying capacitor converter in MATLAB/Simulink environment.

Unlike the modular multilevel converter (MMC) topology operated under the rectifier or inverter modes, control of the balanced state for the submodule (SM) capacitor voltage has emerged as the key issue for DC-DC MMCs. This is mainly caused by no balanced alternative powers being used for balancing SM capacitor voltages, which can be absorbed from the input or output DC sides of the converter. Typically, the alternative voltages and currents should be injected to achieve SM capacitor voltage balance in the DC-DC MMC. However, this solution is based on the cost of adopting the bulky LC filter components. For interconnecting different DC voltages in medium-voltage applications, this paper presents a non-isolated DC-DC MMC equipped with the proposed middle cells. It is intended to achieve DC voltage conversion without adopting bulky passive LC filters. On the one hand, the alternative currents, used for balancing the SM capacitor voltages, are arranged for flowing only within the phase legs of the proposed DC-DC MMC without disturbing the input current. On the other hand, through appropriate control of the middle cells, compensated components can be developed to eliminate the undesirable voltages on the output DC side. The middle cells of the proposed DC-DC MMC are supplied with the function of the active filter, which enables the DC-DC MMC system to escape the bulky LC components. Through theoretical analysis and a control strategy, the proposed DC-DC MMC has been analyzed comprehensively. Finally, the simulation and experimental results are verified to evaluate the effectiveness of the proposed DC-DC MMC.

This work deals with the sizing of submodule capacitors for modular multilevel converters (MMCs) used for variable speed drives for medium voltage and high voltage electric machines. Variable speed drives are required to run over a wide frequency range from zero to a few hundred Hz or even over 1 kHz. Under low frequency operation (<30 Hz), a large current ripple flowing through the MMC submodule capacitors will cause significant capacitor voltage fluctuation. This article proposes an optimal submodule capacitor selection method based on the common-mode voltage injection. The proposed sizing model includes two functions: 1) accurately estimating the capacitor voltage ripple based on the drive system parameters; and 2) identifying the minimum capacitance of the submodule capacitors based on the allowed maximum capacitor voltage fluctuations. Simulation and experimental results show that the error between the actual and estimated values of the voltage fluctuation is within 3% using the proposed model.

Modular Multilevel Converter (MMC) is an emerging converter topology for medium and high voltage applications. Nearest level Modulation (NLM) is the conventional control topology used to control the MMC that produces the N+1 AC output waveform. In previous research work, the Modified NLM has been already proposed, producing a 2N+1 and 4N+1 output waveform while utilizing a half-bridge (HB) submodule (SM) topology. However, half-bridge-based MMC has a similar behavior as two-level Voltage Source Converter (VSC) and cannot block DC fault current in case of DC-side short circuit fault. So, in recent years, full-bridge-based MMC topology is preferably used by manufacturers as it has DC fault blocking capabilities. This paper presents the Modified NLM for Full bridge (FB) SM topology to take the critical benefits of FB SM topology and improve power quality. The proposed method is simpler to implement and produces a 4N+1 AC output waveform. The THD of the output voltage and current reduces to half compared to the conventional NLM method. The proposed method is verified using LabVIEW Multisim co-simulation and as well as real-time simulation.