Search Results (459)

As renewable energy is increasingly integrated via high-voltage direct current (HVDC) transmission, hybrid multi-infeed receiving-end grids containing both line-commutated converters (LCC) and modular multilevel converters (MMC) have become common, and wideband resonance problems in power-electronized networks are growing more prominent. This paper proposes an active resonance analysis and suppression strategy for such systems. First, a wideband current source converter model and a wideband voltage source converter model are adopted to describe the LCC and MMC, respectively, and a positive-sequence s-domain model of the system is established. A two-stage s-domain nodal admittance matrix method is then applied to efficiently determine the wideband resonance modes and the corresponding mode shape eigenvectors. A dual criterion combining the matching degree between resonance frequencies and LCC characteristic harmonics with the modal damping ratio identifies high-risk resonance modes. On this basis, an active damping strategy that realizes a parallel virtual resistance on the AC side through MMC supplementary control is proposed, together with a quantitative design method for the virtual conductance. At the control implementation level, a modulation wave reconstruction bypass injection scheme superimposes the high-frequency damping command directly in the $\alpha \beta$ stationary reference frame, thereby bypassing the PI controller and reducing the amplitude attenuation and phase distortion caused by the high-frequency limitation of the integral path. PSCAD/EMTDC simulation results on an IEEE 9-bus test system demonstrate that the proposed strategy effectively suppresses resonance amplification and wideband power oscillations excited by LCC characteristic harmonics without affecting the fundamental power transmission. Full article

In receiving-end modular multilevel converter (MMC)-based flexible high-voltage direct current (HVDC) grid-connected systems, high-frequency oscillation can significantly increase the peak values of the point of common coupling (PCC) voltage and grid current. To address this issue, this paper proposes a coordinated suppression strategy for high-frequency oscillation in receiving-end MMC grid-connected systems. First, an MMC impedance model is established based on harmonic linearization, and its frequency-domain interaction with the grid impedance is analyzed to clarify the formation mechanism of high-frequency oscillation and its main influencing factors. Then, considering the different roles of the voltage feedforward and current feedback channels in the target frequency band, a coordinated suppression strategy combining band-stop filtering in the voltage feedforward path with low-pass filtering and lead compensation in the current feedback path is designed. Hardware-in-the-loop experimental results show that the proposed method effectively identifies and suppresses high-frequency oscillation. Under the validated operating condition, the oscillation-induced peak increases in the PCC voltage and grid current are limited to within 20% and 12.5%, respectively, thereby suppressing further oscillation growth and reducing the risk of approaching the overvoltage and overcurrent protection thresholds. Full article

The integration of energy dissipation units into the Modular Multilevel Converter (MMC) enables surplus power dissipation without the need for additional energy dissipation devices outside the converter valve, thereby reducing the cost of the surplus power fault ride-through (FRT) solution. The existing topology integrates energy dissipation units in all submodules (SMs) of the MMC, increasing the difficulty of modification. To address this issue, this paper proposes a single-phase integrated configuration of energy dissipation units, in which only the SMs in the outermost phase unit of the converter valve need to be modified. This configuration simplifies on-site construction and facilitates equipment operation and maintenance. This paper studies the control strategy of the topology and determines the corresponding parameters, which can reduce the number of energy dissipation units that need to be retrofitted while suppressing voltage fluctuations. The feasibility of the scheme is verified through simulations and experiments. Finally, a technical and economic analysis is conducted in terms of the investment cost and control performance of the single-phase MMC with integrated energy dissipation equipment (IEDE-MMC). Compared with the three-phase IEDE-MMC, the investment cost is reduced by 33%, the DC voltage fluctuation range is reduced by 66.7%, and the lower limit of the SM capacitor voltage is raised by 10.8%. Full article

This paper addresses the capacitor voltage balancing issue of submodules (SMs) in Modular Multilevel Converters (MMCs) operating under low switching frequencies by proposing a voltage balancing control strategy based on dynamic threshold adjustment. First, a dynamic model of SM capacitor voltage in MMCs is established, and the causes of capacitor voltage imbalance are analyzed. Then, based on the coupling relationship between switching frequency and voltage balancing, and the imbalance model under dynamic operating conditions, a dynamic threshold adjustment strategy is designed. A Fuzzy Logic Controller (FLC) is employed to dynamically adjust the voltage imbalance threshold in real time, ensuring capacitor voltage balance while optimizing the switching frequency and reducing system losses. Simulation results show that the proposed strategy can effectively maintain SM capacitor voltage balance under low-switching-frequency conditions, thereby improving system stability. Full article

The increasing penetration of inverter-based renewable energy resources, especially photovoltaic (PV) systems, has decreased the available system inertia and introduced challenges in maintaining stable grid-forming operation. This paper presents a grid-forming photovoltaic multilevel inverter (MLI) with a modified delta-connected cascaded H-bridge (CHB) multilevel configuration. The proposed system decreases the number of semiconductor switches and provides inherent voltage balancing, while also achieving high power quality, rendering it suitable for grid-forming applications. Each H-bridge cell is connected to an isolated Cúk converter to enable maximum power point tracking (MPPT) of distributed PV modules, allowing for flexible and modular DC-side integration. The proposed MLI operates as a virtual synchronous generator. A control scheme is proposed to attain grid-forming capability, hence providing stable voltage and frequency support. Moreover, a DC-link voltage regulation strategy is also developed to maintain the DC-link voltage at the reference voltage. A detailed mathematical model is developed to characterize the associated dynamics of the proposed MLI and the control system with a grid interface. The model is built in the SIMULINK environment, and the simulation results are presented under variations in solar radiation and grid voltage disturbances to exhibit the functionality of the proposed system and the effectiveness of the control scheme in providing a well-damped frequency response and stable generated voltage and currents. The results demonstrate stable frequency regulation with a settling time of approximately 0.3 s, and the output current exhibits low harmonic distortion, with a Total Harmonic Distortion (THD) of about 0.53%. Simulation results show stable operation and confirm that the proposed approach is a competitive solution for PV-based grid-forming applications. Full article

In practical digital control systems for Modular Multilevel Matrix Converters (M3C), the inherent delays caused by signal sampling and algorithm execution lead to pulse output lags in conventional deadbeat control, subsequently resulting in current tracking deviations and grid-side current distortions. To address this issue, an improved deadbeat inner-loop control strategy based on current state prediction is proposed in this paper. First, the mathematical model of the M3C in the abc coordinate system is established, and a dual αβ0 coordinate transformation is introduced to decouple the system, achieving independent control of the input-side, output-side, and internal electrical quantities. Subsequently, to eliminate the control error caused by the one-step delay, an interpolation prediction method is employed to equivalently evaluate the voltage variation within an ultra-short control period. By predicting the current reference state at the k + 2 instant, the deadbeat control equation is modified to achieve delay compensation. Simulation and experimental results demonstrate that the proposed strategy effectively overcomes the delay limitations inherent in conventional digital control, significantly enhancing the dynamic current tracking accuracy and waveform quality of the M3C system. Full article

To address the insufficient frequency-support capability, the difficulty of multi-terminal power coordination, and the constraints on DC-voltage fluctuations in flexible DC transmission systems under weak-grid interconnection, this paper conducts a simulation-based control strategy study. First, based on the coupling relationship between AC frequency and DC voltage, an inertia-enhanced grid-forming/VSG control method is proposed, enabling converter stations to use DC-link capacitor energy to provide transient frequency support during the initial stage of a disturbance. Second, for multi-terminal flexible DC systems, an adaptive U-P-f dual-droop distributed control strategy is designed to coordinate unbalanced power sharing among multiple converter stations and to limit the DC-voltage deviation generated during frequency support. In this paper, a hybrid half-bridge/full-bridge MMC is adopted as a fixed-converter simulation platform, rather than being treated as an object of systematic topology optimization. Finally, a four-terminal MMC-HVDC simulation model is established in MATLAB/Simulink, and the proposed control strategy is evaluated under weak-grid step-load disturbances, different short-circuit-ratio conditions, and continuous pseudo-random load disturbance scenarios. Simulation results show that, under the tested operating conditions, the proposed method can reduce the maximum frequency deviation, suppress DC-voltage fluctuations, and improve the power-sharing process among multi-terminal converter stations compared with conventional VSG control and fixed-droop control. Full article

In a Modular Multilevel Converter-based Battery Energy Storage System (MMC-BESS), the battery current exhibits significant ripples during normal operation. Traditional circulating current injection suppression methods face limitations, including imprecise modeling, reliance on curve fitting, and non-global optimal injection values. To address these issues, this paper proposes a nonlinear optimization-based battery current ripple suppression method for MMC-BESS. By establishing the state-space equations of the MMC-BESS in the synchronous rotating dq reference frame, precise nonlinear analytical expressions for the fundamental and second-harmonic battery current ripples are derived. Subsequently, a nonlinear optimization module for ripple suppression is developed by designing an objective function and corresponding constraints. This module accounts for the influences of both the arm second-harmonic voltage and the second-harmonic circulating current on the peak-to-peak ripple value, enabling real-time calculation of the optimal reference value for the injected second-harmonic circulating current. This approach eliminates the need for arm energy assumptions and bypasses the requirement for curve fitting, thereby significantly simplifying the design process of the ripple suppression controller. Simulation results verify the effectiveness of the proposed method, demonstrating superior suppression performance compared to traditional circulating current injection methods. Full article

Real-time simulation plays an important role in the development and verification of modular multi-level converter (MMC) systems, especially for the rapid and low-risk evaluation of control and protection functions in medium- and high-voltage applications. However, MMC validation often requires simulation models with different fidelity levels for different testing purposes, while detailed device-level representation further imposes stringent constraints on computational efficiency. To address these issues, this paper develops a multi-level real-time modeling framework for MMCs, in which switch models of different accuracy can be incorporated within a unified architecture and flexibly selected according to the target test scenario. On this basis, a device-level real-time simulation method is further established to capture the nonlinear switching transients of MMCs under the proposed framework. By combining network decoupling with FPGA-oriented implementation, the framework can achieve a minimum simulation step of 50 ns under fully parallel hardware allocation. Considering FPGA resource optimization, the prototype implemented in this work is validated with a 100 ns time-step. A three-phase MMC with four submodules per arm is used as the validation case and implemented on an FPGA platform. Both waveform comparisons and quantitative error analysis demonstrate close agreement between the proposed real-time model and offline reference models. In addition, closed-loop real-time experiments are conducted to further confirm the effectiveness of the developed MMC model in realistic real-time simulation-based testing applications. Full article

This paper presents a voltage compensation algorithm as an addition to the existing improved sorting algorithm for post-fault operation of a modular multilevel converter with integrated batteries, aimed at electric vehicle applications. The work focuses on improving the performance of the sorting algorithm that allows the converter to continue operating without degradation after one transistor fault, by using the faulted module in half-bridge mode while preserving access to its battery. However, the existing sorting algorithm has a limitation during continuous high-power operation, where the faulted module cannot discharge sufficiently. This results in a voltage imbalance between the modules and distortion of the output current waveform. To address this issue, a voltage level compensation algorithm is proposed, which adjusts the module operational limits and the reference signal amplitude based on the measured module voltages. The method compensates the positive and negative half-periods of the output waveform independently, since different modules are active in each half-period during fault conditions. The simulation and experimental results demonstrate that the proposed algorithm compensates the output current successfully, even when the module voltages differ significantly. An FFT analysis confirmed the elimination of the DC offset and the reduction of the low-frequency harmonics, resulting in a total harmonic distortion comparable to normal operating conditions. Full article

Modular multilevel converters (MMCs) are widely used in high-voltage direct current transmission, renewable energy integration, and rail transit. However, most existing MMCs adopt grid-following control, which performs well in strong power grids but easily induces broadband oscillation when interacting with weak power grids, threatening system stability. To address the voltage support and stability issues of weak power grids caused by high-proportion renewable energy integration, grid-forming MMCs are increasingly being adopted, but their stability analysis remains insufficient. To fill this gap, this paper establishes the impedance model of grid-forming MMCs using a multi-harmonic linearization method and analyzes system stability based on the Nyquist stability criterion. To suppress broadband oscillation, a virtual impedance control strategy is introduced, where the parameter selection of virtual impedance directly determines the control performance. Therefore, the grey wolf optimization algorithm is employed to optimize the virtual impedance parameters, achieving effective oscillation suppression and stable system operation. Full article

Due to their superior harmonic profiles and minimal switching energy losses, modular multilevel converters (MMCs) have emerged as the primary topology for high voltage direct current (HVDC) applications. However, traditional Proportional–Integral (PI) control exhibits inferior dynamic performance using MMC-HVDC supplying power in the passive networks. This study proposes a backstepping super-twisting sliding mode control strategy, which significantly improves the dynamic performance of the MMC-HVDC system and mitigates fluctuations in the DC side voltage. First, a mathematical model is established based on the topology of the modular multilevel HVDC transmission system. Then, utilizing the backstepping method, a virtual control law for the current inner loop is designed according to the mathematical model. Subsequently, the super-twisting sliding mode algorithm is introduced based on the backstepping method to form the backstepping super-twisting sliding mode control law. Finally, a comprehensive model is established within the Matlab/Simulink environment, and extensive simulation studies are carried out to evaluate the effectiveness the effectiveness and advantages of the proposed backstepping super-twisting sliding mode control under stable operation, grid voltage sag, and single-phase grounding fault conditions. Comparative evaluations verify that the introduced strategy effectively lowers the total harmonic distortion (THD) of the current and suppresses DC voltage ripples. Moreover, compared to the conventional PI method, the new approach provides enhanced transient robustness with noticeably reduced overshoot with considerably lower overshoot compared to traditional PI control, thereby providing a highly reliable and stable solution for MMC-HVDC systems supplying passive networks. Full article

With the rapid development of offshore wind power, the fractional frequency offshore wind power system based on the modular multilevel matrix converter (M3C) faces severe frequency stability challenges due to the reduced inertia under high wind power penetration. This paper focuses on its frequency control and proposes a set of coordinated strategies. Modified frequency regulation schemes for wind turbines (WTs) under different operating states avoid secondary frequency drop (SFD) and accelerate rotor speed recovery. A coordinated power allocation strategy combining energy storage (ES) and automatic generation control (AGC) suppresses wind-induced power fluctuations, with a reducing pitch angle variation method to extend WTs’ life. Meanwhile, an adaptive virtual inertia control strategy for M3C enhances sustained inertia support. A coordinated frequency control scheme between wind farm, M3C, and ES is further constructed to achieve faster and better frequency stabilization under wind and load variations. Simulation results under a 10.5 MW load disturbance show that, compared with the uncontrolled scheme, the proposed scheme raises the frequency nadir from 49.01 Hz to 49.67 Hz, limits the maximum rate of change of frequency (ROCOF) to 0.583 Hz/s with a 49.8% reduction, fully eliminates SFD, and provides theoretical support for the stable grid integration of fractional frequency offshore wind power. Full article

Given the expanding scale of offshore wind power development, strict spatial constraints on offshore platforms and multi-source power coupling present operational challenges during the collaborative transmission of near-shore and far-offshore wind power through a shared corridor. To address these issues, this paper proposes a collaborative transmission scheme based on the Self-Adaption Statcom and Line-Commutation Converter (SLCC). The technical and economic characteristics of three typical topologies—Modular Multilevel Converter (MMC) onshore grid connection, MMC direct transmission, and SLCC direct transmission—are compared and analyzed. The results demonstrate the advantages of the SLCC scheme in reducing the offshore platform footprint and lowering engineering costs. Furthermore, a hierarchical collaborative control strategy is designed to mitigate the power coupling between near-shore AC wind generation and far-offshore DC wind generation at the converter bus. The bottom layer utilizes a valve-side parallel Static Var Generator (SVG) to achieve reactive power self-balance and quasi-resonant suppression of specific harmonics. In the top layer, an LCC active power-following control strategy based on instantaneous power feedback is implemented. This achieves the logical decoupling of near-shore and far-offshore wind power transmission. The effectiveness of the proposed scheme in managing wind power fluctuations, riding through AC faults, and maintaining stable operation under weak grid conditions is verified using the PSCAD/EMTDC software. Full article

This research paper presents Volvo SmartCell, an AC battery technology that integrates modular multilevel converters and battery cells to form a unified system for electric vehicle propulsion and power supply. The research work addresses the broader challenge of reducing driveline cost and complexity by replacing traditional components such as inverters, onboard chargers, centralized DC/DC converters, vehicle control units and many more. SmartCell uses distributed Cluster Boards comprised of H-bridges which are controlled via wireless communication to generate AC voltage, deliver redundant low voltage power, and support cell level protection mechanisms. The prototype testing demonstrates that the system can supply traction power by engaging clusters according to the required voltage depending on motor speed, achieve AC grid charging by synthesizing sinusoidal voltages without a dedicated charger, and provide autonomous DC/DC operation through cluster level voltage regulation. Simulations further indicate that multilevel voltage generation can reduce switching losses and improve electric machine efficiency compared to conventional systems. Additional benefits include active cell balancing, support for mixed cell chemistries, and high redundancy through multiple independent power branches. Challenges remain in wireless bandwidth limitations and cost optimization of Cluster Boards. Ongoing development aims to enhance communication robustness and validate safety for non-isolated grid charging. Full article