Search Results (34)

Due to the highly demanding operating conditions of the Modular Multilevel Converter (MMC), as well as its inherently complex structure, electric field analysis of the MMC valve is crucial for the safe and stable operation of MMC-HVDC (MMC high voltage direct current) transmission systems. In this paper, a high-speed modeling method for an MMC valve based on data derivation is proposed. Firstly, the relationship between the MMC valve electric field calculation model parameters and the MMC-HVDC transmission system parameters was studied. Based on this, the data derivation system for the electric field calculation model parameters of all components of the MMC valve was established. A modeling parameters database based on empirical knowledge is also used in the calculation process of the data derivation system. The output model parameter matrix includes the geometric parameters, position parameters, and electrical parameters of the components. Based on the output matrix, the electric field calculation model of the converter valve can be quickly generated. Furthermore, to improve the accuracy of the calculations and reduce computation time, a discrete modeling method that combines mesh optimization was proposed. In the process of discretized electric field modeling, a mesh division influence factor based on the accuracy of electric field calculation is proposed. By adjusting the mesh division influence factor during the electric field calculation, the accuracy of the electric field calculation can be optimized rapidly. Finally, the effectiveness and practicality of the high-speed modeling method for the MMC valve are verified through comprehensive case studies conducted on the ±320 kV onshore MMC-HVDC valve. It is demonstrated that the high-speed modeling method proposed in this paper can significantly reduce the modeling time of the converter valve and greatly improve the accuracy of electric field calculation. Full article

Due to system failures and the limited overcurrent capability of semiconductor devices, overcurrent in modular multilevel converters (MMC) is a key factor affecting the safe and stable operation of offshore wind power MMC-HVDC (modular multilevel converter high-voltage direct current) transmission systems. This paper proposes a field–circuit coupling analysis method for overcurrent research in MMC valve. The method integrates the electric field characteristics of valves with the analysis of MMC-HVDC systems. Firstly, the development process and influencing factors of overcurrent in valves in offshore wind power MMC-HVDC systems are analyzed. A field–circuit coupling model and an electric field calculation model for MMC valves are established. The electric field characteristics and stray parameters of MMC valves are analyzed synchronously and the result are incorporated into the field–circuit coupling model. The nonlinear transient parameters of surge arresters are calculated, and the results are incorporated into the field–circuit coupling model. Finally, a reasonable overcurrent suppression strategy for offshore MMC-HVDC valves is proposed based on the proposed method. The effectiveness and practicality of the field–circuit coupling overcurrent analysis method are verified through comprehensive case studies conducted on the ±500kV offshore MMC-HVDC valve overcurrent calculation and suppression. Full article

In recent years, the small-signal stability of modular multilevel converter (MMC)-based high-voltage direct current (HVDC) systems has garnered significant attention. But little attention has been paid to the impact of extreme temperature weather, although it may change the parameters of outdoor devices and lead to oscillations in a weak system. To explore the impact of environmental temperature on the stability of the MMC-HVDC system, this paper firstly establishes a comprehensive harmonic state space (HSS) model, incorporating the effect of temperature on the impedance of AC and DC transmission lines based on the thermal balance equation. By comparing the theoretical and simulation results, the accuracy of the model is validated. Subsequently, the mechanisms through which extreme temperature conditions affect system stability were analyzed. The results indicate that under extreme high-temperature conditions, the impedance of the MMC is significantly affected, weakening system stability and potentially causing small-signal instability. In contrast, extreme low-temperature conditions show no noticeable impact on system stability. Full article

The modular multilevel converter (MMC) high-voltage direct current (HVDC) transmission technology is essential for overcoming the challenges of large-scale renewable energy integration. Line protection is critical for ensuring system safety. However, existing protection methods for MMC-HVDC transmission lines face difficulties in withstanding both high resistance and noise interference, frequently leading to failures in detecting internal high-resistance faults or triggering false operations due to noise. This paper first derives the theoretical expression of the line-mode voltage through analytical methods. By analyzing the second derivative of the line-mode voltage under different fault conditions, this paper constructs a criterion based on the ratio of the integrals of the positive and negative components of the second derivative of the line-mode voltage. This criterion enables effective fault discrimination by utilizing the characteristic differences in the second-derivative waveform. The proposed criterion allows for precise fault identification, requiring only a 0.5 ms time window to detect faults. Additionally, this criterion is highly resistant to transition resistance, remaining unaffected by resistances up to 500 Ω. Moreover, an entropy-based auxiliary criterion is introduced to prevent false operations caused by noise interference. Simulation results using PSCAD/EMTDC demonstrate that the proposed protection scheme can swiftly and reliably detect faults, with a detection time of 0.5 ms and robust performance against both high transition resistance and noise interference. Full article

Over the past few decades, wind energy has expanded to become a widespread, clean, and sustainable energy source. However, integrating offshore wind energy with the onshore AC grids presents many stability and control challenges that hinder the reliability and resilience of AC grids, particularly during faults. To address this issue, current grid codes require offshore wind farms (OWFs) to remain connected during and after faults. This requirement is challenging because, depending on the fault location and power flow direction, DC link over- or under-voltage can occur, potentially leading to the shutdown of converter stations. Therefore, this necessitates the proper understanding of key technical concepts associated with the integration of OWFs. To help fill the gap, this article performs an in-depth investigation of existing alternating current fault ride through (ACFRT) techniques of modular multilevel converter-based high-voltage direct current (MMC-HVDC) for OWFs. These techniques include the use of AC/DC choppers, flywheel energy storage devices (FESDs), power reduction strategies for OWFs, and energy optimization of the MMC. This article covers both scenarios of onshore and offshore AC faults. Given the importance of wind turbines (WTs) in transforming wind energy into mechanical energy, this article also presents an overview of four WT topologies. In addition, this article explores the advanced converter topologies employed in HVDC systems to transform three-phase AC voltages to DC voltages and vice versa at each terminal of the DC link. Finally, this article explores the key stability and control concepts, such as small signal stability and large disturbance stability, followed by future research trends in the development of converter topologies for HVDC transmission such as hybrid HVDC systems, which combine current source converters (CSCs) and voltage source converters (VSCs) and diode rectifier-based HVDC (DR-HVDC) systems. Full article

Modular multilevel converters (MMCs) have gained widespread adoption in high-voltage direct current (HVDC) transmission due to their high voltage levels, low harmonic content, and high scalability. However, conventional control methods such as finite control set model predictive control (FCS-MPC) suffer from a heavy computational burden and sensitivity to system parameter variations, limiting the performance of MMCs. This paper proposes a data-driven approach based on model-free adaptive control with an event-triggered mechanism that demonstrates superior robustness against parameter mismatches and enhanced dynamic performance in response to sudden output changes. Moreover, the introduction of the event-triggered mechanism effectively reduces redundant operations, decreasing the computational burden and switching losses. Finally, the proposed strategy is validated through a MATLAB/Simulink simulation model. Full article

The High Voltage Direct Current (HVDC) transmission technology employing modular multilevel converters (MMCs) can effectively enhance the transmission efficiency and stability of offshore wind farms, thereby aiding the promotion of large−scale utilization of new energy. This holds significant importance for achieving the dual carbon goals. Aiming at the problem of negative sequence current circulation in MMC−HVDC transmission systems, a circulation suppression strategy based on augmented order decoupling linear active disturbance rejection control (LADRC) is proposed in this paper. By introducing new state variables into the traditional ADRC structure, the actual output deviation signal and observation gain signal from the disturbance observation value of the system are used. It can not only realize the decoupling control of disturbance and tracking terms but also enhance the disturbance immunity, robustness and rapidity of the controller. Finally, an 18−level MMC system model is built based on Matlab (9.12.0.1884302 (R2022a)) & Simulink (R2022a), and the circulation suppression effects of stable operation and voltage sudden change are simulated and compared, which verifies the suppression effect of the improved control strategy on negative sequence current circulation, which lays a theoretical and application foundation for the sustainable development of the offshore wind power industry. Full article

The Modular Multilevel Converter High Voltage Direct Current Transmission (MMC-HVDC) technology is considered to be the most feasible choice for high-voltage and high-power transmission systems, and its flexibility and high controllability provide a new solution for renewable energy grid integration. The MMC topology contains a large number of capacitors, which enables it to provide a certain active inertia support for the connected AC system. Different from a synchronous machine, the active inertia control of an MMC can flexibly adjust a system’s inertia-supporting power by changing the control parameters. By introducing a variation of the Sigmoid function with amplitude-limiting capability, this paper proposes an adaptive active inertia control strategy for the MMC-HVDC system. The proposed scheme adjusts the inertia constant adaptively according to the frequency change rate of the AC system, which can better respond to the frequency recovery performance. Finally, the MMC-HVDC simulation model is established in PSCAD/EMTDC to verify the effectiveness of the proposed control strategy. Full article

Renewable energy generation is a manifestation of global economic and societal advancement and serves as a fundamental assurance for humanity’s pursuit of sustainable development. However, recent years have witnessed several instances of high-frequency resonance events in high-voltage direct current (HVDC) transmission systems based on modular multilevel converters (MMC), which have resulted in converter station tripping and significant repercussions on the alternating current (AC) grid. This paper addresses the mid-to-high frequency resonance issues prevalent in flexible DC transmission systems employing modular multilevel converters (MMC-HVDC). To tackle these concerns, an impedance model for MMC’s AC side is established. Utilizing impedance analysis, the essential factors contributing to the negative damping characteristics of MMC are identified as delay and voltage feedforward loops, predominantly causing negative damping in the frequency range exceeding 400 Hz. In response, a suppression strategy is proposed, involving the incorporation of a multi-band stop filter and virtual impedance. This strategy ensures that within the 0–2000 Hz frequency range, only the impedance phase within 230–430 Hz slightly surpasses 90°. Consequently, the phase difference between MMC’s positive-sequence impedance and the AC system impedance is reduced from 222° to 174.7°, thus guaranteeing secure grid operation. Lastly, the accuracy and effectiveness of the theoretical analysis and suppression methodology are verified through the development of an electromagnetic transient model in MATLAB/Simulink, considering delay fluctuations of ±10%. Full article

A modular multilevel converter (MMC) in a high-voltage direct-current (HVDC) transmission system consists of an electric-coupled physical system and a communication-coupled cyber system, leading to a cyber-physical system (CPS). Such a CPS is vulnerable to false data injection attacks (FDIA), which are the main category of cyberattacks. FDIAs can be launched by injecting false data into the control or communication system of the MMC to change the submodule (SM) capacitor voltage seen by the central controller. Consequently, the capacitor voltage of the attacked SM will deviate from its normal value and thus threaten the safe operation of the converter. Stealthy FDIAs characterized by elaborated attack sequences are more dangerous because they can deceive and bypass the attack detector presented in the existing literature for the MMC. To address this issue, this paper proposes a stealthy FDIA detection method to obtain the real SM capacitor voltages. Thus, the attacked SM can be located by comparing its real capacitor voltage with prespecified thresholds. Simulation results validate the effectiveness of the proposed detection and protection strategies. Full article

This paper proposes a decoupled control of a dc-dc modular multilevel converter (MMC) based on a double-T topology intended for multi-terminal high voltage direct current (MT-HVdc) transmission systems or emerging distribution systems operating in medium voltage direct current (MVdc). The aim of the proposed control strategy is to obtain an input current with reduced harmonic content and to eliminate the output ac common-mode voltage, which is not allowed in MT-HVdc systems. The control strategy consists of injecting two circulating ac currents and two dc currents that allow the energy balance between the arms of the converter and the general energy balance of the topology. The dc and ac currents are decoupled and allow control over load variations and reference changes in the dc-links. The proposed topology is mathematically modeled and the control method is then derived. Simulation results are presented to validate the proposed system. Full article

Modular multilevel converter (MMC) is a key device of high-voltage-direct circuit (HVDC) transmission system, the sub-module detection technology of which will directly influence the damage severity, and even the reliability of the whole system. In this paper, the open-circuit fault characteristics of an insulated gate bipolar transistor (IGBT) in a sub-module are analyzed, and a fault-tolerant optimal control strategy is proposed for the redundant hot-reserved MMC based on nearest-level modulation (NLM). A fault sub-module diagnosis and location strategy based on the deviation distance of the capacitor voltages is presented. After the faulty sub-module is removed, due to the asymmetric operation of the MMC, odd-order circulating currents are introduced in the faulty phase, in which the fundamental-frequency circulating current is the major component; the fundamental-frequency voltage related to the redundancy rate is injected into the faulty phase, which effectively suppresses the fundamental-frequency circulating current and harmonics in the faulty phase. The proposed method combines fault detection and fault ride-through steps, so it has the features of high reliability and high compatibility. Based on the Matlab/Simulink simulation model, the effectiveness of the proposed strategy is verified. Full article

Modular multilevel converters (MMC) play a dominant role in integrating remotely located renewable energy resources (RER) over the high-voltage direct current (HVDC) transmission network. The fault ride-through capabilities of the MMC-HVDC network during low-voltage faults and the power fluctuation due to RER intermittency are the major obstacles to the effective integration of renewable energy. In response, this article proposes a local voltage-based combined battery energy control scheme for a PV-wind-battery connected MMC-HVDC system to regulate the HVDC-link voltage during low-voltage faults at the point of common coupling of alternating current grids and to reduce the intermittent RER power fluctuation. The proposed technique removes the dynamic braking resistor from the HVDC-link and smoothly integrates the RER without active power reduction of renewable energy under low-voltage faults. Symmetrical and unsymmetrical low-voltage faults have been conducted to validate the effectiveness of the proposed control scheme for the battery in mitigating surplus energy in the HVDC-link. Additionally, wind speed, solar radiation, and temperature have been changed to confirm the improved performance of the battery energy management system. The complete systems have been simulated and tested in a real-time digital simulator (RTDS) and using dSPACE-based controller hardware in a loop setup. Full article

With the development of power energy technology, flexible high voltage direct current (HVDC) systems with high control degree of freedom flexibility, power supply to passive systems, small footprint, and other advantages stand out in the field of long-distance large-capacity transmission engineering. HVDC transmission technology based on a modular multilevel converter has been widely used in power grids due to its advantages such as large transmission capacity, less harmonic content, low switching loss, and wide application field. In the modular multilevel converter (MMC)-based HVDC system, the protection strategy of converter station internal faults is directly related to the reliability and security of the power transmission system. Starting from the MMC topological structure, this paper establishes the MMC mathematical model in a synchronous rotation coordinate system by combining the working state of sub-modules and the relationship between each variable of the upper and lower bridge arms of each phase of the MMC. It provides a theoretical basis for the design of the MMC-HVDC control system. The causes of the AC system faults and the internal faults of the converter station in the MMC-HVDC system are analyzed, and the sub-module faults and bridge arm reactor faults in the converter station are studied. The sub-module redundancy protection and bridge arm overcurrent protection strategies are designed for the faults, and the correctness of the scheme is verified by Matlab/Simulink. Full article

The circuit topology of a submodule (SM) in an modular multilevel converter (MMC) defines many of the functionalities of the complete power electronics conversion system and the specific applications that a specific MMC configuration can support. Most prominent among all applications for the MMC is its use in high-voltage direct current (HVDC) transmission systems and multiterminal dc grids. The aim of the paper is to provide a comprehensive review and classification of the many different SM circuit topologies that have been proposed for the MMC up to date. Using an 800-MVA, point-to-point MMC-based HVDC transmission system as a benchmark, the presented analysis identifies the limitations and drawbacks of certain SM configurations that limit their broader adoption as MMC SMs. A hybrid model of an MMC arm and appropriate implementations of voltage-balancing algorithms are used for detailed loss comparison of all SMs and to quantify differences among multiple SMs. The review also provides a comprehensive benchmark among all SM configurations, broad recommendations for the benefits and limitations of different SM topologies which can be further expanded based on the requirements of a specific application, and identifies future opportunities. Full article