MMC-HVDC System Research Articles

A Review on Modular Multi-level Converter Based HVDC Systems

Modular Multilevel Converters (MMCs) have emerged as a promising technology for High Voltage Direct Current (HVDC) transmission systems, offering several advantages such as reduced harmonic distortion, high voltage capability and modularity. This paper presents a comprehensive review of MMC-based HVDC systems, focusing on their applications, control strategies and challenges. The paper begins by discussing the basic principles and structure of MMCs, highlighting their modular design and operation. The paper delves into the control strategies employed in MMC-HVDC systems, emphasizing the importance of coordinated control between the AC and DC sides to ensure stable and efficient operation. Furthermore, the paper addresses the challenges associated with MMC-HVDC systems, such as fault ride-through capability, harmonic interactions and grid integration. It highlights recent advancements in control techniques and technologies that are being developed to address these challenges. The paper provides a valuable overview of MMC-based HVDC systems, highlighting their potential benefits and the ongoing research efforts to overcome their challenges. As the demand for renewable energy sources and long-distance power transmission increases, MMC-HVDC systems are poised to play a crucial role in shaping the future of the power grid

Fault Location Method for MMC-HVDC Systems Based on Numerical Laplace Transform

Fault Location Method for MMC-HVDC Systems Based on Numerical Laplace Transform

Valve-Side Single-Phase-to-Ground Fault Analysis in Bipolar MMC-HVDC Systems With Hybrid SMs

Valve-Side Single-Phase-to-Ground Fault Analysis in Bipolar MMC-HVDC Systems With Hybrid SMs

A Frequency Estimation-Based Adaptive Mitigation Approach for High-Frequency Oscillation in MMC-HVDC System

A Frequency Estimation-Based Adaptive Mitigation Approach for High-Frequency Oscillation in MMC-HVDC System

High-Frequency Resonance Suppression Based on Cross-Coupled Filter and Improved Passive Damper for MMC-HVDC System

High-Frequency Resonance Suppression Based on Cross-Coupled Filter and Improved Passive Damper for MMC-HVDC System

Comparison Study of the Wideband Oscillation Risk of MMC between Grid-Following and Grid-Forming Control

Currently, research on the small-signal stability of grid-forming converters under two control modes, grid-following and grid-forming, mainly focuses on low-level converters. However, studies on the differences between grid-following modular multilevel converters (MMC) and grid-forming MMC are limited. A comprehensive analysis of the wideband oscillation risk differences in MMC-HVDC systems under these two control modes from the perspective of impedance characteristics is necessary. Therefore, based on the harmonic state-space (HSS) modeling theory, this article establishes wideband sequence impedance models for grid-forming and grid-following MMC. Subsequently, this article identifies key control parameters affecting the impedance characteristics of MMC under two control modes. The wideband oscillation risks of MMC-HVDC systems under both control modes are compared. The study indicates that when the grid strength weakens, grid-following MMC faces instability risks, while grid-forming MMC can maintain stability. In instances where high-frequency resonance peaks exist in the impedance of the grid, both control modes of MMC may face instability risks. Furthermore, by adjusting the obtained key control parameters of MMC, this article effectively suppresses the oscillation risk discussed above. Finally, the analysis results are verified through an electromagnetic transient simulation.

Response of MMC-HVDC Systems to Geomagnetic Disturbances

Response of MMC-HVDC Systems to Geomagnetic Disturbances

Coupling characteristics between MMC-HVDC and AC transmission lines on the same tower and their impact on protective relaying

Significant coupling exists between DC and AC lines erected on the same tower or sharing transmission corridors. Electromagnetic and electrostatic coupling between DC and AC lines are investigated based on ± 500 kV MMC-HVDC and 500 kV double-circuit AC lines on the same tower. The interactive coupling characteristics and their impacts on protective relaying are analyzed during normal operation and after DC or AC faults. Voltage coupling ratio and current coupling ratio are defined. The 50 Hz components in MMC-HVDC line may be higher than ripple criterion under the coupling of AC line and can be even more significant when a fault occurs on adjacent AC line. The DC reactor with larger inductance can reduce its impact on MMC-HVDC system. Zero-sequence overcurrent relaying and zero-sequence directional relaying for AC lines may mal-operate when a fault occurs on adjacent MMC-HVDC line. And there is also an interruption risk for AC systems. Improvement schemes are proposed to avoid the mal-operation of protective relaying. Simulation case studies verify the correctness of coupling characteristic analysis and the effectiveness of proposed improvement scheme.

An accelerated modelling method of full-bridge modular multilevel converter considering dead-time

To avoid short-circuits of the same side switching of power converters, the dead time is of significance in the pulse trigger gating signal for full bridge modular multilevel converter (FB-MMC). However, the previous equivalent models use the binary resistance switching model to equivalent the switching state which is impossible to simulate the switching with dead time, and the detailed model (DM) with higher levels is time-consuming and inefficient in electromagnetic transient (EMT) simulation. To address this problem, based on the state-space averaging method, a speed-up average value model (AVM) of FB-MMC considering dead time is proposed. Firstly, the working state of FB-MMC is obtained through analysing the switching states with dead time and is classified into three cases according to the capacitor. Secondly, derived from state-space equation, the three cases are integrated into a unified model by introducing the duty cycle, and then the equivalent controlled source is used as the interface circuit between the external grid and capacitor. Finally, a 49-level two-terminal MMC-HVDC system consisting of FB-MMC is constructed in PSCAD / EMTDC simulations, showing that the proposed AVM compared with DM has high accuracy and operation efficiency.

Markov-Based Stochastic Stabilization Control for MMC-HVDC Systems With Inertia Supporting Under Random Disturbances

Markov-Based Stochastic Stabilization Control for MMC-HVDC Systems With Inertia Supporting Under Random Disturbances

Enhancing Fault Detection and Classification in MMC-HVDC Systems: Integrating Harris Hawks Optimization Algorithm with Machine Learning Methods

Accurate fault detection in high-voltage direct current (HVDC) transmission lines plays a pivotal role in enhancing operational efficiency, reducing costs, and ensuring grid reliability. This research aims to develop a cost-effective and high-performance fault detection solution for HVDC systems. The primary objective is to accurately identify and localize faults within the power system. In pursuit of this goal, the paper presents a comparative analysis of current and voltage characteristics between the rectifier and inverter sides of the HVDC transmission system and their associated alternating current (AC) counterparts under various fault conditions. Voltage and current features are extracted and optimized using a metaheuristic approach, specifically Harris Hawk’s optimization method. Leveraging machine learning (ML) and artificial neural networks (ANN), this technique demonstrates its effectiveness in generating a fault locator with exceptional accuracy. With a substantial volume of data employed for learning and training, the Harris Hawks optimization method exhibits faster convergence compared to other metaheuristic methods examined in this study. The research findings are applied to simulate diverse fault types and unknown fault locations at multiple system points. Evaluating the fault detection system’s effectiveness, quantified through metrics such as specificity, accuracy, F1 score, and sensitivity, yields remarkable results, with percentages of 99.01%, 98.69%, 98.64%, and 98.67%, respectively. This research underscores the critical role of accurate fault detection in HVDC systems, offering valuable insights into optimizing grid performance and reliability.

Analysis of high frequency oscillations based on MMC-HVDC system

Modular multilevel converter is widely used in new energy grid-connection and power grid asynchronous interconnection by virtue of its advantages of flexible and easy to expand structure and no commutation failure problem. In recent years, high-frequency oscillations have been observed in several flexible DC transmission projects at home and abroad, while the internal harmonic dynamic characteristics and control system of the MMC system are complex and the mechanism of high-frequency oscillations has not been sufficiently analysed at present. In this paper, the detailed DQ impedance model and the equivalent circuit impedance model on the AC side are firstly derived based on the state-space model of the MMC system, and the impedance sweeping method with superimposed small perturbations is proposed. Meanwhile, the dynamic characteristics of the detailed DQ impedance model are compared with those of the electromagnetic transient model. The amplitude-frequency characteristics of the system impedance under different system delays are further compared, and the mechanism of the high-frequency oscillation of the MMC-HVDC system is revealed.

Small-Signal Stability Analysis of MMC-HVDC System Based on Hybrid Passivity

Small-Signal Stability Analysis of MMC-HVDC System Based on Hybrid Passivity

Fault Handling Capabilities of Grid-Forming Wind Turbines in Offshore Wind Farms Connected With MMC HVDC System

Fault Handling Capabilities of Grid-Forming Wind Turbines in Offshore Wind Farms Connected With MMC HVDC System

A Frequency-Domain Fault Location Method for Underground Cables in MMC-HVDC Systems

A Frequency-Domain Fault Location Method for Underground Cables in MMC-HVDC Systems

High-frequency Resonance Analysis and Impedance Reshaping Control of MMC-HVDC System Based on Frequency Coupling Impedance Model

High-frequency Resonance Analysis and Impedance Reshaping Control of MMC-HVDC System Based on Frequency Coupling Impedance Model

Online Detection and Location Scheme for Aged Cable Segment in MMC-HVDC Systems Based on Admittance Spectrum and Compressed Sensing

Online Detection and Location Scheme for Aged Cable Segment in MMC-HVDC Systems Based on Admittance Spectrum and Compressed Sensing

Inter-pole Cooperative Control Strategy for True Bipolar MMC-HVDC System with Offshore Wind Power

For offshore wind power via a true bipolar MMC-HVDC delivery system, when both poles of the offshore converter stations adopt fixed AC voltage and fixed frequency control (referred to as fixed V/f control), the converter stations are connected to the same bus and control the same target, resulting in unstable operation of the system. Therefore, this paper proposes an inter-pole cooperative control strategy for a true bipolar MMC-HVDC system with offshore wind power. One pole of the offshore converter station adopts fixed V/f control to supply stable AC voltage to the offshore AC system, and the other pole adopts inter-pole cooperative control to achieve balanced power distribution between the two poles, thus improving the system power transmission margin and reducing power losses. Finally, a simulation project is established on the PSCAD/EMTDC to confirm the usefulness of the inter-pole cooperative control strategy.

Loss distribution analysis and accurate calculation method for bulk-power MMC

Accurate evaluation of power losses in a modular multilevel converter (MMC) is very important for circuit component selection, cooling system design, and reliability analysis of power transmission systems. However, the existing converter valve loss calculation methods using the nearest level modulation (NLM) method and the traditional sorting-based capacitor voltage balancing strategy are inaccurate since the submodule (SM) switching logics in the MMC arms are uncertain. To solve this problem, the switching principle of the SMs in the sorting-based voltage balancing strategy is analyzed. An accurate MMC power loss calculation method based on the analysis of loss distribution of various SM topologies, including half-bridge submodule (HBSM), full-bridge submodule (FBSM) and clamp double submodule (CDSM), is proposed in this paper. The method can accurately calculate the losses caused by the extra switching actions during the capacitor voltage balancing process, thus greatly increasing the calculation accuracy of switching losses compared with existing methods. Simulation results based on a practical ± 350 kV/1000 MW MMC-HVDC system with variety of MMC topologies with different voltage balancing strategies demonstrate the effectiveness of the proposed method.

Adaptive Active Inertia Control Strategy of MMC-HVDC Systems for Flexible Frequency Support

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.