Due to the large integration of renewable energy into the power system, frequency control strategy and the frequency support capability which the renewable generation can provide has become a concern for the system operators. This paper studies the collaborative control of the renewable energy base and the VSC-HVDC transmitting system to support the receiving-end grid frequency and proposes a frequency support capability evaluation method. First, a collaborative control architecture for the PV base and two-terminal VSC-HVDC transmitting system is designed to support the receiving-end grid frequency. Subsequently, a method to decouple the primary frequency control (PFC) power and virtual inertia power is proposed, and the evaluation method for the PFC coefficient and virtual inertia time constant are proposed based on the decoupled power. Finally, PSCAD/EMTDC simulations are carried out to verify the effectiveness of the collaborative control and the proposed frequency control parameter evaluation method.

The Hybrid Modular Multilevel Converter with Embedded Supercapacitor Energy Storage System (MMC-SESS) combines the advantages of the Modular Multilevel Converter (MMC) and energy storage system, making it highly suitable for large-scale integration of renewable energy sources. Compared with the conventional Hybrid MMC, the Hybrid MMC-SESS not only retains DC fault ride-through capability but also supports bidirectional active power exchange with the grid, thereby enhancing system stability and reliability. However, the Hybrid MMC-SESS features a complex structure, and its detailed model (DM) is computationally intensive, leading to low simulation efficiency. To accelerate simulation, this paper develops an equivalent model (EM) for the Hybrid MMC-SESS. First, topology analysis of the submodule (SM) is performed, and its companion circuit is established using discretization based on the trapezoidal integration method. Next, EMs of the SM and the arm are established based on the Nested Fast Solution Method (NFSM) and Thévenin equivalent modeling approach. The EM of the Hybrid MMC-SESS is then derived according to series and parallel relationships. Finally, DM is implemented in PSCAD/EMTDC. Comparative verification is conducted between the proposed EM and the DM, including steady-state operation and DC fault scenarios. The results demonstrate that the EM achieves excellent simulation accuracy and computational efficiency.

The paper discusses a frequency support strategy based on MMC-HVDC system, considering the frequency variation and rate of change in the receiving-end grid during load transients. This strategy integrates virtual inertia control and virtual droop control methods to autonomously switch frequency control modes. It is able to reduce control mode oscillations during frequency restoration and providing transient inertia support and short-term frequency support. A simulation model is built in PSCAD/EMTDC, and the frequency control strategy is analyzed based on simulation results. The results indicate that the proposed strategy can provide transient inertia support and short-term frequency support, while also optimizing the impact of load fluctuations on the AC bus voltage.

Line Commutated Converter based High Voltage Direct Current (LCC-HVDC) transmission technology has the advantages of high withstand voltage and large current capability and has been widely used in renewable energy transmission. However, the power intermittency and fluctuation of renewable energy sources (RESs) such as wind and solar power will result in complex and variable operating conditions of the AC power grid. A single converter control strategy is difficult to adapt to all operating conditions, affecting the safety and stability of the system. Therefore, this paper first establishes the state-space model of the LCC-HVDC system with renewable energy at the sending-end and analyzes the small-signal stability and stable operating range of the system under different control strategies. Then, based on the designed control switching principles, a flexible control switching method applicable to the LCC-HVDC system is proposed. This method can not only enhance the stability of the system, but also ensure a smooth switching between different control strategies with minimal disturbance to the system. Finally, a simulation model is built on the PSCAD/EMTDC platform to verify the feasibility of this flexible control switching method. The simulation results show that the disturbance during the control switching is relatively small, and the flexible control switching method can effectively ensure the stable operation of the LCC-HVDC system under different operating conditions.

For LCC-HVDC system with renewable energy integration, the randomness and variability of renewable energy will cause wide variations in the short-circuit capacity provided by the AC system. To effectively assess the supporting capability of the AC system, this paper proposes an active short-circuit capacity identification method for LCC-HVDC system considering the integration of renewable energy. First, an equivalent AC system was established based on Thevenin theorem, and the equivalent electromotive force was calculated. Then, the sensitivity of the voltage at the point of common coupling (PCC) to the active and reactive power flowing through the PCC was computed. Through sensitivity analysis, the key factors affecting the identification of the equivalent resistance and reactance were studied. Based on this, an active short-circuit capacity identification method combining the switching of filters and changes in DC power was proposed. Finally, a simulation model of LCC-HVDC system with grid-following wind power integration was built in PSCAD/EMTDC for verification. The results show that the proposed method is applicable to AC systems with different impedance characteristics. Moreover, with the increase of the grid-following renewable energy, the short-circuit capacity provided by the AC system shows a decreasing trend. The proposed method can actively identify the short-circuit capacity of AC system with renewable energy, thus provides a theoretical basis for the development of control strategies for LCC converter station under different AC system strength.

There are various operating modes of parallel bipo-lar multi-terminal DC transmission systems, among which there is an asymmetric condition of unipolar-bipolar hybrid form. In this case, the distribution of harmonics on the line is different from the normal case, and the equivalent impedance of the mode-domain loops obtained by pole-mode transformation is changed. To solve the problem of harmonic analysis on the DC side under asymmetric conditions, a unified impedance model for bipolar DC overhead lines applicable to various conditions is proposed. Firstly, the decoupling model and equivalent circuit of the mode domain of bipolar symmetrical lines are explained. Then, the limitations of the mode-domain component superposition calcula-tion method in the analysis with asymmetric conditions are re-vealed. On this basis, from the perspective of the dual input and dual output port network of bipolar DC lines, the four-port pole-mode conversion relationship and fourth-order transformation matrix of bipolar DC lines are proposed. A unified impedance model of four-port network for bipolar overhead lines is estab-lished. Finally, the simulation results based on PSCAD show that the proposed model can solve the harmonic transfer analysis and exhibit good reliability, with at least 5% reduction in the average relative error under asymmetric conditions.

Modular Multilevel Converter based High Voltage Direct Current (MMC-HVDC) has been widely used in the large-scale transmission of renewable energy sources (RESs). However, the operating conditions of MMC-HVDC system with RESs are complex and variable, making it challenging to apply a single converter control strategy to different operating conditions, which affects the safe and stable operation of the whole system. In this paper, a flexible switching method of control strategies is proposed for MMC-HVDC converter. Firstly, the state-space model of MMC-HVDC under different control strategies is established, and the small signal stability and stable operation ranges under these control strategies are analyzed. Then, according to the proposed control strategy switching principle, a flexible control switching method for MMC-HVDC converter is proposed. This flexible control switching method ensures a smooth switching between different control strategies and causes a minimal disturbance to the overall system. Finally, a simulation model is established on the PSCAD/EMTDC platform, and the feasibility of the flexible control switching strategy for the MMC-HVDC system is verified. The simulation results show that very small disturbance is observed during the control switching and the flexible control switching strategy can effectively maintain the stable operation of MMC-HVDC system under different operating conditions. The proposed flexible control switching method can be activated according to the change of AC grid strength and it will help improve the stable operation of MMC converter.

In the electricity sector, wind power transmission systems based on Modular Multilevel Converters (MMCs) are increasingly popular due to their potential and growth prospects. However, voltage drops in the receiving-end AC power grid can lead to rapid DC voltage increases due to surplus power accumulation, threatening system stability. This paper proposes a fault ride-through strategy for MMC-based grids, incorporating a supercapacitor energy storage system (SESS) to store excess power and supply reactive power during faults. Simulations show that this strategy effectively curbs DC overvoltage and enhances reactive power supply, improving fault ride-through capability. Compared to the traditional MMC current-limiting strategy, the proposed method reduces DC voltage deviation during faults (from +18.88% to +9.27%) and narrows post-fault voltage fluctuation (-6.49% to +14.77% vs. - 19.79% to +37.39%). It also increases active power output (181.04 kW vs. 154.27 kW) and reactive power output (62.17 kVar vs. 0.8 kVar) during faults, ensuring higher power absorption and voltage support. This strategy thus offers a robust solution for enhancing the stability and efficiency of large-scale wind power transmission systems.

This paper addresses the critical need to determine the stable operating limit of modular multilevel converter-based high voltage direct current (MMC-HVDC) systems, particularly concerning the integration of extensive renewable energy sources. To achieve this, the steady-state mathematical model and state-space model of bundled hydropower and photovoltaic integration through MMC-HVDC systems are established. A novel methodology considering steady-state and small-signal stability constraints is proposed to compute the stable operating region of the system. The quantitative assessment reveals that diminishing AC system short-circuit capacities amplify restrictions from small-signal stability constraints, thereby reducing the system's stable operating region. Eigenvalue and participation factor analyses shed light on the pivotal factors affecting small-signal stability in weak AC systems. To expand the system's stable operating region, a supplementary frequency damping control strategy is proposed. The theoretical analysis and calculation results are validated by building a simulation model for the bundled hydropower and photovoltaic integration through MMC-HVDC systems in PSCAD/EMTDC.

Most distributed power sources in microgrids lack the necessary inertia, resulting in poor system frequency stability. Therefore, virtual synchronous machines (VSG) are used to simulate the characteristics of synchronous motors to provide inertia support, but at the same time, problems such as active power oscillation are introduced, especially when multiple VSGs are operated in parallel. This problem is more serious. This paper proposes a mutual damping control strategy based on a distributed communication architecture. Through the mutual damping control between adjacent VSGs, the VSGs in the system tend to the same output frequency at all times under disturbance, which improves the dynamic characteristics of VSGs and effectively suppresses power oscillations. In addition, this paper proves the stability of the improved system by constructing a small signal model to draw the system pole distribution, and verifies the effectiveness of the proposed control strategy through Matlab/Simulink simulation.

In order to analyze the influence of harmonics on the stability of line commutated converter based high voltage direct current(LCC-HVDC)system.It is urgent to establish an accurate model of LCC-HVDC considering harmonic coupling.Based on the theory of harmonic state space(HSS),an impedance model of 12-pulse LCC is established considering frequency coupling effect and control system.The constructed AC/DC harmonic impedance model can match the sweep frequency results in a wider frequency band.Finally,the correctness of the proposed LCC-HSS impedance model is verified by comparing the PSCAD electromagnetic transient simulation results with the HSS impedance model calculation results.The LCC-HSS impedance modeling method improves the accuracy of mathematical modeling for LCC converter stations and can adapt to impedance modeling of LCC converter stations in various modes,providing a more accurate model for stability analysis and parameter optimization of LCC systems.

The flexible DC converter valve power supply is the core device of power supply to the power module drive board, in which there are a lot of ceramic capacitors to ensure its stable work. However, ceramic capacitors can easily reduce reliability under high voltage and strong field environment, and can cause failure accidents of energy taking board. This study mainly studies the short-circuit failure characteristics and mechanism of ceramic capacitors. Firstly, it analyzes the voltage and temperature factors that are easy to cause the failure of ceramic capacitors in energy taking power supply, and then explores the pollution source of failed ceramic capacitors and analyzes the failure mechanism of ceramic capacitors through microscopic testing. The results show that ceramic capacitors are prone to short-circuit burning under different external environment, which is closely related to material selection. The research in this paper provides reference data for the selection of ceramic capacitors and failure analysis of energy taking power supply, so as to improve the reliability of energy taking power supply.

Grid-forming control demonstrates higher stability compared to grid-following control in expanding inverter-dominated power grids. However, parameters are not harmoniously adjusted for large-scale inverter-dominated grids. For instance, while the damping factor can enhance the stability of local inverters, it may lead to instability in other inverters. Furthermore, existing literature fails to consider other performance metrics such as response speed and overshoot. Therefore, this letter proposes an online adaptive control based on reinforcement learning. Leveraging the flexible computing paradigm of cloud-edge-terminal systems, the proposed approach comprehensively assesses the performance of grid-forming inverters.

换流器作为高压交直流输电系统中的重要元件,因其存在复杂的电力电子器件而产生大量频次的谐波严重影响到输电系统的安全稳定,获取交直流输电系统交流侧谐波电流是谐波治理的基础,因此交流侧谐波电流的精确计算具有重要意义.本文基于时域分段法的思想,充分考虑到直流纹波电流的影响,提出了内外双环迭代的计算方法:内环采用时域分段法计算直流侧端口电压瞬时量、采用相量法计算直流侧纹波电流、利用直流纹波电流和直流侧端口电压耦合迭代计算的方法计算直流侧纹波电流;外环利用换相过程中退出换相的相电流在换相结束时刻电流过零的特点,结合纹波电流迭代计算换相角的方法精确获取换相角和交流侧谐波电流.与PSCAD/EMTDC模型计算结果对比表明:此方法既保证了谐波电流计算结果的精确度和计算效率,也充分计及了工频负序电压、背景谐波电压、触发角不等距、换相电感不平衡、变比误差、测量误差、纹波电流等各种非理想因素的影响,同时易于程序实现.

针对兴安直流工程出现的谐振放大问题,研究了谐振抑制机理.基于同时考虑送、受端交流系统和直流系统的阻抗等值网络,通过频率扫描法研究等值网络的阻抗特性和谐振抑制稳定性,结果证实兴安直流工程交直流系统在特定频率附近存在谐振风险,与暂态仿真结果和现场振荡现象一致.对直流振荡特性的影响因素进行了仿真分析,提出改造直流侧网络阻抗的谐振抑制方法.仿真结果验证了所提策略对谐振抑制的有效性.最后,将所提策略应用于兴安直流工程,典型运行方式下未再出现谐振现象.

The decrease of the strength of AC power grid will adversely affect the stable operation of the modular multi-level converter HVDC (MMC-HVDC) transmission system. To avoid the potential safe and stable operation risk brought by the weak AC power grid to the AC/DC system, and to realize the rapid online monitoring of the AC power grid strength under complex and changeable working conditions, this paper proposes an online identification method of strength based on disturbance of Control System of MMC Station. Firstly, the principle and identification method of disturbance and calculation method of AC power grid are introduced from the theoretical perspective. Then the influence of the frequency variation characteristics of the transmission line and the amount of reactive power injection on the accuracy of the power grid strength identification results are introduced respectively. Finally, the feasibility of the proposed method and the constraint of the identification accuracy are verified in simulation.

The equivalent impedance of the AC power grid reflects the strength of the AC system, and the reduction of the system strength will adversely affect the stable operation of the modular multilevel converter based high voltage direct current（MMCHVDC）transmission system. In order to effectively avoid the safe operation risk of the AC/DC systems brought by high impedance/low strength and realize the rapid and accurate detection of the equivalent impedance of complex and changeable power grids, this paper proposes an online identification method of the equivalent impedance based on reactive power disturbance injection for the AC power grid with modular multilevel converters at both ends. First, the principle of impedance identification with reactive power injection is described from the theoretical perspective, and the influence of the resistance and measurement meter error on the impedance identification accuracy is analyzed. Then, an evaluation index for impedance identification accuracy based on AC bus voltage information is established. Based on this index, this paper proposes the principle to design the volume of reactive power disturbance injection and the online identification method for the equivalent impedance of the AC power grid based on the identification accuracy constraint. Finally, the feasibility of the proposed method and the controllability of the impedance identification accuracy are verified by simulation.

With a large amount of new energy access and the wide application of high voltage direct current (HVDC) transmission technology based on line commutated converter (LCC), it is of great significance to study the impedance frequency characteristics of the converter to analyze the stability of the LCC-HVDC system. Due to the complexity and inefficiency of the existing modeling techniques, in this paper, a simplified impedance modeling method for the DC side of LCC based on the simplified switching function is proposed. Firstly, the commutation process of LCC is deduced according to the converter valve opening sequence and circuit principle, and the simplified switching function of LCC is proposed. Next, based on the dynamic phasor method, through the analysis and calculation of harmonic transmission between the AC and DC sides of LCC, the simplified impedance of the DC side of LCC is obtained. Then, a simulation model is established in PSCAD, and the impedance scanning results of the simulation model show that the error is small between the method proposed in this paper and the method without simplification, which verifies the validity of the proposed model. The proposed method can reduce modeling complexity and provide a fast calculation model of the DC side impedance of LCC for analyzing the stability of large complex transmission systems. (c) 2023 The Author(s). Published by Elsevier Ltd. This is an open access article under theCCBY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

With the increasing complexity of AC system, it is difficult to obtain the admittance matrix of the whole system. From the perspective of automation, universality and precision, this paper proposes a method for constructing admittance matrix of arbitrary complex AC system. Firstly, the rated parameters of AC system are obtained by batch/automatic simulation software, and the equivalent ways of each component are classified and discussed. Secondly, this paper proposes to use frequency sweep method to judge the equivalent value of complex components by node position, and the equivalent admittance of the components is included into the matrix. Then the element is used as the minimum system admittance and the traversal algorithm is used to complete the traversal of AC system nodes. By using the ideas of power flow correction, step simplification and batch processing, the admittance matrix of the whole network and the corresponding simplified condition can be formed quickly and accurately. Finally, in order to verify the correctness of the proposed method, this paper takes IEEE9 node system as an example to verify the correctness of the admittance matrix.