Transient performance comparison of grid-forming converters with different FRT control strategies

Grid-forming converters(GFMs)are faced with the threat of transient inrush current and synchronization instability issues when subjected to grid faults.Instead of disconnecting from the grid unintentionally,GFMs are required to have fault ride through(FRT)capability to maintain safe and stable operation in grid-connected mode during grid fault periods.In recent studies,different FRT control strategies with distinguishing features and that are feasible for different operation conditions have been proposed for GFMs.To determine their application scope,an intuitive comparison of the transient performance of different FRT control strategies is presented in this paper.First,three typical FRT control strategies(virtual impedance,current limiters,and mode-switching control)are introduced and transient mathematical models are established.A detailed comparison analysis on transient inrush current and transient synchronization stability is then presented.The results will be useful for guiding the selection and design of FRT control strategies.Finally,simulation results based on PSCAD/EMTDC are considered to verify the correctness of the theoretical analysis.

不同故障状态下电网形成变换器的故障穿越控制

电网形成GFM(grid-forming converter)变换器作为高渗透率电力电子设备电力系统的主要元件之一,其故障穿越FRT(fault-ridethrough)能力是保证高度电力电子化电力系统稳定运行的重要基础。基于此,提出1种适用于GFM变换器的FRT策略,既考虑了变换器的硬件约束(即电流约束),又可在发生对称和非对称故障时使GFM在保护模式下运行。首先,详细分析了GFM变换器的FRT相关问题;然后,建立了1种适当的FRT模型及控制方法;最后,通过功率硬件在环仿真与实验验证所提方法的有效性。结果表明,与电网跟随变换器相比,所提控制方法可保障GFM变换器在故障时瞬时注入无功电流,以防止变换器出现过流问题,并最终实现GFM变换器在严重故障条件下仍可容错运行。

构网型风电机组的惯量阻尼特性分析

构网型变流器具有较好的惯量阻尼特性,目前已经成为新能源方面的研究焦点。为此,首先研究构网型风电变流器的阻尼特性,从阻抗的角度分析其对外等效特性,并利用伯德判据分析其在并网时的稳定性;其次从惯量的角度研究构网型风电变流器对电网的频率支撑特性,通过仿真分析,对比构网型风电变流器与跟网型风电变流器在电网频率变化及风机侧功率变化时的支撑能力。研究结果表明:构网型风电变流器对外阻抗特性呈阻感特性,并网稳定性较好;构网型风电变流器在电网故障时能够提供一定的功率支撑,等效增加了系统的惯量,并网特性较好。

计及功率控制型构网变流器接入的交流系统暂态稳定性仿真研究

为研究功率控制型构网变流器接入交流系统后的暂态稳定性演化趋势,基于IEEE 39节点交流测试系统,将发电机分别等容量替换为功率控制型构网变流器与跟网型变流器,并改变其在交流系统中接入数量、并网位置进行电磁暂态仿真,结果表明:功率控制型构网变流器对接入区域的暂态稳定性提升能力更优,尤其当所替换发电机相邻时,提升效果显著;随着功率控制型构网变流器接入系统比例增加,系统暂态稳定性越强;功率控制型构网变流器内部等效惯性系数越大,系统同步稳定性越弱,其内部阻尼系数越大,系统同步稳定性越强。

构网型VSC暂态稳定机理及改进限幅策略

针对构网型变流器(grid-forming voltage source converter,GFM-VSC)系统在大扰动下暂态稳定问题,现有研究未能充分考虑电力电子电源暂态快速响应与控制可塑的特点。为此,以GFM-VSC为对象,借助等面积法原理与相平面图法,从能量角度揭示了其暂态响应机制与传统同步机系统的差异,分析了控制塑造下GFM-VSC系统的暂态稳定机理;然后,针对大扰动下易于触发的限幅环节,分析了系统无法自主退出限幅而失稳的机制,并提出了附带电流分配系数的改进限幅策略,有效增强了系统暂态稳定性。最后,通过仿真验证了理论分析与改进方法的正确性。

适用于HVDC和HVAC高效稳定互联的MMC双端口电网形成控制

针对不同类型电网互联时互联电力变换器IPC(interconnecting power converter)控制模式复杂、控制难度大等问题,提出一种用于互联多个高压直流和高压交流子电网的IPC新型电网形成GFM(grid-forming)控制方法。该方法利用模块化多电平变换器MMC(modular multilevel converter)同时控制其AC和DC端电压,并提出2个双端口GFM MMC控制策略。针对单端口GFM控制和所提双端口GFM控制进行仿真对比,结果表明,与单端口GFM控制相比,双端口GFM控制方法对突发事件(如线路和发电机停运等)的处理更具弹性,且不需为电网中的IPC端口选择GFM或电网跟随GFL(grid-following)的控制方式。

Use of Voltage Limits for Current Limitations in Grid-forming Converters

Renewable generation interfaced through gridforming converters are proposed as a replacement for synchronous generators in power systems.However,compared to the synchronous generator,the power electronics converter has a strict limit on the current to avoid overcurrent damage.The gridforming converter acts like a voltage source,directily controlling the voltage.This conflicts with the operation of the conventional current limit control,which is applied to a current source.The switch between the voltage control and current control aimed to impose the current limit leads to synchronization instability.This paper proposes a novel control scheme which can be applied to the grid forming voltage control in order to enforce current limits.The proposed method has been verified through simulation and hardware tests in both symmetrical and asymmetrical faults to perform current suppression while maintaining synchronization stability in the voltage control mode.

Controller Design-oriented Analysis of Grid-forming Converters for Stability Robustness Enhancement

Grid-forming converters can suffer from control interaction problems in grid connections that can result in small-signal instability.Their inner-loop voltage controller tends to interact with the outer-loop power controller,rendering the controller design more difficult.To conduct a design-oriented analysis,a control-loop decomposition approach for grid-forming converters is proposed.Combined with impedance-based stability analysis,the control-loop decomposition approach can reveal how different control loops affect the converter-grid interaction.This results in a robust controller design enabling grid-forming converters to operate within a wider range of grid short-circuit ratios.Finally,simulation and experimental results,which validate the approach,are presented.

基于定向电流指令的构网变流器故障限流策略

构网控制技术可使变流器实现类似同步发电机的频率和电压支撑响应,有助于提高新型电力系统稳定性,其难点之一是在故障限流的同时提供支撑。为此,提出一种基于端口电压和内电势的压差进行电流指令定向的构网变流器故障限流策略。在该方法中,正、负序电流指令的角度分别滞后于正、负序压差90°,电流指令合成向量的最大幅值为变流器的过流能力,相当于通过控制分别产生正、负序自适应虚拟电感。该方法不需要直接计算虚拟电感值,且能够发挥变流器的最大支撑能力,实现类似同步发电机的故障电流特性。通过PSCAD/EMTDC搭建仿真系统,对所提出的故障限流策略进行验证,结果表明该策略能够在限制故障电流的同时对交流系统提供有效支撑。

Stability Comparison Between Grid-forming and Grid-following Based Wind Farms Integrated MMC-HVDC

Grid-forming(GFM)control based high-voltage DC(HVDC)systems and renewable energy sources(RESs)provide support for enhancing the stability of power systems.However,the interaction and coordination of frequency support between the GFM-based modular multilevel converter based HVDC(MMC-HVDC)and grid-following(GFL)based RESs or GFM-based RESs have not been fully investigated,which are examined in this study.First,the detailed AC-and DC-side impedances of GFM-based MMC-HVDC are analyzed.The impedance characteristics of GFL-and GFM-based wind turbines are next analyzed.Then,the influences of GFL-and GFM-based wind farms(WFs)on the DC-and AC-side stabilities of WF-integrated MMC-HVDC systems are compared and evaluated.The results show that the GFM-based wind turbine performs better than the GFL-based wind turbine.Accordingly,to support a receiving-end AC system,the corresponding frequency supporting strategies are proposed based on the GFM control for WF-integrated MMC-HVDC systems.The GFM-based WF outperforms the GFL-based WF in terms of stability and response time.Simulations in PSCAD/EMTDC demonstrate the DC-and AC-side stability issues and seamless grid support from the RESs,i.e.,WFs,to the receiving-end AC system.

Impedance Analysis of Grid Forming Control Based Modular Multilevel Converters

Grid-forming control(GFC)is promising for power electronics based power systems with high renewable energy penetration.Naturally,the impedance modeling for GFC is necessary and has gained significant attention recently.However,most of the impedance analyses for GFC are based on a twolevel converter(TLC)rather than a modular multilevel converter(MMC).MMC differs from TLC with respect to its dominant multi-frequency response.It is necessary to analyze the impedance of GFC-based MMC owing to its superiority in highvoltage direct current(HVDC)transmission to interlink two weak AC systems with high renewable energy penetration.As the main contribution,this paper presents the AC-and DC-side impedance analyses for the GFC-based MMC with both power and DC voltage control using the harmonic transfer function(HTF),and compares the impedances of GFC-based MMC and TLC.It is inferred that although the impedance is mainly influenced within 200 Hz,the instability still could occur owing to negative resistance triggered by relatively larger parameters.The difference in AC-side impedance with power and DC voltage control is not apparent with proper parameters,while the DC-side impedance differs significantly.The generalized Nyquist criterion is necessary for AC-side stability owing to the relatively large coupling terms under GFC.Moreover,the coupling between AC-and DC-side impedances is noneligible,especially considering the DC-side resonance around the system resonant peak.The effects of parameters,system strength,and virtual impedance on the impedance shaping are analyzed and verified through simulations.

电力系统变流器构网控制技术的现状与发展趋势

随着新能源和电力电子设备的渗透率增加,电力系统有着惯性减小、系统强度变弱的趋势,稳定性问题愈发严重。构网(grid-forming,GFM)控制技术可以提高变流器的电压、频率支撑能力,增强电力系统稳定性。对构网控制的现状与发展趋势进行了综述。首先,比较了构网控制技术与跟网型(grid-following,GFL)控制技术的差异,介绍了4类构网控制策略。然后,分析了构网控制技术在电力系统中的作用,介绍了具体的应用场景。最后,指出了该技术仍面临的挑战及可能的解决思路。