Grid-forming (GFM) control has been considered a promising solution for accommodating large-scale power electronics converters into modern power grids thanks to its grid-friendly dynamics, in particular, voltage source behavior on the AC side. The voltage source behavior of GFM converters can provide voltage support for the power grid, and therefore enhance the power grid (voltage) strength. However, grid-following (GFL) converters can also perform constant AC voltage magnitude control by properly regulating their reactive current, which may also behave like a voltage source. Currently, it still remains unclear what are the essential differences between the voltage source behaviors of GFL and GFM converters, and which type of voltage source behavior can enhance the power grid strength. In this paper, we will demonstrate that only GFM converters can provide effective voltage source behavior and enhance the power grid strength in terms of small signal dynamics. Based on our analysis, we further study the problem of how to configure GFM converters in the grid and how many GFM converters we will need. We investigate how the capacity ratio between GFM and GFL converters affects the equivalent power grid strength and thus the small signal stability of the system. We give guidelines on how to choose this ratio to achieve a desired stability margin. We validate our analysis using high-fidelity simulations.

With the development of renewable energy,the voltage support strength of multi-infeed systems with renewable energy is gradually decreasing,and there is an urgent need for strength evaluation methods.Considering that devices are all at the rated operation condition points,accurate quantification of strength can be achieved by utilizing existing power grid strength indices and threshold.However,the actual operation conditions of devices are diverse and complex,which disrupts the existing criteria for defining indices and causes existing methods to fail.Therefore,this paper focuses on the quantification problem of voltage support strength under actual operation conditions.Based on the perturbation theory of eigen-subspaces,an equivalent single-infeed system that can approximately characterize the stability of the actual system is solved.On this basis,the definition and physical significance of the generalized operational short-circuit ratio are provided,so as to rigorously quantify the strength under actual operation conditions using the generalized operational short-circuit ratio and the critical operational short-circuit ratio for devices.In addition,this paper reveals the specific mechanism of the actual operation conditions on strength with different stable forms.Finally,the effectiveness of the proposed method is verified through cases.

The large-scale integration of renewable energy into the power grid leads to the decrease of system voltage support strength,which is usually characterized by its short circuit ratio(SCR).This results in stability issues such as static voltage stability problems and sub/super synchronous oscillation.Grid-forming(GFM)converters have effective voltage support capability,and thus equipping renewable energy stations with a certain proportion of GFM converters can improve the stability of the system.However,the analytical relationship between the capacity of GFM converters and the stability margin is unclear,which makes it difficult to estimate the required proportions of GFM converters theoretically.To fill this gap,this paper discusses the estimation method for the required capacity ratios between GFM converters and grid-following(GFL)converters to maintain small signal stability.First,based on the voltage-source equivalent analysis of GFM converters,the influence of the capacity ratios on the system strength and stability margin is analyzed through the generalized short circuit ratio(gSCR)index.Then,considering the practical engineering scenario of updating some wind turbines to GFM control or installing new GFM converters,the typical values of the capacity ratios are discussed based on typical parameters of step-up transformers and the related industry standards as boundary conditions.The validity of the conclusion is verified by simulation results of a typical multi-wind-farm system.

The ever-increasing adoption of power electronic converters for renewable integration and energy-saving applications has weaken grid and caused new types of small-signal stability issues, such as sub-/super-synchronous oscillations, resulting from the interaction between the fast-acting converter control and the power network. It is challenging to analyze the small-signal stability issues by grid strength assessment due to the complex interaction between many converter-interfaced generators (CIGs) and the power network in a multi-CIG system (MCIGS). This complexity is further increased when considering different non-rated operating conditions of each CIG, such as their actual power injections and terminal voltages, which manifest the heterogeneity in the MCIGS. Moreover, in such a heterogeneous MCIGS, it is difficult to characterize the small-signal stability boundary under non-rated operating conditions. To address the challenges, this paper derives generalized operational short-circuit ratio (gOSCR) and critical gOSCR (i.e., CgOSCR) by the small-signal stability analysis in such a heterogeneous MCIGS. Based on gOSCR and CgOSCR, a method is proposed for grid strength assessment to identify the small-signal stability issues in such a heterogeneous MCIGS. This proposed method can be used when CIGs are modeled by either black-box or white-box modeling in a MCIGS under non-rated operating conditions. More importantly, the proposed method can also be used to assess grid strength in terms of the static voltage stability in a MCIGS under non-rated operating conditions, since the static voltage stability issue is a special case of the small-signal stability issue with focus on zero frequency band for converter controls. The efficacy of the proposed method is demonstrated in the IEEE 39-bus system and a practical power system with a large-scale renewable integration.

With the large-scale integration of renewable energy sources, the system voltage support strength (hereinafter referred to as "system strength") gradually decreases, leading to an increased risk of system instability. In order to improve the stability of the system, energy storage systems (ESSs) based on grid-following (GFL) converters have been widely integrated. However, it is still unclear how ESSs affect the system strength of heterogeneous systems integrated with heterogeneous converter-based-renewables (CBRs) (hereinafter referred to as "heterogeneous multi-CBR systems"), leading to a lack of effective optimization site selection methods of ESSs. This paper focuses on the ESS site selection method in the heterogeneous multi-CBR system. Firstly, based on the perturbation theory, we solved and obtained the equivalent single-converter subsystem, which can represent the system strength of the heterogeneous multi-CBR system containing ESSs. On this basis, we reveal the mechanism by which ESSs affect the heterogeneous system strength. Furthermore, an optimization site selection method of ESSs based on a sensitivity index is proposed to better enhance the heterogeneous system strength.

随着新能源及电力电子设备的大规模接入，系统电压支撑强度降低，新型电力系统的安全稳定风险增加。在同构电力电子设备接入场景下，利用电网广义短路比和设备临界短路比可以形成理论严谨、与稳定强相关的系统电压支撑强度量化方法；在弱异构场景下，基于设备和电网一些特殊动态特性，利用广义短路比和设备临界短路比的一阶近似可以实现强度量化，但缺乏通用的计算原理。为此，本文聚焦小扰动稳定维度下电压支撑强度的量化问题，首先利用电力电子多馈入系统可近似解耦为多个低维系统的规律，提出特征子系统的概念和近似计算方法，并诠释了其物理意义；其次，基于特征子系统，提出了广义短路比及其临界短路比的通用计算原理，实现了异构多馈入系统的强度量化。此外，针对跟网型电力电子设备在非额定运行点、有功功率反向以及包含构网型电力电子设备等几种典型场景，给出了具体计算公式。最后，算例验证了所提原理和方法的正确性和有效性。

新能源发电设备的大量接入显著改变了现代电网的动态特性,给电网的安全稳定带来了挑战,其中由锁相环带宽与电网强度不匹配导致的次/超同步振荡问题尤为严重.已有研究表明,外特性表现近似为电压源的构网型变流器能够减少由锁相环主导的新能源电力系统小干扰稳定问题,并且为系统提供较强的电压支撑能力,但基于构网型变流器的储能系统对多新能源馈入系统稳定性的作用机理及其选址定容优化方法仍不清晰.为此,文中揭示了基于构网型变流器的储能系统对多新能源馈入系统小干扰稳定性的提升作用机理,从提升系统稳定裕度的角度研究了基于构网型变流器的储能系统选址定容优化方法.最后,基于算例验证了所提方法的有效性.

系统电压支撑强度(简称系统强度)常用于描述系统电压响应性能和量化安全稳定水平,其中基于短路比的强度量化指标在描述新能源送出极限时具有简单直观的特点.然而,现有基于短路比的分析基础是以同步机提供短路容量或电压支撑为前提,难以适用于无同步机支撑的新能源基地柔性直流送出系统.为此,该文从小扰动角度探讨新能源基地柔性直流送出系统的强度评估问题,首先推导了该系统中多端口电流对母线电压的灵敏度传递函数矩阵,阐述了受扰后母线电压响应与系统静态电压稳定/小扰动同步稳定的定性关系;然后,基于送端柔直电压源等值思路,并结合新能源设备临界短路比提出了源-网分离的系统强度评估方法,将广义短路比推广应用到全电力电子系统,所提出的方法能快速分析在系统运行点处的静态电压稳定/小扰动同步稳定裕度,确定系统强度的薄弱环节以及系统强度提升的优化路径;最后,多风电场经柔性直流送出系统算例验证了评估方法的有效性.

System strength is generally used to describe the voltage response performance under a disturbance and quantify the stability margin. The system strength indicated by short-circuit ratio (SCR) has provided a simple and intuitive reference for grid operators. However, the existing SCR-based methods rely on the premise that synchronous generators provide short-circuit capacity and voltage support. Due to this premise, these methods are unsuitable for renewables delivery systems with voltage source converter-based high voltage direct current (VSC-HVDC), where all apparatuses are power-electronic interfaces. This paper aims to respond the system strength evaluation problem in terms of small-disturbance analysis. Firstly, the sensitivity transfer function matrix of the bus voltage to the renewables multi-feed current is derived and the relationship between the voltage performance and stability is illustrated; Secondly, the generalized short-circuit ratio is extended into the renewables VSC-HVDC delivery system based on the voltage-source equivalent modeling of VSC-HVDC; Then, by combing the apparatus critical SCR and generalized short-circuit ratio, a source-grid separation method can be further proposed to quantify the system strength of such a system. The proposed method can assess the static voltage stability margin or small-disturbance synchronous stability margin at the operating point and determine the critical bus of system strength and the optimization path of system strength improvement. Finally, the proposed method is verified by simulation in multiple wind plants with VSC-HVDC.

随着电力电子设备的并网规模增加,电力系统电网强度相对降低,小干扰失稳风险增加.调相机可为大规模电力电子设备馈入的电力系统提供电压支撑,降低系统失稳风险,但量化调相机对系统稳定性提升效果的研究尚不成熟,选址规划方法缺乏.为此,文中首先建立了含调相机的多馈入系统动态模型;其次,揭示了调相机提升系统电网强度和提高系统稳定裕度的机理;最后,从提升系统稳定裕度的角度研究了调相机优化选址的方法.通过改进的2区4机系统和39节点系统的仿真算例验证了所提方法的有效性.