Subjects: Dynamic and Electric Engineering >> Electrical Engineering submitted time 2024-03-07
Abstract： The large-scale integration of renewable energy into the power grid leads to the decrease of short circuit ratio (SCR) and system voltage support strength, which results in stability issues such as static voltage stability problems and sub/super synchronous oscillation. Grid-forming (GFM) converters have effective voltage support capacity and 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 is difficult to estimate the required proportions of GFM converters theoretically. Therefore, from the perspective of small signal stability, this paper discusses the estimation method and typical values of the capacity ratios between GFM converters and grid-following (GFL) converters. Firstly, 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. Secondly, considering changing some of the wind turbines into GFM control and installing new GFM converters, the typical values of the capacity ratios in practical engineering are discussed based on typical parameters of step-up transformers and using relevant industry standards as boundary conditions. The validity of the conclusion is verified by simulation results of the multi-wind-farm system.
Subjects: Dynamic and Electric Engineering >> Electrical Engineering submitted time 2023-12-20
Abstract： With the large-scale integration of renewable energy and power electronic devices, system voltage support strength decreases, thus threatening the system's safe and stable operation. In the homogeneous scenario where power electronic devices integrated into the system have the same dynamics, a theoretically rigorous and highly stable quantitative method for quantifying system voltage support strength can be formed based on the generalized short-circuit ratio(gSCR) and the device critical short-circuit ratio(SCR0) In the heterogeneous scenario where power electronic devices integrated into the system have weakly different dynamics, system voltage support strength can be quantified by the first-order approximation of gSCR and SCR0, based on some special dynamic characteristics of devices and power grid. However, there is a lack of unified calculation principles at the theoretical level. To this end, this paper focuses on quantifying system voltage support strength under small-signal stability and discovers that the multi-infeed system can be approximately decoupled into multiple low-dimensional systems. On this basis, this paper proposes the concept and approximate calculation method of eigen-subsystems and interprets their physical significance. Based on the concept of eigen-subsystems, the general calculation principle of gSCR and SCR0 were unified, which achieves strength quantification of heterogeneous multi-infeed systems. Additionally, the specific calculation methods for gSCR and SCR0 are provided in typical scenarios, such as grid-following converters under non-rated operating conditions, reverse active power output, and considering grid-forming devices. Finally, the effectiveness of the calculation principles and methods is verified in several cases.
Peer Review Status:
Awaiting Review
Subjects: Dynamic and Electric Engineering >> Electrical Engineering submitted time 2023-11-29
Abstract： With the development of renewable energy, the voltage support strength of renewable energy multi-infeed systems is gradually decreasing, and there is an urgent need for strength evaluation methods. If considering that converters are all at the rated operating conditions, accurate quantification of strength can be achieved by utilizing existing power grid strength indices and threshold; However, the actual operating conditions are diverse and complex, which disrupts the existing criteria for defining indices and leads to the failure of existing methods. Therefore, this paper focuses on the quantification of voltage support strength under actual operating conditions. Firstly, the mapping relationship between the dynamic characteristics of grid-following converters, operating conditions, and parameters is discovered. Secondly, based on the perturbation theory of characteristics subspaces, an equivalent single-infeed system that can approximate the stability of the actual system is solved. On this basis, the operational generalized short circuit ratio(OgSCR) are defined. A rigorous method for quantifying voltage support strength can be formed based on the OgSCR and the device critical operational short-circuit ratio (OSCR0). In addition, this paper reveals the effect of actual operating conditions on strength. Finally, a numerical example is used to verify the feasibility of the proposed method
Peer Review Status:Awaiting Review
Subjects: Physics >> General Physics: Statistical and Quantum Mechanics, Quantum Information, etc. submitted time 2023-08-31 Cooperative journals: 《中国全科医学》
Abstract：背景 抗生素滥用导致的耐药问题已成全球重大公共卫生议题，减少公众上呼吸道感染的抗生素不合理使用是我国治理抗生素滥用的重要策略，识别上呼吸道感染疾病病症特点及其对抗生素不合理使用行为的影响有助于临床医生设计更为精准的干预政策。目的 定量分析公众上呼吸道感染症状的潜在模式，探究其对抗生素使用行为的影响。方法 本研究采用整群随机抽样调查，于 2022-07-2008-02 选取重庆市三个县（区）的公众作为调查对象。调查公众上呼吸道感染疾病症状、公众上呼吸道感染抗生素使用行为、公众抗生素使用知识及人口学特征。采用潜在类别分析方法鉴别公众上呼吸道感染疾病症状的潜在模式，采用多因素 Logistic 回归分析探索不同潜在症状模式对抗生素使用行为的影响。结果 815 位公众参与本研究。其中，30.06%（245/815）受访者存在无处方从药店购买抗生素，14.72%（120/815）受访者使用过抗生素自我药疗以应对上呼吸道感染。公众抗生素合理使用知识水平较低〔（2.31.7）分〕。上呼吸道感染疾病发病模式方面，潜在类别分析共识别 4 种症状模式，包括多样症状组 39 例（11.41%）、全身症状组 124 例（15.21%）、鼻咽症状组 282 例（34.60%）和轻微症状组 316 例（38.77%）。多因素 Logistic 回归分析显示：相较于轻微症状组，鼻咽症状组出现无处方购买抗生素行为的概率更高（OR=1.538，P<0.05），结果在调整知识与人口学变量后仍显著。除个体疾病症状潜在模式外，年龄和医保类型也对公众无处方抗生素购药行为有显著影响（P<0.05）。抗生素使用知识水平对抗生素自我药疗行为有显著影响（OR=0.869，P<0.05），对公众无处方抗生素购药行为也有显著影响（OR=1.155，P<0.05）。结论 公众上呼吸道感染症状存在 4 种潜在模式，疾病病症模式显著影响公众抗生素的合理使用，应着重关注出现鼻咽症状患者的抗生素不合理使用行为。
Subjects: Dynamic and Electric Engineering >> Electrical Engineering submitted time 2023-06-06
Abstract： 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.
Peer Review Status:Awaiting Review
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