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: Physics >> General Physics: Statistical and Quantum Mechanics, Quantum Information, etc. submitted time 2023-08-12
Abstract： With the integration of large-scale new energy and power electronics, the power system has gradually formed a form of interconnection between networking equipment and network following equipment. The networking equipment mainly includes virtual synchronous machines and synchronous machines, while the network following equipment mainly consists of phase-locked loop converters. Synchronous stability is crucial as the foundation for the operation of large power grids, but due to the significant differences in synchronization mechanisms between networking devices and network following devices, it is extremely difficult to analyze the synchronous stability of interconnected systems. Therefore, this article utilizes the concept of dual excitation winding synchronous machines in physical understanding to construct an equivalent structure that can unify network equipment and network following equipment. In mathematics, a universal modeling approach is proposed to describe the synchronization characteristics of equipment, and a synchronization stability analysis model for interconnected systems based on current sources is established. Furthermore, the mathematical definition of synchronization stability in interconnected systems and several sufficient conditions for system synchronization stability are given. Finally, a simulation example was built in MATLAB/Simulink to verify the rationality of the model.
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
Subjects: Dynamic and Electric Engineering >> Electrical Engineering submitted time 2023-04-25
Abstract： There are many impedance-based methods to analyze the oscillation of grid-connected converter system (GCS) via dq frame, sequence frame and polar frame, which can be transformed into each other and are equivalent in judging the stability of the system. However, the discrepancy and adaptation of these methods has not been discussed theoretically. Therefore, the impedance models of GCS are established in above three kinds of coordinates respectively, then the mathematical and physical characteristics of them are analyzed. Secondly, the decoupling of impedance models in different frames are analyzed, and the differences of the impedance criteria in stability analysis and mechanism interpretation are given. Furthermore, the sub-synchronous oscillation caused by the phase-locked loop considering converter four quadrant operation scenario are illustrated, and the existence of open-loop transfer function right half plane poles and its sensitivity to uncertain parameters are analyzed from the nominal performance and robustness of the impedance criteria. On this basis, the adaptation of several impedance criteria of GCS are analyzed. Theoretical analysis and simulations confirm the validity of the proposed conclusion.
Subjects: Dynamic and Electric Engineering >> Electrical Engineering submitted time 2022-12-08
Abstract：多个外特性类似的直流和新能源等电力电子设备/场群同时馈入交流电网后形成了多电力电子馈入系统（简称多馈入系统），是新型电力系统中很典型的一种形态。系统电压支撑强度（简称系统强度）常用于描述该多馈入系统受到扰动后考虑了设备动态的电压响应性能，而电网强度则是描述不考虑设备动态的开环交流电网性能的术语。现有研究一般认为电网强度与系统强度之间存在正相关性，却并未揭示电网强度与系统强度之间的内在联系。为此，本文聚焦于电网受到小扰动后的系统强度问题，从抗扰性、小扰动稳定性以及静态电压稳定性三个角度分析了基于广义短路比的电网强度指标与系统强度指标之间的解析关系。研究表明，广义短路比本质是一种多端口网络的电压电流和电压无功最大灵敏度，是描述多个设备并网点与交流电网等效中心之间的一种综合电气距离指标，它与设备耐受电网的临界短路比之差反映了系统的稳定裕度和抗扰裕度，从而可用于量化系统强度。为了便于应用与实际操作，给出了广义短路比判据的若干个充分条件，并提出了多种从母线视角下量化电网强度的节点广义短路比指标，同时证明了传统CIGRE多馈入短路比本质是一种特殊的节点广义短路比，其保守性较强且场景扩展性较差。最后，对短路比的计算方法和适用场景等进行了探讨，并基于算例验证所提指标的有效性。
Peer Review Status:Awaiting Review
Subjects: Dynamic and Electric Engineering >> Electrical Engineering submitted time 2022-10-11
Abstract： The temporary overvoltage (TOV) during fault recovery process of renewable energy system seriously restricts the development of renewable energy. However, the existing research lacks quantitative indicators of the TOV. Therefore, based on the mechanism of TOV, this paper proposes a method to quantify the TOV risk of multiple renewable energy station system (MRESS) considering the reactive power saturation characteristic. Firstly, based on the TOV mechanism of MRESS, the analysis model under different factors during fault recovery is established, and then the rationality of TOV assessment based on phasor model is demonstrated. After fully considering the saturation characteristics of converters after fault and the interaction between renewable energy generators, an evaluation method for TOV of MRESS is derived. Through analyzing the relationship between TOV of the system and short-circuit ratio, the temporary overvoltage short-circuit ratio (TOVSCR) of MRESS and its application method are proposed. Simulation results show that the proposed method can effectively evaluate the TOV risk and safety margin of the MRESS.
Peer Review Status:Awaiting Review
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