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: Library Science，Information Science >> Information Science submitted time 2023-04-01 Cooperative journals: 《图书情报工作》
Abstract： [Purpose/significance] The demand for readers to access library information services through the mobile has become increasingly prominent, but the development of mobile library service models has fallen into problems such as single development form, lack of humanistic features, and poor service results. Through case study and analysis, this paper seeks for the construction and development strategy of mobile library service mode in colleges and universities, and provides important reference for the construction of mobile library service.[Method/process] Taking 42 "double first-rate" universities' mobile libraries as the research object, and using the method of network, literature research and case analysis. Studying the current status and development strategies of APP service, mobile webpage and WeChat platform.[Result/conclusion] The research found that both APP and mobile webpage use the fixed templates generally. The content and function homogenization phenomenon is serious, which lead to poor user experience. The management of WeChat Official Accounts is unevenness of quality, and the penetration rate of Mini Program is not high. Focus on some part of mobile library, instead of providing all services. Develop WeChat library and perfect service promotion. These will become the important measures to improve the mobile library services.
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