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.
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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.
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