考虑实际工况的新能源经MMC-HVDC送出系统强度评估方法

doi:10.19595/j.cnki.1000-6753.tces.241215

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Abstract

Large-scale renewable energy transmission via high voltage direct current system (HVDC) has become a typical scenario in modern power system. The evaluation method of voltage support strength (referred to as system strength) based on the generalized short-circuit ratio index can describe the stability margin of renewable energy transmission system via two-level converter HVDC. However, modular multilevel converter high voltage direct current system (MMC-HVDC) is more commonly utilized in power transmission of renewable bases. Compared with the two-level converter HVDC, MMC-HVDC is more complicated in dynamical characteristic because of complex harmonic coupling. Consequently, it remains unclear how to quantify the voltage support capability of the ac grid containing MMC-HVDC, especially when analyzing small-disturbance voltage stability. Furthermore, the operating conditions of renewables are changing in the actual operating system. Operating conditions have a significant impact on the stability of the system, further increasing the difficulty of assessing the system strength. To solve the challenge mentioned above, this paper proposes the system strength assessment method considering operating conditions, focusing on small-disturbance voltage stability.
Firstly, the frequency-domain closed-loop transfer function matrix of the renewables MMC-HVDC transmission system (RMTS) is established by linearization of the state equations at the stable operating point. The sensitivity relationship between bus voltage response and small-disturbance current is described by impedance matrix. Based on the transfer function matrix, the closed-loop characteristic equation of RMTS is derived. Secondly, the small-disturbance voltage support capability of MMC-HVDC is analyzed equivalently based on the maximum singular value of its impedance matrix, and an equivalent model of MMC-HVDC is proposed accordingly. This equivalence results in modification to the ac grid admittance matrix. Then, the definition of generalized operational short-circuit ratio (gOSCR) is extended to RMTS considering operating conditions. The relative difference between gOSCR and critical gOSCR is defined as the stability margin of RMTS, as performing the evaluation of the system strength. Finally, the effectiveness of the proposed methods is verified by electromagnetic transient simulation of 4-infeed RMTS in varied operating conditions. The results show that the evaluation results correspond with time-domain simulation waveform.
The following conclusion can be drawn: (1) The critical gOSCR reflects the tolerance of renewable energy devices to the ac grid strength supported by MMC-HVDC, which is related to the characteristics of devices. The gOSCR characterizes the voltage support capability of ac grids with MMC-HVDC. By comparing the gOSCR and critical gOSCR, the small-disturbance voltage stability of RMTS can be assessed in a rapid way, avoiding massive work of electromagnetic transient modeling. The proposed method can be applied in both the operational scenario and the planning phase. (2) The variation of power output and bus voltage of renewable energy are reflected in gOSCR. When the output of the renewables increases, the bus voltage of the renewables gradually decreases, leading to reduction in the system stability margin of RMTS and thus a decrease in system stability margin. (3) The gOSCR is also determined by the voltage support dynamic characteristics of MMC-HVDC. When the voltage support dynamic of MMC-HVDC approaches ideal voltage source, the gOSCR increases, resulting in increment of RMTS stability margin.

Key words： Renewables MMC-HVDC system strength generalized operational short-circuit ratio

Received: 10 July 2024

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TM712

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	Wang Xiangyu
	Xin Huanhai
	Fu Chuang
	Ma Fuyilong
	Hou Zhixian

Cite this article:

Wang Xiangyu,Xin Huanhai,Fu Chuang等. System Voltage Support Strength Evaluation Method for Renewables MMC-HVDC Transmission System Considering Operating Condition[J]. Transactions of China Electrotechnical Society, 0, (): 20241215-20241215.

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https://dgjsxb.ces-transaction.com/EN/10.19595/j.cnki.1000-6753.tces.241215 OR https://dgjsxb.ces-transaction.com/EN/Y0/V/I/20241215

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