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ISSN 1000-0585
CN 11-1848/P
Started in 1982
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  • Table of Content
      , Volume 61 Issue 5 Previous Issue    Next Issue
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    SMALL-SIGNAL STABILITY
    A robust small-signal stability criterion for microgrid with distributed energy and an adaptive cover algorithm for the parametric security region
    MA Qianli, WEI Wei, MAO Hangyin, MEI Shengwei
    Journal of Tsinghua University(Science and Technology). 2021, 61 (5): 385-394.   DOI: 10.16511/j.cnki.qhdxxb.2021.21.016
    Abstract   HTML   PDF (5433KB) ( 204 )
    With the increasing proportion of distributed energy sources and power electronic devices in microgrids, the stability of multisource microgrids is influenced by the control parameters, thereby bringing great challenges to the safe and stable operation of a system. Aiming at a linear time-invariant system with uncertain parameters that change in given intervals, this paper gives a robust stability criterion based on the Kronecker matrix and Kronecker-related matrix to estimate the maximum variation range of the parameters that ensure a small-signal stability of the system, called the robust stability radius. The parameter space is further sampled. For stable parameters, the robust stability radius is used to estimate the neighborhood that ensures the small-signal stability of the system, and for unstable parameters, parameter sensitivity is used to estimate the neighborhood that is not small-signal stable. This paper proposes an adaptive coverage algorithm that can acquire a set of parameters that guarantees system stability through a hypercube coverage, called the small-signal stability region (SSSR). The proposed algorithm can adjust the size of the hypercube adaptively, thereby avoiding redundant searching calculations and improving calculation efficiency. A case study on a microgrid with generator-based and inverter-based energy resources is used to verify the effectiveness of the proposed algorithm. This algorithm can provide useful information for microgrid control parameter design and stability analysis.
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    Small-signal modeling of LCC-HVDC systems with switching for electromagnetic timescale stability analyses of power systems
    JIANG Kezheng, ZHU Jianhang, HU Jiabing, WANG Haijiao
    Journal of Tsinghua University(Science and Technology). 2021, 61 (5): 395-402.   DOI: 10.16511/j.cnki.qhdxxb.2021.21.011
    Abstract   HTML   PDF (2700KB) ( 244 )
    The wide application of power electronic equipment in power systems has caused significant changes in the dynamic behavior of modern power systems. As an important power system component, line commutated converter based high voltage direct current (LCC-HVDC) systems have an important impact on the safe, stable operation of power systems. Since the switching process in LCC-HVDC systems has intermittent time-varying characteristics and the switching frequency is equivalent to the electromagnetic timescale constant, these intermittent time-varying switching characteristics are the key to the stability of LCC-HVDC systems. This paper presents an electromagnetic timescale, small-signal model for LCC-HVDC systems that includes the switching process with a stability analysis. The model includes the nonlinear characteristics and the inherent periodic time-varying characteristics of LCC-HVDC systems for modeling the key challenges in the control system. Then, linear time-periodic theory is used to develop an electromagnetic timescale, small-signal model of the LCC-HVDC system. Finally, this model is compared with an existing model that does not consider the time-varying characteristics of the switching process to show the influence of the switching on the system stability with the results verified by time-domain simulations.
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    LARGE-DISTURBANCE STABILITY
    Large-disturbance stability of power systems with high penetration of renewables and inverters: Phenomena, challenges, and perspectives
    YANG Peng, LIU Feng, JIANG Qirong, MAO Hangyin
    Journal of Tsinghua University(Science and Technology). 2021, 61 (5): 403-414.   DOI: 10.16511/j.cnki.qhdxxb.2021.21.010
    Abstract   HTML   PDF (4034KB) ( 453 )
    The green energy revolution is leading to power systems with high penetrations of renewable energy sources and high penetrations of inverter-interfaced devices. The dynamic characteristics of these power systems are very different from conventional systems, which has led to many new stability problems. This paper focuses on large-disturbance stability issues and the large-disturbance instability phenomena that appear in these “double-high” power systems. This paper also discusses the shortcomings of existing methods and upcoming challenges. Finally, this paper presents our perspectives on future theoretical models and methods to provide large-disturbance stability in “double-high” power systems.
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    Electromagnetic transient synchronization stability with grid-connected inverters
    JIANG Qirong, ZHAO Chongbin
    Journal of Tsinghua University(Science and Technology). 2021, 61 (5): 415-428.   DOI: 10.16511/j.cnki.qhdxxb.2021.21.012
    Abstract   HTML   PDF (7459KB) ( 245 )
    The integration of new energy sources through grid-connected inverters (GCI) is changing the dynamic characteristics of modern power systems. Electromechanical transient stability analyses for systems dominated by synchronous generators are no longer comprehensive since they do not take the electromagnetic transients into account. This paper presents an overview of electromagnetic transient synchronous stability issues of GCI during fault ride-throughs of new energy generation units caused by large disturbances in which the GCI loses synchronization with the main system which causes the corresponding generator to go offline. This paper describes the GCI current source and voltage source control and the synchronization mechanism compared with that of the synchronous generator. This paper then introduces the key factors characterizing the power electronics and simplifications that give fast accurate results. Then, this paper introduces the typical modeling-analysis process and stability improvement strategies. Finally, further research challenges are identified to improve the power system stability.
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    FREQUENCY STABILITY
    Control strategy for primary frequency regulation with the participation of a quick response energy storage
    JIA Tianxia, CHEN Lei, MIN Yong, XU Fei, XIONG Xuejun, ZHAO Le, FENG Yuyao
    Journal of Tsinghua University(Science and Technology). 2021, 61 (5): 429-436.   DOI: 10.16511/j.cnki.qhdxxb.2021.21.015
    Abstract   HTML   PDF (4341KB) ( 233 )
    With the emerging frequency security problem of power systems, the application of quick response energy storage devices to the primary frequency control is an effective measure to ensure frequency security. This paper proposes a control strategy for primary frequency regulation with the participation of a quick response energy storage. The core idea is to design a whole transfer function based on the expected frequency response of the system and to design a primary frequency control strategy of the energy storage based on the whole transfer function. The demand for energy storage capacity is minimized with the optimization of the parameter. The control strategy is beyond the frame of traditional integrated inertia control and thus has a wide universality. The proposed strategy is applied in a simplified system to minimize the maximum frequency difference with the goal of obtaining a frequency response curve without overshoot. Results of the study verify that the control strategy proposed in this paper can significantly improve the frequency response characteristics compared with the traditional integrated inertia control strategy.
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    Virtual inertia configuration analysis considering small-signal stability and frequency stability
    HUA Yunyue, YANG Chaoran, HE Guoqing, XIN Huanhai
    Journal of Tsinghua University(Science and Technology). 2021, 61 (5): 437-445.   DOI: 10.16511/j.cnki.qhdxxb.2021.21.017
    Abstract   HTML   PDF (5036KB) ( 158 )
    With the increase in the proportion of new energy generation, the inertia of power system decreases. A virtual synchronous machine can provide inertia for the power system. At present, most of the parameter configuration of the virtual inertia is analyzed from the perspective of small-signal or frequency stability. These two factors have been simultaneously considered by a few studies to conduct virtual inertia configuration. Moreover, research on the configuration of virtual inertia mainly focuses on the voltage-source virtual synchronous machine, while a current-source virtual synchronous machine is rarely studied. To solve this problem, the influence of virtual inertia parameters on the small-signal stability and frequency stability is analyzed by establishing synchronous dominant loop models for voltage-source and current-source virtual synchronous machines. Results show that the small-signal stability of the system can be improved by decreasing the virtual inertia. However, under power perturbation, the output frequency of the current-source virtual synchronous machine will superimpose the transient component of Vq dominated by the virtual inertia, resulting in overshoot. If the virtual inertia parameter is too small, the frequency will not meet the grid-connected operation standard. Based on this, for the system to simultaneously exhibit good small-signal stability and frequency stability, the configuration of virtual inertia needs to be restricted by the two kinds of stability. Finally, the conclusion of this study is verified through the simulation of the inertia configuration of a single infinite machine system and an island two-machine system.
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    OSCILLATION STABILITY
    Sub-synchronous oscillation in wind farm integrated power system considering static var compensator
    HUANG Biyue, CHEN Yahao, SUN Haishun, MAO Yujie, HAN Yingsheng, WANG Dongze
    Journal of Tsinghua University(Science and Technology). 2021, 61 (5): 446-456.   DOI: 10.16511/j.cnki.qhdxxb.2021.21.018
    Abstract   HTML   PDF (3794KB) ( 126 )
    Recently, the long-standing sub-synchronous oscillation (SSO) in wind farms has greatly affected the operation of power systems. Large-scale wind farms comprise various types of wind turbine generators (WTGs) and dynamic reactive power compensation devices, further increasing the complication. Based on the practical phenomenon for further study, this paper focuses on the interactions among the direct-drive permanent synchronous generator (D-PMSG)-based wind farm, static var compensator (SVC), and connected grid. First, the impedance-frequency characteristics of SVC are studied based on the time-domain model of SVC and the frequency scanning method. Moreover, a state-space model of the D-PMSG-based wind farm equipped with an SVC is established to calculate the modes and their participation factors. The sub-synchronous interaction between the SVC and the D-PMSG-based wind farm will then be observed. Results indicate that the SVC may weaken the damping of the wind farm integrated power system and increase the risk of SSO. Furthermore, there is an obvious interaction between the SVC and the D-PMSG-based wind farm, which will lead to oscillation. System strength and control parameters of the SVC and D-PMSG-based wind farms can influence the oscillation characteristics. Theoretical results are verified by simulation.
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    Wide-area measurement and early warning system for wide-band oscillations in “double-high” power systems
    MA Ningning, XIE Xiaorong, TANG Jian, CHEN Lei
    Journal of Tsinghua University(Science and Technology). 2021, 61 (5): 457-464.   DOI: 10.16511/j.cnki.qhdxxb.2021.21.014
    Abstract   HTML   PDF (3069KB) ( 398 )
    The wide-band oscillation problem caused by the high penetration of renewable energy sources and the high proportion of power electronic equipment, “double-high” systems, seriously affects normal power equipment operation and power system stability. Such systems require on-line monitoring and analyses of the multi-mode and time-varying wide-band oscillation frequency. Traditional wide-area measurement systems (WAMS) can monitor low-frequency oscillations in power grids in real-time. However, they cannot monitor wide-band electromagnetic oscillations. This paper presents a wide-area measurement and early warning system (WAMWS) for monitoring wide-band oscillations in “double-high” power systems. This system has all the functions in the existing WAMS while monitoring wide-band oscillations in “double-high” power systems. The warning system provides wide-band state estimates, oscillation source identification, and security and stability evaluations of the wide-band oscillations. The effectiveness of this system for monitoring wide-band oscillations is verified in simulations. Finally, this paper considers applications of WAMWS.
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    PROTECTION
    Converter station transmission characteristics for protecting hybrid AC/DC power grids
    SONG Guobing, ZHANG Yuxuan, ZHANG Chenhao, HOU Junjie, XU Ruidong
    Journal of Tsinghua University(Science and Technology). 2021, 61 (5): 465-477.   DOI: 10.16511/j.cnki.qhdxxb.2021.21.009
    Abstract   HTML   PDF (1184KB) ( 104 )
    China has built the world's largest high voltage hybrid AC/DC power grid. The key component of the hybrid AC/DC system is the converter connecting the AC and DC power grids. The converter regulator is non-linear and time-varying with complex interactions between the AC and DC grids. For relay protection research, it is important to study the relationship of the electric quantities between both sides of the converter and the coordination of the protection configured in the AC/DC system. This paper introduces a mathematical model of the converter station transmission characteristics and its fault analysis limitations. Then, protection methods for the AC and DC systems are evaluated based on the transmission characteristics. A commutation failure on the DC side can be transmitted to the AC side which will influence the adaptability of the AC system protection. In addition, an AC system disturbance can be transmitted to the DC system, which will affect the DC system protection. Solutions are given to correct these incorrect AC/DC protection responses. Finally, additional research ideas are given for fault analyses and protection principles based on the converter transmission characteristics.
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    Single terminal waveform characteristic protection of flexible DC lines based on deep learning
    HE Jinghan, ZHANG Kexin, LI Meng, NIE Ming, SONG Yuanwei
    Journal of Tsinghua University(Science and Technology). 2021, 61 (5): 478-486.   DOI: 10.16511/j.cnki.qhdxxb.2021.21.013
    Abstract   HTML   PDF (4211KB) ( 92 )
    Multi-terminal flexible DC grids are a significant development trend for future smart grids, but they have significant line protection problems. Control systems must coordinate existing flexible DC line protection, complicated threshold settings and imperfect protection functions. This paper shows that the waveform characteristics of the reverse traveling wave contain abundant fault location information that can be used to develop a deep learning protection scheme. The reverse traveling wave is calculated based on a polar mode transformation and the line frequency parameters. Then, a normalized wave is used as the input to a deep learning model that adaptively mines the waveform characteristics. This then gives a bipolar fault diagnosis and single pole fault selection in the protected area. Simulations show that this scheme can provide a fault diagnosis within 2 ms and can withstand a 200 Ω transition resistance.
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