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: Electronics and Communication Technology >> Electron Technology Subjects: Dynamic and Electric Engineering >> Electrical Engineering submitted time 2024-01-30
Abstract： In the scope of our investigation, we have solved the stability problem of the boost converter. Ouyang Changlian's doctoral dissertation made me realize the importance of duty cycle in control system.
Peer Review Status:
Awaiting Review
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: Dynamic and Electric Engineering >> Electrical Engineering submitted time 2023-11-29
Abstract： With the development of renewable energy, the voltage support strength of renewable energy multi-infeed systems is gradually decreasing, and there is an urgent need for strength evaluation methods. If considering that converters are all at the rated operating conditions, accurate quantification of strength can be achieved by utilizing existing power grid strength indices and threshold; However, the actual operating conditions are diverse and complex, which disrupts the existing criteria for defining indices and leads to the failure of existing methods. Therefore, this paper focuses on the quantification of voltage support strength under actual operating conditions. Firstly, the mapping relationship between the dynamic characteristics of grid-following converters, operating conditions, and parameters is discovered. Secondly, based on the perturbation theory of characteristics subspaces, an equivalent single-infeed system that can approximate the stability of the actual system is solved. On this basis, the operational generalized short circuit ratio(OgSCR) are defined. A rigorous method for quantifying voltage support strength can be formed based on the OgSCR and the device critical operational short-circuit ratio (OSCR0). In addition, this paper reveals the effect of actual operating conditions on strength. Finally, a numerical example is used to verify the feasibility of the proposed method
Peer Review Status:Awaiting Review
Subjects: Dynamic and Electric Engineering >> Electrical Engineering submitted time 2023-07-28
Abstract： The high penetration of the renewable energy generation brings significant volatility and uncertainty for the power system operation. To address this issue, it is necessary to involve a significant amount of flexible resources for grid dispatch and ancillary services. The numerous, widely distributed, and diverse characteristics of these flexibility resources make cluster-based management difficult, requiring a quantitative evaluation of their power flexibility range. This article proposes a quantification metrics system for aggregating flexible resources based on their application requirements in grid dispatch and the power market. By classifying the energy resources in the cluster into generator-type and storage-type, they are respectively aggregated into equivalent generator and equivalent storage devices as the reference model. The parameters of these two equivalent devices serve as the quantification metrics, possessing intuitive physical meanings, robust nested characteristics, and applicability across multiple time scales. These metrics can be widely applied in many scenarios, such as the asynchronous dispatch of distributed resource clusters, the coordinated optimization dispatch of integrated energy systems, and bidding of virtual power plants participating in the power market.
Peer Review Status:Awaiting Review
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
Subjects: Dynamic and Electric Engineering >> Electrical Engineering Subjects: Mathematics >> Control and Optimization. submitted time 2022-08-07
Xiaodong Zheng
Baorong Zhou
Xiuli Wang
Bo Zeng
Jizhong Zhu
Haoyong Chen
Peizheng Xuan
Waisheng Zheng
Abstract： Quick-start generation units are critical devices and flexible resources to ensure a high penetration level of renewable energy in power systems. By considering the wind uncertainty, and both binary and continuous decisions of quick-start units within the intraday dispatch, we develop a Wasserstein-metric-based distributionally robust optimization model for the day-ahead network-constrained unit commitment (NCUC) problem with mixed integer recourse. We propose two feasible frameworks for solving the optimization problem. One approximates the continuous support of random wind power with finitely many events, the other leverages the extremal distributions instead. Both solution frameworks rely on the classic nested column-and-constraint generation (C&CG) method. It is shown that due to the sparsity of L1-norm Wasserstein metric, the continuous support of wind power generation could be represented by a discrete one with a small number of events, and the extremal distributions rendered are sparse as well. With this reduction, the distributionally robust NCUC model with complicated mixed-integer recourse problems can be efficiently handled by both solution frameworks. Numerical studies are carried out, demonstrating that the model considering quick-start generation units ensures unit commitment (UC) schedules to be more robust and cost effective, and the distributionally robust optimization method captures the wind uncertainty well in terms of out-of-sample tests.
Subjects: Dynamic and Electric Engineering >> Electrical Engineering Subjects: Mathematics >> Control and Optimization. submitted time 2022-08-07
Abstract： In this paper, a study of the day-ahead unit commitment problem with stochastic wind power generation is presented, which considers conditional and correlated wind power forecast errors through a distributionally robust optimization approach. Firstly, to capture the characteristics of random wind power forecast errors, the least absolute shrinkage and selection operator (Lasso) is utilized to develop a robust conditional error estimator, while an unbiased estimator is used to obtain the covariance matrix. The conditional error and the covariance matrix are then used to construct an enhanced ambiguity set. Secondly, we develop an equivalent mixed integer semidefinite programming (MISDP) formulation of the two-stage distributionally robust unit commitment model with a polyhedral support of random variables. Further, to efficiently solve this problem, a novel cutting plane algorithm that makes use of the extremal distributions identified from the second-stage semidefinite programming (SDP) problems is introduced. Finally, numerical case studies show the advantage of the proposed model in capturing the spatiotemporal correlation in wind power generation, as well as the economic efficiency and robustness of dispatch decisions.
Subjects: Dynamic and Electric Engineering >> Electrical Engineering submitted time 2022-05-29
Abstract：
Many methods and criteria of stability assessment have been proposed due to the complex dynamics of renewable power system. The physical implications of these methods/criteria and their scopes of adaptation are different. It is difficult to theoretically answer that, for certain stability issue, which stability criterion is suitable and whether the corresponding physical interpretation is reasonable. This paper was divided into two parts. The methods for assessing the adaptation of stability criteria were put forward, and the question whether the corresponding physical interpretations were reasonable were answered in part I. While the adaptation of each stability criterion was analyzed by applying the method and the physical interpretation of each stability problem was derived in part II. In part I, the derivation process of the existing stability criteria and their underlying physical significance were reviewed at first. Secondly, the selection principles of stability criteria were proposed from 3 aspects, namely stability equivalence, nominal performance and robust stability. And the quantitative index named loop gain sensitivity was proposed. Finally, the adaptation of several impedance criteria of grid-connected converters were analyzed.?It was shown that the nominal performance and robustness of these impedance criteria were not equivalent in frequency domain modal analysis. The adaptation of other typical stability criteria and the corresponding physical interpretation were analyzed in part II.
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Peer Review Status:Awaiting Review
Subjects: Dynamic and Electric Engineering >> Electrical Engineering submitted time 2022-05-29
Abstract：
From the perspective of stability equivalence, nominal performance and robustness, the qualitative principles and quantitative evaluation index of stability criterion adaptation are proposed in Part Ⅰ. In addition, the adaptation of the existing stability criteria and the physical interpretation of instability corresponding to the suitable stability criterion are analyzed for the typical equipment in power system, such as synchronous machines, doubly-fed induction generators (DFIG) and converters in Part II. Thereby the stability classification of the renewable power system is discussed based on dominant output variable. Firstly, aiming at low-frequency and sub-synchronous oscillation/resonance (SSO/SSR) of synchronous machine, the rationality of the existing stability analysis methods and physical interpretation is verified by using the proposed evaluation index. Secondly, the proposed evaluation index is used to analyze the oscillation of converters and DFIG, and the suitable stability criterion, derivation mechanism and dominant output variable are further discussed. Finally, a novel idea of equipment stability classification based on physical mechanism and dominant output variables are proposed, which classifies equipment stability from the vector perspective into three categories: phase-dominated synchronous stability, amplitude-dominated voltage stability, and electrical resonance formed by their special combination. Issues such as the concept of wide-band oscillation, the distinction between equipment stability and system stability, and the extensibility of their classification are further explored.
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Peer Review Status:Awaiting Review
Subjects: Dynamic and Electric Engineering >> Electrical Engineering Subjects: Dynamic and Electric Engineering >> Engineering Thermophysics submitted time 2022-04-10
Abstract：
Exergy is an index to measure energy quality, which reveals the essence of work capacity loss in the process of energy transfer. In order to study the dynamic evolution mechanism of exergy in time-varying energy networks, the constitutive relations between physical quantities in energy networks are listed based on the energy network theory, and the generalized expressions of exergy are given, including the generalized description and loss equation of exergy based on the second law of thermodynamics, and the energy level factor is introduced to evaluate the energy quality. The dynamic evolution process of different forms of exergy in the energy transfer tube (line) is analyzed, including electric exergy, thermal exergy and pressure exergy, and the output equation and efficiency calculation method of exergy are given. The dynamic evolution process of exergy in energy conversion equipment is analyzed, and the loss, storage and efficiency of exergy are calculated. Finally, the dynamic evolution process of exergy in an integrated energy system is simulated by a specific example. The study of this paper can fully tap the energy efficiency potential of the integrated energy system, and lay a solid theoretical foundation for better realization of energy cascade utilization.
Peer Review Status:Awaiting Review
Subjects: Energy Science >> Engineering of Energy Sources System Subjects: Dynamic and Electric Engineering >> Electrical Engineering submitted time 2019-07-09
Abstract： The challenges of multi-area power system optimal dispatch exist on how to be in line with certain dispatching mode as well as on the modeling/solution. In this article, a multi-area system day-ahead scheduling, with a provincial systems-reducing method and a generation variances-minimizing objective, is proposed, based on the hierarchical dispatching organization and the long-term bilateral energy contract mode in China. Non-iteratively, the sub-optimal scheduling of ac/dc tie-lines and independent generation units can be derived from solving the model, which also considers some practical constraints like discrete constraints of dc power profiles and power flow limits of local ac interface, etc. Case study is carried out based on the realistic data from the multi-area hybrid ac/dc southern China power system, to verify the effectiveness and feasibility of the proposed model. It is demonstrated that with this method, the upper-level dispatching institution can generate an optimized and reliable transmission power plan with limited information, which decreases the peak-valley difference and standard variance of generation series, relieves the burden of peak regulation of its sub-systems, and hence improves the economic efficiency. The approach suits the dispatching mode of power systems in China well, handles various operation scenarios, and thus has been implemented in the system operator.
Peer Review Status:Awaiting Review
Subjects: Dynamic and Electric Engineering >> Electrical Engineering submitted time 2019-03-05 Cooperative journals: 《电气工程学报》
Abstract：针对变频空调用功率因数校正（PFC）电路中的电感在运行时存在局部过 热、温升较高的情况，本文采用电磁热耦合仿真对其进行了优化设计。通过有限元分 析，搭建了 PFC 电感三维电磁热场的仿真模型，利用电磁仿真计算得出 PFC 电感的 功率损耗，并将其作为热源进行了热场模拟。通过改变绕组的匝数和磁心的尺寸，模 拟了四组不同的设计方案，分别进行了磁场 - 热耦合分析，从中给出了优化设计方案。 实际运行表明，优化后的 PFC 电感有效降低了温升和损耗，设计方法对于其他电感器 件的设计具有参考价值。
Subjects: Dynamic and Electric Engineering >> Electrical Engineering submitted time 2019-03-05 Cooperative journals: 《电气工程学报》
Abstract：不均匀气隙结构的异步起动永磁体同步电机是针对普通异步起动永磁同步 电机由于气隙磁场谐波复杂而导致的运行过程中转矩脉动大、电磁噪声高等问题提出 的，此结构电机的永磁体退磁情况与普通电机有较大不同。本文基于永磁体内各处的 磁场矢量基本相同的原则，利用永磁体内部一点的磁密表征整个永磁体在运行过程中 磁密的变化，对不均匀气隙结构异步起动永磁体同步电机永磁体工作点展开了理论研 究与有限元仿真。分析了影响电机退磁的因素，研究了施加不同负载与转动惯量时永 磁体的磁密变化情况，得到电机稳态运行时和起动过程中永磁体工作点的变化规律， 提出了获得永磁体磁密最小工作点的研究方法。
Subjects: Dynamic and Electric Engineering >> Electrical Engineering submitted time 2019-03-05 Cooperative journals: 《电气工程学报》
Abstract：为了提高风力发电机叶片故障的识别率，利用支持向量机建立风力发电机 叶片故障和特征参数之间的非线性关系。在蜂群算法中引入一种动态柯西因子，动态 调节蜂群寻优过程中的搜索步长，提高蜂群算法的扰动能力，避免蜂群陷入局部搜索， 采用这种动态柯西蜂群算法对支持向量机的参数寻优，建立动态柯西蜂群算法优化的 支持向量机模型。采集南方某风场风力发电机叶片的四种工况下的特征数据训练此模 型并进行故障诊断，诊断结果表明改进后的蜂群算法优化支持向量机模型能够提高风 力发电机叶片的故障识别率，具有一定的工程参考意义。
Subjects: Dynamic and Electric Engineering >> Electrical Engineering submitted time 2019-03-05 Cooperative journals: 《电气工程学报》
Abstract：传统的感应耦合电能传输（ICPT）系统中，松耦合变压器接收线圈 Rx 的 半径与发射线圈 Tx 到接收线圈 Rx 之间的距离 h 的最佳比例系数，即径距比 γ 的求取， 是通过系统建模来推出 Tx 和 Rx 间的互感值，并辅以大量实验来获得的。针对这种缺 少理论依据并浪费人力、物力的问题，本文提出了一种通过仿真观察 Rx 中电流密度变 化规律来获得 Rx 半径与 h 比例系数的最佳值的设计方法。首先求取单管 ICPT 系统之 一次侧并联、二次侧并联（PP）结构下的系统去耦等效电路模型，据此推导出 Rx 上 的电流密度和互感 M、系统传输功率的关系；并通过公式分析，确定 Rx 电感 L1 与 Tx 电感 L2 的比值 a 和系统的耦合系数 λ 的取值，来确定在不同频率下 L1 和 L2 的值，从 而建立仿真模型，利用有限元仿真软件对 γ 的最优值进行研究。相比于通过线圈互感 值来优化线圈的方法，本文中的电流密度可以通过软件直接观测到，形象而且直观，节省时间和成本，有效提高设计效率。综合仿真结果确定了松耦合线圈最佳径距比γ，该参数与企业通过生产实践总结出的经验值相吻合。
Subjects: Dynamic and Electric Engineering >> Electrical Engineering submitted time 2019-03-05 Cooperative journals: 《电气工程学报》
Abstract：针对 IEEE 次同步谐振第一标准测试系统，提出抑制次同步谐振的可控串补 （TCSC）方案。利用 PSCAD/EMTDC 仿真软件，建立 TCSC 及其控制仿真模型，仿真 研究得到能抑制次同步谐振的可控串补和固定串补组合方案。采用开环阻抗控制策略 对 TCSC 抑制次同步谐振的性能进行仿真验证。
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