受静态电压稳定约束的新能源临界渗透率计算方法

随着大规模风电、光伏等新能源接入电网,新能源机组已开始影响电力系统的稳定性,同时受新能源接入电网的网架特性约束以及新能源装备自身安全限制,新能源并网的电压问题更加突出。主要研究了受静态电压稳定约束的新能源临界渗透率。首先分析了新能源机组并网的数学模型,并基于静态电压稳定约束提出新能源临界渗透率的工程实用计算方法;最后以实际电网为例,通过该方法计算新能源临界渗透率,并分析了影响新能源渗透率的相关因素。文中研究成果能够为新能源发电的最大消纳及区域新能源规划提供重要依据。     

高渗透率新能源并网对电网频率特性参数的影响分析

新能源发电不具备惯量能力和一次调频能力,随着电网中新能源渗透率的不断增加,会严重影响系统的频率稳定性。在MATLAB/Simulink中建立考虑新能源发电接入的电力系统频率响应模型,在此基础上理论分析了频率响应过程中各参数的解析表达式,并利用频率响应综合评估模型对不同新能源渗透率下的电网频率响应进行仿真,从新能源渗透率角度分析了电力系统频率特性指标受影响程度,为从新能源渗透率角度分析电力系统频率特性指标提供了参考。     

基于虚拟同步控制的双馈风电并网系统暂态功角稳定研究综述与展望

随着新能源渗透率增加,电力系统惯量、阻尼逐渐降低,稳定性问题日益凸显。虚拟同步发电机(virtual synchronous generator,VSG)技术作为一种提高新能源发电系统主动支撑电网能力的有效手段而获得广泛关注与应用,VSG并网系统暂态功角稳定问题也成为研究热点。首先介绍了电力系统暂态功角稳定分析方法,然后分析了常规双馈风电机组(doubly-fed induction generator,DFIG)接入对电力系统暂态功角稳定的影响,在此基础上,分析了采用不同技术路线的风电VSG并网系统暂态功角稳定分析的关键问题与研究现状,并对VSG并网系统暂态功角稳定的进一步研究进行了展望。     

考虑构网型与跟网型逆变器交互的孤岛微电网小信号稳定性分析

随着新能源在电力系统中的渗透率越来越高,低惯性成为以新能源为主体的新型电力系统的重要特征,极大影响了系统的同步稳定运行能力。由于缺少大电网的频率支撑,孤岛微电网在故障扰动下将产生频率偏移现象。同时考虑新能源渗透率和频率偏移的影响,研究低惯量孤岛微电网的小信号稳定性。低惯量微电网由构网型逆变器和跟网型逆变器组成,其中构网型逆变器采用虚拟同步机控制策略。首先建立了孤岛微电网的全阶小信号模型。应用特征值分析法,揭示了2种类型逆变器的功率渗透率对微电网系统小信号稳定性的影响规律。进一步利用参与因子法分析了系统参数和控制参数对微电网系统稳定性的影响程度。最后仿真结果验证了理论分析的准确性。研究成果为低惯量微电网中2类逆变器的容量规划、参数设计和控制器优化提供了理论支撑,旨在提高微电网的小扰动同步稳定性。     

考虑稳态和动态有功平衡约束的新能源消纳综合评估方法

随着新能源占比的增大,电网调峰能力和频率稳定问题已成为新能源消纳的主要制约因素,提出一种考虑调峰、频率稳定以及机组调节性能等稳态、动态平衡条件约束下的新能源消纳能力综合评估方法。考虑联络线调峰,计算电网的新能源渗透率。通过应力和寿命之间的关系计算出机组频繁调节下的寿命损耗,分析对新能源接纳能力的影响。并把频率稳定作为重要约束条件,分析不同类型扰动例如一般扰动和严重故障限制下的新能源消纳水平。分析结果揭示了不同因素下的交互制约关系,提高了评估新能源消纳能力的系统性和适应性,并给出了相关建议。     

低调频容量高比例新能源主导局部电网低频失稳分析

随着局部电网新能源发电比例的提高及调频容量的降低,系统低频段阻抗变化及频率维持特性削弱引发了局部电网的低频失稳现象。现有弱电网假设及固定基频的分析方法与电网调频特性不对应,导致系统低频振荡耦合频率波动失稳机理分析受限。针对含柴油同步发电机组及并网逆变器的局部电网,构建了含电压、电流、基频扰动的涵盖阻抗外特性与基频扰动特性的新能源主导发电模型框架。基于所建立的电网强度与渗透率、网侧等效阻抗的耦合关系,分析了强电网弱化后源网端低频段阻抗变化及衍生低频失稳诱因。构建了含基频扰动项的回比矩阵,分析了频率特性项对系统低频失稳的潜在影响并预判了不同渗透率下系统稳定情况。最后,结合硬件在环实验结果,在临界失稳工况下与传统固定基频分析方法进行对比,所提模型可准确预判低调频容量高比例新能源主导局部电网的低频失稳,验证了上述理论分析的正确性。     

计及SSSC的高渗透率新能源电网静态电压稳定特征研究

随着大规模风电、光伏等新能源接入电网,高渗透率新能源逐渐影响到电力系统的安全稳定运行。同时受新能源接入电网的网架特性约束以及新能源装备自身安全限制,新能源并网后的静态电压问题更加突出。探讨分析了计及静态同步串联补偿器（SSSC）的高渗透率新能源电网系统静态电压稳定特征,从系统静态电压特征方面评估SSSC发挥的潮流控制效能。首先分析了SSSC的等效电路及工作原理,并基于SSSC的等效功率注入模型得到潮流计算方程;其次,基于潮流计算方程提出能够反映系统静态电压稳定特征的效能评估指标,并计算分析接入不同容量新能源对静态电压稳定特征的影响,以及SSSC对新能源并入电网后薄弱节点静态电压稳定特征的影响;最后通过仿真验证,安装SSSC后能够改善新能源电网薄弱节点的电压稳定问题,提升电力系统运行时的安全稳定性。所提方法具有快速简洁的特点,并且具有一定的工程应用价值。     

基于机电扰动传播特性的电网惯量分布辨识方法

大规模新能源通过电力电子设备并入电网,影响电力系统的动态运行特性。惯量是反映电网抗频率扰动能力的重要特性指标。基于机电扰动传播特性建立数学模型,通过分析扰动传播速度与惯量之间的映射关系,提出了电网母线节点的惯量分布辨识方法,实现对电网不同节点的频率抗扰动能力的在线评估。基于惯量分布结果,进一步评估了新能源接入对电力系统惯量特性的影响。通过IEEE 39节点系统仿真验证了所提惯量分布辨识方法的有效性,分析了新能源接入对电网惯量分布的影响。     

面向区域间振荡模式的电力系统小扰动惯量域构建

为提升低惯量电力系统的小干扰稳定性,给出保障适应新能源发展的合理惯量裕度,并明确惯量分布对于互联系统小干扰稳定性的影响规律。研究了惯量分布对系统区域间振荡模式的影响规律,构建了面向区域间振荡的电力系统小扰动惯量域(Small Signal Inertia Region, SSIR)。首先,基于部分惯量中心(Part of the Center of Inertia,PCOI)等值方法和多项式Leverrier解法,构建了等值两机系统区间振荡模式阻尼比解析式。在此基础上,依据临界阻尼比构建了计及小干扰稳定约束的惯量域。进而,对区域互联系统惯量分布和小干扰稳定性之间的联系进行解析。最后,对惯量域进行全面分析,并在新能源系统中进行了初步验证。仿真结果表明,所提出的计算方法能够实现电力系统小扰动惯量域的准确、快速构建,增强低惯量电力系统小干扰稳定性的评估和监控能力,并为新能源惯量域的构建提供理论指导。     

新能源电力系统中需求侧响应关键问题及未来研究展望

化石能源短缺和节能减排的双重压力促使中国能源发展方式亟待转型调整。随着新能源发电的规模化并网应用,传统电网正在逐步向着新能源电力系统方向演变,并对其运行控制带来显著的影响。作为一类虚拟可控资源,在新能源电力系统中考虑需求侧响应(DR)可有效克服新能源发电的间歇性问题,提高电网对新能源的利用效率,实现源荷互动与协同增效。文中首先简要介绍了新能源电力系统的基本特征及内涵理念;其次,根据资源的分类及特点,总结了不同类型DR对新能源电力系统的潜在贡献及作用影响;在此基础上,从规划、运行、控制、评价这4个维度对新能源电力系统中DR问题的研究情况进行了总结,并就上述领域值得进一步关注的研究方向给出了相关建议;最后,结合当前国情,对推动新能源电力背景下DR应用及其保障机制建设提出了一些建议。     

Robust gain-scheduled control for vibration suppression

Limited control authority is a key issue in the field of structural control and is a major research area since most of the practical control problems are dominated by constraints on the control signal. The paper presents a simple and practical gain-scheduled controller design procedure for active vibration suppression of a three-storey flexible structure. First, system identification experiments are performed and the plants uncertainty is derived. Next, robust controller design with constraint on the control signal is presented. For a better trade-off between control performance and control constraint a gain-scheduling approach is investigated. Stability analysis of the gain-scheduled controller is analysed using a parameter-independent Lyapunov function (quadratic stability) as well as a parameter-dependent Lyapunov function (biquadratic stability). Finally, the gain-scheduled controller is tested experimentally when the flexible structure is excited with a scaled historical earthquake record (1940 El Centro record). Successful experimental results show that the proposed robust gain-scheduled control approach offers good performance in the case of control authority limitation.Nomenclature M mass matrix - C damping matrix - K stiffness matrix - q relative displacement vector to base - m_a active mass - active mass acceleration - ground (base) acceleration - K_gap transfer gain of the displacement sensor - K_acc transfer gain of the acceleration sensor - K_AMD transfer gain of the active mass damper - _id frequency range of system identification experiments - r control reference - u control signal - d external disturbance - y control output - e control error - x state vector - p(t) time-varying parameter - N_p parameter boxs dimension - V(x) Lyapunov function - V(x,p) parameter-dependent Lyapunov function - largest parameter box where quadratic stability holds - S(s),T(s) sensitivity and complementary sensitivity transfer functions - G_n(s),G(s) transfer function of nominal and real plant - K_rate rate feedback gain - P_n(s) transfer function of nominal plant modified by rate feedback - P(s) transfer function of real plant modified by rate feedback - G_red transfer function of the reduced-order plant - _m(s) multiplicative uncertainty - W_S(s),W_T(s) performance and robustness weighting functions - G_c(s) controllers transfer function - G_c1(s),G_c2(s) transfer function of robust controller for vertex 1 and vertex 2 - G_cs(p,s) transfer function of the gain-scheduled controller - u_A amplitude of the control signal - K_min,K_max minimum and maximum controller gain - K(p) scheduled controller gain - J₁,J₂,J₃,J₄,J₅ performance evaluation parameters     

The localness of electromechanical oscillations in power systems

An innovative index, indicative of the relative localness of electromechanical oscillations in electric power systems, is introduced in this paper. The L_index is calculated using the normalized participation factors obtained from a small signal analysis of the system. With the help of simple representative examples the efficacy of the index to understand power system dynamic behavior, like coherency identification is established.     

Research on voltage sensitivity of vibration accelerometer based on cymbal transducer

The cymbal piezocomposite transducer was used as the sensitive element of a new type of vibration accelerometer. In this paper voltage sensitivity, S _v, of the accelerometer was researched. Voltage sensitivity, S _v, as a function of operation frequency, f, and voltage sensitivity as a function of preinstalled mass, M, was analyzed. The dimensions of cymbal transducer effecting on voltage sensitivity were also discussed. The experimental results are shown that the voltage sensitivity of this new kind of accelerometer is more 30 times than that of conventional piezoelectric vibration accelerometer which sensitive element is PZT-5A disk.     

Evaluation of the reduction in CO2 emissions by applying Micro‐Grid to home energy supply system

Dispersed generators such as wind power systems, photovoltaic systems, and cogeneration systems are expected to mitigate the environmental burden of energy consumption, and their installation has been promoted recently. Micro‐Grid is focused on as a method to solve some problems in a commercial electric power line when installing a large number of dispersed generators, and some demonstrative research on Micro‐Grid for large‐scale systems is being carried out now. Also, small cogeneration systems for houses, such as gas engines and fuel cells, are expected to improve CO₂ emissions. However, if the power and heat demand of a family are relatively small or are unbalanced, the cogeneration system does not operate effectively. The authors have studied the application of Micro‐Grid for home energy supply, and have developed a control system to solve this problem. The system achieves a reduction of CO₂ emissions and energy costs by sharing electric power and heat among some houses with cogeneration systems. This paper presents an outline of the newly developed system, and in particular describes the effect of the reduction in CO₂ emissions compared with a conventional energy supply method, and the case in which dispersed generators are installed in some houses and operate independently. © 2009 Wiley Periodicals, Inc. Electr Eng Jpn, 170(3): 19–27, 2010; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20879     

Magnetic core properties and their thermal stability in a BCC Fe-Zr-B-Cu alloy with nanoscale grain size

A mostly single bcc phase with nanoscale grain size of 10 to 20 nm has been found to form by annealing amorphous Fe-Zr-B, Fe-Hf-B and Fe-M-B-Cu (M = Ti, Zr, Hf, Nb and Ta) alloys. It has further been clarified that the newly developed nanocrystalline bcc alloys exhibit high permeability (μ_e) combined with high saturation magnetization (B_s). Subsequently, the possible application potentials of the bcc alloys were investigated. The core loss is as small as 66 mW/kg at 1 T and 50 Hz for a nanocrystalline bcc Fe₈₆Zr₇B₆Cu₁ alloy obtained by annealing for 3.6 ks at 873 K. This value is 45 and 95 percent smaller than those for amorphous Fe₈₆Si₉B₁₃ alloy and Fe-3.5 percent Si alloy, respectively, which are presently in use as core materials in electric power transformers. The frequency dependence of the core loss for the bcc alloy at 0.2 T is almost the same as that for an amorphous Co_70.5Fe_4.5Si₁₀B₁₅ alloy with zero magnetostriction. In the frequency range of 10 to 300 kHz, the core loss for the bcc alloy is slightly smaller than the Co base amorphous which has been used as core material in high-frequency transformers. Furthermore, the core losses of the nanocrystalline Fe₈₆Zr₇B₆Cu₁ alloy also were found to have high stability against thermal aging. Thus, nanocrystalline bcc Fe-Zr-B-Cu alloys with the advantages of high B_s, high μ_e, and low core loss is expected to be used as a core material in various transformers.     

考虑工频过电压的海上风电场无功配置方案研究

随着海上风电能源的不断开发,海上风电场的过电压问题对系统的安全运行极为重要,而无功补偿问题影响着系统的电能质量,因此在确定无功补偿方案的同时也应该考虑过电压的限制。本文基于海上风电场含有架空线路和海底电缆混合输电线路的风电场电磁暂态模型,提出了一种考虑工频过电压的无功配置方案,利用ATP/EMTP仿真软件分析了海上风电场工频过电压,并在此基础上对风机不同出力情况下的风电场无功容量需求进行计算,从而得到符合系统安全运行范围的海上风电场无功容量配置方案,并以某海上风电场为例,最后利用MATLAB/Simulink仿真验证了所提方案的可行性。     

PHASE TRANSITION IN Pb(Mg1/3Nb2/3)0.68TiO0.32O3 SINGLE CRYSTAL

ABSTRACT

Phase transition of (001)-cut Pb(Mg_1/3Nb_2/3)_0.68Ti_0.32O₃(PMNT32%) single crystal have been investigated by polarizing light microscopy in the temperature ranged form room temperature to 200 K. The transformation from monoclinic (M_A) to cubic (C) phase through an intermedial monoclinic (M_C) phase, i.e. M_A-M_C-C, was observed in the heating run, and the C-M_C-M_A phase transition sequence was observed in the cooling run. The temperature-induced rotation of extinction position, which was the result of polarization rotation, was observed in M_C phase. It is interesting that tetragonal (T) phase has not been reached before the M_C phase turn into cubic phase. The existing phase diagram for PMNT system was improved by this experimental result. In addition, it was found the phase transition processes of M_C-M_A and cubic-M_C show distinct characters though both of they are first-order.     

基于双重熵值修正指标权重的电能表状态评价研究

对电能表的状态进行全面、准确的综合评价,有助于指导实际检修工作,减少资源浪费。提出了一种基于双重熵值修正指标权重的电能表状态评价方法。具体地：首先,根据指标建立原则,梳理并构建了较为完备的电能表状态评价指标体系。其次,对传统熵值法进行了改进,给出了一种基于双重熵值确定指标权重的方法,兼顾了评价指标的主观经验性和客观规律性。然后,提出了一种电能表状态评价模型,并在某省级电网中应用,分析了不同类别和不同批次的电能表状态特征;并与传统评价方法对比,突出了所提方法的有效性和优越性。最后,结合现场经验和电能表状态评价结果,给出了一种灵活的电能表周期性检验策略,可以减少物力和人力消耗。     

Improved estimation of tissue hydration and bound water fraction in rat liver tissue

Based on recent improvements in the field, biexponential data from fresh rat liver and monoexponential data from cold storage experiments allow quantification of three distinct relaxation components in liver tissue: bound water (4.2%, R₁=12.0 ± 1.7 s^–1, R₂=440 ± 180 s^–1); structured water (59%, R₁ 3.3 ± 0.07 s^–1, R₂ 24.9 ±1.1 s^–1); and free water (37%, R₁=R₂ 0.4 s^–1). However, only the relaxation rates of the structured water component change with water content: R_1A (s^–1)=6.53 * M_s/M_w – 0.77 (r²=0.911); R_2A (s^–1)=71.15 * M_s/M_w – 3.09 (r²=0.956), respectively. This suggests a slow exchange between bound and structured water in liver cells.