For a multi-inverter grid-connected system, the stability of the point of common coupling (PCC) voltage is evaluated considering the distribution parameters of the transmission lines. First, the systems on both sides of the PCC are equalized, a small-signal equivalent circuit similar to the “current source-grid” is established, and a mathematical model for the voltage of the PCC is derived. Then, using Euler's formula and Nyquist stability criterion, the PCC voltage stability of the grid-connected system is evaluated by the impedance analysis method under the premise that the single-side excitation is stable. In addition, the grid-connected conditions causing PCC voltage instability are studied. A phase compensation method based on an impedance phase compensation control strategy is introduced. The stability of the grid-connected system is improved by compensating the phase margin at the equivalent impedance crossover-section frequency on both sides of the grid-connected system PCC. Finally, a simulation circuit is built to simulate and analyze the proposed model and phase compensation method. The simulation results verify the accuracy and effectiveness of the theoretical analysis.

In power systems, dense frequency signals (DFSs) comprise several adjacent harmonics or inter-harmonics derived from power electronic devices. Signals that contain two or more significant frequency components within any 15 Hz frequency band are considered DFSs. Existing frequency detection methods cannot detect DFSs with precision in a 10-cycle-period. To solve this issue, an improved dense frequency signal detection (DFSD) method is proposed. Firstly, this paper presents a dense frequency model by which a signal can be decomposed into a series of intrinsic mode functions via complete ensemble empirical mode decomposition (CEEMD). Then, singular value decomposition (SVD) is performed to reduce the dimensions and eliminate noise. Finally, the intensive frequency model is solved via an independent component analysis (ICA), and the parameters of each frequency component are calculated. Simulation and experimental results demonstrate that the proposed method can accurately calculate dense frequency parameters. Considering asynchronous sampling, the proposed method has higher accuracy for harmonic and inter-harmonic detection under the IEC framework.

针对非同步采样条件下密集频率信号无法准确测量的问题,提出一种密集型频率信号检测方法.基于离散傅里叶变换建立密集型频率信号模型,然后对单通道观测信号做周期延拓,形成多路信号.使用可变主成分分析法进行去相关性处理.再基于极大似然盲源分离法求解密集型频率信号模型.最后根据比例系数校正各频率成分参数.仿真和实验结果表明,所提方法具有较高的精度和一定的抗噪声能力.

针对电缆分布参数在并网系统中引发的稳定性问题,提出一种考虑线路分布参数的输入导纳修正判据.首先,通过多层变量代换与适当简化形成实数域的并网系统电导和电纳模型,建立考虑线路分布参数的线路双曲函数并网系统简化模型;其次,验证输入导纳判据对于考虑线路分布参数的并网系统稳定性分析的适应性,分析系统短路比、线路长度、线路参数对于并网系统稳定性的影响;最后,通过Simulink仿真与传统方法的对比分析,验证了所提方法的正确性和有效性.

针对传统多点电力负荷预测方法未进行数据校正问题,提出离群数据挖掘的多点电力负荷预测方法.通过计算离群数据点与其邻近点的距离,进行离群数据的挖掘;依据离散傅里叶转换数据集合,实现离散数据点集合求取;采用储存间距获取数据点与其邻近点的距离;节点与整体单位设定单层或多层节点,根据整体辅助节点负荷构建多点预测模型;利用信息熵准则选取聚类的代表性点与合并点,通过负荷特征曲线达成数据校正,从而实现多点电力负荷的预测.仿真结果表明,提出的电力负荷预测方法不仅具有精准预测的有效性,噪声去除效果最佳,而且预测效率较高.

Among the methods used for analysis of harmonic resonance, modal analysis theory and modal sensitivity can be used to illuminate the nature and properties of this phenomenon, and are thus widely applied. In this paper, an improved modal sensitivity-based method is proposed to reliably suppress the harmonic resonance. First, we show that the current resonance analysis schemes that are guided by traditional modal sensitivity are not always robust in some cases. Then, through a theoretical analysis, the roots of the shortcomings of the traditional modal sensitivity are determined. The key factor is that the modal sensitivity of a network component is not always constant but tends to vary with the adjustment of the component parameters. Finally, to quantify this sensitivity variation and improve the robustness of the analysis results, a second-order modal sensitivity and a critical index are proposed. The results of the IEEE 14-bus system and a field case involving high voltage DC (HVDC) systems indicate that the proposed indexes can accurately show the effects of each network component on the resonance, thus are useful in the design of the corresponding analysis scheme.

当实测数据相位角缺失时,现有计算谐波阻抗方法通常失效.由此,该文提出一种无需测量数据相位角而仅需测量点谐波电压、电流幅值及其相位差的系统侧谐波阻抗计算新方法.所提方法首先依据系统短路容量估算系统侧谐波阻抗初值并反解出相应的背景谐波电压,再排序分段该背景谐波电压.据此,依据各段背景谐波电压与相应测量数据之间的弱相关性计算各段数据对应的谐波阻抗,并根据谐波阻抗波动最小准则求取真实谐波阻抗.所提方法不依赖于阻抗初值的选取,当用户侧谐波阻抗非远大于系统侧谐波阻抗且背景谐波波动较大时,所提方法仍能取得好的评估效果,有效解决了实测数据相位缺失时系统侧谐波阻抗计算难题.仿真和实测数据验证了所提方法的有效性.

针对电压暂降评估问题,提出一种基于互信息与改进灰靶理论的电压暂降严重度数据挖掘分析方法.提出暂降距离和交叉影响域指标,考虑电压暂降的多种特征属性建立严重度评估框架;构建互信息网络模型对电压暂降进行数据挖掘,利用连接权提取关联规则构成电压暂降知识库,对全网电压暂降严重度水平进行初步分析,避免了基于支持度和置信度的传统数据挖掘方法产生的冲突规则问题;提出靶心度的改进计算方法,结合数据挖掘所得的全网暂降严重度水平,建立电压暂降严重度条件搜索模型进行变电站暂降严重度评估,解决了匹配结果多重化的问题,提高了电压暂降评估的准确性.算例分析验证了所提方法的可行性.

城市配电网的高电缆化率,光伏系统接入时易引发谐波谐振问题;光伏系统并网运行时会向电网注入谐波,造成谐波污染.提出了一种基于相关向量机回归的光伏系统谐波发射水平计算方法,首先建立光伏电站诺顿等效电路;将公共连接点测量数据作为输入向量,利用相关向量机回归模型得到系统谐波阻抗,进而计算光伏系统谐波发射水平.克服了支持向量机方法,其核函数必须满足Mercer条件、参数难以确定等问题.仿真分析和实测数据计算验证了所提方法的有效性.

Evaluating the harmonic contributions of each nonlinear customer is important for harmonic mitigation in a power system with diverse and complex harmonic sources. The existing evaluation methods have two shortcomings: (1) the calculation accuracy is easily affected by background harmonics fluctuation; and (2) they rely on Global Positioning System (GPS) measurements, which is not economic when widely applied. In this paper, based on the properties of asynchronous measurements, we propose a model for evaluating harmonic contributions without GPS technology. In addition, based on the Gaussianity of the measured harmonic data, a mixed entropy screening mechanism is proposed to assess the fluctuation degree of the background harmonics for each data segment. Only the segments with relatively stable background harmonics are chosen for calculation, which reduces the impacts of the background harmonics in a certain degree. Additionally, complex independent component analysis, as a potential method to this field, is improved in this paper. During the calculation process, the sparseness of the mixed matrix in this method is used to reduce the optimization dimension and enhance the evaluation accuracy. The validity and the effectiveness of the proposed methods are verified through simulations and field case studies.

The sparse distribution characteristics of renewable energy resources can lead to there being tens of kilometers of transmission lines between a grid-connected inverter and the actual grid. Accurate analysis of the stability of such grid-connected inverter systems currently involves using a complex hyperbolic function to shaped model of the transmission line circuit. This has proved to be problematic, so, drawing upon the distribution parameter characteristics of transmission lines, this paper looks at how to use impedance-based stability criteria to assess the stability of multi-paralleled grid-connected inverters. First, the topology of multi-paralleled inverters connected to the grid via transmission lines is established, using each transmission line terminal as a grid connection point. Each grid-connected system is taken to be equivalent to a small-signal circuit model of the "current source-grid". Euler's formula and the Nyquist stability criterion are combined to assess the stability of the associated grid-connected current transfer functions and evaluate the stability of the grid-connected current. Finally, a simulation analysis circuit is constructed to verify whether power line intervention will cause stability problems in the grid-connected system. Overall, it is found that long-distance transmission lines are more likely to cause unstable output of the grid-connected current. It is also found that the number of grid-connected inverters, the short-circuit ratio (SCR), the distorted grid and the inverter parameters can all have a significant impact on the stability of the grid-connected current.

风机滤波器在某些谐波频率呈现低阻抗,估计谐波发射水平误差大,因此提出基于改进协方差特性估计永磁直驱风电场谐波发射水平的方法.根据风机滤波器和风电场网络结构,聚合各元件阻抗用于简化永磁直驱风电场侧附加谐波阻抗计算.在此基础上,利用改进协方差特性建立了公共连接点处谐波电压和电流的偏差量与系统侧和风电场侧的谐波阻抗的表达式,得到系统谐波阻抗,进而求出谐波发射水平.该方法克服了公共连接点处谐波电流与背景谐波相关性带来的误差.仿真分析和实测数据计算验证了所提方法的有效性.

Distributed parameter characteristics of Ultra-long Distance Transmission Lines (UDTLs) are prone to cause harmonic amplification. Amplified harmonic components deteriorate the quality of energy delivery, and consequently affect the safety and operations of a power grid. This paper proposes a method based on Frequency-Length Factor (FLF) to investigate the harmonic transmission characteristics (HTCs) for UDTLs, including Half-Wavelength Transmission Lines (HWTLs). The proposed method considers the impact of line loss and reveals the comprehensive effects of line length, operation mode, and frequency on HTCs analysis for UDTLs. It is proved that only inter-harmonics can be amplified and cause resonance in lossy standard HWTLs propagation. Using the proposed method, the severity of harmonic amplification is quantitatively calculated. Additionally, this paper provides a fast evaluation approach for potential resonance frequencies of UDTLs, mitigating power quality issues caused by harmonic amplification. The effectiveness of the proposed method is verified by PSCAD simulation.

现有分散式多谐波源责任量化方法均采用谐波源等值电路(常用诺顿等效模型)的外部端口电流(统称为"谐波源端口电流")来计算该谐波源对关注母线的谐波贡献度.由于谐波源端口电流受基尔霍夫定律约束,只要电网中某谐波源端口电流发生变化,所有谐波源对关注母线的谐波贡献度会随之改变,但该电流通常是谐波源和背景谐波耦合作用的结果,由此计算的谐波贡献度不能真实反映该谐波源的发射水平.基于此,文中运用谐波源电流本身而非谐波源端口电流来计算谐波贡献度,所提方法直接计算各谐波源对关注母线的谐波电压矢量贡献,而无须求解各谐波源对关注母线的谐波转移阻抗,减少了误差累积效应,可更为公平合理地反映各谐波源责任.仿真和实验算例验证了所提方法的有效性和准确性.

With the large-scale deployment of power electronic equipment, harmonic sources in the power system are gradually increasing. Traditional harmonic source location methods are inaccurate and they usually require prior information of the location, which is difficult to obtain. This study presents a novel method of harmonic source location that can use less prior information to accurately locate the location of harmonic sources. First, a linear screening criterion is constructed using sparse component analysis. Then, the source signals separated by complex independent component analysis are filtered by linear and non-Gaussian secondary screening, improving the accuracy of source signals and mixing matrix. Finally, the precise location of the harmonic is obtained by matching the mixing matrix with the columns of the matrix, which related to the system topology. Three IEEE systems are used to validate the proposed method. The simulation results show the high accuracy of the proposed method in the harmonic source location.

Aiming at the fact that the independent component analysis algorithm requires more measurement points and cannot solve the problem of harmonic source location under underdetermined conditions, a new method based on sparse component analysis and minimum conditional entropy for identifying multiple harmonic source locations in a distribution system is proposed. Under the condition that the network impedance is unknown and the number of harmonic sources is undetermined, the measurement node configuration algorithm selects the node position to make the separated harmonic current more accurate. Then, using the harmonic voltage data of the selected node as the input, the sparse component analysis is used to solve the harmonic current waveform under underdetermination. Finally, the conditional entropy between the harmonic current and the system node is calculated, and the node corresponding to the minimum condition entropy is the location of the harmonic source. In order to verify the effectiveness and accuracy of the proposed method, the simulation was performed in an IEEE 14-node system. Moreover, compared with the results of independent component analysis algorithms. Simulation results verify the correctness and effectiveness of the proposed algorithm.

在非同步采样条件下,若电网采样信号中谐波和间谐波相邻,会出现严重的频谱干涉问题,且无法识别出信号中实际频率成分.针对以上问题,提出了一种基于快速独立分量分析(FastICA)的频谱分离算法测量谐波和间谐波参数.首先构建了多频率成分模型,将频谱中的谱线表示为多个频率成分分量的叠加,然后利用FastICA算法和最小二乘法得到频率成分参数,最终实现了对相邻多频率成分的测量.仿真结果表明,该算法可以在需求谱线数较少的情况下准确识别频率成分并保持较好的测量精度,且具有一定的抗噪能力.

To analyze the distribution characteristics of voltage and current along half-wavelength transmission lines (HWTLs) in the cases with or without short circuit in the steady state, the method based on the frequency-length factor (FLF) for lossy lines is proposed. Firstly, according to the pole condition of the FLF, the distribution characteristics of power-frequency waves along HWTLs are analyzed. Then, the comprehensive effects of the system parameters and fault resistance are explored, revealing the mechanism of the power-frequency resonance caused by nonmetallic short circuit. Meanwhile, unbalanced short-circuit fault is studied by exploiting additional impedance. The results show that the distribution of the maximum value of power-frequency resonance voltage is related to the system parameters but not to the fault impedance. When a HWTL is short circuited at 2640 km∼2930 km, the resonance voltage can reach to 21 p.u. In relation to symmetrical short circuit, the resonance voltage appears at 1469 km from the short-circuit point, while the position moves towards the short-circuit point with the increase of additional impedance in asymmetrical short-circuit conditions. Additionally, the model theoretically proves that the power-frequency overvoltage induced by short circuit does not appear on a line whose length is less than 1469 km. Finally, cases are studied on PSCAD to verify the accuracy of the model.

提出了一种基于云模型的谐波负荷状态监测方法.以谐波电流历史正常监测数据建立一年的总体样本;根据总体样本的统计规律计算正态云的3个数字特征-期望、熵和超熵;将其转换到标准正态云下计算95％概率值和最大值两个评估指标的偏高和异常隶属度;最后计算未来的谐波电流每天95％概率值和最大值在标准正态云下的隶属度,并与模型所确定的隶属度进行比较,从而对谐波运行状态做出评判.介绍了应用于四川省电能质量全景数据平台的监测结果.

非侵入式负荷识别可以实现电网和用户的灵活双向互动,对智能电网的发展具有重大意义,而神经网络因其自学习能力及计算复杂度低等优点越来越多地应用在非侵入式负荷识别中.针对现有BP神经网络方法容易陷入局部最优、收敛速度慢的问题,文章提出了一种基于广义回归神经网络(GRNN)的非侵入式负荷识别方法.该方法使用负荷投切过程的功率、谐波、投切时间等暂态特征作为输入,应用Parzen非参数估计方法搭建网络结构,利用模拟退火算法的全局搜索能力对光滑因子进行寻优,从而建立GRNN网络模型进行负荷识别.实验结果表明,相对于BP神经网络,文中方法具有更好的识别精度和训练速度.