Islanding detection using wavelet transform and rate of change of frequency relay method in presence of different distributed generation technologies

In this paper, islanding detection in the presence of different technologies of distributed generations (DGs) is discussed. Three types of DGs, namely a synchronous generator (SG), a wind energy conversion system (WECS), and a photovoltaic (PV) system, with distribution network parameters as well as linear and nonlinear loads are simulated using the Power System Computer Aided Design/Electromagnetic Transients Including DC (PSCAD/EMTDC) software. To detect the island state, a comprehensive islanding detection algorithm based on wavelet transform (WT) and rate of change of frequency (ROCOF) relay method is considered. To determine the islanding threshold value, the voltage and frequency values of sensitive buses are sampled at regular intervals, and the signals are analyzed with WT using MATLAB. The proposed algorithm could detect the exact island state definitively with the consideration of various disturbance signals such as capacitor bank switching, induction motor switching, and the sudden disconnection of power sources and loads from different buses of the distribution network. The results of the simulation are validated with the logic of ROCOF relay islanding detection method, which determines the accuracy threshold value when the power generation and load consumption difference is negligible. In addition, the proposed algorithm based on the rested DG's output power in the islanding state can supply prioritized loads based on thier importance. © 2016 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

计及分布式电源的配电网可靠性研究

阐述了分布式电源(DG)的几种运行方式和功率输出模型,依据孤岛划分原则建立了划分模型,结合广度和深度优先搜索法制定出计划孤岛划分策略,并采用配电网评估的最小路算法进行了DG配电网可靠性指标计算.算例仿真与分析结果表明,DG接入配电网可以在一定程度上提高电网供电的可靠性.     

分布式电源并网后的自适应负载反孤岛方法

大量分布式光伏电源并网后,孤岛问题愈加突出。先分析了现有的一些反孤岛方案,并分析了其优缺点。随后,提出了采用自适应负载反孤岛的方案,即利用负载的自动投切,改变整体负载的特性,使得被动式反孤岛无死区。并采用PSCAD对该方案进行了仿真,验证了该方案的实用性。为尽量减少分布式光伏电源并网的投资,该方案依托于新一代的分布式光伏并网接口及反孤岛装置实现。随后介绍了反孤岛系统的整体构架及功能配置。该系统能解决大量分布式光伏电源发生孤岛运行后的快速离网问题,为分布式光伏电源的大面积并网打下坚实基础。     

Optimized restoration of combined ac/dc shipboard power systems including distributed generation and islanding techniques

Reconfiguration involves changing the status (OFF/ON) of switches, and reconfiguration for restoration involves changing the switch status to maximize the supply to loads that are left unsupplied after fault removal. Shipboard Power Systems (SPS) need automated reconfiguration for restoration schemes to restore vital loads quickly and efficiently in order to improve fight-through and survivability capabilities. The restoration in this paper is achieved using optimization with multiple objectives—maximizing the restored load and giving priority to vital loads. A restoration scheme for SPS with an integrated power system (IPS) and distributed generation (DG) involving islanding has been developed. This formulation includes a hybrid power system that has both ac and dc parts. The restoration formulation in this paper also considers the unbalanced nature of SPS operation with mutual coupling.     

脉冲序列调制-脉冲跳周期调制Buck变换器研究

脉冲序列调制(Pulse Train Modulation,简称PTM)采用高、低能量脉冲对开关变换器的输出电压进行调整,其动态响应速度快,控制器设计简单,但同时开关变换器轻载效率低,输出电压纹波大。脉冲跳周期调制(Pulse Skipping Modulation,简称PSM)采用ON/OFF控制对输出电压进行调整,虽然提高了开关变换器的轻载效率,但也存在输出电压纹波大的缺点。结合PTM与PSM调制技术,提出了开关变换器的PTM-PSM调制技术,在保留PSM调制轻载效率高的同时,降低了输出电压的纹波。最后设计了基于PTM-PSM调制的Buck变换器,通过实验验证了分析结果。     

An active islanding detection scheme for inverter-based DG with frequency dependent ZIP–Exponential static load model

In this paper an analytical design method is proposed for constant voltage controlled inverter based DG with frequency dependent ZIP–Exponential static load model to obtain proper inverter output control and successful islanding detection. The proposed technique is aimed to eliminate the dependency of DG operation point on anti-islanding performance which is unavoidable in case of traditional control strategy. The generalized formulae are derived for calculating the upper and lower limits of the voltage gain, and it is recommended to select the voltage gain within the designed range for effective islanding detection. The proposed approach is tested in MATLAB/Simulink environment and result shows that the proposed scheme is found to be highly effective in islanding detection considering complex network configurations with single and multiple DG systems. The proposed control scheme is able to detect islanding within 10 ms (half cycle) from the event inception, showing the fastness of the developed technique. It is also observed that the percentage of voltage and current THD of the proposed scheme is within permissible limits, indicate that the power quality of the inverter is not degraded by the proposed control strategy, and thus overcomes the drawback of existing active anti-islanding techniques.     

Measurement of dynamic magnetic loss parameter in ferrites

The measurement of the iron loss in ferrites is important for developing high‐efficiency switching power supplies. The authors have proposed the dynamic magnetic loss parameter, λ_f, for evaluating the iron loss in ferrites. In previous studies, the parameter was assumed to be a constant value for an individual ferrite material and defined for one period of a small B–H loop. In this paper, assuming that λ_f is a function of the time derivative of the magnetic flux density, dB/dt, a novel measurement method of λ_f of a Ni‐Zn ferrite is proposed using rectangular wave voltage excitation and the Fourier expansion of the exciting current. In order to realize an iron loss measurement system with the rectangular wave voltage excitation, a high‐frequency FET inverter has been developed. The results of measuring λ_f show that it is uniquely determined by dB/dt regardless of the B–H loop size. The measured dB/dt characteristics of λ_f are applied to practical examples for switching power supplies and sinusoidal wave voltage excitations. Their experimental and computational results coincide and it is clarified that the measured dB/dt characteristics are effective and useful. © 2006 Wiley Periodicals, Inc. Electr Eng Jpn, 156(1): 1–6, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20101     

光伏发电系统并网点谐波电压波动特征孤岛检测法

光伏发电系统并网点处的谐波电压由系统扰动、本地负荷和光伏发电系统共同确定,基于波动量法提出一种以并网点谐波电压波动量为特征的光伏发电系统孤岛检测方法。测量公共联结点谐波电压,用小波变换法提取测量电压中包含的高次谐波分量,以高次谐波电压波动量变化规律作为孤岛识别和检测指标,无需外加扰动,可消除检测盲区。基于IEEE.Std 1547标准,对理想光伏发电并网系统进行的仿真证明,利用并网点测量电压中包含的谐波电压波动量进行孤岛检测,当测量点处的电压和频率在正常范围内时,不仅可快速、准确地检测孤岛,还能准确识别电网故障引起的电压暂降、投切负荷等干扰,避免伪孤岛引起的检测错误。     

一种快速小盲区的主动移频式孤岛检测方法研究

针对光伏PV并网发电系统中孤岛检测速度、检测盲区等问题,提出了一种快速小盲区的带正反馈的主动频率偏移检测方法。该方法根据第0个检测周期的公共耦合点PCC的电压频率与电网电压额定频率之差来确定初始斩波因子及施加扰动方向;并对第i个检测周期以前PCC点电压频率变化量分别取绝对值后进行累加,将累加值作为正反馈量施加至PCC点,在规定的检测周期内,完成孤岛的检测。采用Qf0×Cnorm坐标系对算法的检测盲区进行描述,利用Matlab/Simulink对孤岛检测性能进行仿真分析。结果表明:相比AFDPF法,新方法在满足并网电能质量的同时,具有更小的检测盲区;对感性负载的孤岛检测时间缩短至0.06 s,对容性负载只需0.04 s即可检测出孤岛;孤岛检测结果符合我国孤岛检测标准要求。     

基于分布式并网发电的新型孤岛检测研究

提出了一种新的基于分布式并网发电系统的主动式孤岛检测方法,进行了仿真和实验研究.在PWM正弦调制波前后半周期产生一个微小的不对称,使并网逆变器输出电流也产生不对称,从而引起逆变器输出功率在连续的两个半周期内产生规律性的变化.并网时,输出功率的变化并不明显,而一旦孤岛出现时,输出功率的变化将导致并网公共点(PCC)频率的变化,以该频率的变化为依据,实现分布式发电系统的孤岛检测.在光伏并网发电系统的实验平台上进行实验验证,结果表明,该方法能有效检测出孤岛,且算法简单,易于实现.     

一种新型非谐振型软开关交错并联Boost电路

提出一种新型非谐振型交错并联Boost零电压转换(ZVT)电路。在传统交错并联Boost拓扑基础上添加了一组由一个电感、两个电容、一个开关管、四个二极管组成的辅助网络,令主开关管实现了零电压开通与关断,辅助开关管实现了零电流开通与部分零电压关断,降低了开关损耗,提升了电路变换效率。软开关可在宽工作范围内有效实现,电路工作在连续电流模式(CCM),控制方式简明易行,辅助网络的引入没有给主开关管带来额外电流应力。通过复用部分辅助元件,提高了辅助网络利用率,减少了体积与费用;降低了开关过程中的dv/dt、di/dt,抑制了开关噪声。详细分析了电路拓扑结构、工作原理,并对主要参数进行了优化选取,最后通过实验验证了理论分析的正确性。     

盲区小的燃料电池并网系统孤岛检测方法

为保护系统及人身安全、保障用户供电质量,燃料电池并网系统必须具备孤岛检测功能。文中提出一种检测速度快、盲区小的新型孤岛检测方法——ASMS_Q法。该方法对滑模频率偏移法进行改进,在偏离工频较大区域加入加速增益,并结合q轴无功电流扰动法,在电网断开后,进行更快速地频率扰动,使其超出正常范围,达到过/欠频保护。而且电网正常工作时,给电网注入扰动量很小,对逆变器输出电能质量没有影响。在Q_f-f_0和Q_(f_0)-C_(norm)坐标系下,对比分析了ASMS_Q法及两种传统方法的盲区。仿真和实验结果表明,ASMS_Q法比传统的主动式检测方法盲区更小,检测速度更快,适用于高品质因素的负载。     

电能存储装置测试系统中Boost软开关变换器研究

提出一种应用于电能存储装置测试系统中的新型交错并联Boost软开关变换器。该变换器采用一组部分复用的辅助网络,令主开关管实现了零电压(zero-voltage-transition,ZVT)开通与关断,辅助开关管实现了零电流(zero-current-transition,ZCT)开通与部分零电压关断,降低了开关损耗,提升了电路效率。开关过程中的dv/dt、di/dt得以降低,从噪声源上改善了变换器电磁兼容(electromagnetic compatibility,EMC)性能,对高频段噪声抑制效果明显。对拓扑结构、工作过程进行了分析,对宽范围工作的有效性与EMC性能的改善进行了论证,并搭建了1.5 k W原理样机进行试验研究,试验结果验证了理论分析的正确性。     

Intelligent islanding detection of a synchronous distributed generation using governor signal clustering

Detection of intentional and unintentional islanding of distributed generation units is one of the major protection issues of the distribution networks. Regarding the safety and reliable operation of a modern distribution network, an expert diagnosis apparatus is required to distinguish network cut off from variety of normal occurrences. Automatic load-frequency controller (ALFC) is an indispensable component of the synchronous generators. Simulation results show that input signal of the governor includes somewhat singular characteristics for each possible phenomenon or disturbance. Therefore, a new method based on Self-Organizing Map (SOM) neural network is proposed using input signal to the governor to cluster various occurrences into islanding and non-islanding categories. Simulation results presented in this paper shows that the input signal of the governor employed by a SOM can cluster a majority of occurrences of the system and distinguishes the islanding phenomenon with high confidence.     

变频空调用新型高性能开关电源的设计

采用单端反激式拓扑设计了一种基于高性能单片控制芯片TNY-268P的新型开关电源,包括开关变压器设计,具有变频调制、开/关控制和关断时频率抖动等特点。实验结果表明,该电源具有结构紧凑、成本低、效率高和EMI减小等优点,可用于采用隔离或非隔离硬件设计的变频空调等家用电器。     

分布式发电条件下的配电网孤岛划分算法

在分布式发电条件下,孤岛可作为提高配电网供电可靠性的一种重要运行方式。为在配电网发生故障时及时制定出优化的孤岛划分方案,提出了孤岛划分算法。该算法利用配电网简化模型,根据孤岛运行时的功率平衡要求以及各种负荷对供电可靠性的不同要求,采取了启发式的搜索策略, 能够在短时间内得到可行的孤岛划分方案。文中还给出了2 个典型的孤岛划分实例。理论和实例分析表明,该算法可在配电网发生故障以后,根据故障前的实际运行情况动态地生成优化的孤岛划分方案。     

基于谐波电压与基波频率的被动式孤岛检测方法

针对光伏电站独立防孤岛保护装置现有的被动式孤岛检测方法盲区较大,同时频率判据无法区分电网低频与孤岛低频的问题,结合电网实际需求,提出了一种基于并网点谐波电压与基波频率的新型被动式孤岛检测方法。首先,对该方法进行理论分析及防误动措施设计;其次,在孤岛和并网的多种情况下对所述方法进行仿真验证;最后,通过实际发生的孤岛数据验证该方法的有效性。研究表明,该检测方法简单可靠,不仅缩小了检测盲区,而且可区分电网低频和孤岛低频。     

提高开关电源可靠性电路的研究

小义介绍了一种蓄电池充电开关电源,重点研究了其中软启动的设计,硬件消除开关机电路抖动干扰设计,输入、输出过流保护等提高开关电源可稚性的电路方案。通过设计了几种保护电路,不仅很好地提高了蓄电池充电电源的可靠性,还在一定范围内提高充电电源的效率。文章最后通过两款可快速对24V／300Ah蓄电池充电的开关电源样机实验进行了验证。     

计及衰减直流分量的基频电压半周期检测

衰减直流(DDC)分量、高次谐波等干扰信号的存在,使得对电网畸变信号中基频分量的幅值、相位检测存在一定误差,其中DDC分量的时间常数通常超过45 ms,持续时间较长.为此,文中首先针对畸变信号中DDC分量提出一种半周期四点采样检测算法,缩短了DDC分量的检测响应时间.其次,针对同时含有DDC分量与高次谐波的畸变信号,提出将畸变信号进行半周期积分后,在dq坐标系下将上述DDC分量检测算法与高次谐波检测算法进行组合,在工频半周期中可同时滤除干扰信号的影响,准确检测到畸变信号中的基频分量.最后,搭建了MATLAB/Simulink半实物实时仿真模型,从检测精度、响应时间等方面验证了所提算法的有效性.     

基于小波变换与BP神经网络的光伏电站孤岛检测研究

随着光伏电站规模不断增大,并网光伏系统对原有供电网络的影响越来越大,孤岛检测成为光伏电站必须深入研究的问题。针对现有孤岛检测方法的不足,提出了一种基于小波变换与BP神经网络的新型被动式孤岛检测法。该法通过小波变换获得有关信号孤岛发生前后的特征信息,再由BP神经网络根据这些特征信息实施孤岛检测和孤岛保护行为。仿真研究的结果表明,所述新型被动式孤岛检测方法检测速度快,检测盲区小,在多个负载品质因数、谐波等扰动情况下,不会出现孤岛检测的误判行为。