一种新的分布式发电孤岛检测方法

分布式发电系统的孤岛运行可能会给系统、用电设备、维修人员等造成伤害,必须采取一定的措施监控分布式发电系统的运行状态,迅速检测出孤岛的产生并将其切离主电网。在传统被动式孤岛检测方法ROCOF法和VS法的基础上,结合两者的优势,通过适当的整定及其配合,提出了一种新的孤岛检测新方法,并利用PSCAD/EMTDC进行了建模仿真验证。结果表明,该方法在孤岛发生时可以有效地做出判断,并且分析了它在各种可能情况下的准确性和可靠性。     

Adaptive notch filter for synchronization and islanding detection using negative‐sequence impedance measurement

Dispersed generation (DG) has been found promising for satisfying the requirements of high power quality in distributed systems. One operation situation, namely, an island, is formed when one or more DG systems and an aggregate of local loads are disconnected from the main grid and remain operational as an islanded entity. Islanding is either due to intentional events, e.g. maintenance outage, or due to unintentional events, such as faults, and their subsequent switching actions. Islanding is usually undesirable because of the potential harm it causes to the existing equipment, human safety, power reliability and quality, etc. Accordingly, anti‐islanding schemes are used to immediately detach a DG system from the feeder after islanding. This paper presents an active method of detecting islanding for DG systems by injecting a small negative voltage into the point of common coupling, and then measuring the negative system impedance. In this study, an adaptive notch filter is introduced as a synchronous part instead of a phase‐locked loop and as a signal processing unit as well. The proposed control strategy allows DG systems to detect properly the occurrence of islanding in a balanced distribution system. Simulation results show the overall system performance including synchronization, power control, and islanding detection capability of the simulated DG system. © 2012 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

含多种分布式电源的微电网控制策略

针对微电网的并网与孤岛运行方式以及2种运行方式之间的转换,提出了一种含多种分布式电源的微电网控制策略。该控制策略中微电网中心控制器连续监测微电网和大电网的运行状态并对微电网进行统一的协调控制:对于并网运行的微电网,当检测到孤岛状态时立即切换到孤岛运行控制方式;对于孤岛运行的微电网,通过选择主调频电源实现微电网频率的无差调节,避免了下垂控制产生的频率偏差;微电网重新并网时,通过采用电压灵敏度分析方法调节并网接口处的电压幅值并监视与大电网的电压相位差,实现微电网运行方式的平稳切换。采用PSCAD/EMTDC软件对含多种分布式电源的微电网进行仿真分析。仿真结果表明,提出的控制策略能够维持微电网的稳定运行,并能实现微电网并网与孤岛运行方式的平稳过渡。     

Integration of a hybrid fuel cell-battery system to a distribution grid

In order to integrate a proton exchange membrane type (PEM) fuel cell system (FCS) combined with a battery bank to a distribution grid; this paper proposes a local controller based on fuzzy logic. The proposed system provides primary frequency control and local bus voltage support to the local grid. This opposes the passive distributed generation of the present that do not provide auxiliary services, such as back-up power, voltage support and reliability of supply as they operate under constant power factor equal to 1 at all times. During network disturbances, the distributed generations of the present are disconnected until normal operation is reestablished. When the distributed generation penetration is high this may lead to system instability. The microgrid concept is the effective solution for the control and quality improvement of grids with high level of DG penetration. So, the proposed system, also, can be an active controllable microsource of a microgrid in the future that cooperates with other microsources in order to cover the local load demands for active and reactive power either under grid-connected mode or under islanding operating mode. In cases where the distribution grid (working as microgrid) is forced to operate in islanded mode, the hybrid system provides the demanded active and reactive power. The FCS is connected to a weak distribution grid so that the system performance is studied under the worst conditions. The simulation results are obtained using MATLAB software under a severe step load change where the grid is still connected and under islanded operation. In both cases the system presents a good performance.     

Under-Frequency Load Shedding scheme for islanded distribution network connected with mini hydro

Under-Frequency Load Shedding (UFLS) is a common technique to maintain power system stability by removing the overload in some part of the system. The existing UFLS schemes operate successfully in the interconnected grid system. A particular approach of UFLS scheme is required for an islanded system which is known to be not as strong as the grid. This paper presents a new approach of UFLS scheme for the implementation in an islanded distribution system. The scheme is based on a combination of adaptive and intelligent techniques. The disturbance magnitude for response based and event based is treated differently: (1) event based is based on the power imbalance and (2) response based is based on the swing equation which relies on the frequency and rate of change of frequency measurement. Dynamic simulation on an intentional islanding operation is performed on an existing Malaysian network interconnected with a mini hydro generation. The objective is to investigate the effectiveness of the proposed scheme. The simulation results demonstrate that the proposed scheme is capable of improving the frequency response of the islanded system. A significant finding is that the scheme can successfully reduce the effect of power deficiency following the grid connected to islanding operation transition.     

Two level islanding detection method for distributed generators in distribution networks

Use of distributed generators in distribution networks is proved highly reliable and economical. In utility practice at the present time, accidental islanding is an undesirable mode of operation as it can harm the personnel and other connected systems. Therefore, islanding must be detected and the islanded distributed generators must be disconnected from the rest of the system. This paper proposes a new islanding detection method which provides a reliable detection in two levels. At first level wavelet energy entropy is used as an indicator to suspect islanding and activate second level method. Second level method is an active frequency drift method with positive feedback and pulsating chopping fraction that vary the output voltage frequency of distributed generators to realize the anti islanding protections. The performance of the proposed method is simulated in MATLAB environment. The proposed algorithm provides fast islanding detection with least disturbance in inverter’s grid connected operation.     

用于分布式发电系统孤岛检测的偶次谐波电流扰动法

孤岛检测是分布式并网发电系统必须具备的能力,但频率或相位偏移等传统主动式孤岛检测方法在并网逆变器输出与本地负载功率平衡时难以有效检测出孤岛现象。针对已有孤岛保护方法的不足,本文分析了不同电流扰动信号对孤岛检测能力的影响,找到了适合多机并网系统快速孤岛检测的偶次谐波扰动信号,在此基础上结合过零点频率检测法,将电流谐波扰动的正反馈引入孤岛检测与保护中。并网逆变器的输出电流参考信号受到微弱扰动信号的影响,孤岛发生时,形成扰动信号的正反馈回路,并迅速累积使公共耦合点电压频率超出限定阈值,从而实现孤岛保护。该方法具有便于实现、对电网污染小、检测速度快及适合多机系统等优点。仿真和实验结果验证了该方法的有效性与可靠性。     

微网运行模式及控制策略研究

微网技术解决了分布式发电大规模并网的问题,提高了可再生能源的效能,是分布式发电的高级利用形式。介绍了微网和并网开关的基本结构,分析了微网的运行模式,包括并网运行时微网的并网标准、微网与大电网的相互作用、孤岛运行时孤岛检测与孤岛划分。针对微网在不同运行模式下,其内部分布式电源运行特性不同的特点,对微网的控制策略展开深入的研究。     

基于频率-功率因数角组合判据的新型孤岛检测法

为解决传统的孤岛检测方法难以兼顾电能质量和检测效率的问题,在综合分析孤岛前后孤岛内公共耦合点的频率和功率因数角之间变化关系的基础上,推导出了两者的定量关系,提出了一种基于组合判据的新型孤岛检测方法。该方法将频率变化与功率因数角的变化相结合,提高了判据的灵敏性,并且通过延时提高了新方法的可靠性。仿真证明,相对于传统的被动式孤岛检测法,在不同性质的本地负荷下,新方法具有较高的检测效率,且在系统发生扰动时,有较高的鲁棒性。新方法是一种被动法,在系统并网运行时不会影响电能质量。     

H‐infinity controller for frequency and voltage regulation in grid‐connected and islanded microgrid

This paper presents a study on a grid‐connected and islanded multiple distributed generation (DG) system for frequency and voltage regulation. The multiple DG system includes solar cells, wind turbine, fuel cell, and battery storage. The H‐infinity controller is used whose weighting parameters are optimized to minimize voltage and frequency deviation. The performance of the system is analyzed under different conditions for both grid‐connected and islanded modes of operation. In case of the load variations, the inner voltage and current loop react based on the H‐ infinity control strategies. The outer power loop uses the droop characteristic controller. The design is simulated using MATLAB/SIMULINK. The simulation results show that the multiple DG system can supply high‐quality power both in grid‐connected and islanded modes. Also, we show that the proposed control methodology will make the system to transit smoothly between the islanded mode and the grid‐connected mode. The results indicate that the frequency and voltage deviations meet the nominal values as per IEEE standard. © 2015 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

Shifting of research trends in islanding detection method - a comprehensive survey

The augmentation in electricity demand, power system privatization as well as efficacy of renewable resources has paved the way for power system companies and researchers to exploit the field of grid connected distributed generation (DG) and its issues, islanding being a dominant one. Several research works have been conducted to mitigate the issues of islanding detection (ID). In context of this, the paper gives a comprehensive review of islanding issues, standard test systems, criteria and shifting of research trends in islanding detection methods (IDMs). The significant contributions pertain to categorization of IDMs, evaluation of non-detection zone (NDZ) for each test system, disquisition on evolution and advancement of IDMs and its comparisons based on criteria such as NDZ, run on time, nuisance tripping percentage, applicability in multi DG system and implementation cost to draw out the strength and shortcomings of individual methods that will come to aid to the companies or researchers for establishing the applicability and appropriateness of such method for their concerned domain.     

微电网基于下垂控制的改进孤岛检测技术研究

由于微电网要求能够在并网和孤岛两种模式下运行,孤岛检测技术必不可少。为此,利用逆变器下垂控制方法中频率和有功的线性关系,在参考功率上添加有功扰动,使逆变器的频率在电网故障时偏离出正常范围,以达到孤岛检测的目的。在此基础上,改进了有功扰动的选取,通过判断频率连续偏移次数来给定相应的有功扰动系数,在系数中引入频率正反馈,加快孤岛检测速度,减少对微电网并网运行时的电能质量的影响。同时,分析微电网结构特点,给出了孤岛检测算法在其中的应用方案。通过仿真,证明了孤岛检测算法用于微电网的有效性。     

基于贪心算法的自适应解列断面自动搜索方法：(一)解列指标体系与算法设计

现有的解列断面搜索方法仅能考虑发电机同调、孤岛功率平衡等少数约束条件,在大系统的解列断面搜索过程中需要事先人工化简系统,无法实现从在线调度数据出发自动获得解列断面。文中以交直流系统的自适应解列控制为研究目标,提出了交直流电网解列指标体系,并设计了基于贪心算法的解列断面自动搜索方法。该方法根据系统发电机慢同调分群结果,确定解列孤岛数量及孤岛初始搜索节点,然后,对各孤岛一级相连节点进行评分,确定应该并入各孤岛的节点;最后,设计了解列策略校验环节。     

Shifting of research trends in islanding detection method - a comprehensive survey

The augmentation in electricity demand, power system privatization as well as efficacy of renewable resources has paved the way for power system companies and researchers to exploit the field of grid connected distributed generation (DG) and its issues, islanding being a dominant one. Several research works have been conducted to mitigate the issues of islanding detection (ID). In context of this, the paper gives a comprehensive review of islanding issues, standard test systems, criteria and shifting of research trends in islanding detection methods (IDMs). The significant contributions pertain to categorization of IDMs, evaluation of non-detection zone (NDZ) for each test system, disquisition on evolution and advancement of IDMs and its comparisons based on criteria such as NDZ, run on time, nuisance tripping percentage, applicability in multi DG system and implementation cost to draw out the strength and shortcomings of individual methods that will come to aid to the companies or researchers for establishing the applicability and appropriateness of such method for their concerned domain.     

Shifting of research trends in islandingdetection method - a comprehensivesurvey

The augmentation in electricity demand, power system privatization as well as efficacy of renewable resources has paved the way for power system companies and researchers to exploit the field of grid connected distributed generation (DG) and its issues, islanding being a dominant one. Several research works have been conducted to mitigate the issues of islanding detection (ID). In context of this, the paper gives a comprehensive review of islanding issues, standard test systems, criteria and shifting of research trends in islanding detection methods (IDMs). The significant contributions pertain to categorization of IDMs, evaluation of non-detection zone (NDZ) for each test system, disquisition on evolution and advancement of IDMs and its comparisons based on criteria such as NDZ, run on time, nuisance tripping percentage, applicability in multi DG system and implementation cost to draw out the strength and shortcomings of individual methods that will come to aid to the companies or researchers for establishing the applicability and appropriateness of such method for their concerned domain.     

UK scenario of islanded operation of active distribution networks with renewable distributed generators

This paper reports on the current UK scenario of islanded operation of active distribution networks with renewable distributed generators (RDGs). Different surveys indicate that the present scenario does not economically justify islanding operation of active distribution networks with RDGs. With rising DG penetration, much benefit would be lost if the DGs are not allowed to island only due to conventional operational requirement of utilities. Technical studies clearly indicate the need to review parts of the Electricity Safety, Quality and Continuity Regulations (ESQCR) for successful islanded operations. Commercial viability of islanding operation must be assessed in relation to enhancement of power quality, system reliability and supply of potential ancillary services through network support. Demonstration projects under Registered Power Zone and Technical Architecture Projects should be initiated to investigate the utility of DG islanding. However these efforts should be compounded with a realistic judgement of the associated technical and economic issues for the development of future power networks beyond 2010.     

Power quality enhancement in autonomous microgrid operation using Particle Swarm Optimization

This paper presents an optimal power control strategy for an autonomous microgrid operation based on a real-time self-tuning method. The purpose of this work is to improve the quality of power supply of the microgrid where some Distributed Generation (DG) units are connected to the grid. Voltage and frequency regulation, and power sharing are the main performance parameters which are considered in this work, particularly during the transition from grid-connected to islanding operation mode and also during load change. In this work, two typical DG units are connected in parallel to configure the microgrid. The controller scheme is composed of an inner current control loop and an outer power control loop based on a synchronous reference frame and the conventional PI regulators. The power controller employs two typical strategies: active–reactive power (PQ) control strategy and voltage–frequency (Vf) control strategy. Particle Swarm Optimization (PSO) is an intelligent searching algorithm that is applied for real-time self-tuning of the power control parameters. The proposed strategy in this paper is that both DG units adopt the Vf control mode once the microgrid is islanded in order to regulate the microgrid voltage and frequency, whereas during the load change, only the second DG unit invokes the PQ control mode to ensure maximum power exportation. The results show that the proposed controller offers an excellent response to satisfy the power quality requirements and proves the validity of the proposed strategy.     

基于非特征频率突变量阻抗的多机集群孤岛检测方法

微电网中多类型电源并存的多机集群复杂工况对传统阻抗式孤岛检测法的应用提出挑战。提出了基于非特征频率突变量阻抗的孤岛检测方法。该方法采用外部集中扰动的方式注入扰动信号,并利用快速傅里叶变换计算公共耦合点处非特征频率突变量阻抗,同时将断路器流经的工频电流与相邻多机集群的非特征频率突变量阻抗作为辅助判据,以实现可靠有效的孤岛检测。通过理论分析与仿真验证可知,本文所提的孤岛检测算法不存在检测盲区,能够可靠应对负荷投切等系统暂态扰动,适应于多类型电源并存的多机集群微电网工况。     

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。     

含分布式电源的智能配电网保护控制方案

分布式电源(DR)在配电网中的高度渗透,使传统的配网保护与控制面临极大的困难,对此提出了一种基于高级馈线终端单元(Advanced Feeder Terminal Unit,AFTU)的保护控制技术。根据DR容量和负荷大小,预先将含DR的配电网络划分为若干能孤岛运行的区域。在馈线出口、DR接入位置以及区域边界处装设断路器和AFTU,AFTU之间通过光纤网络互联。各AFTU实时检测本地信息,并与相邻的AFTU进行信息交互。根据本地及临近的信息,可快速定位故障区段,实现故障的隔离及孤岛运行。利用相关的信息,设计了重合闸和孤岛再并网控制策略。案例分析说明了所提分布式保护控制方案的可行性。