Optimal DG Allocation in Radial Distribution Systems with High Penetration of Non-linear Loads

This paper addresses the optimal distributed generation sizing and siting for voltage profile improvement, power losses, and total harmonic distortion (THD) reduction in a distribution network with high penetration of non-linear loads. The proposed planning methodology takes into consideration the load profile, the frequency spectrum of non-linear loads, and the technical constraints such as voltage limits at different buses (slack and load buses) of the system, feeder capacity, THD limits, and maximum penetration limit of DG units. The optimization process is based on the Genetic Algorithm (GA) method with three scenarios of objective function: system power losses, THD, and multi-objective function-based power losses and THD. This method is executed on the IEEE 31-bus system under sinusoidal and non-sinusoidal (harmonics) operating conditions including load variations within the 24-hr period. The simulation results using Matlab environment show the robustness of this method in optimal sizing and siting of DG, efficiency for improvement of voltage profile, reduction of power losses, and THD. A comparison with particle swarm optimization (PSO) method shows that the proposed method is better than PSO in reducing the power losses and THD in all suggested scenarios.     

Online Droop Tuning of a Multi-DG Microgrid Using Cuckoo Search Algorithm

This article presents an intelligent strategy to achieve appropriate real power sharing among distributed generators in a microgrid. The presented strategy employs two droop-based control methods and automatically adjusts their parameters. The first method is unit power control, which has specifications similar to the conventional droop method, and the second is feeder flow control, showing significant characteristics in both grid-connected and islanded modes operation of a microgrid. A combination of unit power control and feeder flow control methods is used for a multi-distributed generator microgrid. The microgrid operation mode passes from the grid-connected to the islanded through a transition. A new evolutionary algorithm called cuckoo search is employed to coordinate the power management of distributed generators within an on-line droop tuning. In comparison to the predecessor evolutionary algorithms, the cuckoo search algorithm represents more effective random processes with fewer parameters. Using the proposed control strategy, while the distributed generators contribute to load demand provision based on their rated powers, their powers are optimized in terms of overshoot and settling time. Digital time-domain simulation studies are carried out in the MATLAB/SIMULINK (The MathWorks, Natick, Massachusetts, USA) environment to verify the performance of the proposed control system.     

Study of Resonances in 1 × 25 kV AC Traction Systems

AC traction systems are 1 × 25 or 2 × 25 kV 50-Hz single-phase, non-linear, time-varying loads that can cause power quality problems. One of the main concerns about these systems is voltage distortion, because adjustable-speed drives for trains may inject harmonic currents of frequencies below 2 kHz. Since the presence of parallel resonances in the contact feeder section of the traction circuit worsens the scenario, traction system resonance phenomena should be analyzed to prevent problems. Several works addressed these phenomena, but they only drew weak numerical conclusions based on the frequency scan method. This article studies 1 × 25 kV traction system resonances at pantograph terminals and provides more effective analytical expressions to locate them and determine the impact of traction system parameters on them. These expressions are validated from several traction systems in the literature.     

低压配电网无功补偿分散配置优化方法

针对低压配电网集中配置无功补偿装置无法解决电压质量问题,提出了一种低压配电网无功补偿分散配置优化方法。优化模型的目标函数为最小化配电网的网损,约束条件包括各个节点电压的上下限和无功补偿点补偿容量的上下限。先以所有负荷节点作为候选补偿点,采用内嵌二次罚函数处理离散变量的非线性原对偶内点法求解优化模型,得到理想分散无功补偿配置方案,同时获得配电网的大致无功补偿容量需求,在此基础上综合考虑无功补偿装置的投资费用以及便于装置的运行管理等要求,选定几个重要补偿点再次求解优化模型,获得最终的实用分散无功补偿配置方案。对几个实际低压配电网台区进行的分析计算验证了方法的有效性和实用性。     

2015年3月20日日食对德国电网的影响

由于德国的可再生能源发电占比很高,2015年3月20日发生的日偏食使德国光伏出力在短时间内大幅度波动,对德国的电力平衡、电网安全造成了威胁,也影响了电力交易。但最终德国没有发生任何电力短缺或停电,并且电网频率波动甚至比平常还要小。通过分析日食当日与非日食日的电力数据,以及电网公司采取的措施,探讨了光伏大规模接入电网后遇到极端自然现象时的经验与预防措施。结果表明,电网的合理规划设计、适当的虚拟电厂以及需求侧管理技术和调度部门的协作对于维持供电安全有着重要意义。     

考虑环境效益的抽水蓄能电站日运行方式优化

随着能源短缺和气候环境的恶化,节能减排和可持续发展已逐步深入到电力系统生产运行的各个方面。分析了抽水蓄能电站的环境效益,以系统发电成本和污染物排放的综合费用最低为目标,建立了计及环境效益的抽水蓄能电站日运行方式优化模型。通过对抽水蓄能电站运行容量和备用容量的划分,以最大化调峰效益安排其抽水发电位置,并求解各运行方式下的系统运行成本,寻得最优运行方式。最后通过算例验证了本文模型和方法的合理性。     

四桥臂APF定频滞环电流控制

提出了一种基于空间电压矢量的三相四桥臂APF定频滞环电流控制方法。该方法通过一组外滞环比较器,快速判定参考电压区域,将参考电压矢量空间分成四个平行六面体。通过一组內滞环比较器,根据参考电压所在区域,选择适当的三相开关独立地控制相应的相间电流,解耦后实现定频滞环控制。该方法还考虑了误差电流相位调节,使误差电流达到最优,提高了控制精度,同时保持了稳定的开关频率。最后应用PSCAD/EMTDC电力暂态仿真软件验证了本文方法的有效性。     

不平衡负载的一种更加经济的补偿方法

针对低压配电网不平衡负载降损的问题,会联想到电容补偿器,其在供电系统中起到提高电网的功率因数与降低供电变压器及输送线路的损耗,提高供电效率以及改善供电环境的作用。传统方法只需计算出负载所需要补偿的导纳值,然后配置相应的电容器,比较新的一种导纳值计算方法如平衡分量法。提出一种"线路调整法"的降损补偿方法,进行负荷三相平衡线路调整,仅需对用户集表箱的接入相做调整。提出一种基于优化算法的智能换相策略,不需要增加额外投资,通过算例分析验证了此方法的正确性和有效性。因此,"线路调整法"是一种更简单、更经济的降损方法。     

基于电感电压反馈和输入整形技术的LC滤波器混合阻尼控制

电流型PWM整流器采用LC环节来滤除谐波及帮助器件换流,LC型滤波器呈二阶特性,存在谐振尖峰,且电容阻尼较小,在控制系统存在扰动的情况下,容易引起电流波形畸变和振荡。提出了一种采用输入整形技术和电感电压反馈结合的控制策略,输入整形技术能够有效地抑制小阻尼系统的振荡,电感电压反馈控制能够在外部干扰抑制网侧电流波形畸变。结合两者的优点,可有效地解决电流型脉宽调制整流器网侧电流振荡和畸变。仿真验证了所提出的控制策略能够有效减少电流畸变,增加系统的稳定性,并具有一定工程实用性。     

高温超导变压器研发进展综述

与传统常规变压器相比,超导变压器具有很多优势,如体积小、重量轻、损耗低、无污染火灾隐患等。近十几年来,随着高温超导线材取得重要的研究进展,国际上相继开展了高温超导变压器的研发,已有三台样机成功地完成了挂网试验。简要介绍了近年来国内外高温超导变压器（包括电力变压器和机车变压器）的研发情况以及近期几个基于第二代高温超导线材的高温超导变压器项目,并指出随着高温超导线材的技术发展,限流变压器的研究值得关注。