Transposition and Voltage Unbalance in High Phase Order Power Transmission Systems

—High phase order systems have been proposed at the early inception of power transmission engineering, but few direct applications have been made. High phase order transmission should be considered as an alternative in the case of high power density applications. In this article, an analysis of transposition of high phase order overhead transmission lines is presented and voltage unbalance in high phase order systems is considered. Definitions are presented for “fully transposed” and “roll transposed” along with advantages and disadvantages of each. A generalized voltage unbalance factor is introduced and utilized to determine the benefits of transposition. The generalized voltage unbalance factor is compared with three other possible unbalance factors to determine if the generalized voltage unbalance factor is an appropriate indication of unbalance. Exemplary results are presented for 6-phase and 12-phase designs. Conclusions show that the generalized voltage unbalance factor is a good indication of transmission line voltage unbalance and certain configurations may not need full rotation transposition to minimize the unbalance factor. The transposition analysis and voltage unbalance are considerations in the assessment of high phase order as a high power transmission alternative.     

Induction machine fault detection using stray flux EMF measurement and neural network-based decision

The aim of this paper is to present the performances of voltage unbalance and rotor fault detections using an external stray flux sensor in a working three-phase induction machine. The automatic classification and fault severity degree evaluation are realized by using a neural network approach based on a multi-layer perceptron (MLP) structure. In this paper, it is proved that a simple external stray flux sensor is more efficient than the classical stator current sensor to detect rotor broken bar and voltage unbalance, using data processing at low-frequency resolution.     

基于有效值的台区电流不平衡度计算方法研究*

为解决在相位信息缺失条件下三相电流不平衡度准确计算的难题,提出了一种基于基波电流有效值的低压台区电流不平衡度计算方法。文章分析了基于对称分量法的电流不平衡度计算方法,以及在相位信息缺失条件下,4种常用三相电流不平衡度工程计算方法的差异性;基于对称分量法分解规律,将三相不平衡电流相量分解为两组相位对称的电流相量。在相位对称条件下,推出基于基波电流有效值的电流不平衡度计算式,确定不同工况下两组电流不平衡度的主从关系;结合低压台区负荷电流的不平衡特性,分析了计算方法在低压台区三相电流不平衡度计算中的适用性及差异性,并对低压台区运行数据进行实测验证。与4种常用工程计算方法相比,所提方法与对称分量法计算结果更加接近,且基本一致。     

Mitigation of voltage unbalance by using static load transfer switch in bipolar low voltage DC distribution system

In this paper, a method is proposed to mitigate voltage unbalance and to reduce power loss due to neutral current in a bipolar Low-Voltage DC (LVDC) distribution system by using a static load transfer switch (SLTS). Furthermore, an algorithm to determine the proper position of loads by using the SLTS is presented; this algorithm is developed by considering the neutral current at each load point. The underlying strategy of the SLTS method is that a local substation generates switching signals by using the proposed algorithm on the basis of data measured by a DC/DC converter; thereafter, SLTSs, which are placed in the DC/DC converter, are operated to reconfigure the structure of loads. To evaluate the performance of the proposed method, the conventional method for calculating the percent voltage unbalance for an AC system is modified and made applicable for a DC system. The modeling of a 1500 V bipolar LVDC distribution system carried out using the ElectroMagnetic Transients Program (EMTP) is also presented herein. Finally, a simulation carried out by employing the SLTS method under various load conditions is presented; the results show a decrease in power loss and mitigation of voltage unbalance.     

超高压直流系统中的直流滤波器保护

介绍了超高压直流输电系统中的直流滤波器保护,分析了直流滤波器的特点以及超高压直流输电的各种运行工况对直流滤波器保护带来的影响,提出了直流滤波器保护的总体设计思想、相应的保护原理与方案,重点讨论了差动保护和不平衡保护的实现及其应该注意的问题。     

双级矩阵变换器输入电压不平衡控制策略

双级矩阵变换器(又称为间接矩阵变换器)是一种新的矩阵变换器拓扑,存在中间直流环节,但不包含储能的电容或者电感.因此,当输入侧电压不平衡时,输入输出电流波形将直接受到影响,降低了双级矩阵变换器的性能.针对这一问题,本文提出了一种基于瞬时无功理论的补偿方法.在输入电压不平衡情况下,该方法控制输入侧有功为恒定,构造输入电流矢量参考值,并对整流级电路和逆变级电路分别采用空间矢量脉宽调制,从而保证输出性能的稳定.实验结果表明,该方法有效地减小了输入输出电流中的谐波含量,改善了变换器的性能.     

The impact of single-phase grid-connected distributed photovoltaic systems on the distribution network using P-Q and P-V models

The gradual emergence of photovoltaic (PV) systems as the most common distributed generation interconnected with the electric power system calls for a detailed power flow analysis with different models especially in the evolving unbalanced active distribution network. This paper is an extension of a previous study carried out on the performance evaluation of a 10 kWp grid-connected PV system deployed in a school, with the grid providing a virtual storage and access to upstream markets. For increased adoption of such systems in the unbalanced distribution network, it is pivotal to understand the mode of operation and the type of connection to the system. This article presents an impact analysis of such utility interactive single-phase PV systems distributed on all the single-phase load nodes of the traditional IEEE-13 bus distribution test feeder. The PV distributed generation (PV-DG) can be modelled as constant P-Q or P-V nodes with varied impacts in power flow studies for the unbalanced active distribution network. Results from these models are compared in terms of their impacts on voltage profiles at load buses, voltage unbalance, equipment loading, power losses and the total number of iterations for a converged power flow solution.     

单/三相混合微电网中虚拟组合式三相变流器功率协调控制

针对单/三相混合微电网系统孤岛运行时单相电源和负载引起的三相电压不平衡问题,提出了虚拟组合式三相变流器及其功率协调控制。详细阐述了基于虚拟三相瞬时功率的下垂控制和虚拟阻抗控制,实现同相并联单元的功率分配。针对组合式三相变流器相间同步控制难点,提出了新型120°鉴相器和基于锁相环原理的相间同步控制,提高三相电压平衡度。在此基础上,给出了电压补偿控制,进一步改善供电质量。同相及相间功率协调控制无需附加任何同步芯片或通信线,原理简单,控制灵活,可靠性高。最后,仿真和实验结果表明所提控制策略可以实现动稳态工况下同相并联单元间功率按照设定比例分配以及不同相间相位和幅值的同步控制,100%不平衡负载下电压不平衡度为0.1%,供电质量较高。     

微网系统不平衡电压补偿控制策略

微网系统中负荷不平衡会造成系统中电压的不平衡,影响微网系统的供电质量。针对这一问题,文中首先介绍了微网系统逆变器的控制方法,然后基于下垂控制,分析了微网系统电压不平衡的原因。为保证微网逆变器端口和公共耦合点电压能同时满足供电质量标准的要求,提出一种采用微网中储能逆变器对逆变器端口和微网公共耦合点不平衡电压进行同时补偿的控制策略,对所提补偿策略进行了详细的阐述,最后通过仿真和实验分析验证了所提方法的有效性。     

Voltage unbalance assessment in secondary radial distribution networks with single-phase photovoltaic systems

This research describes a way to analyze voltage unbalance sensitivity for different maximum sizes of a single-phase photovoltaic system (SPPVS) with multiple PV penetration levels in a typical secondary radial distribution network (SRDN) in Spain. This analysis effectively assesses current requirements as specified in regulations concerning maximum size to be connected. It thus helps distribution network operators to define optimal limits, depending on their context. A stochastic assessment method is proposed to account for any random combination of SPPVSs in an SRDN. In addition, this method evaluates weekly voltage unbalance during a one-year time period, on the basis of 10-min intervals. More specifically, the voltage unbalance in SRDNs with SPPVSs is assessed for each 10-min interval by means of a probabilistic radial three-phase load flow (RTPLF). The results obtained show the maximum sizes of the SPPVS to be connected as a function of the PV penetration level in the SRDN, where high PV penetrations can produce voltage unbalance problems.     

电压不平衡度的加窗FFT快速测量方法

电压不平衡度是电能质量的重要参数之一,其准确测量一方面依赖于三相电压基波相量(幅值和相位)的精确估计,另一方面需要快速求取相序分量。但是,频谱泄漏和栅栏效应会严重影响电压幅值和相位的估计精度,而相序分量计算过程中的开方运算又降低了不平衡度测量的实时性。提出电压不平衡度加窗快速傅里叶变换(FFT)测量方法。首先采用加窗FFT实现三相电压相量的准确计算,然后根据几何原理将方均根计算简化为多项式计算,从而实现相序分量及电压不平衡度的快速测量。仿真结果表明,所提方法可以达到与GB/T 15543—2008推荐方法相同的精度等级。实际测量实验进一步验证了所提算法的有效性及可行性。     

电力系统谐波与不平衡污染程度评估指标的研究

在电力系统中,谐波与不平衡会导致线损增加、设备被损坏、电能计量不准确等诸多问题。但是目前并没有一套固定的指标来对谐波与不平衡污染程度进行评估。针对该问题,根据IEEE Std 1459-2010功率理论和其对视在功率的分解方法,提出了一套适用于非正弦不平衡条件下新的谐波与不平衡污染程度评估指标。对传统评估指标的合理性进行了分析;针对传统评估指标的缺点,分别给出了单相系统和三相系统中新评估指标的定义;最后在Matlab/Simulink中搭建仿真模型,比较传统指标与新指标的评估效果。分析结果表明,新指标能够准确和全面地对谐波和不平衡污染程度进行评估。     

直流系统控制方式对大扰动后交直流混合系统电压和功率恢复的影响

使用电力系统暂态安全评估工具TSAT基于南方电网2005年规划数据研究了直流控制方式和低压限流器参数对大扰动后交直流混合系统电压和功率恢复的影响.结果表明,常规的整流侧定电流、逆变侧定直流电压控制比整流侧定功率、逆变侧定熄弧角控制有利于大扰动后交流换流母线电压的恢复;为缓解大扰动后有功功率和无功功率失衡的问题,低压限流器参数的大小应取折中值.     

基于无线负控信道的电能质量管理系统

本文探讨了一种在原有的负荷管理系统平台上增加电能质量监测功能的方法。通过对远程终端硬件的改造和原有系统软件的升级，在配电网中增加了谐波、电压、三相不平衡的监测和管理，保证了电能质量。在软件中采取了数据仓库技术，提高了数据操作的响应速度。     

基于动态博弈的配电网单相电力用户用电行为分析

针对配电网三相负荷不平衡及电力尖峰负荷持续攀高问题,利用博弈论信号传递机理,建立基于动态博弈的配电网单相电力用户用电非合作博弈模型。该模型以A,B,C各相电力用户为博弈参与者,各参与者以一个用电周期用电费用最低为优化目标,其费用函数同时考虑了三相不平衡的抑制及负荷的削峰填谷。利用粒子群优化算法求解该问题的纳什均衡。仿真结果表明,所提方法可通过博弈联动在抑制三相负荷不平衡的同时,实现削峰填谷和降低峰谷差。     

可同时补偿电压不平衡的串联型有源电力滤波器

文章中的串联型有源滤波器是与无源滤波器配合使用的,它除了可以改善无源滤波器的滤波效果之外,还可以同时补偿三相中点接地系统中的电源电压不平衡.提出的检测电流谐波和电压中负序零序分量的方法都基于瞬时无功功率理论,检测算法较为简单并且实时性好.最后仿真结果验证了检测算法的正确性.     

供电电压不平衡时系统中周期性负荷能耗和温升的试验研究

为了研究供电电压不平衡对系统中周期性负荷能耗及温升的影响,设计了相应试验研究方案．将带不同类型负载的异步电机相互组合实现模拟系统中的周期性负荷,并系统研究了供电电压不平衡情况对此类负荷能耗及温升的影响。研究结果表明：供电电压不平衡引起电机最大输出功率降低的同时还会导致能耗增加．在电压不平衡度为2％和4％时,电机带载能力分别降低17％和29％;当带80％额定负荷时,2种不平衡度下的能耗分别增加1．6％和3。5％;此外,通过对电压不平衡对电机温升影响的研究,发现当不平衡度为4％、电机带额定负荷运行时,其温升比正常运行时高约100℃,这将导致电机使用寿命降低约50％。     

区域电网电能质量问题及治理关键技术综述

本文对区域电网影响程度最大,影响范围最广的稳态电能质量问题的现状、分析方法和治理技术开展了综述。分析了区域电网中典型的谐波源及其谐波特征,简述了谐波对区域电网的影响,分主动和被动两类介绍了主要的谐波治理措施。分析了区域电网三相不平衡问题的来源、现状及危害,重点介绍了相关标准及不平衡度计算方法,介绍了解决三相不平衡问题的运行措施和治理设备。分析了区域电网低电压问题的现状、产生机理及危害,分析了低电压监测存在的问题并总结了相应的治理技术。最后指出了治理区域电网稳态电能质量问题的关键。     

变压器空投引起风机脱网分析

随着风电并网容量的大规模增加,风电机组脱网问题对电网安全稳定运行产生了较大威胁。从变压器空投时造成风机脱网的实例出发开展研究,对比分析变压器空投引起的系统电压波动、有功功率和无功功率变化的过程,计算脱网风电场电压不平衡度和电流不平衡度,探寻了对周边风场冲击大小的影响因素,初步判断出了风机脱网的原因。