Effects of open-wye/open-delta transformers on the operation of distribution systems

Transformers with open-wye/open-delta connections have been widely used in electrical distribution systems, the advantage being that both three-phase and single-phase loads may be supplied by using only two single-phase transformers. In this paper, the equivalent circuit of the transformer bank was derived for the computer simulation of a distribution system. The models were verified with laboratory experiments. A sample feeder with this type of transformer was used to demonstrate its effect on the operation of a distribution system. The power flow and voltage imbalance which were induced by this transformer were analyzed. The relationship between the system imbalance and the power factor of a three-phase load was also investigated.     

三相变四相电力变压器研究

对3种不同的三相变四相电力变压器接线原理及结构特点进行比较并总结了各自的优缺点;建立了3种不同变压器的仿真模型并进行了在不同负荷条件下的仿真实验,比较了3种不同变压器在三相与四相交流电转换过程中的性能特点。研究表明:阻抗匹配三相变四相平衡变压器是利用绕组间的阻抗约束关系实现绕组间电磁关系平衡,绕组间无曲折连接,材料利用率较高,在三相与四相不对称转换过程中具有明显的优越性。     

Network modeling of 24-pulse rectifier transformers for rigorous simulation of rail transit power systems

This paper presents network models of rectifier transformers for rigorous simulation of rail transit power systems. The performance of traction power systems can be evaluated in detail by applying the EMTP, PSPICE or three-phase power flow programs, when accompanied by the proposed models. The proposed admittance models of the rectifier transformers consider winding connections, off-nominal tappings, and phase-angle displacement between primary and secondary voltages. The proposed models are presented for a 24-pulse rectifier transformer with two four-winding Delta/Delta-Wye connections. One of them has +7.5° phase-shifting in its primary side and the other −7.5° instead.     

A new approach to modeling three-phase transformer connections

The modeling of three-phase transformer connections for power-flow and short-circuit studies can take on many forms, depending upon the assumptions made. Too many times, simplifying assumptions lead to answers that are totally wrong. Exact models of the various connections must be used if correct results are to be achieved. The exact models must satisfy Kirchhoff's voltage and current laws, and the ideal relationship between the voltages and currents on the two sides of the transformer windings and the models must represent any phase shift that is a result of the particular connection. New models for all common three-phase transformer connections have been developed that satisfy all of the requirements stated above. This paper limits the discussion to the ungrounded wye-delta connection and the model developed for use in power-flow and short-circuit studies     

三相三柱变压器对偏磁电流的耐受特性研究

为了分析三相三柱电力变压器在高压直流输电系统大地回流作用下运行性能的变化情况,采用电路—磁路耦合的数值方法建立了包括变压器内部主要漏磁回路在内的该型式变压器详细模型,并利用该模型通过改变漏磁磁阻值重点讨论了漏磁回路对变压器耐受直流电流偏磁能力的影响。仿真分析结果表明,三相三柱变压器内部漏磁回路特性对变压器受直流电流偏磁作用的影响不可忽视。当漏磁磁阻减小至一定程度时,该种型式变压器仍然会受到直流偏磁电流的影响,因此在三相三柱变压器设计制造环节应对其漏磁回路加以充分考虑。     

Causes and effects of unbalanced voltages serving an inductionmotor

A common requirement of a distribution system is to serve a combination of single-phase lighting loads and a three-phase induction motor. Two transformer connections for providing this combination service are the ungrounded wye-delta and the open wye-open delta. Each connection has its good points and bad points. Of concern are the three-phase voltages provided the induction motor. If the voltages are unbalanced, depending upon the degree of unbalance, the induction motor is subject to overheating. Sources of the voltage unbalance can be the unequal spacing between conductors of the primary and secondary lines, the transformer ratings and connection, and the amount of single-phase load relative to the three-phase load. This paper utilizes previously published models of the primary and secondary lines, transformer connections, and induction motors. A simple system is developed to demonstrate the voltage unbalance created by these components under normal and abnormal conditions and the effect of the resulting unbalances on the induction motor. The system model also provides an insight into the voltages that can appear across the transformer terminals during abnormal operating conditions     

电气化铁路三相电压不平衡分析算法研究

通过对牵引供电系统结构的分析,推导了三相-两相系统变换方法,将牵引供电系统的两相参数转换为三相系统参数,使牵引供电系统与三相电力系统在拓扑结构上统一起来,从而可以分析牵引负荷引起的电网三相电压不平衡。该算法能够在计算中考虑牵引变压器自身阻抗的影响。使用PSCAD软件与本算法进行了验证,证明了本算法的正确性。     

Development of Three-Phase Unbalanced Power Flow Using PV and PQ Models for Distributed Generation and Study of the Impact of DG Models

With the increased installations of distributed generators (DGs) within power systems, load flow analysis of distribution systems needs special models and algorithms to handle multiple sources. In this paper, the development of an unbalanced three-phase load flow algorithm that can handle multiple sources is described. This software is capable of switching the DG mode of operation from constant voltage to constant power factor. The algorithm to achieve this in the presence of multiple DGs is proposed. Shipboard power systems (SPS) have other special characteristics apart from multiple sources, which make the load flow difficult to converge. The developed software is verified for a distribution system without DG using the Radial Distribution Analysis Package (RDAP). The developed software analyzes an IEEE test case and an icebreaker ship system. System studies for the IEEE 37-node feeder without the regulator show the effect of different models and varying DG penetration related to the increase in loading. System losses and voltage deviations are compared.     

Power system applications for PASC converter systems

This paper shows, using EMTP computer simulations, some preliminary results of applying pulse amplitude synthesis and control (PASC) technology to single-source voltage converter systems. The method can be applied to any single terminal pair source with appropriate modifications in power extraction interface and computer control program to match source and load impedance characteristics. The PASC realization as discussed employs banks of transformers, one bank per phase, in which the primaries are connected in parallel through a switch matrix to the DC source. Two opposite polarity primaries per transformer are pulsed alternatively in time to produce an oscillatory sinusoidal output waveform. PASC conversion system capabilities to produce both leading and lagging power factor power output in single-phase and three-phase Δ or Y configurations are illustrated. EMTP simulations are used to demonstrate the converter capabilities. Also included are discussions regarding harmonics and potential control strategies to adapt the converter to an application or to minimize harmonics     

电气化铁路对电力系统谐波的影响研究

提出利用PSCAD仿真软件建立不同接线方式牵引变压器的模型,并在恒电阻负载下验证了模型的正确性。阐述了将模型应用到某17节点电网进行牵引负荷分析,研究采用不同接线方式的牵引变压器时牵引负荷给电网带来的谐波和负序的影响。通过仿真分析得出当电力系统短路容量一定时,5种接线方式下牵引负荷在电力系统侧引起的电压畸变率由大至小依次为:单相接线、Yn/d11接线、V/v接线、阻抗匹配平衡接线和Scott接线,但5种接线牵引变压器之间的电压畸变率差别不大。     

On influence of load modelling for undervoltage load sheddingstudies

This paper explores the influence of load models on decisions of undervoltage load shedding in power systems. A controlled load rejection can be used as an emergency countermeasure to avoid widespread blackout when system voltages are unstable. In this paper, dynamic simulations of a small power system using both static and dynamic load models are presented. When using a static load model, the system includes an explicit model of a transformer with load tap changer. The aim is to demonstrate how different load models influence the analysis and calculation of the amount of load needed to be shed to stabilise the system voltage     

影响油浸式电力变压器振动因素的研究

变压器振动信号在运行过程中会受到各种因素影响。论文采用理论结合实验的方法研究了振动传感器安装位置、铁芯和绕组压紧程度、负载功率因数、负载三相不平衡度、绝缘油粘度对油浸式电力变压器振动信号的影响。研究结果表明：顶部测试位置可较好反映振动信号特性,铁芯和绕组压紧程度与振动强度呈反比,三相不平衡度和振动强度呈正比,功率因数影响振动信号频谱复杂度,绝缘油粘度对振动信号影响较小。     

电气化铁道应用三相变四相电力变压器的理论分析

三相变四相电力变压器是一种新型平衡变压器，由此可以实现四相输电方式，也可以用于电气化铁道的AT供电方式。该文基于该变压器的结构、磁势平衡方程、中性点接地和三相输入阻抗平衡条件，导出了该变压器四相侧的等值电路。分析了AT牵引网不同区段发生故障时变压器的负载阻抗特性，得到变电所处不设自耦变压器会影响三相侧的对称性和增加变压器的设计容量的结论。为了准确反映电力机车的负载电流或短路电流，应将接触导线和正馈线上的电流互感器次边差接。     

Evaluation of load division among transformers of different capacities in the grounded wye–delta and open wye–open delta banks under balanced loading and various power factor conditions

This paper evaluates the load division among transformers of different capacities in the grounded wye–delta and open wye–open delta banks under balanced loading and various power factor conditions. In some contingency cases two or three single-phase transformers of different kVA ratings are connected as a three-phase transformer bank to provide three-phase service for critical customers. In these cases, the load division among transformers becomes unequal and results in an unbalanced distribution system. The results of this study corroborate power company experience. Even when both three-phase sources and loads of the bank are balanced, the load division among transformers is unequal if transformers of different capacities are used in a bank, or an open wye–open delta connection is adopted.     

The analysis of an ungrounded Wye-Delta transformer bank serving aninduction motor and single-phase lighting loads

This paper documents the detailed analysis of the three-phase ungrounded wye-delta transformer bank serving a combination three-phase induction motor and single-phase lighting load. A 3×3 transfer function matrix is developed that makes it possible to determine the load line-to-line voltages with a knowledge of the primary voltages of the transformer bank. Included in the transfer function matrix are the transformer impedances, unsymmetrical secondary impedance matrix, and the impedances of the motor and single-phase lighting load     

Wyes and wye nots of three-phase distribution transformerconnections

The advantages and disadvantages of various three-phase transformer connections are summarized. Recommendations are provided regarding the standard types of connections a utility should offer for various service requirements. Warehousing costs, ferroresonance, neutral grounding, core construction, overvoltages, unbalanced loading, zero sequence source, and through faults are considered. Emphasis is placed on the exchange of information between customer and utility. An Application for Three-Phase Service form is provided. It can be used to determine service requirements including voltage, grounded, ungrounded, three-phase load, single-phase load, motor characteristics, and fault current interruption rating of secondary breakers and switchgear. Suggestions on utility-supplied information such as maximum available three-phase and line-to-ground fault currents and a discussion of the operation of ungrounded systems are also provided     

应用于三相潮流程序的变压器模型的实现

介绍一种三相潮流程序中的变压器模型的实现方法。其思路是将变压器分左右两块实现:左侧部分包含一次侧的联结组别和三相的变比值,右侧部分包含二次侧联结组别和变压器阻抗值,并分别以子程序实现。计算中根据实际变压器类型调用相应子程序,通过组合得到完整的变压器模型。该方法思路简洁,易于编程实现,能疗便地实现多种不同接法的变压器模型,可以很好地应用于三相潮流程序。通过一个简单算例对模型的有效性进行验证和分析。     

新型三相变四相阻抗匹配平衡变压器原理

在已有的三相变两相平衡变压器基础上,从阻抗匹配原理出发提出并讨论了一种新型的三相变四相阻抗匹配平衡变压器。详细分析了四相侧各绕组阻抗匹配关系以及三相侧与四相侧安匝平衡关系,功率平衡关系,电压、电流变换关系等,论证了该新型阻抗匹配平衡变压器在四相侧负载变化时对一次的零序电流没有影响。通过Matlab/Simulink仿真实验验证了三相变四相阻抗匹配平衡变压器原理分析的正确性,表明该新型阻抗匹配平衡变压器能实现三相与四相输变电的平衡对称转换。     

Harmonic cancellation by mixing nonlinear single-phase andthree-phase loads

The voltage on the distribution line is, in most cases, distorted even at no load of the transformer. This is due to the “background” distortion on the medium-voltage line caused by the large number of single-phase nonlinear loads, such as PCs, TVs, VCRs, etc. This paper proposes a method to mix single-phase and three-phase nonlinear loads and reduce the harmonic currents significantly. The dependence of the phase angle of the harmonic currents as a function of the short-circuit impedance is investigated using SABER for the three-phase and the single-phase diode rectifier both with and without DC-link inductance. The phase angle of the fifth harmonic current of a three-phase diode rectifier is always in counterphase with the fifth harmonic current of a single-phase diode rectifier. This leads to the conclusion that adding three-phase rectifier load can actually improve the power quality at the transformer. This is also validated by a number of on-site measurements in several applications of three-phase adjustable-speed drives     

负荷模型对电力系统动态稳定的影响

随着大区电网的互联,低频振荡在我国电力系统中已成为突出的问题.本文研究电力系统的负荷模型对电力系统动态稳定的影响.通过搭建单机无穷大系统,理论上研究送端负荷分别采用恒功率,恒电流及恒阻抗模型时,分析其与自动电压调节器及电力系统稳定器的相互作用对电力系统动态稳定的影响.通过对一个两区域四机系统的典型实例,利用PSD-BP...