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     

Experimental analysis of a DC bucking motor blockinggeomagnetically induced currents

An experimental study investigates the performance of a DC bucking motor for the blocking of geomagnetically induced currents (GICs) within a 3 kVA/416 V three-phase distribution feeder. This blocking method consists of connecting a separately excited DC motor between neutral and ground of a Y-grounded transformer consisting of a bank of three single-phase transformers. Losses and currents of the transformer and the blocking device are measured at no-load, rated-load, unbalanced-load and line-to-neutral short circuit conditions without and with GICs. Thereafter, the DC, AC and total λ-i characteristics of one single-phase transformer of the above-mentioned transformer bank are simultaneously measured using an arbitrary waveform generator, a power amplifier and a digital data acquisition network. These λ-i characteristics (where the DC bias current is a parameter) are then used to model a single-phase transformer under DC bias current conditions (e.g., assumed GICs) and to calculate voltages and currents of a single-phase transformer when subjected to GICs. The simulated signals are then compared with measured time functions     

基于平衡变压器的三相-单相对称供电条件研究及参数匹配

提出一种在平衡变压器三相变两相的基础上,根据移相原理匹配以无功补偿元件,实现三相系统对单相负荷平衡供电的三相-单相对称供电方式。该供电方式利用平衡变压器输出两相电压的正交特性,从而简化了匹配元件的计算;讨论了其三相-单相变换的拓扑结构,针对纯阻性、阻感性、阻容性3种不同性质的负荷提出了3种不同的接线方式及其匹配元件参数的计算公式;并对由匹配元件参数的误差和负荷在一定范围内变化所引起的三相侧电流负序分量增大情况进行了分析,给出了抑制此负序电流的一种解决办法;最后在一台2 kVA的平衡变压器样机上验证了所推计算方法的正确性和实用性。     

单相变压器供电的高压钠灯降压节电试验

城市道路照明负荷是单相负荷,将原三相变压器供电改由单相变压器供电,组成V／V0-V/V单相变压器-高压钠灯节电照明系统,降低了低压配网损耗,工程造价相对减少,提高了供电可靠性,同时可以达到下半夜路灯自动降压节电的节电效果。     

Scott接线变压器后备距离保护研究

为了满足牵引变压器后备保护的特殊需求,针对Scott接线变压器,构造了一种基于相间电压和相电流后备距离保护。从数学模型出发,讨论了Scott变压器高低压侧之间的电压和电流变换关系,并分析了变压器的等值漏阻抗。理论分析表明,三相-两相变换导致Scott变压器高压侧的相地阻抗和相间阻抗无法直接反映低压侧故障。由于无中性点接地,Scott变压器高压侧无法测得相电压,根据电压关系用相间电压和相电流构成了适用于Scott接线变压器的距离保护。仿真结果表明,基于相间电压和相电流构成的距离保护能够明确区分短路阻抗和负荷阻抗,从而正确动作于故障。     

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.     

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.     

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     

新型YNvd接线平衡变压器电气分析及其仿真模型

YNvd接线变压器是应用于电气化铁路的一种新型三相一两相平衡变压器.为了实现其理想的三相一两相平衡变换,从磁势平衡方程、绕组接线方程、电压传递方程出发分析了YNvd接线变压器的运行特性,采用绕组间短路阻抗和等值阻抗表达阻抗匹配关系.并给出了YNvd接线变压器的二次侧两相端口的电压输出方程、两相等值电路和相坐标下的节点导...     

Parameter Estimation for Power Transformer Models from Time-Domain Data

The existence of power system harmonics has been realized for many years. With the advent of new technologies and more solid state loads, power system harmonics are certain to increase both in amplitude and frequency bandwidth. Signal propagation in power systems depends on transformer impedances. Transformer impedances, on the other hand, are functions of signal frequency as well as input voltage and loading condition. Present power transformer models with single-valued parameters obtained from tests cannot adequately represent the transformer in the presence of harmonics. Accurate modeling of transformers must account for changes in signal frequencies and loading conditions as they occur. This paper presents algorithms for estimation of the parameters of a structurally known transformer model. These algorithms use time-domain data and continuously update the estimates, thereby accounting for changes in operating conditions. The techniques presented here can be applied to three-phase and single-phase N-winding transformers. The results of model identification for a single-phase two-winding transformer is given. The data acquisition system used and the problem of identification in a noisy environment are discussed. The simulated response of the identified model is compared with the transformer actual response.     

Space vector modulated three-phase to three-phase matrix converterwith input power factor correction

Analysis, design, and implementation of the space vector modulated three-phase to three-phase matrix converter with input power factor correction are presented. The majority of published research results on the matrix converter control are given an overview, and the one which employs simultaneous output-voltage and input-current space vector modulation, is systematically reviewed. The modulation algorithm is theoretically derived from the desired average transfer functions, using the indirect transfer function approach. The algorithm is verified through implementation of a 2 kVA experimental matrix converter driving a standard induction motor as a load. The switching frequency is 20 kHz. The modulator is implemented with a digital signal processor. The resultant output voltages and input currents are sinusoidal, practically without low-frequency harmonics. The input power factor is above 0.99 in the whole operating range     

劈相机和单相三相变压器的原理及应用

分析了单相三相变压器的原理,给出了单相 三相变压器补偿电容和电感值的计算方法。通过对劈相机和单相 三相变压器性能的比较,指出在需要将单相电源变为三相电源时,采用单相 三相变压器更为有利     

华东电网500 kV主变备用适用性的初步研究

以浙江电网1000 MVA容量的500 kV主变乔司2号变为例,介绍以现有的250 MVA容量500 kV单相变压器备品更换存在缺陷的333 MVA容量500 kV单相变压器的可行性和适用性.选取华东全网作为研究和计算对象,采用德国西门子公司电力系统仿真软件包NETOMAC,在NETOMAC平台上,开发出通用的三相三绕组变压器模型及算法用于三相不平衡潮流计算.     

Causes and effects of single-phasing induction motors

It is well known that a three-phase induction motor will continue to operate when a disturbance of some sort causes the voltages supplied to the motor to become single phased. Single phasing can occur as a result of a fuse blowing or a protective device opening on one phase of the motor. Other possibilities include feeder or step-down transformer fuses blowing. Even though the motor will continue to operate in this condition, the motor will heat up very quickly, and it is essential that the motor be removed from service by the opening of a motor circuit breaker or some other type of protective device. This paper will describe three different ways in which an induction motor will operate in a single-phase condition. For purposes of this paper, "single phase" will include any condition in which the three line-to-line voltage phasors appear on the same line.     

三相负载不平衡对变压器振动特性的影响研究

三相负载不平衡是配网中较为常见的问题。为研究三相负载不平衡工况下配电变压器振动特性,本文理论分析了铁芯和绕组振动特性及绕组振动传播特性;利用有限元软件分析了三相负载不平衡前后铁芯磁感应强度及绕组电磁力变化情况;搭建了变压器振动测式平台,分别采集变压器在三相不同位置单相带额定负载、两相带额定负载工况下的振动信号。研究结果表明：三相负载不平衡工况下铁芯中磁通密度基本不变,负载不平衡不会对变压器振动信号高频分量产生影响;不同位置的绕组振动对同一振动测试点影响程度不一样;三相负载不平衡程度相同,不平衡相发生的位置不同,对同一测点的影响程度不一样     

基于逆阻式IGBT的三相/单相矩阵式变换器

针对三相/单相矩阵式变换器系统,提出一种平均电压脉宽调制方法,通过实时检测三相输入相电压,经过简单计算即可对输出进行正弦调制.采用新型开关器件逆阻式IGBT(RB-IGBT)实现了一台3kW的三相/单相矩阵式变换器样机,并针对RB-IGBT独特的漏电流特性,提出一种基于输入电压大小检测、适用于RB-IGBT的两步换流方法.实验结果表明,该样机可以实现正弦的输出波形.从而证明了本文提出的控制策略和换流方法的有效性和可靠性.     

Analysis of three-phase wound motors under unbalanced conditions

The analysis of three-phase wound motors with unbalanced supply voltages and unbalanced rotor-phase impedances is presented. The rotating field theory and coupling impedances approach are used in modelling the motor in the steady state. The effect of rotor skewing is considered. The relationships between stator and rotor quantities, in a general form, are obtained. The analysis is verified by comparison with the conventional equations for currents and torque.     

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     

Control of Electronic Power Transformer with Star Configuration under Unbalanced Load Conditions

The electronic power transformer has many advantages, such as reactive power compensation, voltage regulation, harmonic suppression, and power factor correction. When a three-phase electronic power transformer with a star configuration is applied in the distribution network, three-phase load unbalance will cause three-phase DC voltage unbalance if the electronic power transformer controller generates improper asymmetric voltage signals, which may trigger over-voltage protection. The effects of three-phase load unbalance to the work condition of the electronic power transformer are analyzed, and a control algorithm is proposed to balance three-phase DC voltages in this article. Meanwhile, input stage control, isolation stage control, output stage control, and individual DC voltage balancing control are presented. Simulation and lab experimental results show that the electronic power transformer still works properly even under serious unbalanced load conditions.     

一种基于回路电流法的有源配电网潮流算法

提出一种基于回路电流法的主动配电网三相潮流算法,并提出风机等多种分布式电源在该算法中的计算模型。首先建立配电网络对应的图,然后将基本回路电流、变压器原边支路电压、非恒阻抗负荷支路电压、分布式电源支路电压、异步电机正序负序电压、转差率作为未知量,列写回路KVL方程、变压器原副边电流方程、负荷功率平衡方程以及分布式电源相关方程,推导Jacobian矩阵,并利用牛顿法求解方程。该方法不需要PV节点转化为PQ节点的过程,也不需要将环路解列及复杂的节点编号,没有对Jacobian矩阵进行简化和近似,具有二阶收敛性。算例表明,所提方法计算速度快,能够处理所有常见的分布式电源,具有较强环路处理能力,且比前推回推法有更好的收敛性。