Analysis of the voltage phasors characteristics for motor unbalanced supplies under constant voltage level

This article presents an analytical and graphical study of the supply characteristics of a three-phase induction motor operating under constant average voltage in unbalanced conditions. Till now, researchers have addressed the unbalance analysis of induction motors without paying attention to the accuracy on the voltage level issue. This work develops the loci of voltage phasors and their symmetrical components, and calculates the variation ranges of voltages and voltage unbalance factor (IEC) for an induction motor operating under constant line average voltage and constant percent voltage unbalance (NEMA). The results are applied to the variation limits established in the electrotechnical standards.     

The effect of fast changes in voltage unbalance on vibration characteristics of asynchronous motors

The effect of voltage unbalance in electrical networks that supply electrified traction loads on the operating regimes of large asynchronous motors (AMs) is considered. Based on the method of symmetrical components and analysis of hodographs of magnetic fields of AMs with a short squirrel-cage rotor, the influence of various types of voltage unbalance (continuous and intermittent) on its vibration characteristics is studied. It is shown that an increase in magnetic induction in the air gap of an AM causes an increase in radial vibration generating forces in the case of voltage unbalance. The obtained results indicate that alternating voltage unbalance can be riskier for AM operation than is constant voltage unbalance, in addition to having increased levels of AM vibrations. It is concluded that the use of new parameters of electric-energy quality allowing determination of instantaneous values of unbalance amplitude and the angle between symmetrical components of voltage of positive and negative phase-sequence is reasonable. The relevance of this research is confirmed by typical cases of AM shutoff at an oil-pumping station due to tripping of vibration-protection sensors.     

A quantative approach to estimate the life expectancy of motorinsulation systems

Electric motors represent the largest single load in most power systems. With continuing increase in nonlinear loads, the effects of harmonic contents of power systems on various components is becoming more important. This paper is concerned with the effects of power system harmonics and voltage unbalance on the useful life of electric motors. A simple equivalent circuit is used to calculate the additional heat losses caused by supply distortions. A lumped-parameter thermal network is proposed to calculate the resulting heat distribution within the machine. An Arrhenius chart is then used to estimate motor insulation remaining life. The proposed approach is used to determine the effect of supply harmonics and voltage unbalance on the useful life of a three phase induction motor and the results are verified experimentally     

Design aspects of synchronous PWM rectifier-inverter systems underunbalanced input voltage conditions

A complete analysis of a synchronous front-end PWM rectifier-inverter system under unbalanced input voltage conditions is presented. The analysis includes the harmonic assessment of input/output current and voltage waveforms. It is shown both theoretically and experimentally that input voltage unbalance generates uncharacteristic low-frequency harmonic components in the input and output currents. Special emphasis is given to the generation of a DC component in the inverter output voltage in the case in which the output frequency is twice the input frequency. System design curves for different degrees of unbalance for all major system components are reported. Key predicted results are confirmed experimentally     

三相四臂对逆变电源控制方法的研究

针对三相四臂对逆变电源提出了一种新的控制方法,即在对三相输出电压不平衡机理分析的基础上,通过设计负序、零序控制器,消除了三相四臂对逆变电源中负序和零序分量的影响,从而保证三相相电压在不平衡负载下的对称输出,仿真结果验证了该方法的可行性.     

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。     

Evaluating power system unbalance in the presence of harmonic distortion

This paper presents a new method to evaluate power system unbalance in the presence of harmonic distortion. The balanced, the first unbalanced, and the second unbalanced components are obtained from the original three-phase current and voltage phasors of each harmonic component. The equivalent root-mean-square (RMS) value of three-phase voltage and current is then decomposed into the balanced fundamental, the unbalanced fundamental, the balanced harmonic, and the unbalanced harmonic components. The same decomposition is applied to the apparent power, and the system unbalance can then be evaluated through the proposed system total unbalance distortion factor. The unbalanced fundamental, the balanced harmonic, and the unbalanced harmonic distortion factors are defined. These distortion factors clarify the interleaving between the unbalance and harmonic distortion, and are suitable for unbalance evaluation in the presence of harmonics. Numerical examples are also presented to illustrate the application of the proposed method on the study and characterization of unbalance in power systems.     

Effect of Unbalanced Voltage on the Operation of Polyphase Induction Motors

Three-phase electric power systems generally provide voltage supply at the generating station that is well balanced in both magnitude and displacement. For varying reasons, however, some degree of voltage unbalance occurs at the point of utilization, and, depending upon the amount of unbalance, this can have an adverse effect upon the operation of polyphase induction motors. A major cause of unbalanced voltage at the point of utilization is that single-phase loads on a system are not uniformly applied to all three phases. This is particularly true for some rural electric power systems, and it can also occur in large urban power systems where heavy single-phase demands are imposed by facilities such as extensive shopping plazas. A large manufacturing plant may have well balanced incoming supply voltage, but unbalance can develop within the plant from its own single-phase power requirements if these are not uniformly spread among the three phases. Regardless of the cause, unbalanced voltage is harmful to three-phase induction motors. The paper to follow concentrates upon that aspect of unbalanced voltage.     

Analysis of the neutral conductor current in a three-phase supplied network with nonlinear single-phase loads

This paper describes what factors (i.e., load and supply) have an important effect on the neutral conductor current. It is shown that an asymmetry up to 10/spl deg/ or an unbalance of 10% in the power supply has only a minor effect on the RMS value of the neutral conductor current. An unbalance in load conditions increases the neutral conductor current. Harmonics in the power supply voltage highly affect the RMS value of the neutral conductor current.     

张北数据港柔性变电站DC/AC变流器关键控制策略

柔性直流输电是实现新能源并网和直流电网的极具潜力的输电方式。文中在张北数据港设计构造了一台柔性直流输电系统用DC/AC变流器。所设计的变流器采用多变流器并联+z型接地变压器结构。为抑制离网下由于负荷特性而造成的输出电压不平衡与输出电压畸变问题,分别提出了变流器输出电压不平衡控制策略与输出电压谐波抑制策略,以保证设备的供电质量。为保证设备的不间断供电并提高设备的供电可靠性,提出一种主动限流控制策略,在设备离网供电模式下电力系统发生短路故障时进行主动限流。最后,搭建了2.5 MW DC/AC变流器进行实验研究。实验结果验证了所提控制策略的有效性。目前,该装置已应用于张北数据港柔性变电站。     

Criteria to estimate the voltage unbalances due to high-speedrailway demands

This paper has presented simple criteria to estimate the voltage unbalances due to high-speed railway demands that are generally single-phase loads. Feeding traction loads from the public power system may lead to some voltage unbalance on the latter and consequently affect the operation of its energy supply system and other equipments connected with it. Three transformer connection schemes that are commonly used in power supply systems for high-speed railways are discussed and compared. The estimating criteria have been derived and represented by simple formulae that can be easily applied to evaluate this voltage unbalance. The results are of value to related engineers and consultants especially during periods of planning and design     

三相电压不平衡对开口三角保护的影响

目前大量采用开口三角保护作为10 kV并联电容器成套装置的不平衡保护,本文分析开口三角保护与电源三相不平衡的关系,主要通过对开口三角保护公式的推导计算,表明只有当电容器装置的三相阻抗不平衡时才能产生开口三角保护电压。当三相阻抗不平衡时,三相电压不平衡可能引起开口三角电压放大。     

基于电网电压定向矢量变换的SVC平衡化补偿策略

冲击性负荷会引起供电系统严重的电压波动和三相不平衡,对其进行平衡化补偿具有重要的意义。将矢量控制思想移植到动态无功补偿中,建立晶闸管控制电抗器(TCR)型静止无功补偿器(SVC)平衡化补偿策略。该策略通过在电网电压稳定条件下对负荷电流进行矢量变换,实现其基波正序、负序分量的分离以及基波有功、无功分量的解耦,最后利用低通滤波器检出有用直流信号并计算出补偿电纳。仿真及实验表明,该策略具有较高的精度和优良的动态性能,其补偿效果不受负荷谐波电流的影响。     

级联STATCOM阀组直流侧电压不平衡现象及原因分析

在级联H桥静止同步补偿器（STATCOM）研究实验过程中,发现各相阀组间存在直流侧电压不平衡,且阀组间直流侧电压不平衡度随着补偿电流的增大有增大的趋势。阀组间的直流侧电压平衡,对于级联H桥STATCOM的安全运行以及性能均具有重要意义,但尚未有阀组间直流侧电压不平衡的原因分析。文中通过对系统电压的正负序分解,分析了级联...     

Novel frequency-domain-based technique to detect stator interturnfaults in induction machines using stator-induced voltages afterswitch-off

Traditionally, for medium- and high-voltage motors and generators, condition-based monitoring of stator faults is performed by measuring partial discharge activities. For low-voltage machines, negative-sequence impedance or currents are measured for the same. Such diagnostic schemes should be carefully implemented as supply voltage unbalance, manufacturing-related asymmetry, etc., also produce negative-sequence voltages. A few approaches based on motor current signature analysis have already been proposed to detect stator interturn faults. However, little or no physical insight was provided to explain the occurrence of certain harmonics in the line current or the influence of voltage unbalance on these harmonics. Also, in at least one of these papers, a large portion of the stator winding was shorted to emulate the faults. The method proposed in this paper monitors certain rotor-slot-related harmonics at the terminal voltage of the machine, once it is switched off. In the absence of supply voltage, issues such as voltage unbalance, time harmonics do not influence the measurements except as initial conditions, which is a very desirable feature when the machine is fed from an adjustable-speed drive. Satisfactory simulation and experimental results have been obtained with only about 1.5% (5/324) of the total number of turns shorted     

基于粒子群优化算法的孤岛微电网电压不平衡补偿协调控制

微电网系统孤岛运行条件下,其系统电压完全由网内众微电网逆变器协调运行提供支撑,微电网逆变器控制性能将决定网内供电电压质量的优劣。考虑到微电网中大量单相负荷的存在,系统负荷实则通常表征为不平衡,容易引起系统电压的不平衡。针对这一问题,以同时兼顾微电网逆变器端口电压及公共连接点(PCC)电压不平衡度控制为目标,从分析电压不平衡机理入手,提出一种基于粒子群优化算法的负序电压补偿算法。补偿算法以逆变器端口电压和PCC电压不平衡度作为约束条件建立目标函数,同时考虑逆变器间的环流问题,在实现逆变器端口及PCC电压不平衡控制的同时减小逆变器间的环流。最后,分别基于MATLAB及某电力电子实时仿真平台搭建了系统仿真模型和半实物实时仿真模型,仿真及实验结果验证了所提方法的正确性和有效性。     

基于比例积分谐振的微网逆变器电压均衡控制策略研究

对于工作在离网模式下的微网逆变器,在可能会出现的负载不对称或者单相负载运行场合,采用传统的电压电流双闭环控制方法会导致输出电压严重不平衡,从而危害供电设备以及整个系统。基于分裂电容式的三相四线逆变器系统,针对逆变器在离网模式下带不平衡负载运行的问题,建立相关数学模型分析抑制不平衡问题,分析传统比例积分控制的缺陷,并在此基础上采用正负序分离独立控制和比例积分准谐振控制策略。同时针对微电网运行状态平滑切换的要求,给出一种基于间接电流控制策略的平滑切换控制策略。仿真结果证明该控制策略的正确性。最后在微电网逆变器实验平台上通过实验验证了该方案的实用性和可行性。     

配电系统大功率交流电弧炉电能质量问题及方案治理研究

根据典型参数搭建电弧炉供配电系统，采用电弧炉现场录波数据作为负荷模型，以此为平台对电弧炉运行时给系统带来的电能质量问题进行详细分析，包括负荷谐波、不平衡以及母线电压波动等。以电能质量治理为目标，综合设计补偿方案：无源电力滤波器用于滤除负荷谐波电流，降低连接母线电压的总谐波畸变(total harmonic distortion, THD)，而变压器隔离的链式DSTATCOM配合瞬时电流分相控制方法，能够分相快速对负荷相电流进行控制，补偿负荷不平PSCAD/EMTDC中构建仿真系统，对基于现场数据模型的衡、抑制母线电压波动和闪变现象的产生。在电弧炉系统和补偿方案进行研究，最后给出了仿真结果用以表明所提治理方案的有效性。     

一种适用于直流配电网的双向稳压型电压平衡器

直流配电网是一种新型供电系统,面对三线制直流配电网中负荷不平衡问题及其需为用户端提供稳定可靠供电的要求,提出一种适用于新型直流配电网的双向稳压型电压平衡器。将双向稳压Buck/Boost电压转换电路运用于电压平衡器之中,这样不仅能解决三线制网络中正负极电压平衡的问题,而且可以实现平衡输出端电压波动,维持输出端的电压稳定。首先针对该双向稳压型电压平衡器进行数学建模分析,利用状态平均法建立小信号状态模型;之后基于所提出的平衡器特性与其建模分析,在双向互补脉冲宽度调制（pulse width modulation,PWM）控制技术的基础上提出并分析控制策略;最后利用PSCAD仿真软件进行存在电压波动与不平衡负荷情况下的2种负荷类型的稳压平衡器仿真验证,仿真结果验证了其稳压及平衡电压能力,说明了理论分析的正确性。     

A method integrating deterministic and stochastic approaches forthe simulation of voltage unbalance in electric power distributionsystems

This paper deals with modeling, analysis and Monte Carlo simulation of three-phase voltage unbalance based on data measured from a 69/11-kV distribution substation. Random variation of the voltage unbalance factor is modeled with the aid of correlated Gaussian random variables that represent random variations in three-phase active and reactive powers. Also presented in this paper is a network reduction technique using multiple-port Thevenin equivalent circuits that allows the Monte Carlo simulation to be carried out faster. Comparison of simulated and recorded probability distributions of voltage unbalance factors is given, and good agreement has been obtained