Effects of momentary voltage dips on the operation of induction andsynchronous motors

Simplified stability calculations of induction and synchronous motors on voltage dips are presented. The voltage dip withstand characteristics of induction and synchronous motors rated at the same horsepower and driving the same inertial loads are calculated. The motor characteristics influencing stability are: the type of motor selected (synchronous or induction); the pull-out torque of synchronous motors and the breakdown torque of induction motors; a constant excitation controller or an excitation boost on voltage dips for the synchronous motors; load and motor inertia; and load characteristics. Synchronous motors are not suitable for fast autoclosing or bus transfer, although these can be autoresynchronized. Transient torques and current surges must be considered when induction motors are connected for fast bus transfer. Due to varying power systems, motors and load characteristics, and much interaction between these, an analysis of avoiding a shutdown on voltage dips has to be performed on a case-by-case basis. This may include a computer-based study. The stiffness of the power system in relation to motor loads, the probable type of faults and fault clearance times, and motor protection and controls are some other concerns requiring an analysis in this context     

电压稳定分析中异步电动机动态负荷建模探讨

探讨电压稳定分析中异步电动机(IM)负荷的建模问题。分析一般的动态负荷模型(GDLM)不能捕捉IM动态行为的机理。分别以IM的滑差模型和滑差磁链模型为基础，严格地导出了适用于电压稳定分析的简化一阶(SIM)和详细三阶(DIM)动态负荷模型，同时还说明了SIM和DIM模型实质上是对GDLM模型进行了不同程度的改进。对SIM和DIM模型在暂态电压稳定分析和静态电压稳定分析中的适用性进行分析比较，仿真结果表明：SIM和DIM分别以不同的准确度适用于不同场合的电压稳定分析。     

感应电动机参数对厂用电切换方式的影响

厂用电快速切换是提高负荷供电可靠性的一种重要手段.鉴于现代负荷以感应电动机为主,文中以一个典型发电厂为例,采用PSCAD/EMTDC深入分析感应电动机参数对快速切换、同期捕捉切换和残压切换的三种电源切换方式的影响.研究发现电动机惯性常数和负荷转矩负载率主要影响快速切换和同期捕捉切换;而残压切换方式主要受电动机转子电阻,励磁电抗和负荷转矩负载率的影响.该文对于研制更为有效的厂用电切换装置具有一定的参考意义.     

电动机节能的智能控制

在分析异步电动机节能原理的基础上 ,提出了一种以单片机为核心的新型异步电动机节能控制装置 ,整个过程实现了智能控制。实时监测异步电动机的负载大小 ,自动控制其端电压 ,使其与负载理想匹配 ,达到节能的目的     

应用综合负荷模型的电容补偿配电网感应电动机起动方法

以目前在国家电网公司推广应用的考虑配电网的综合负荷模型为平台,建立了从系统的角度综合考虑配电网及负荷的电容补偿电动机起动分析系统。分析了电容补偿电动机起动过程中母线电压达最低要求对配电网高压母线短路容量﹑配电网阻抗的要求,以及电动机容量﹑配电网高压母线短路容量﹑配电网阻抗对电容容量的影响,为电容补偿配电网感应电动机起动过程中母线电压满足最低要求的条件以及电容容量的选择提供理论支持。     

考虑动态负荷机械转矩参数的节点暂态电压评估

动态负荷的主要成分是电动机,其负荷特性是电力系统节点暂态电压稳定性的重要影响因素,但目前相关研究未考虑电动机负荷机械转矩特性。文中在简化感应电动机模型基础上,结合电动机负荷机械转矩变化,提出了极限切除时间解析方法。得到的极限切除时间指标能反映负荷节点暂态电压稳定状态,可以作为节点暂态电压稳定性指标。IEEE30节点系统的仿真结果表明,所述方法计算出的极限切除时间能真实反映电动机负荷的实际运行特性,是评估系统暂态电压稳定性的良好指标。     

VSC-HVDC适应感应电动机负荷稳定性研究

日益增长电网负荷中心含有的感应电动机负荷,因其高度非线性和动态特性,给电力系统带来严重的电压失稳甚至崩溃的风险。文章通过研究感应电动机负荷特性,分析其与电力系统稳定的关联性。同时基于柔性直流系统功率传输可控特性,研究了一种柔直紧急支援控制策略,通过适当改变控制指令值调节交直流线路间的交换功率,从而实现负荷以及系统的稳定。最后通过PSCAD/EMTDC平台搭建某城市局部交直流系统模型,针对不同感应电动机负荷比例进行仿真分析。经验证,柔性直流系统采用该控制策略能有效提升电网感应电动机负荷稳定水平。     

Effects of various unbalanced voltages on the operation performance of an induction motor under the same voltage unbalance factor condition

This paper investigates the effects of different unbalanced voltages with the same voltage unbalance factor (VUF) on the performance of a three-phase induction motor and its feedback influence on the power system. Under eight different voltage unbalance conditions, the performance of a three-phase induction motor was measured through a real load test. The loss, efficiency, power factor and temperature rise of the motor under different unbalanced voltages are investigated. According to the test results and analysis, the importance of the positive-sequence voltage on the motor’s apparent performance and the negative-sequence voltage in the hidden damage were pointed out. Finally, it is strongly suggested that (1) the derating and temperature rise curves and related regulations should consider the VUF percent and the magnitude of positive-sequence voltage, and (2) the effects of unbalanced voltage on induction motors should also be modeled in power system voltage stability studies.     

一种改进的电动机静态聚合方法

电动机参数是电力系统电压稳定、暂态稳定的关键因素之一.在基于统计综合法的负荷建模中,如何提高电动机参数的聚合精度是重要课题.针对传统电动机聚合方法没有考虑聚合前每台电动机稳态滑差的不足,提出了一种改进的电动机的静态聚合方法.该方法首先根据聚合前各电动机的惯性时间常数、暂态电抗、转子回路时间常数、临界滑差定义了一个电动机的特性指标,并按照该指标对电动机进行分组.然后将每组中的电动机根据其稳态等值电路转化为3种阻抗并联的形式,在对3种阻抗分别聚合后再还原为一台电动机稳态等值电路.算例表明该方法能够有效提高仿真精度.     

基于节点电动机最大自起动量的配电网低压减载

低压减载是电力系统安全稳定第三道防线的重要组成部分,而减载方案是低压减载的重要研究内容。感应电动机是造成电网电压崩溃的主要因素,也是低压减载的主要负荷。基于配电网及感应电动机有功特性、无功特性、转子运动方程等在Matlab搭建模型进行时域仿真分析。结果表明低压减载后的电网存在感应电动机的自起动过程,且以感应电动机为减载对象时,自起动的最大允许量与最小减载量有紧密联系。从而提出一种基于节点电动机最大自起动量的减载量整定计算方法以及减载策略。即通过不同初始转速下的节点电动机最大自起动量计算出每轮减载量,考虑减载速度对电压稳定性的影响,可在每轮减载中引入基于电压变化率的加速判据,实现有选择性地加速切除负荷。最后对传统减载策略和所提减载策略进行对比仿真,验证了所提减载方案在保证节点电压快速稳定的同时可实现对减载量的有效控制,从而保证了系统和重要电动机负荷的安全稳定运行。     

含多感应电动机负荷的微网电压稳定性仿真分析

微网中含大量高度非线性的感应电动机负荷,对微网孤岛运行时的电压稳定性构成严重威胁。为此,针对含多感应电动机负荷的微网,根据电力系统功率平衡的原则,从理论上推导了微网各电动机负荷转差与系统电压的动态耦合关系,进而说明感应电动机负荷更加敏感的原因及多感应电动机负荷的转差和系统电压之间的动态耦合特性与微网稳定性相关。基于MATLAB/Simulink仿真平台,搭建了风光柴储多源主从控制微网,通过仿真研究了不同条件下多机叠加启动及微网故障两种情况下微网的电压稳定性,进而提出了提高含多感应电动机负荷的微网电压稳定性的措施。仿真结果表明,多感应电动机负荷之间的相互作用对微网电压稳定性具有重大影响,且串联主从微网的临界故障清除时间受三相短路故障点离感应电动机负荷点的距离和故障点离主控单元的距离双重制约,采用叠加启动策略和故障点快速切机等措施可有效提高微网电压稳定性。所得结论为含多感应电动机负荷的微网稳定性研究与控制提供了理论参考。     

考虑带泵类负载感应电动机影响的静态电压稳定性分析

为了更准确计及泵类负载机械特性对配电网静态电压稳定的影响,建立了一种将感应电动机T型等效电路与泵类负载机械特性相结合的感应电动机负荷模型。将其用于配电网潮流计算,采用前推回代法求解。进而将其用于连续潮流计算,建立了带泵类负载多台相同感应电动机的等效负荷模型,分析负荷连续增长对配电网静态电压稳定的影响。采用IEEE33节点算例对提出的模型与方法进行仿真验证,并与负荷按恒功率因数增加模型的连续潮流进行了对比。仿真计算验证了所提方法的合理性和有效性,表明采用该模型所得的静态电压稳定裕度要小于采用恒功率因数负荷计算结果,更接近工程实际情况。     

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     

异步电动机降压运行时的一种计算方法

给出了一种计算异步电动机降压运行时各损耗功率及定子电流电压的方法。以驱动某一风机负载为例 ,当其以降压调速方式调节风机流量时 ,应用MATLAB语言 ,对电动机的各损耗功率、定子电流电压及其实际运行效率等进行较为详尽地计算 ,计算结果以数据表格和图形方式给出     

STS and induction motors

Industrial plants with a high amount of induction motor load can run into problems following short interruptions of the supply or voltage sags. After the voltage has recovered, the high current drawn by the motors can cause voltage collapse. The case study presented in this article shows that the static transfer switch (STS) can provide a solution to this problem by transferring the loads to an alternative healthy supply within some milliseconds from the occurrence of the disturbance. However, this case study has also shown that the time needed for transferring the load is dependent on the type of disturbance and the instant when it occurs, and the transfer time is, in general, higher with an induction motor load, as compared with static load. For plants with a high percentage of motor load, a specific performance study should be carried out to assess if the installation of the STS actually improves the power quality for the plant.     

Computer Analysis of 3-Phase Induction Motor Operation on Rural Open-Delta Distribution Systems

Open-delta/open-delta and open-wye/open-delta transformer banks are often used as an economic means of supplying simultaneous single-phase and 3-phase loads in rural areas. If the 3-phase load is an induction motor, the inherent voltage unbalance causes increased losses and uneven heating that may lead to motor failure. often these motors are submersible or deep well pump motors located in remote unattended areas or air conditioning motors which may not have sufficient over-design to handle the voltage unbalance. The theory analyzing the open-delta distribution system has existed for many years, however, because of the tedious and almost infinite differences in distribution topology, the problem has been neglected. The purpose in this endeavor is to present a simple general approach to the solution of the problem by utilizing existing motor and network concepts and theories. Two previously published theories are used to predict motor heating and the motor derating that is necessary to prevent insulation failure due to the unbalanced voltages. A general motor model has been developed. Motor heating and derating are obtained from the model through a generalized digital computer program.     

Characterization of voltage sags in industrial distribution systems

This paper describes the various characteristics of voltage sags experienced by customers within industrial distribution systems. Special emphasis is paid to the influence of the induction motor load on the characterization of voltage sags. During a fault, an induction motor operates as a generator for a short period of time and causes an increase in sag magnitude. Its reacceleration after the fault clearance results in an extended post-fault voltage sag. The influence of the induction motor on the imbalanced sags caused by single line-to-ground faults (SLGFs) and line-to-line faults (LLFs) has been analyzed in detail. For an imbalanced fault, the induction motor current contains only positive- and negative-sequence components. Induction motors create a low impedance path for the negative-sequence voltage due to an imbalanced fault. This causes a small sustained nonzero voltage with large phase-angle jump in the faulted phase and a voltage drop in the nonfaulted phases with a small phase-angle jump. The symmetrical components of the induction motor during the imbalanced sags have been studied. The results show that induction motor behavior is determined by positive- and negative-sequence voltages during the imbalanced sag     

计及负载率和临界滑差的改进感应电动机加权聚合

在暂态稳定仿真中所用的等值感应电动机的模型参数对仿真精度的影响很大。为了获取这些参数,大多采用基于容量加权的模型参数聚合法。感应电动机群中的各电动机初始滑差不同对聚合结果有重要影响,而该初始滑差由负载率和临界滑差联合决定。文中据此提出了利用负载率和临界滑差来修正的感应电动机加权聚合法。算例表明,依据该方法所聚合出的等值感应电动机能更好地模拟原感应电动机群对暂态稳定性的影响。研究还表明,由于实际系统中参与聚合的感应电动机个数很多,且其负载率分布具有随机性,用统计综合法得到的等值感应电动机模型参数基本可用。     

Assessment of nonintrusive motor efficiency estimators

Three instruments have been evaluated for their ability to estimate the load conditions and efficiency of operating motors based solely on measurements of terminal voltage and current. Laboratory tests, compared to a high-accuracy dynamometer system were used to assess the instruments' accuracy. Field tests were conducted in a paper mill to determine the ease of use and lack of intrusion. In both laboratory and field tests the instruments were applied to 50-hp, 100-hp, and 200-hp 460-V induction motors. Data of the relative accuracies of the three instruments, as functions of motor load and supply voltage, are presented together with observations of ease and lack of intrusion in use.     

Features of a Power System Incorporating Large AC Motors/Captive Transformers

Both synchronous and induction motors driving fans, pumps, and compressors (variable torque types of loads) may prove to be more economical and reliable when utilizing a lower voltage motor plus a captive transformer rather than a 13.8-kV motor. The economic factors, matching motor capabilities to load requirements, power system considerations, relaying and grounding systems possibilities, and operating experience with installed drives in Atlantic Richfield's Philadelphia Refinery are covered.