基于振荡边界圆的失磁保护新判据研究

提出了一种新的失磁保护判据——振荡边界圆判据,来解决传统的失磁保护阻抗元件无法区分失磁与振荡这一问题。推导了发生振荡时机端阻抗轨迹的方程,经过分析得到系统发生低励磁振荡时,低励程度越深,机端阻抗轨迹圆越小,并且形成包含关系。根据这一结论,提出振荡边界圆判据,当机端测量阻抗轨迹进入振荡边界圆内,认为发生了失磁故障,保护直接动作,通过仿真验证了该判据的正确性。针对振荡边界圆判据,提出了一套新的失磁保护方案,并通过实例分析,验证了新失磁保护方案的有效性。     

The Minimum Input Power Conditions for Induction Motors Fed from Variable Frequency Sources

ABSTRACT

A simplified analytical method, based on the equivalent circuit of an induction motor, is presented for computing the frequency and voltage corresponding to the minimum input power. This optimum operating condition is determined for a specified torque and within a specified speed range. It is shown that the optimal per unit slip lies between the limits of zero and r₂/x₂     

The modeling of switched reluctance machines with magneticallycoupled windings

This paper presents a method of modeling switched reluctance motors with strongly coupled windings, such as switched reluctance machines (SRMs) with fully pitched windings or with commutating windings. These machines are particularly difficult to model because, unlike a conventional SRM, the flux linking any one winding of the machine is a nonlinear function of rotor position and all phase currents. The problem is overcome by breaking down the instantaneous flux linkages and currents in each phase to give the flux and magnetomotive force (MMF) in each tooth of the machine. The flux and MMF per tooth can then be linked in the model by a simple lookup table, determined from either simulation or measurement. The paper describes the simulation method in detail and then gives extensive comparisons between measurement and simulation results for an SRM with fully pitched windings, operating over a range of operating conditions. The results presented also serve to give an insight into the method of operation of these rather complex machines     

A novel analytical approach for a vector‐controlled,adjustable‐speed synchronous machine

A novel approach for operation analysis of a vector control synchronous machine has been described. To draw the vector diagram under adjustable speed operation, the concept of normalized voltage has been introduced. It is to divide the voltage value by angular frequency of the machine, then the voltage vectors, which contain angular frequency immanently, will not change their length under adjustable speed operation, and the vector diagram will keep its size unchanged for the whole speed range. Also, reactance drop locus and its accompanying yardstick circle for determining current phase angle, in corresponding to the variation of the given armature current vector, were deduced. Then, the operation of the machine for the whole adjustable speed range can be represented in single diagram. As the accompanying matters, the methods for calculating the operating characteristics of maximum torque control and 100% power factor control were shown. © 2002 Wiley Periodicals, Inc. Electr Eng Jpn, 141(2): 48–56, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.10058     

Generic study of the power capability of a cascaded doubly fed induction machine

The paper deals with the steady state power operating margins of a cascaded doubly fed induction machine. A generic analytic method, based on a per unit representation, is suggested to derive systematically the active-reactive power domain. Therefore, the study can be applied to different ranges of doubly fed induction machines. The limit curves are defined in terms of the rated quantities of the machine. The effect of magnetic circuit saturation is further investigated. It is shown that the power capability is determined by the stator current maximum values and is subject to several limitations. The analytical approach is tested and validated by experimental measurements.     

Determination of equivalent circuit constants of synchronous machines from the extended Dalton‐Cameron method

This paper presents a method of determining the equivalent circuit constants which accord with the physical construction of synchronous machines, using the dc decay testing method with the rotor in arbitrary position (proposed by the authors and called the extended Dalton–Cameron method). The conventional Dalton–Cameron method calculates the direct‐ and quadrature‐axis subtransient reactance from a standstill response test in any arbitrary rotor position using a single‐phase power supply. The extended Dalton–Cameron method determines the direct‐ and quadrature‐axis operational impedances for each slip from a standstill response test using a small‐capacity dc power supply. The direct‐ and quadrature‐axis operational impedance loci thus obtained synchronous machine constants (subtransient, transient, and synchronous reactances) are used to estimate the direct‐ and quadrature‐axis equivalent circuit constants which accord with the physical construction of synchronous machines. As an example, equivalent circuit constants are determined for a 10‐kW laminated salient‐pole‐type synchronous machine with damper winding. The validity of the equivalent circuit constants is confirmed by comparing the calculated resistance and leakage reactance of the field winding determined from the operational impedance when the terminals are short‐circuited, to those when the terminals are connected to an external resistance. © 2001 Scripta Technica, Electr Eng Jpn, 138(1): 56–67, 2002     

Motor design slip performance on cyclic loads

Cyclic loads such as eccentric pumping units cause a varying torque on the driver. Determination of the most efficient induction motor for the load is discussed. Ultrahigh-slip motors as well as NEMA (National Electric Manufacturers Association) design motors are evaluated. Comparisons of operating costs and investments are presented. The following is concluded: (1) any motor design will work on a pumping unit; (2) the motor must be carefully sized to provide minimum operating cost; (3) the lowest operating cost and the least investment come from the motors with the least slip; (4) the lower-slip motors cause an increase in peak torque on the unit; and (5) an accurate computer model with an easy-to-use program that allows what-if options is highly desirable     

An aid for teaching doubly fed synchronous machines based on thecircle diagram

This paper discusses with the extension of the circle diagram to the doubly fed synchronous machine (DFSM). Beyond the slip, this graphical approach is characterized by two leading parameters: the rotor voltage root-mean-square value and the angle between the stator voltage phasor and the rotor voltage phasor referred to as the stator windings. The generalization of the torque line in the case of the DFSM has allowed the machine power flow analysis     

低励限制整定原则与调试方法

电力系统在枯水小方式下普遍存在系统运行电压偏高的情况,调整发电机从滞相到进相运行,是解决运行电压的有效途径之一,为了保证机组政党进相运行,其励磁控制系统中的低励限制单元不仅要按要求投运,而且要正确整定,文中介绍了采用静态失步圆特性低励限制的整定原则,计算方法及调试方法。     

发电机进相运行时失磁保护动作特性分析

分析了发电机失磁保护阻抗判据的特点,提出了将发电机低励限制曲线转化为阻抗平面的方法,以及阻抗平面上阻抗圆与低励限制曲线的配合关系,分析进相试验时机端测量阻抗的轨迹与阻抗圆之间的关系,提出进相试验时,防止失磁保护误动的方法。     

INDUCTION MOTOR DRIVE STABILIZATION WITH STATOR CURRENT FEEDBACK

ABSTRACT

The use of state feedback to control Induction motor drive systems is examined. Using the machine currents and speed as the system states, a general linearized formulation of the problem is derived. This formulation has the advantages of previous forms based on complex variable analysis, but is not subject to the limitations of that method.

The proposed model Is used to study machine control with stator current feedback. The results show that stator current feedback can be used to improve the dynamic characteristics of the drive system. Comparisons of stator current control with the uncontrolled machine and with the slip controlled machine are presented over the range of operating speed, load, and inertia. In all cases, advantages of current controlled operation can be seen.     

A method for wide frequency range modeling of power transformersand rotating machines

Wide frequency range representation of power system components such as power transformers, reactors and rotating machines is mainly needed for the analysis of electromagnetic transients in power systems. In this paper, a method is presented for state space formulation of machine windings. The described procedure is completely based on external terminal impedance measurements and data related to the internal structure is not required. First, the wide frequency behavior of machine windings is reviewed and the nonlinearity effects of the iron core are discussed. A numerical method is given for calculation of the state equation parameters directly from the measured terminal impedance characteristics. The presented modeling method is applicable to any transformer, reactor or rotating machine in service where sufficient information on the internal design is not available. Besides, the canonical structure of the model offers advantages in terms of computer memory and computation effort in time domain analyses of complex power systems     

Evaluation of losses in DC machines-a statistical approach

The power ratings of the drive systems used in cyclic duty applications such as flying-die and cut-to-length shears are dictated by the thermal considerations associated with the losses produced by the machine during an operation cycle and the specified production rate. Since speed and torque are both transient functions with no steady-state operating point for such systems, a study was conducted to determine an expression that would describe these losses for a DC machine as a function of its operating speed and torque. A multiple regression technique was applied to the data collected from a set of sample machines for this purpose. Starting with the analysis of individual expressions for each of the components of losses in a DC machine, two regression models are presented. The first, based on a sample size of ten machines, provides an expression for the determination of operating point losses at any specified speed and torque per unit of rated machine losses (at rated speed and torque). The second model, based on data on 64 machines, provides an expression for the determination of rated losses in percent of machine power rating as a function of rated motor speed in r/min, armature volume in cubic units (DSQL), and percent armature droop. Mean estimates are also presented for the determination of percent droop and armature volume for a range of power ratings from 50 hp to 800 hp machines     

Minimizing Induction Motor Losses by Excitation Control in Variable Frequency Drives

For a given induction motor under a specified load condition, a combination of stator voltage and frequency exists which results in minimum power losses. This optimal operating point is calculated over the complete speed and torque range using a model of the induction machine including saturation, stray losses, and skin effect. It is shown that substantial power savings can be achieved compared to conventional programmed volts/hertz control and that main flux saturation has a dominant role in determining the optimal operating condition. A detailed analysis of the mechanisms yielding power savings is presented as well as an experimental confirmation of the theoretical results. These results are utilized to study thetinfluence of machine size, rated slip, and other factors on the optimal performance.     

双馈风机定子侧串阻抗的阻值优化

当电网受到大扰动导致电压骤降时,双馈风机需配合硬件改造措施以实现机组的低电压穿越。针对双馈风机采用定子侧串联阻抗的低电压穿越策略中存在的阻值整定问题,首先推导了双馈风机的转子暂态电流和开路电动势表达式,再以故障期间换流器的运行限制分析其有功无功耦合特性,最后结合上述分析提出了一种基于无功最优定子侧串阻抗的阻值优化方法。通过转子暂态电流约束和转子开路电动势约束确定阻抗选取范围,以暂态期间最大无功出力为目标,优选串联的阻抗值。仿真结果表明,所提方法优选出的阻抗值不仅能够有效抑制双馈风机所受到的暂态冲击,还能保证机组在故障期间提供无功功率,为电网电压提供支撑,提升机组故障运行能力。     

异步电机不对称运行时电容值的确定

借助网络图论，用回路电流法列写了便于计算机辅助分析计算的网络方程，计算了单相电源供电的三相异步电机稳态运行时的各支路电流。基于支路电流，推导了对应于正序与逆序旋转磁场的正序与逆序等效电流。以逆序电流有效值与正序电流有效值之比作为不对称运行异步电机运行状态的判据，该判据最小为优化的目标函数，运行电容值为一维变量，用黄金分割法求出某一转差率下的理想运行电容值。算例的计算结果表明，在运行范围内理想电容值变化不大，这为选择一合理的固定运行电容值提供了理论依据。     

抽水蓄能机组RTDS仿真与失磁保护改进研究

抽水蓄能机组具备调峰、调频、调相以及事故备用等功能,对电网的安全稳定运行有重要的作用。抽蓄机组容量大、结构复杂、造价昂贵,在现场和物理模型上均无法进行大量的故障及异常运行试验。基于RTDS搭建了抽水蓄能机组的仿真模型,模拟机组在多种工况、多种负荷水平下的机组区内外故障及失磁等异常运行状态。证明在抽水调相工况下发生失磁时,现有的以定子阻抗判据为主的失磁保护有缺陷,机组励磁不能及时切换,不利于系统的稳定运行。对仿真数据进行分析,提出了新的改进失磁保护的KP阻抗圆判据,实现了机组全工况无死区的失磁保护。该判据算法对相关继电保护从业人员具有一定的参考价值。     

计及电压测量特性的MMC中高频阻抗建模及稳定性分析

在建立MMC阻抗模型时,现有研究未计及电容式电压互感器(CVT)测量特性对MMC阻抗中、高频段的影响,可能会降低柔性直流输电系统稳定性分析的准确度.以渝鄂柔直工程南通道单元为研究对象,在分析杂散电容对500 kV速饱和型CVT测量特性影响的基础上,基于多谐波线性化原理提出了一种计及CVT宽频测量特性的MMC阻抗模型建立...     

Diagnosis of Broken-Bar Fault in Induction Machines Using Discrete Wavelet Transform Without Slip Estimation

The aim of this paper is to present a wavelet-based method for broken-bar detection in squirrel-cage induction machines. The frequency-domain methods, which are commonly used, need speed information or accurate slip estimation for frequency-component localization in any spectrum. Nevertheless, the fault frequency bandwidth can be well defined for any squirrel-cage induction machine due to numerous previous investigations. The proposed approach consists in the energy evaluation of a known bandwidth with time-scale analysis using the discrete wavelet transform. This new technique has been applied to the stator-current space-vector magnitude and the instantaneous magnitude of the stator-current signal for different broken-bar fault severities and load levels.     

电机定子结构及其绝缘介电常数的频率特性对定子绕组电容值的影响

确定电机定子线棒的高频参数对于研究局部放电信号在电机绕组中的传播特性及研究PWM逆变器驱动的交流电机的定子绕组电压分布有重要意义。文中对大型汽轮发电机定子线棒的电容做了比较详细的研究，内容包括：对电机定子线棒复合绝缘等值介电常数的频率特性的测试研究、采用数值计算方法计算出用铝箔纸裹住的电机定子线棒的多导体传输线模型的单位长度电容、线棒股线间薄绝缘对电容的影响、对铁心硅钢片叠片间隙对电容的影响等。其数值计算结果与之前测量得到的电容值比较吻合，这也证明了通过测量开路阻抗、短路阻抗来得到电机定子线棒分布参数的试验方法是有效的。