Measurement of eddy-current loss coefficient PEC-R,derating of single-phase transformers, and comparison with K-factorapproach

A power amplifier is used to supply sinusoidal currents of different frequencies for measuring eddy-current losses of a 25 kVA single-phase transformer under short-circuit condition. Measured data show that eddy-current loss is a linear function of frequency with power of 2 and the eddy-current loss coefficient P_EC-Rlinear is computed. New measurement techniques are applied to determine the derating of single-phase transformers with full-wave diode and thyristor rectifier loads. The derating of transformers has been defined such that for the (apparent, real) power transfer of a transformer the total losses are identical to the rated losses at rated temperature. A relation between apparent power, derating and K-factor is given taking into account iron-core and stray-power losses. Measured derating values are compared with computed results based on the eddy-current losses, iron-core losses, stray-power losses, and K-factors. The eddy-current loss coefficient P_{EC-Rnonlinear} is computed from harmonics caused by diode/thyristor bridge loads     

Transformer magnetizing current and iron-core losses in harmonicpower flow

The problem of modifying the harmonic power flow analysis to permit the inclusion of nonlinear anisotropic transformers and computation of their iron-core (and copper) losses are discussed. The nonlinear model used for transformers is capable of simulating saturation of iron-cores, anisotropy of laminations, and the iron-core and copper losses associated with anisotropic transformers, but simple enough to be included in the harmonic power flow algorithm without deteriorating its convergence properties. The dependency of iron-core losses on the maximum value of the total (fundamental and harmonic) flux density and the fact that iron-core losses are a function of the waveform of the induced voltage-that is its harmonic phase shifts with respect to the fundamental phase angle-are included. The induced voltage is transformed from frequency domain to time domain: the instantaneous induced voltage and the computed λ-i characteristics are employed to compute the instantaneous magnetizing and core-loss currents. Thereafter a transformation is made from time domain back to frequency domain to compute the fundamental and harmonic components of the above-mentioned currents. Therefore, transformer harmonic couplings are properly modeled and included in the analysis. The modified harmonic load flow formulation is applied to a balanced three-phase feeder consisting of a grounded-wye, grounded-wye nonlinear anisotropic transformer and linear and nonlinear loads     

Computation of no-load iron losses including spatial harmonics and tooth pulsation components for cage motor

The contribution of spatial harmonics and tooth pulsation in iron losses of three phase squirrel cage induction motor has been calculated using lumped circuit approach. An equivalent circuit of the three phase induction motor considering stator magnetomotive harmonics has been used for the calculation of the same. An empirical formula has been developed and validated for determination of tooth pulsation losses. The usage of lumped parameters circuit considering magnetomotive force harmonics for predicting the iron losses components has not appeared in the literature. Here iron losses due to fundamental, spatial harmonics components and tooth pulsation for various motors with different designs and polarities have been deter-mined and compared with experimental results. It is found that the iron losses due to the stator magnetomotive harmonics is in same range as that due to fundamental component, the tooth pulsation losses are in the range of 25 to 35% for 2 pole, and 20 to 30% of total iron losses for 4 pole induction motors. The percentage errors between computed and average experimental results for total iron losses are within acceptable limits (±5%) without taking too much of computation time compared to the numerical methods.     

Effects of direct currents in power transformers Part I. A general theoretical approach

The DC component of current circulating in power transformers produces a certain degree of core saturation. The waveshape of the excitation current displays a pulse of high magnitude at each cycle. The width of the pulse depends on the period of the cycle under saturation. This excitation current induces a drop of potential across the primary winding impedance thus reducing the magnetizing voltage and injecting harmonics into the network. Moreover, it is demonstrated that this condition of operation will increase copper and stray losses but will tend to decrease iron losses.In order to study the detrimental effect of direct current, the equivalent circuit of a power transformer has been reviewed to confirm its applicability under partial saturation of the core. For single-phase transformers, the mathematical model developed allows calculation of rms and crest values of magnetizing current, core flux and magnetizing voltage, along with copper and core losses.     

非晶合金铁心变压器的特点分析

非晶合金应用于电力变压器,将会成为一种能够大幅度降低变压器空载损耗的节能材料,介绍了非晶合金材料的优缺点,并对非晶合金铁心变压器与硅钢片铁心变压器的性能进行了分析比较,为研制非晶合金铁心变压器提供了参考。     

Performance characteristics of a triac-controlled single-phase induction motor having space harmonics in its magnetic field

The analysis of the steady-state performance characteristics of a triac-controlled single-phase induction motor having space harmonics in its magnetic field is presented. The analysis is based on the independent consideration of the fundamental and harmonic content of the motor applied chopped sinusoidal voltage. Each harmonic component is applied individually to the equivalent circuit of the motor having space harmonics in its field. The computed characteristics are checked experimentally.     

Computer-aided testing of electrical apparatus supplying nonlinearloads

A computer-aided testing program for electrical apparatus (CATEA) is designed to test the performance of single and polyphase electrical devices. It has been applied to measure the losses of single- and three-phase AC and DC electrical machines, single- and three-phase transformers with linear and nonlinear loads. On-line measurements of iron-core and copper losses of singleand three-phase transformers with rectifier load are presented and compared with the results of the open- and short-circuit tests     

变压器谐波损耗计算及经济性评估

随着先进的基于微电子控制技术的自动化生产流水线大量应用于现代工业,企业的生产效率不断提高;与此同时企业对电力系统的电能质量问题也越发关注.为了掌握谐波对企业造成的损失情况,携带电能质量分析仪——Fluke435对该企业的每个测量点进行了24小时连续不间断的测量;在实测数据的基础上,结合变压器谐波损耗的经典理论对谐波造成的直接损失和间接损失进行了经济性评估.对工业用户评估及计算电力系统谐波造成的经济损失有一定的指导意义和参考价值.     

电力谐波对电能计量的影响

介绍了电力谐波产生的原因及测量方法,从频率响应特性入手,分析了谐波电能产生的原理以及谐波对感应式电能表和电子式电能表电能计量的影响,提出了分别计量基波电能和谐波电能并记录谐波电能方向的计量方式。这种计量方式计费合理,使非线性用户在经济上承担了对谐波这一公害的责任,但有较大的实施难度。     

多重化技术的几种拓扑结构和控制策略

近年来，多电平大功率变频器在电力传动领域越来越受到重视，因为它具有良好的输出波形，谐波少、开关损耗小。本文将介绍几种多重化拓扑结构和控制策略，其中某些拓扑结构通过优化配置H桥中间直流电源比率，输出端就可以得到最大数量的输出电平，从而提高波形质量；另外一些拓扑结构致力于电压等级和容量的提高，减少谐波。最后给出了几种拓扑结构的仿真波形。     

Optimum Shunt Capacitor for Power Factor Correction at Busses with Lightly Distorted Voltage

The goal of this paper is to determine the effect of time variation of system impedances and voltage harmonics on the value of optimum capacitor for power factor correction at busses with nonsinusoidal voltage. Two types of 24 hours time-variation of voltage harmonices and Thevenin impedance are assumed. The equivalent load impedance is also considered time variable and assumed to contain a large proportion of induction motors. The daily energy losses are computed and graphed in function of the shunt capacitance used for power factor correction. The results of this study indicate that in order to avoid resonances and to find the optimum capacitor the time variation of harmonics and system impedances must be known as precise as possible.     

An experimental study of the operating characteristics of an asynchronous motor with pulse supply

The fundamentals of design procedure of performance data of controlled asynchronous motors (AMs) supplied with pulse voltage are stated in this article. The proposed model is based on a L-shaped equivalent circuit for each of harmonics, on which the curve of applied to a phase voltage is displayed. It is proposed to find the increase of magnetic iron loss caused by hysteresis in proportion to the peak value of the pulse voltage. The influence of PWM voltage on magnetic hysteresis loss is determined by using a coefficient depending on the effective values of fundamental and higher harmonics of the EMF and voltages, as well as the coefficient taking into account the increase of electrical resistance of iron due to the skin effect. In a similar way, it is proposed to take into account magnetic losses in the rotor that are occasioned only by the influence of higher harmonics. It is proposed to determine the increase of resistance and decrease of the inductance of short-circuited rotor of the influence of currents of higher harmonics by the well-known Interelectro methodology. The resulting values of consumption current, power factor, effective power, and consumption power of an asynchronous motor with pulse voltage are calculated taking into account the total effect of currents and voltages of fundamental and higher harmonics. Based on the methodology that we have laid out, a program was written and modeling was carried out of the operational characteristics of an asynchronous motor with a power of 180 W supplied by a PWM inverter with a fundamental frequency of 50 Hz. In addition, the harmonic composition of pulse voltage was obtained by processing the oscillograms taken from phase windings of the motor supplied by the frequency converter. To experimentally check of the proposed model, an experimental plant for loading the asynchronous motor was assembled and the operational characteristics of asynchronous motor with the fundamental harmonic of 50 Hz were determined. The experimental results showed satisfactory convergence with the calculated results that makes it possible to recommend the proposed methodology to evaluate the energy indices of frequencycontrolled asynchronous motors.     

Technique for determining power capability of an AC motor operating in a nonlinear network

The influence of time harmonics that result from the operation of an ac motor in a nonlinear network, e.g., from a frequency converter based on a current inverter, on the power capability of the ac motor is studied. A calculation expression for this capability is obtained when an allowance is made for the influence of harmonics on fundamental and additional losses in stator winding, stator and rotor’s active steel, as well as the rotor’s squirrel cage. A practical example of implementation of this technique is given.     

谐波平衡有限元法用于饱和铁心的损耗分析

利用一种新的有效的谐波平衡有限元算法 ,结合Lavers等人提出的考虑谐波时迭片铁心的磁滞损耗和涡流损耗的模型 ,提出了一种较准确求解饱和铁心损耗的计算方法。并与传统的计算方法进行了比较。以铁磁型三倍频变压器的空载情况为例进行了仿真计算和物理实验。结果表明 ,考虑谐波磁通密度影响时的铁心损耗值与传统的仅考虑基波磁通密度时的值相比有显著的增加。物理实验验证了所提算法的正确性。为设计工作在高度饱和状态下的电器设备估算铁心损耗提供了有效的分析方法     

On the economic selection of medium voltage cable sizes innonsinusoidal conditions

Selection of cable size in the nonsinusoidal conditions is only based on ampacity considerations without any attention to the cost of the losses that will be suffered in the cable life. Since the cost of these losses (fundamental plus harmonics) can assume significant values, the selection of a cross section higher than required for ampacity considerations can result in a large reduction of cost. This paper proposes a method which allows the optimal economic selection of medium-voltage cables in nonsinusoidal operating conditions; it takes into account the initial investment costs and the Joule losses costs, including the additional costs due to current harmonics. It employs simplified expressions similar to those adopted by the IEC Standard in sinusoidal conditions, being the harmonic presence taken into account by a proper definition of a harmonic loss factor and by the introduction of harmonic coefficients to be predicted. Numerical applications to medium-voltage cables are developed and discussed in order to show the sensitivity of the cable optimum size to variations in the coefficients that characterize the harmonic presence     

电力线路谐波测量仪的研究

一文设计的以８０９８单片机为核心的谐波测量仪，采用基波和谐波分通道测量的方法，用软件实现对系统频率的跟踪，有效地提高了测量精度，简化了结构，降低了成本。     

A new scheme for inrush identification in transformer protection

The second harmonic component is commonly used for blocking differential relay in power transformers. The altitudes of harmonics and fundamental are computed by discrete Fourier transform (DFT). However, this method is not effective in some cases. A new scheme to discriminate fault current and inrush current is presented in this paper. The ratio of the power spectrum (PS) of second harmonic to the PS of fundamental based on autoregressive process is used for inrush identification. Test results with the sampled data from a prototype device on a dynamic power system model verify the effectiveness of the proposed scheme.     

Current Transformer Model

This paper presents a current-transformer (CT) model that is useful for low-frequency applications. To describe the iron-core magnetic behavior, a hysteresis model is proposed, which is able to generate minor asymmetric loops and remanent flux. The effects of classic eddy current losses and anomalous losses are represented by linear and nonlinear resistors, respectively. The obtained results are compared with those calculated by the Preisach's model and measured in the laboratory. This model may be applied in power system protection studies, as it is the case of numeric correction of distorted secondary currents in current transformers (CTs)     

带桥式交流器的凸极同步电机的稳态特性分析

本文给出了利用复数坐标变量系统解带桥式变流器的凸极同步电机稳态特性的计算方法,并对一台实际电机作了具体计算。计算波形和实验波形的一致性良好。文中还利用FFT对各电压及电流进行了诸波分析,并详细讨论了等效逆序电流的计算。最后,给出了计及谐波电流后的定子磁势。     

一种二极管箝位型多电平逆变器控制方法研究及DSP实现

为改善高频应用中二极管箝位型多电平逆变器的输出电压波形质量,基于阶梯波PWM法原理,利用波形合成理论,分析了逆变器实现谐波控制的工作方法。通过控制开关器件的导通时间,实现了逆变器输出电压低次谐波的消除及输出电压基波幅值的调节。设计了基于数字信号处理器DSP的逆变器数字化控制方案,并给出了实验结果。