Radial basis probabilistic neural network for differential protection of power transformer

Protection of medium- and large-power transformers has always remained an area of interest of relaying engineers. Conventionally, the protection is done making use of magnitude of various frequency components in differential current. A novel technique to distinguish between magnetising inrush and internal fault condition of a power transformer based on the difference in the current wave shape is developed. The proposed differential algorithm makes use of radial basis probabilistic neural network (RBPNN) instead of the conventional harmonic restraint- based differential relaying technique. A comparison of performance between RBPNN and heteroscedastic-type probabilistic neural network (PNN) is made. The optimal smoothing factor of heteroscedastic-type PNN is obtained by particle swarm optimisation technique. The results demonstrate the capability of RBPNN in terms of accuracy with respect to classification of differential current of the power transformer. For the verification of the developed algorithm, relaying signals for various operating conditions of the transformer, including internal faults and external faults, were obtained through PSCAD/EMTDC. The proposed algorithm has been implemented in MATLAB.     

High power universal piezoelectric transformer

This study describes a multilayer piezoelectric voltage and power transformer that has one direction poling, operates in a wide-frequency range and delivers both step-up and step-down voltages by inverting the electrical connections. In this design, the input and output electrodes are on the same side of the disk and are isolated from each other by a fixed isolation gap. The electrode pattern is a ring/dot structure such that it uses radial mode for both input and output part that are built-in on the same ceramic disk. A prototype transformer was fabricated of size 15 x 2.78 mm2 having mass of 3.8 gm. In the step-down configuration at the constant output power of 6 W, the transformer characteristics across a 100 omega load were found to be efficiency = 92%, gain = 0.21 input voltage = 110 V(rms), and temperature rise = 20 degrees C from the room temperature. In the step-up configuration at the constant output power of 5 W, the transformer characteristics across a 5 komega load were found to be efficiency = 97%, gain = 9.5, input voltage = 16 V(rms), and temperature rise = 8 degrees C from the room temperature. A detailed equivalent circuit analysis of the transformer was done, and the results were found to be in excellent agreement with the experimental results.     

High power universal piezoelectric transformer.

This study describes a multilayer piezoelectric voltage and power transformer that has one direction poling, operates in a wide-frequency range and delivers both step-up and step-down voltages by inverting the electrical connections. In this design, the input and output electrodes are on the same side of the disk and are isolated from each other by a fixed isolation gap. The electrode pattern is a ring/dot structure such that it uses radial mode for both input and output part that are built-in on the same ceramic disk. A prototype transformer was fabricated of size 15 x 2.78 mm2 having mass of 3.8 gm. In the step-down configuration at the constant output power of 6 W, the transformer characteristics across a 100 ohms load were found to be efficiency = 92%, gain = 0.21 input voltage = 110 Vrms, and temperature rise = 20 degrees C from the room temperature. In the step-up configuration at the constant output power of 5 W, the transformer characteristics across a 5 kohms load were found to be efficiency = 97%, gain = 9.5, input voltage = 16 Vrms, and temperature rise = 8 degrees C from the room temperature. A detailed equivalent circuit analysis of the transformer was done, and the results were found to be in excellent agreement with the experimental results.     

Modelling of electronic power transformer and its application to power system

Mathematical models of an electronic power transformer (EPT) are established for steady-state and transient stability investigations. To simplify oscillation damping analysis, a simplified dynamic model is developed by neglecting transients of a high-frequency transformer and filter inductors. Comparison between the simplified dynamic model and the detailed model is carried out by simulations using numerical computation and Matlab/Simulink to verify the accuracy of the developed model. For the purpose of the EPT controller design, a small-signal model of the EPT in a d-q synchronous rotating reference frame is established by linearisation about the operating point. According to the developed models, linearised Phillips-Heffron models of the power systems installed with the EPT are presented. Afterwards, as a case study, based upon the Phillips-Heffron models, the damping function of EPT is discussed and its performance is verified by numerical simulation.     

小型电源变压器的型式试验要点

一、关于认证样品的一般要求小型电源变压器作为自愿性产品认证目录中的一类,在申请认证时,首先要对所申请单元的典型样品进行型式试验.在整个试验期间,每一个变压器样品应当符合所有相关试验.     

Excitation current and power loss characteristics for mitered joint power transformer cores

The 45° mitered-overlap joint is commonly used in the construction of large power transformer cores. There are several variables which affect the performance of such cores with respect to core loss and exciting current. The major variables investigated in this paper are 1) overlap distance, 2) stagger layer thickness, and 3) overlap method, namely, simple staggering versus step-lap. Also investigated to a lesser degree were the effect of frequency and the effect of series air gaps at the corners. One set of results in this paper is not in agreement with a previous paper and reasons for this are speculated. The other sets of results are new, and indicate that staggering beyond two laminations per stagger is not advisable, that the difference between one and two laminations per stagger is marginal, and that the use of a step-lap joint reduces exciting current requirements but not core loss.     

Subspace-based identification of power transformer models fromfrequency response data

A recent frequency-domain, subspace-based algorithm is used in the identification of two power transformers. The results indicate that the subspace-based identification algorithms can be used without modification even when the dynamic range of frequency response data is large     

500kV电力变压器直流偏磁故障分析

针对电力变压器发生直流偏磁的现象,采用ANSYS软件研究其内部磁场和励磁电流的畸变率变化情况。为分析某电厂变压器所受到的影响,该文建立大型电力变压器的二维瞬态场数学模型。在考虑到初始条件和边界条件下,利用场路耦合有限元法研究变压器在不同直流入侵后铁心内部磁场的变化,并且计算不同直流偏磁下励磁电流的总谐波畸变率。理论分析和仿真结果表明:变压器铁心饱和程度随着直流偏磁的增大而增大,励磁电流谐波量也随之增加,畸变率也越来越大。     

Heat transfer in power transformer windings with oil-forced cooling

Power transformer outages have a considerable economic impact on the operation of an electrical network. One of the most important parameters governing a transformer?s life expectancy is the hot-spot temperature (HST) value. The classical approach has been to consider the HST as the sum of the ambient temperature, the top-oil temperature rise and the hot-spot-to-top-oil temperature gradient. Temperature distribution is solved using the heat conduction equation. Finite element method is used in the numerical solution. The transformer selected for simulation is 32 MVA transformer with non-directed oil-forced cooling and directed oil-forced cooling. Comparing the results with those obtained from finite integral transform and experimental test checks the validity and accuracy of the proposed method.     

A precision transformer used for an audio-frequency power standard

A precision current transformer for the audio-frequency band in which an auxiliary current transformer equalizes the potential along the primary and secondary windings of the main current transformer is discussed. The arrangement results in a dramatic reduction of capacitive leakage current. In this way, a ratio error of less than 10 p.p.m. and phase angle error of less than 10 μrad is achieved at frequencies up to 10000 Hz     

Influence of current transformer saturation on line current differential protection algorithms

The performance of line current differential protection algorithms during current transformer (CT) saturation is analysed. The study covers popularly used algorithms and presents their degree of immunity to CT saturation. The degree of error in phasor measurement due to CT saturation is estimated through simulation studies and this information is used to decide the CT dimensioning for the relay     

一种反激式LED驱动电源变压器设计方法

变压器作为LED驱动电源的核心部分,其设计方法较多,但是大部分设计方法都有公式多、计算困难等缺陷.研究提出一种反激式LED驱动电源变压器设计方法,从电感的电学特性引入变压器相关参数的计算,并结合工程实践给出了相关参数计算步骤.最终通过相关实例对该方法进行了验证,并且与电源设计软件PI Expert相比较.结果表明,该方法有效可行,且对参数的选择更具有灵活性.     

Conversion of current transformer errors at unity power factor burden, to 0.8 power factor-equivalent

The paper contains a full derivation of formulas that enable the conversion of current transformer errors measured at unity power factor burden to equivalent errors at 0.8 power factor lagging burden, required by IEC 60044-1, as well as errors at any other burden. The assumptions and limits of the formulas are discussed.     

Flux distribution and power loss in the mitered overlap joint in power transformer cores

The design of the joints in a power transformer core has a marked influence on the efficiency of the core as a whole. Two experimental cores have been built such that the lengths of their 45° mitered overlaps could be varied from 0 to 2.0 cm. The larger core, with 1.0 m limbs, was used for investigating flux and loss variations and the second, smaller core was used to study how the results might be affected by changes in dimensions. For the larger core, the power loss was measured in two ways for a range of overlaps: first, using the localized power loss technique, and then by measuring the total power loss with a precision wattmeter. In both cases, a minimum power loss was found when the core was built with an overlap of 0.5 cm. The overlap length was varied in the smaller core and again a minimum power loss was found, but with a 1.0 cm overlap. A change in loss of over 20% was found for the range of overlap lengths used, and the optimum overlap length was independent of flux density over the range from 1.0 T to 1.8 T. The special flux distribution was determined from an array of search coils. The variations in flux distribution enables a qualitative explanation of the occurrence of the minimum in power loss.     

Application of probabilistic neural network for differential relaying of power transformer

Investigations towards the applicability of probabilistic neural networks (PNNs) as core classifiers to discriminate between magnetising inrush and internal fault of power transformer are made. An algorithm has been developed around the theme of conventional differential protection of transformer. It makes use of the ratio of the voltage-to-frequency and the amplitude of differential current for the detection of the operating condition of the transformer. The PNN has a significant advantage in terms of a much faster learning capability because it is constructed with a single pass of exemplar pattern set and without any iteration for weight adaptation. For the evaluation of the developed algorithm, transformer modelling and simulation of fault are carried out in power system computer-aided designing PSCAD/EMTDC. The operating condition detection algorithm is implemented in MATLAB     

A novel method of reducing iron losses in power transformer cores

A method is described in which the iron loss of single phase transformer cores constructed from grain-oriented silicon-iron can be reduced. It is shown in two different sized cores that by assembling the limbs with laminations cut at small angles to the rolling direction (RD) of the steel, and stacked in a certain way, the loss can be reduced. Reductions of up to 6 percent were achieved in cores assembled from high permeability steel.     

Busbar differential protection in conjunction with a current transformer compensating algorithm

The design, evaluation and implementation of a busbar differential protection relay that operates in conjunction with a current transformer (CT) compensating algorithm are described. Prior to saturation, the secondary current of a CT is not compensated. The compensating algorithm detects the start of first saturation on the basis of the third-difference function of the current and estimates the core flux at the first saturation start by inserting the negative value of the third- difference function of the current into the magnetisation curve of a CT. Thereafter, it calculates the core flux and then the corresponding magnetising current in conjunction with the magnetisation curve. The calculated magnetising current is added to the measured secondary current to obtain the correct secondary current. The algorithm can estimate the correct current irrespective of the level of the remanent flux. In the proposed busbar protection scheme, a current differential relay with the single-slope operating characteristic is used on the basis of the compensated current of the saturated CT. Test results indicate that the relay shows satisfactory performance for the various external and internal faults with CT saturation, particularly in the case of a progressive fault from a feeder fault to a busbar fault. The algorithm is implemented in a prototype relay based on a digital signal processor. The relay achieves greater stability on external faults, enhanced sensitivity on internal faults and fast operation on internal faults with CT saturation.     

Algorithm to identify the excitation inductance of power transformer with wye-delta connection

An algorithm to identify the excitation inductances of three-phase power transformer with wye-delta connection is proposed. Existing methods of determining the excitation inductances of three-phase transformers require that all winding currents to be known, making them impractical on some wye-delta transformers where the delta winding currents are not measured. Based on the transformer equivalent circuit, the proposed algorithm eliminates the influence of the delta circulating current, allowing the excitation inductances to be calculated using the line currents of the delta connection side directly. The algorithm?s accuracy has been verified by electromagnetic transients program including direct current (EMTDC) simulations, which show that the proposed algorithm is able to differentiate accurately and sensitively between transformer inrush and fault conditions. This lends itself to applications in the area of transformer protection.