Partial discharge location in power transformers using wideband RF detection

This paper presents a method for partial discharge (PD) source location in power transformers based on detecting electromagnetic (EM) radiation emitted from PD. The key problem of PD location by EM waves is the difficulty in getting the accurate time arrivals of the signals from the source to multiple sensors. Several measures are taken to improve the time arrival resolution. Firstly, a measuring frequency band (1 GHz-5 GHz) within RF band is proposed to accurately pick up the fast rise time of the signals. Secondly, an antenna array with diamond-shaped configuration is designed to improve the correlation coefficient of the waveforms from multiple sensors. Thirdly, two antenna arrays are proposed to eliminate the influences of the EM wave scattering caused by the iron core. Fourthly, an algorithm is proposed to increase the signal to noise ratio and to measure the time arrivals automatically through cross-correlation and averaging operation. Lastly, a hyperboloid-genetic algorithm is developed to search the PD source. Experiments on a single-phase transformer model were conducted in the laboratory. The tested results proved the validity of the proposed method and showed that the location error in 16 cm, mostly is about several cm     

Multiturn high-frequency coaxial winding power transformers

Previous papers on coaxial winding transformers have focused on designs that use a copper tube to form a single-turn outer winding, and litz wire for a multiple-turn inner winding. In high-power applications, the increased resistance of a solid outer winding due to skin effects can be a limiting factor in achieving good performance, especially at frequencies above several hundred kilohertz. A single-turn outer winding leads to large core cross section areas at lower frequencies and the turns ratio is limited to a ratio of n:1, where n is an integer. This paper demonstrates several methods for achieving multiturn outer windings to improve design flexibility for the coaxial winding power transformer. Experimental measurements on transformers that have multiturn outer windings are included to confirm analytical results and demonstrate the modified designs     

A simple method for calculating winding temperature gradient in power transformers

This paper presents a new method for calculating winding temperature gradient in large power transformers. A general formulation of the problem is made using the electrical analogy. An expression for the thermal resistance of the solid insulation is found using analytical methods. Expressions for the thermal resistance of the oil boundary layer for natural and directed oil flow are then found through correlation of test results from actual transformers. A comparison of the results of the new method and previous methods is presented. The new method is found to be a significant improvement.     

Temperature distribution in foil winding for ventilated dry-type power transformers

Predicting temperatures of transformers is important in order to prevent the deterioration of electrical insulation since the life of the transformers corresponds with that of the insulation. Reports of thermal behavior of ventilated dry-type transformers are rare in the literature. In particular, foil winding has received little attention despite their wide usage in practice. In this study, a thermal model for foil winding was proposed and temperature distributions were determined by the finite element method (FEM). In order to cope with the non-uniformity of the heat fluxes in the foil winding due to induced currents, different convection coefficients and varying air temperature along the vertical height of the foil winding were suggested and applied to the thermal model. The thermal model was solved by coupling it with the electromagnetic model to calculate the non-uniform power losses. The model was applied to a ventilated dry-type power transformer rated at 2000 kVA. Experimental temperatures were measured with thermocouples, and used to verify the finite element results. They showed reasonable agreement and will provide a useful tool for transformer engineers.     

Helical winding’s magnetic field in power transformers

Helical winding is typically used for low voltage and high current winding of large generator transformers. Due to its distinct design (spiral form, small number of turns and high current), some additional eddy-current losses may be expected. Differences in magnetic field produced by helical and cylinder windings are presented in this paper. A simple closed-form expression for estimating the loss, which is based on numerical modeling, is proposed.     

A hybrid transformer model for determination of partial discharge location in transformer winding

Partial discharges are well known as a source for insulation degradation in power transformers. A hybrid transformer model is introduced to simulate the transformer winding transient response. Transformer structural data is used to determine the hybrid model parameters. Calculations of the hybrid transient model parameters are based on the parameters of the lumped parameter equivalent transformer model and electromagnetic rules. Modern computation techniques and optimizations are employed beside this model for PD location using the multi conductor transmission line model and also to analyze its propagation aimed at achieving (i) more reliable simulation results (ii) less computational time (iii) accurate results for a wide range of frequency. The simulation results on a 66 kV, 25 MVA fully interleaved winding are presented. The measurement results on this winding are employed to validate this model     

On-line partial discharge calibration and monitoring for power transformers

Partial discharge (PD) is one of the major factors causing insulation damage of power transformers. This paper describes the development of on-line partial discharge monitoring system for transformers. The developed set can monitor up to four power transformers continuously. It uses various methods to reject interference and enhances monitoring sensitivity of PD effectively. The optical cable is applied to communicate between measurement system near transformers and the upper computer in the main control building. On-line calibration of partial discharge quantity is requisite for on-line detection or monitoring. This paper describes an on-line calibration method using pulse injection through tap of high voltage transformer bushings. It also discusses the influence of limit inductance and bushing capacitance, from tap of bushing to ground, in on-line calibration circuits. Influences of multi-grounding of the transformer tank for monitoring and calibration are also discussed. Simulation in laboratory and site test shows that the method is practically valid for on-line PD calibration of power transformers within permissible engineering error range under certain defined conditions. This system has been in operation steadily in 500 kV substations for more than 2 years.     

An experimental approach for investigating low-level interturn winding faults in power transformers

Interturn faults are one of the most prevalent and potentially destructive electrical faults in power transformers. In spite of an extensive interturn fault detection literature, the issue still constitutes an open problem. The difficulty is mainly caused by lacking of enough information on the characteristic signatures associated with interturn faults which were not clearly brought out by the previous studies. This contribution is aimed at obtaining a better understanding of the physical behavior of the power transformers in the presence of interturn faults as well as extracting several features which would be useful to specify the transformers interturn faults. The approach keeps at disposal a 100 kVA distribution transformer on which interturn faults were imposed and an experimental setup consisting of an instrumentation system and suitable transducers which enables monitoring selected characteristics of the transformer. The paper examines the faulted transformer performance under various fault and transformer operating conditions. Unique characteristic fault-signatures derived from the experiments will provide an important basis for developing more reliable and sensitive methods to detect interturn faults on the transformer windings well before such faults lead to a catastrophic failure with serious damage to the windings.     

JFY-3型变压器局部放电在线监测系统

ＪＦＹ－３型变压器局部放电在线监测系统能连续监测多台大型变压器的局部放电,并具有事故追忆、故障报警和定位等功能。它采用芝带多通道、大容量、高采样率数据采样;运用多种数字信号处理方法抑制干扰;采取电－声联合监测方式,以实现放电点定位。主控室的上位机与现场的数据采集装置之间的通信由网络舆系统实现,提高了信号传输的速率和可靠性,从而提高了系统的抗干扰能力。本文主要介绍了下位机相关的硬件部分。     

Design of the winding–bushing interconnections in large power transformers

The paper presents the improvement in design of the conductor connecting the windings and bushings in oil power transformers. This is a sensitive part of large transformers and there is a need to optimise its manufacturing time and costs. The thermal problem of the heating of this conductor with increased insulation thickness on a part of it is treated using a non-linear two-dimensional thermal model. The experiments performed made determining of some problematic parameters (heat resistance of oil-paper insulation and the paper to oil convection heat transfer coefficient) of the thermal model feasible. The results obtained are of practical interest in the design practice of interconnections, but also affect the important parameters of heat transfer by devices with oil immersed paper insulation.     

电力变压器绕组轴向短路力的研究

针对一台带独立调压绕组的电力变压器,用ANSYS软件建立了该变压器漏磁场的磁路耦合模型,得出了绕组漏磁场和电动力分布,应用有限元法对其漏磁场和轴向短路电动力进行了计算分析,计算结果符合电力变压器的基本特性.     

A comparison of trans-admittance and characteristic impedance as metrics for detection of winding displacements in power transformers

Transfer function signatures are commonly used in condition monitoring analysis to give early indications of winding movement in power transformers. This paper presents a comparison of the use of trans-admittance (frequency response analysis, FRA), presently the industry standard, and characteristic impedance (transmission line diagnostics, TLD), a new method, as signatures for the detection of winding displacements in power transformers. To facilitate the comparison, a computer based winding model based on a multi-phase transmission line system was developed and different winding deformations were analyzed. The results show that TLD has a higher degree of sensitivity and can be used to augment the ability to classify different types of distortions currently diagnosed by FRA.     

A probabilistic feature to determine type and extent of winding mechanical defects in power transformers

Frequency response analysis (FRA) is a widely-used method to detect axial displacement (AD) and radial deformation (RD) in windings of power transformers, because of its high sensitivity to small amount of mechanical defects. Interpretation of frequency response curves has been the most intricate problem of FRA method. To solve it, different numerical indices have been introduced by researchers to evaluate the frequency response of power transformers, but (1) the researchers have not discussed on genesis origin of the proposed indices and (2) most of these indices can not present a regular and linear behavior. In this paper, a probabilistic feature has been utilized to demonstrate how an efficient index originates, to diagnosis axial displacement and radial deformation in windings of power transformers. The resulted index presents a nearly linear mapping between frequency response curves and their related defect extent. To verify usefulness of this index, it has been applied to several sets of measured frequency response curves. It is illustrated that the extracted index is able to determine extent of the defects. To discriminate between axial displacement and radial deformation, local behavior of TF curves has been used.     

一种基于MIA的油浸式变压器放电性故障定位新方法

为解决油浸式电力变压器中低能放电、高能放电等放电性故障的定位问题,提出了基于油中金属分析(Metal In-Oil Analysis,MIA)的放电性故障定位方法。通过对变压器内部高故障概率构件进行表面处理,将潜在的故障信息源预置于构件表面,并应用示位金属(Metal for Position Indication,MPI)进行发生故障构件的确定。在此基础上,结合已有的局部放电、油中溶解气体分析等在线监测系统进行软、硬件的整合,可以实现较为完善的变压器放电性故障的诊断与定位。研究结果表明,该方法在提高放电性故障定位精度的同时,还可以降低对原有某种特定故障定位方法在精度方面的要求,并通过连续监测使运行维护人员对变压器的潜伏性故障信息有更为全面的掌握,为变压器状态检修的实现提供了新的技术支撑。     

A new method for comparing the transfer function of transformers in order to detect the location and amount of winding faults

Since the transfer function (TF) is an acknowledged method for detecting power transformer faults, introduction of a proper manner to compare these TFs is necessary. Although a lot of effort has been made, proposed methods are not reliable. This paper presents a new approach based on weight functions, compared with all other methods. This comparison shows that the weight function method is much more effective than the others.     

Thermal modeling of disc-type winding for ventilated dry-type transformers

A thermal model for disc-type winding of ventilated dry-type power transformers is presented. The proposed model was based on fundamental heat transfer mechanisms, i.e. heat conduction, convection and radiation. All cooling surfaces were identified, and heat transfer coefficients for each identified surface were presented. The thermal model was applied to a ventilated dry-type power transformer rated at 2000 kV A under different load conditions. Temperature distributions were calculated, and the hottest-spots were located accordingly. The temperatures for the predicted hottest-spots were measured by an infrared thermometer, and they were compared with the temperatures calculated by the model. They are in good agreement.     

高压电力变压器的在线、早期报警、远方局部放电监视器

简介 局部放电是变压器内部绝缘击穿的有效预兆,如若能够在变电站的众多噪音中被可靠检测出来,可用来进行早期报警.新型在线连续监测系统通过采用对局部放电所产生的无线电频率和超声波信号的统计采样进行局部放电检测.随着信号达到四级水平,将在变电站本地报警,并通过调制解调器通讯进行远程报警.此监测系统已经在澳大利亚的新变压器上安装和在旧变压器上进行了翻新改进.早期报警节约了维护大修费用,延长了高压电力变压器的寿命,提高了安全性,避免了环境污染.     

电力变压器局部放电检测技术的现状和发展

本文概述了电力变压器局部放电检测技术的研究现状,着重论述了近几年发展起来的数学化测量和超高频（UHF）检测,并对电力变压器局部放电检测技术的发展前景作了探讨。     

Development of a partial discharge location method for power cables

Insulation deterioration of power cables can be measured by the method of partial discharge. If a large partial discharge is detected in a cable, the cable is considered to be defective and a partial discharge is located to find the position of the defect. However, partial discharge location methods are generally disadvantageous in that they require much skill for operation and have inferior sensitivity and accuracy. To solve these problems, the authors developed a locating method using two-phase cable and an automatic partial discharge locator which automatically localizes the generating site of discharge immediately after making partial discharge measurement by simple operations. Field measurements are made on SL (separately lead-covered) cables and other cables with the locator to test its accuracy and reliability, and it was found that the device is effective and useful.