A transformer model for winding fault studies

This paper deals with a method of modeling internal faults in a power transformer. The method leads to a model which is entirely compatible with EMTP software. It enables simulation of faults between any turn and the earth or between any two turns of the transformer windings. Implementation of the proposed method assumes knowledge of how to evaluate the leakage factors between the various coils of the transformer. A very simple method is proposed to evaluate these leakage factors. At last, an experimental validation of the model allows the estimation of its accuracy     

High frequency model of transformer winding

A high frequency model of transformer winding is used to analyze the voltage oscillations due to various excitations such as the very fast transient overvoltage which occurs at the time of disconnecting switch operations. Usually, a circuit of interlinked inductances and capacitances is used for this purpose, in which circuit parameters have to be properly determined. Previously, those constants have been estimated taking the coil section pair as a unit. In the method proposed here, the section pair can be further subdivided. The time‐domain calculation is conducted combining the frequency analysis and FFT technique. The voltage oscillations of the winding subjected to the lightning impulse are calculated. The correspondence with the experimental results is satisfactory. The response to a chopped impulse shows this method's applicability to high frequency analysis. Since the constants are calculated directly from the design parameters of transformer winding, this technique is particularly useful in developing and designing transformers. © 2003 Wiley Periodicals, Inc. Electr Eng Jpn, 146(3): 8–16, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.10280     

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     

Locating faults in a transformer winding: An experimental study

Based on terminal measurements on a single, continuous-disc winding of a transformer, it is demonstrated how faults introduced at different positions along the winding could be localized with reasonable accuracy. Fault in the present context represents a discrete change, e.g. short-circuiting a few turns within a disc (i.e. predominantly an inductive change) and/or addition of some tens of pico-Farad capacitance between a disc and ground (i.e. predominantly a capacitive change). Open-circuit and short-circuit natural frequencies are determined by sweep frequency measurements, in addition to measuring effective resistance, shunt capacitance and inductance, at the terminals. The proposed method aims at utilizing the measured data to iteratively synthesize a lumped-parameter ladder network, corresponding to each set of measurement. Comparison of such synthesized circuits with a reference (or fault-free) circuit reveals the location, quantum, and nature of fault. Results presented demonstrate the potential of this method.     

Development and calibration of a network-based streamingelectrification model for core-form transformer structures

Streaming electrification occurs in large power transformers when oil circulated for cooling purposes acts to separate charges from the Debye layer on the cellulose insulation surfaces. This phenomenon is believed to have caused several catastrophic transformer failures and remains to be satisfactorily understood in realistic geometries. A network-based streaming electrification model is presented which utilizes single winding ducts as the basic element to investigate a core-form structure. The network is first analyzed using hydraulic duct parameters to calculate the oil flow distributions within the structure. This data is then provided to the electrification solver which predicts the charge densities at each duct outlet. The charge density results are ultimately used to determine the streaming potentials at each node based on an a suitable matrix of leakage resistances. Development of the network model is presented along with empirically-based calibration data for a realistic section of core-form transformer geometry     

Development and calibration of a streaming electrification modelfor a cellulose duct

Streaming electrification occurs in large power transformers where insulating oil circulated for cooling purposes acts to separate charge from the interfacial double layer present on a multitude of cellulose insulation surfaces. In this paper, the electrification phenomenon is addressed from a microscopic viewpoint. An empirically calibrated analytical model is provided for calculating the amount of charge separated in a core-form transformer duct. The model incorporates a boundary layer approach in approximating the wall charge distribution which provides a critical length scale for the analysis. This length scale is used to specify the appropriate model for charge development under turbulent flow conditions. The duct model considers both laminar and turbulent flows subject to ac electric fields and varying temperature. This concept forms the elemental building block of a network-based methodology for examining electrification in a complete transformer geometry     

dc field effects on streaming electrification in insulating oils

The results of an experimental investigation into the influence of impurities on electrification in insulating liquids in a tube model under an external dc field effect is described. Both conduction and streaming currents were measured as a function of temperature and flow rate using a paper-pressboard model with concentric cylindrical electrode geometry. Positive or negative dc voltage was applied to the inner electrode. The streaming current was measured from a shielded test chamber and the conduction current was measured from the ground electrode. Energization by positive dc voltage always enhanced the streaming electrification; however, when the applied voltage was negative, the polarity of streaming current was field strength dependent. At low fields (<0.52 kV/mm), the streaming current with negative dc field I_sn^c increased with increased field and with field strength >0.52 kV/mm, polarity reversal was observed. The field at which current crossed the zero value has been defined as the zero cross over point (ZCOP). Flow rate did not have any influence on the ZCOP; however, the additive concentration, moisture content and the temperature affected this ZCOP. In the presence of the corrosion inhibitor carbazole, the streaming current with negative dc field, I_sn attained zero at low fields. The conduction currents corresponding to fields where streaming currents were zero were also relatively low. It is therefore possible that by applying a moderate dc field of appropriate polarity the streaming current can be reduced to very low values by adjusting the carbazole concentration in the test liquid     

Improvement of dielectric strength on transformer winding using new aramid paper

Large power transformers using perfluorocarbon liquid have advantages of high dielectric strength, excellent cooling ability, and nonflammability. To make the transformer more compact, it is very important to improve the insulation material. Recently, the authors succeeded in developing a new type of aramid insulation paper which has no fluff on its surface to avoid field concentration. In this study, an outline of the developed paper and the experimental results of the insulation strength on turn-to-turn and intercoil models are described. It was found that the new aramid paper increases the insulation strength 1.2 (turn-to-case) to 1.7 (intercoil case) times that of the existing insulating paper.     

变压器绕组参数在线计算方法

在变压器等值回路方程的基础上,提出一种新的变压器绕组匝数比和漏电感参数的计算模型,该模型以回路方程差值最小为目标。采用现代内点算法求解各参数并利用MATLAB编程实现。该方法仅需利用变压器正常运行时的电压、电流信息,不用获取变压器绕组结构参数以及有载调压分接开关位置信息,易于实现。利用ATP软件建立变压器仿真系统,模拟变压器各种正常运行状态,并利用动模实验数据对所提方法进行验证。仿真实验结果表明,所提方法不受变压器运行状态及其三相参数不平衡的影响,具有较高的计算精度。     

一种高效的电力变压器绕组轴向固有振动频率计算模型

在电力变压器设计过程中,绕组的固有振动频率是一个非常重要的性能参数。本文基于弹性动力学理论,根据绕组周向垫块弹性支撑等效原则,提出了一种高效的计算电力变压器绕组轴向固有振动频率的二维有限元模型,其固有振动频率的计算具有与三维模型一致的结果,但其计算效率得到大大的提高。同时研究表明,增加垫块的宽度和数量,以及增大预紧力都可以提高绕组轴向的固有振动频率,是避免绕组共振的一种有效应对方法。最后通过对固有振动频率计算的实例校核,验证了本文所提二维模型方法的有效性。     

电缆绕组变压器绕组温度场的二维数值计算

对一个电缆绕组变压器的绕组，应用传热学和流体力学的原理建立其温度场和附近流场的有限元方程，通过求解方程，得到温度场和流场 分布，与实测温度值进行比较，误差均在3K范围内，从而证明了此方法的正确性，为研究该类变压器的温度特性提供了方便。     

变压器绕组局部放电故障的定位

频率响应分析法只能检测变压器绕组变形故障 ,而不能实现对局部放电故障的定位。在频率响应分析法的基础上 ,对变压器绕组的模拟电路进行了分析研究 ,利用绕组在一定频率范围内呈现纯容性的特性 ,给出了实现变压器绕组局部放电故障定位的新思路。经仿真计算 ,验证了这种局部放电故障定位的有效性。     

变压器绕组局部放电故障的定位

频率响应分析法只能检测变压器绕组变形故障,而不能实现对局部放电故障的定位.在频率响应分析法的基础上,对变压器绕组的模拟电路进行了分析研究,利用绕组在一定频率范围内呈现纯容性的特性,给出了实现变压器绕组局部放电故障定位的新思路.经仿真计算,验证了这种局部放电故障定位的有效性.     

Influence of solid insulating phase on streaming electrification oftransformer oil

Due to increased demand of electrical power, the trend of the capacity and voltage rating of power transformers increases with reliable design; especially in large oil-cooled power transformers. Consequently, the use of polymer blended and polymer coated insulations in such transformers are the most recent advances in modern technology. The consequences of the type of solid insulating material, in contact with the streaming oil, on the ECT (electrostatic charging tendency) of oil are the principal aim of this paper. ECT of transformer oil is experimentally investigated using different types of oils and solid insulting materials. Two laboratory analog cycles are used; namely, a closed and an open cycle. In the closed cycle, the investigated factors are the type and polarity of the applied voltage (±10 kV and 10 kV_rms at 50 Hz), solid insulating material (polymeric and non-polymeric insulation), and oil type, temperature (20 to 70°C) and velocity (1.5 to 3.25 m/s). Good agreement is achieved between the measured data under ac and dc voltages of both the conduction and streaming currents, and the derived formulas. The currents increase exponentially with oil temperature, velocity and applied voltage. All solid dielectrics used show quantitative as well as qualitative changes in oil electrification especially at low temperatures. In the open cycle, both the rotating cylinder speed (430 rpm) and the oil volume are kept constant, and the oil temperature is raised to 120°C for different types of oils and solid dielectrics. ECT depends on the oil type and is activated with temperature. Cellulose-based dielectrics give the highest ECT and coating of these dielectrics with epoxy resin enhances the ECT. Different suppression characteristic is obtained using Theophylline in different fresh oils     

变压器绕组变形的综合分析方法

气相色谱法、频响法及短路阻抗法是测试变压器绕组是否变形的重要手段,但单一的试验方法和试验数据对判断绕组变形存在一定的局限性。结合现场实际,提出应用色谱法进行检测后,再结合频响法和短路阻抗法进行分析的综合方法。应用结果表明:变压器绕组变形的综合分析方法能够有效、准确判断绕组的状态。     

Flow electrification of transformer oil effects of mixed fields

Static electrification in transformer oil is investigated experimentally using a laboratory synthetic closed cycle, where the oil is pumped in a coaxial electrode arrangement. The electrode system is electrically energized radially with dc and/or ac, and the electrostatic charging tendency (ECT) of the oil is quantified by measuring the streaming current. The results indicate that for the same voltage ratio (K=Vˆ/(Vˆ+V)), increasing the ac or the dc voltage component leads to higher ECT of oil, although the ECT under only ac field is much lower than that under dc field. The unenergized streaming current is inherently affected by the electrode material and configuration; where electrodes having a lower work function give higher positive ECT of oil at high temperature. The energized streaming current increases with oil temperature, oil velocity and electric field; where negative dc voltage application to the outer electrode gives both higher conduction and streaming currents. While the conduction current decreases with oil velocity, increasing the frequency of mixed ac voltage has no significant effect on streaming current. Moreover, the effect of combined radial electric (ac or dc) and axial magnetic (ac or dc) fields on ECT of oil is investigated, and the results reveal that the ECT is enhanced by the magnetic field while the radial conduction current decreases. Derived formulas for the streaming current for unenergized and ac energized cases are also presented together with expressions of the volume and surface charge densities for the coaxial electrode system