Large Power Transformers Research Articles

Temperature distribution in ONAN power transformer windings with finite element method

AbstractSince large power transformers belong to the most valuable assets in electrical power networks, it is suitable to pay higher attention to these operating resources. Thermal impact leads not only to long‐term oil/paper‐insulation degradation; it is also a limiting factor for the transformer operation. Therefore, the knowledge of the temperature, especially the hot‐spot temperature (HST), is of high interest. The calculation of current thermal stress helps to avoid unexpected outages. This paper presents the temperature distribution in windings of the oil power transformers. In this paper, energy (thermal) equation is solved until temperature is obtained. Oil in the transformer is assumed nearly incompressible and oil parameters such as thermal conductivity, special heat, viscosity, and density vary with temperature. For numerical solution of above equation, finite element method is used. The selected models for simulation are 300 KVA and 22.5 MVA ONAN transformers. Copyright © 2008 John Wiley & Sons, Ltd.

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.

Design of a New Oil Spraying Device for Hot Spot Cooling in Large Scale Electric Power Transformers

Hot spot temperature (HST) is the most important parameter in the operation of power transformers. The HST has to be held under a prescribed limit. HST has a considerable effect on the insulation aging. Therefore detecting, monitoring and removing the HST could be a very important and necessary action for utilities. A new design of oil spraying and its effect, along with a thermal management in a transformer cooling system has been studied in this paper. The effect of oil fluid flow on the HST problem has been considered in this paper; and the calculations and simulation have been performed by Ants algorithm. The simulation results have been validated based on a 230/63/20 kV, 250MVA transformer at the Sari substation in Iran, and the results indicate that the new design could mitigate the limitations of transformer loading due to the HST problem. The Ants algorithm have been proposed and applied for accomplishing this task and to give an improved level of accuracy.

Innovation on Energy Power Technology (11)A History on Improvement of Disassembled Transportation Technology and Development of Large Power Transformer

The restrictions of transportation are one of the major parameters for transformer design and affect the available capacity, various characteristics and even reliability. With the progress of increasing power demand, transformer size and weight have been increasing. However, in Japan the restrictions for size and weight in rail and road are strict, and several kinds of technology for transformer construction and transportation method have been developed to meet these requirements.One technology is the disassembled transportation method for transformer. Fortunately we have experienced three type disassembled transportation methods for Kamishiiba power station for approximately 50 years. This paper introduces a history on improvement of disassembled transportation technology and development of large power transformer.

Investigation of load noise generation of large power transformer by means of coupled 3D FEM analysis

PurposeThis paper aims to present a 3D numerical analysis of the load noise generation associated with large, oil immersed three‐phase power transformers.Design/methodology/approachAfter studying the mechanical behavior of the winding structures of transformers, the results of coupled magneto‐mechanical simulations are presented.FindingsAn appropriate modeling strategy of the vibratory winding structures of transformers is necessary to reduce complexity and computational resources.Originality/valueThe presented model setup describes a fully transient, 3D coupled magneto‐mechanical simulation of the vibratory winding structure of large power transformers.

An investigation of parameters affecting the temperature rise in windings with zigzag cooling flow path

Disk type windings are widely used in large power transformers. Block washers are used within some designs of this winding type to help to direct the oil flow. These washers increase the heat transfer coefficient, decrease hot spot temperature and consequently make the transformer smaller. A proper model is helpful to calculate the temperature distribution inside a natural convective cooled disk type winding including block washers. Therefore, in this paper a mathematical thermal model is presented and its validity and accuracy is verified with experimental measured data. Meanwhile, the effects of some parameters such as heat loss, number of washers, height of radiators and channel geometry are investigated using the model.

Internal faults in power transformers

The paper deals with internal faults in large power transformers. The explanation of such faults is followed by the description of an analytical method proposed for the calculation of the power flow during an interturn fault, including the current supplied by the power grid, the current in a group of short-circuited turns and the current in the spot of short-circuiting. The currents are calculated for cylindrical windings as a function of the fault area and its location with respect to the winding height for different structural configurations. An appropriate modification of Rogowski's coefficient is introduced. Computational results based on theoretical considerations are shown to be in good agreement with measurements carried out on a realistic model.

06/01855 An interpretation of neural networks as inference engines with application to transformer failure diagnosis: Garcez Castro, A. R. and Miranda, V. International Journal of Electrical Power & Energy Systems, 2005, 27, (9–10), 620–626

Disk type windings are widely used in large power transformers. Block washers are used within some designs of this winding type to help to direct the oil flow. These washers increase the heat transfer coefficient, decrease hot spot temperature and consequently make the transformer smaller.A proper model is helpful to calculate the temperature distribution inside a natural convective cooled disk type winding including block washers. Therefore, in this paper a mathematical thermal model is presented and its validity and accuracy is verified with experimental measured data. Meanwhile, the effects of some parameters such as heat loss, number of washers, height of radiators and channel geometry are investigated using the model.

Reducing the magnetizing inrush current by means of controlled energization and de-energization of large power transformers

Energizing unloaded transformers may result in magnetizing inrush current of high amplitude. These currents have many unfavorable effects including differential protection maloperation, deterioration of the insulation material and mechanical support structure of windings, voltage sag and other power quality issues on the HV, as well as on lower voltage terminal(s). If no point-on-wave controller is applied the energization of the transformer may occur at any time on the sinusoidal wave producing high inrush current peaks if the transformer core is driven into saturation. The point-on-wave control methodology presented in this paper has been elaborated to eliminate the inrush currents of 132/15 kV, Y/delta connected generator step-up transformers, which are switched very often in quick start, gas-turbine power stations. A new controller has been elaborated to operate the mechanically staggered, common drive type circuit breakers which are characterized by three-pole operated spring drive and fixed time delay between the operating poles. The paper proposes a new method to minimize the residual flux in the core by means of controlled de-energization. The new concept has been proved by several field tests and the new controllers have been put into service in several power stations in Hungary.

Great geomagnetic storms and extreme impulsive geomagnetic field disturbance events – An analysis of observational evidence including the great storm of May 1921

Impulsive geomagnetic field disturbances are an important aspect of the geomagnetic storm environment for electric power grids and other ground-based infrastructures that can be impacted by geomagnetically induced currents (GIC). Significant power grid impacts in present day networks have been observed at relatively low levels of intensity, for example the Quebec grid blackout during the March 1989 storm occurred at a peak intensity of ∼480 nT/min and permanent damage to large power transformers have occurred at even lower intensity levels. An analysis of both contemporary and historic storm data and records indicates d Bh/d t impulsive disturbances larger than 2000 nT/min have been observed at latitudes of concern for power grid infrastructures. In extreme scenarios available data suggests that disturbance levels as high as ∼5000 nT/min may have occurred during the great geomagnetic storm of May 1921. A comparative evaluation of this storm using morphology-based techniques indicates that large electrojet intensifications exceeding those observed during the March 1989 superstorm were present, further that the Dst range for the May 1921 storm could be as much as −900 nT or ∼50% larger than the March 1989 Dst ranking which is the largest on record.

An overview of the impulsive geomagnetic field disturbances and power grid impacts associated with the violent Sun‐Earth connection events of 29–31 October 2003 and a comparative evaluation with other contemporary storms

An overview is provided of the geomagnetic storms and impacts on electric power grids associated with the violent Sun‐Earth events of October 2003. During the period from 29 to 31 October 2003, two large geomagnetic storms were observed, as measured by periods of high Kp, Ap, and Dst indices. In fact, these storms had Ap rankings of 6th and 16th all time. This ranking would suggest that the October 2003 storms would be significant with regard to geomagnetically induced currents (GIC) in power grids. However, the resulting geomagnetic storms were much lower in delta B and dB/dt intensity than other historically large geomagnetic storms. A variety of geomagnetic storm processes drove observed GIC. For example, ground observations indicated the presence of large dB/dt pulsations and GIC at North American midlatitude locations on 29 October 2003 that may be due to unusually intense Kelvin‐Helmholtz shearing. Sustained disturbance conditions at low‐latitude and equatorial latitude locations that are likely linked to ring current intensifications may be the source of sustained GIC at these locations and the cause of large power transformer failures. Comparative evaluations will be provided for the 29–31 October 2003 storms and other important and contemporary storms, such as those observed on 13–14 March 1989, 13–14 July 1982, and 15–16 July 2001. Rather than an index‐based evaluation method, the comparative evaluations presented in this paper are based on comparisons of storm morphology. This approach provides a more meaningful comparison of geomagnetic field disturbance dynamics that are important to characterize large GIC threats to power grid infrastructures.

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.

A fuzzy ARTMAP fault classifier for impulse testing of power transformers

The work presents an artificial intelligence (AI) based impulse test technique for oil filled power transformers. Determination of exact nature and location of faults, during impulse testing of large power transformer is of practical importance to the transformer manufacturers as well as designers. The presently available impulse test techniques more or less depend on expertise of the test personnel, and in many cases lead to ambiguity and controversy. The new AI approach presented in the paper overcomes the limitations of conventional test methods. This new technique relies on high discrimination power and excellent generalization ability of fuzzy neural networks in complex pattern classification problem. The proposed method employs a fuzzy ARTMAP pattern recognition technique to recognize the frequency responses of the winding admittance of high voltage transformers under healthy and different faulty conditions of winding insulation, and learns to establish the correlations between the nature and physical location of occurrence of an internal insulation fault in a transformer winding and its associated frequency response. The technique was tested on the winding model of typical high voltage transformer and yielded high diagnostic accuracy by successful detection and discrimination of faults of different nature and different site of occurrence in the high voltage winding.

A Simple Method for Calculating Core Temperature Rise in Power Transformers

This paper presents a new method for calculating core temperature rise in large power transformers. A general formulation of the problem is made using the electrical analogy. This formulation extends to the L- and T-joints. An expression for the thermal resistance of the core material is found using analytical methods. A comparison of the results of the new method with measured values on two power transformers is presented.

Numerical Simulation of Transient Force and Eddy Current Loss in a 720-MVA Power Transformer

The transient eddy current fields are short-circuit forces upon the coils of large power transformers are analyzed. The proposed approach is based on the A-V-A formulation in which the eddy current field and the electric circuit equations are solved simultaneously. The effectiveness of a magnetic bypass plate near the coils ends for reducing the eddy current loss and the local overheating is discussed. A simulation to study the transient short-circuit behavior of a 720-MVA power transformer using the proposed coupled approach is reported.

Fuzzy logic-based relaying for large power transformer protection

Power transformer protective relay should block the tripping during magnetizing inrush and rapidly operate the tripping during internal faults. The frequency environment of power system has been made more complicated and the quantity of 2nd frequency component in inrush state has been decreased because of the improvement of core steel. Then, traditional approaches will likely be maloperated in the case of magnetizing inrush with low second harmonic component and internal faults with high second harmonic component. This paper proposes a new relaying algorithm to enhance the fault detection sensitivities of conventional techniques by using a fuzzy logic approach. The proposed fuzzy-based relaying algorithm consists of flux-differential current derivative curve, harmonic restraint, and percentage differential characteristic curve. The proposed relaying was tested with relaying signals obtained from Salford EMTP simulation package and showed a fast and accurate trip operation.

Thermal overload tests on a 400-MVA power transformer with a special 2.5-p.u. Short time loading capability

Comprehensive load /thermal testing was performed on a 400/400/125-MVA transformer representing a new design generation for large power transformers with a short time emergency loadability of 2.5 p.u. This particular overloadability factor required special design features to control the very high leakage flux resulting from such a loading condition and hence ensure that the transformer satisfies existing standards limits on hot-spot. This paper presents these design features along with a description of a full-size transformer which was built for the sake of developing these flux control measures. Hot-spot temperature measurements in the windings and in the outer core packets and structural parts under a number of overloading conditions are presented and discussed. The results confirm the soundness of the design to handle the 2.5-p.u. overloadability while satisfying the hot-spot requirement according to the existing loading guides for 1.5-p.u. overloading. The measurements also show that the hot-spots will rise much faster than what an exponential function based on the time constant of top oil would predict.

Temperature Responses to Step Changes in the Load Current of Power Transformers

Comprehensive load/thermal testing was performed on a 400/400/125 MVA, ONAF-cooled transformer. It was shown that the local hot-spots in the windings, core, and structural parts rise much faster at load increase than what an exponential function based on the time constant of top oil would predict. This paper analyzes these fast rises more in detail for hot-spots in windings, core, yoke clamps, and tie plates. An alternative mathematical model of the winding hot-spot response is presented. Different cooling modes, like ONAF and OFAF, are dealt with. Also different modes of oil circulation through the windings, in horizontal (zig-zag) or axial cooling ducts, are compared. The results are verified by fiberoptic installations and tests on several large power transformers in the range of 250 - 650 MVA. Hot-spot responses predicted by the IEEE Loading Guide, Annex G, are also compared to measured values.

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.

Development of a fibre-optic sensor for partial discharges in oil-filled power transformers

Abstract Determination of partial discharge locations in large oil-filled power transformers is unreliable and difficult using external acoustic sensors, and internal sensors would need to be non-metallic. Thus the use of a fibre-optic coil as an acoustic sensor would allow the detection and location of partial discharges with potentially a much greater sensitivity. This paper describes the design of such a sensor and its signal processing requirements, its construction and its initial testing. The sensor was found to be able to detect 1-Pa acoustic pressure waves under optimum conditions; and to detect partial discharges in small artificial voids in an oil-bath.