Oil-filled Distribution Transformer Research Articles

Quasi-one-dimensional approach to heat transfer: General formulation and its application to transformer windings

A quasi-one-dimensional approach to conduction heat transfer with convective boundary conditions is developed for bodies of variable geometry. The novelty of the method is demonstrated on the windings in oil-filled distribution transformers with partial cooling ducts. We obtain analytical solution for the windings average temperature rise and tangential temperature distribution and find them in an excellent agreement with the FEM simulation, numerically confirming the validity of the reduction from 2D to 1D. When the quasi-one-dimensional approach is applied to calculation of temperature rise for the low and high voltage windings in industrial setting we obtain 1% and 5% discrepancy with the industrial tests respectively. Hence, the method offers reliable estimates of the temperature rise, and temperature distribution in a chosen direction, in bodies of variable geometry.

A Combined Computational Fluid Dynamics and Thermal–Hydraulic Modeling Approach for Improving the Thermal Performance of Corrugated Tank Transformers: A Comparative Study

In this study, we present a complete analysis of the thermal behavior of an oil-filled distribution transformer using two different approaches: numerically, by applying computational fluid dynamics (CFD) simulations; and analytically, by using thermal–hydraulic modelling (THM). The THM method becomes challenging when the corrugated transformer tank wall structures are complex, and therefore CFD simulations are required to provide in-depth details of the winding oil thermal–hydraulic behavior and to generate parameters for improving the THM calculation. The numerical and analytical thermal performance results were then compared with heat-run measurements of a case study transformer and the accuracy of both approaches on different thermal performance parameters was validated. This study has certain reference value for improving the thermal performance and operation efficiency of distribution transformers, and eventually aims to provide engineers with an effective tool to design the most efficient and reliable distribution transformers with corrugated tank walls for various applications.

Average temperature of oil-filled transformer windings with partial cooling ducts

Abstract An effective one-dimensional model is presented that describes the temperature profile of a winding of an oil-filled distribution transformer with an arbitrary number of partial cooling ducts. An analytical solution of the model is applied to a specific example — a low voltage winding of a 400 kVA distribution transformer with one or two partial cooling ducts. Starting from the exact solution, a simple and practical formula for the temperature rise of similar windings has been derived that is suitable for transformer designers.

Modelling of Acoustic Wave Propagation Due to Partial Discharge and Its Detection and Localization in an Oil-Filled Distribution Transformer

AbstractPartial discharge (PD) is the main cause of the insulation decay and hence periodical testing of the insulation condition of a distribution transformer is necessary. This paper presents a model of PD acoustic wave propagation, detection, and localization in an oil-filled distribution transformer using finite element method supported by COMSOL Multiphysics software. Using an acoustic module and AC/DC module of COMSOL Multiphysics software, oil filled distribution transformer, and the acoustic piezoelectric sensor are simulated to analyze and detect the PD inside the transformer. PD is numerically simulated in the transformer windings, core, and oil ducts that produce acoustic wave signal. The distribution of the acoustic pressure wave inside the model transformer is analyzed first. Next, the acoustic piezoelectric sensors are modelled at four different locations of the model transformer to detect the pressure acoustic wave signal induced due to PD in the transformer. Finally, using an artificial neural network (ANN), the localization, and identification of PD in various parts of the transformer have been analyzed. The results obtained for location and detection are quite encouraging.

Effects of voltage harmonic on losses and temperature rise in distribution transformers

The effects of harmonic pollution in the power system on distribution transformer losses and temperature rise have become an increasing concern. The pollution can be in the form of current and/or voltage harmonics present in the supply networks. Since the impact of voltage harmonics has hardly been investigated, this study examines the effects of distorted voltage excitation on oil-filled distribution transformers. Short-circuit and open-circuit tests were conducted on two distribution transformers. The results revealed significant increase in both losses and temperature. An equation is developed to quantify the power loss. This study also derives an equation to evaluate the resistance corresponding to the eddy-current loss.

Experimental Investigations and Ageing Studies on Effects of Insulating Barriers in Transformer Oil during High Frequency High Voltage Transients in Inhomogeneous Field

Insulating barriers utilized in complex high voltage insulation systems has been exploited widely by manufacturers of power apparatus. Barriers have found utility in a variety of power apparatus for protection against direct lightning strokes, as an interface at the boundaries of composite insulation materials etc. Though investigations have been carried out by researchers on the breakdown characteristics, barrier position in the gap, influence of space charge etc these characteristics have been analyzed based on aspects pertinent to classical testing procedures only. Advent of high speed switching devices in power systems provides avenues to investigate the effect of insulation performance due to High Frequency High Voltage (HFHV) also. Further, uncharacteristic failures in oil filled distribution transformers have been reported indicated fast transients as the major cause for failures which was not revealed during classical laboratory testing. This research focuses on the influence and effects of solid insulating barriers placed in transformer oil due to HFHV in inhomogeneous fields. Test cell arrangement has been fabricated for carrying out experimental investigations to adhere international standards. Exhaustive experimentation has been performed to infer the influence of the position of barrier, nature of field and breakdown voltage, erosion characteristics of the barrier at an appropriate range of frequencies. A detailed comparison between HFHV and classical power frequency voltage has been carried out. The nature of ionization at the barrier interface and the subsequent increase in the breakdown voltage as explained by Double Electric Layer (DEL) theory has been cross validated. In addition, a study has also been carried out to assess and characterize the ageing of oil based on Dissolved Gas Analysis (DGA) analysis using Weibull distribution.

Thermal–fluid transient two-dimensional characteristic-based-split finite-element model of a distribution transformer

This study presents a two-dimensional (2D) finite-element (FE) linear model for predicting the thermal and fluid dynamics of an oil-filled distribution transformer. The model is based on the solution of the Navier–Stokes and Maxwell equations which are weakly coupled. Two electromagnetic FE models are used to calculate the load and no load losses of the transformer. The thermal–fluid equations are solved using first-order 2D FE and the characteristic-based-split (CBS) scheme. The CBS is used for the first time in the thermal fluid modelling of a transformer thereby is the proposed contribution of this study. The CBS algorithm is applicable to all ranges of flow and gives the same results as specialised methods such as the Petrov–Galerkin and Taylor–Galerkin methods. A natural convection analysis for an incompressible Newtonian flow in a distribution transformer is presented. Convection, radiation and symmetry boundary conditions are taken into account in the model. The model predictions are compared against those obtained with a commercial software and good agreement was obtained. The proposed model was programmed in C language.

Hybrid Optimum Design of a Distribution Transformer Based on 2-D FE and a Manufacturer Design Methodology

Integrating finite elements (FEs) into the design procedure of electromagnetic devices has always been a challenge for researchers and design engineering community. This paper presents the application of a hybrid method that involves the application of a conventional design methodology commonly employed by transformer manufacturers and 2-D FEs. The optimization is carried out using a genetic algorithm (GA) which allows finding the global minimum and then a deterministic algorithm is used to find the final solution. The methodology is illustrated in the design of a 1500-kVA, 13.2/0.22-kV, 60-Hz, delta-wye oil-filled distribution transformer. It is demonstrated that computation time can be reduced by using the design proposal and at the same time the FE is kept in the design process.

Design of distribution transformers based on a knowledge-based system and 2D finite elements

This paper presents the development of a hybrid system for the design of distribution transformers. It is based on a knowledge-based system (KBS) and two-dimensional (2D) finite-element (FE) models, which are used to compute the electromagnetic fields within the transformer. A KBS is a software product based on established artificial intelligence techniques that has been successfully applied in many areas, whereas the FE method is a sound numerical technique for solving partial differential equations. Both methods are combined to offer transformer designers a computational tool for designing distribution transformers. First order elements are employed in two FE models, an axi-symmetric one is used to compute leakage reactance and eddy losses in transformer windings and a 2D cartesian model is used to calculate the iron loss and magnetizing current. The KBS mimics the designers experience through the representation of heuristic, procedural and graphical knowledge. The proposed design system is applied to the design of a 1500 kVA, 13.2 kV–220 V, delta-star, oil-filled distribution transformer.

Temperature rise of small oil-filled distribution transformers supplying nonsinusoidal load currents

Temperature rise tests on standard single-phase, oil-filled distribution transformers are compared for sinusoidal and nonsinusoidal current conditions. In addition to a stock 50 kVA unit, two 25 kVA transformers were specifically constructed with embedded thermocouples that permitted the true hot-spot temperatures to be measured under load. Test results show that when subjected to full-load currents having low-order harmonic distortion values up to 40 percent THD, the average winding temperature rise is 2 to 7/spl deg/C hotter than for full-load sinusoidal currents. The greatest difference recorded between the average winding temperature and the hot-spot temperature was 7/spl deg/C, significantly less than the 15/spl deg/C allowance given in the standards. This study indicates that the present standard for sizing small distribution transformers supplying nonsinusoidal currents appears to provide good estimations of load capability.

Optimum Use of Oil-Filled Distribution Transformers to Design Resistance Ground Sources for Medium-Voltage Delta-Connected Power Sources

A procedure is developed to design a resistance ground source for medium-voltage power systems with delta-connected source transformers. The optimum connection is shown to be three distribution transformers in wye/broken delta configuration with a 600-V resistor in the secondary circuit. The assumptions are that the transformer banks are used solely as a source of ground fault current, that the power system has no loads at line-to-neutral voltage, and that the ratio of source transformer to distribution transformer three-phase kVA is greater than 25.

Gas Bubbles In Distribution Transformers

Gas bubbles are known to have a deleterious effect on the dielectric strength of transformer oil. The possibility that such bubbles may develop in oil-filled distribution transformers and may cause excessive failure rates is thus of concern to both manufacturer and user. The paper presents new information relative to failure mechanisms in transformers. It describes some of the operating conditions which may lead to gas bubble formation and it gives limited data showing the effect of bubbles on insulation breakdown strength.

Static pressures developed in distribution transformers due to internal arcing under oil

In this paper equations are derived to calculate static pressures which may occur within faulted oil filled distribution transformers as a result of arcing and gas generation, under known conditions such as tank dimensions, air space, specific fuses, and specific pressure relief devices; and unknown quantities such as actual fault current, arc length, arc location, and gas temperatures. The behavior of arcs under oil, the character of pressures in transformers, and the effect on distribution transformer pressure of various fusing means are discussed in the paper. Calculated transformer static pressure values are useful not only for the design of the transformer enclosure, but also for the design of the distribution system, the selection of protective fusing, and the formulation of operating procedures. The paper is largely based on Electric Power Research Institute (E.P.R.I.) project and report RP 325-1.