The aim of this paper is to introduce hot-spot and top-oil temperature thermal models for more accurate temperature calculations during transient states based on data received in a normal heat run test (i.e., the top oil in the tank of the transformer and the average winding-to-average oil gradient). Oil viscosity changes and loss variation with temperature are taken into account. The new thermal models will be validated using experimental (fiber-optic test) results obtained at varying load current on a 250-MVA-ONAF-cooled unit, a 400-MVA-ONAF-cooled unit and a 605-MVA-OFAF-cooled unit. The results are also compared with the IEEE-Loading guide (1995) Annex G method.

Dynamic thermal modelling of power transformers

With the fast-paced changing technologies in the power industry, new references addressing new technologies are coming to the market. Based on this fact, there is an urgent need to keep track of international experiences and activities taking place in the field of modern transformer design. The complexity of transformer design demands reliable and rigorous solution methods. A survey of current research reveals the continued interest in application of advanced techniques for transformer design optimization. This paper conducts a literature survey and reveals general backgrounds of research and developments in the field of transformer design and optimization for the past 35 years, based on more than 420 published articles, 50 transformer books, and 65 standards.

Transformer Design and Optimization: A Literature Survey

The IEEE Transformer Loading Guide equations use the top oil temperature rise over ambient to determine the winding hottest spot temperature during an overload. Recent investigations by the author and others have shown that during overloads there is a time lag between the top oil temperature rise and the oil temperature rise in the winding cooling ducts. This phenomena results in winding hottest spot temperatures greater than predicted by the current (1981) IEEE Loading Guide equations. Accurate predictions of the winding hottest spot temperature requires the use of the temperature of the oil entering and exiting the winding cooling ducts. Low flammability liquids such as silicone or high temperature hydrocarbons are not covered in the present Loading Guide. Resistance change with temperature and liquid viscosity are not considered in current Loading Guide equations. Loading Guide equations also assume a constant ambient air temperature during a load cycle. Improved loading equations based on analysis and testing are presented in this paper. The improved equations consider type of liquid, cooling mode, winding duct oil temperature rise, resistance and viscosity changes, and ambient temperature and load changes during a load cycle. A PC BASIC computer program to perform the calculations was developed and will be included in the next edition of the IEEE Loading Guide for Mineral Oil Immersed Transformers. >

Predicting liquid filled transformer loading capability

This paper presents new temperature calculation methods based on heat-transfer theory, the application of the lumped capacitance method, the thermal-electrical analogy, and a new definition of nonlinear thermal resistances at different locations within a power transformer. The methods presented in this paper take into account oil viscosity changes and loss variation with temperature. The changes in transformer time constants due to changes in the oil viscosity are also accounted for in the thermal models. The models are validated using experimental results, which have been obtained from a series of thermal tests performed on a range of power transformers. The results are also compared with the IEEE-Loading guide (1995) Annex G method

Dynamic thermal modeling of power transformers: further Development-part II

In the authors' previous work (1997), an original thermal model of transformers with ONAN cooling was developed. The model takes into account the influence of nonlinear thermal characteristics in transient thermal processes; instead of exponential functions and time constants, the numerical solution of differential equations is used. The model delivers one characteristic temperature in copper (solid insulation) and one characteristic temperature in oil. In this paper, the analysis stresses the definition of the hot-spot temperature of the solid insulation by using easily measurable quantities. Then, the parameters of the model can be precisely determined from inexpensive measurements in a short-circuit heating experiment and the model can deliver the hot-spot temperature. The experimental base of this research are the measurements on a 630-kVA, 3 /spl times/ 10 kV/3 /spl times/ 6 kV ONAN transformer equipped with 112 temperature sensors (102 inside the central positioned 10-kV winding).

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A new method for the calculation of the hot-spot temperature in power transformers with ONAN cooling

Manufacturers may be required to design transformers to overload specifications. Oil viscosity changes and loss variation with temperature are not accounted for in the industry loading guide. A comprehensive test program was performed using a winding assembly with embedded thermocouples. Steady-state and transient tests were performed for natural and nondirected forced oil calculation. The temperature distribution in the winding was not linear with height as usually assumed. A large number of temperature sensors is required to determine the hot spot location. The location of the hot spot was found to be different and less predictable for natural oil calculation than for nondirected forced oil circulation, especially during overload conditions. Improved loaded guide equations were developed to account for oil viscosity, resistance change, cooling duct oil rise and location of the hot spot. >

An investigation of the thermal performance of an oil filled transformer winding

A comprehensive thermal test program was conducted on a 2000 kVA cast-resin dry type transformer with 300 embedded thermocouples to determine the hottest spot temperature allowance and to obtain data to develop mathematical models. Nineteen thermal test runs were performed at different loads, tap positions, and core excitations under natural and forced air operating conditions. The test data indicated that the hottest spot temperature allowance used in the IEC standard was too low. In the IEEE standard, the hottest spot temperature allowance appears conservative up to about 115 degrees C average winding temperature rise. The locations of the hottest spot for the primary and secondary winding were determined. Conclusions about thermal testing and loading are presented. >

L.W. Pierce

Papers

An investigation of the thermal performance of an oil filled transformer winding

An investigation of the temperature distribution in cast-resin transformer windings