Experimental Investigation of Hot Spot Factor for Assessing Hot Spot Temperature in Transformers

Ageing of paper insulation within oil immersed power transformers is the determining factor for the transformer expected life. A thermal diagram as shown in the IEC 60076-2:2011 standard specified a hot spot factor method to estimate the hot spot temperature based on heat run test data. The hot spot factor is split into a Q factor, which depends on loss distribution within winding, and an S factor, which depends on oil flow distribution within winding cooling ducts. In this paper, an experiment is conducted on a disc type winding model to study the hot spot factor under fixed winding geometries subjected to wide range of inlet flow rates and uniform or non-uniform power loss injections. The hot spot factor is calculated from measured temperatures within the winding model using thermocouples. The hot spot factor was decoupled by applying first uniform losses to calculate the S factor and then nonuniform losses to calculate the Q factor. It was observed that the hot spot factor is lower for more uniform temperature profiles. It was observed that the S and Q factors are interdependent and cannot be decoupled.<br/

Flow and temperature distributions in a disc type winding-part I: Forced and directed cooling modes

© 2019 The Authors The thermal design of a power transformer determines its lifetime and loading capability, so correct modelling of liquid flow and temperature distributions in the winding is of vital importance. Existing standards and widely used simple calculation models can seriously under/overestimate winding hotspot temperatures under certain circumstances. In this paper, liquid flow and temperature distributions in a physical model representing a disc-type winding in a liquid forced and directed cooling mode are investigated experimentally using Particle Image Velocimetry (PIV) and a temperature measurement system and numerically using Computational Fluid Dynamics (CFD). Dimensional analysis is used to generalise the results into a form useful for design review. The operating conditions investigated include different liquid inlet velocities, inlet temperatures, power losses in individual disc segments, and the effect of alternative liquids. It is shown that hotspot temperature and position within the winding are a non-linear function of liquid inlet velocity, with stagnation and reverse flow demonstrated in both experiments and CFD models. Comparisons of liquid flow and temperature distributions between measurements and CFD simulation results show that 2D CFD results are representative when there are no reverse flows and 3D CFD simulations are needed when reverse flows occur. The results are presented first in dimensional forms to show the effect of each parameter, and then in non-dimensional forms to provide a generalised insight into transformer thermal design

Techniques of variational analysis

Variational arguments are classical techniques whose use can be traced back to the early development of the calculus of variations and further. Rooted in the physical principle of least action they have wide applications in diverse fields. This book provides a concise account of the essential tools of infinite-dimensional first-order variational analysis illustrated by applications in many areas of analysis, optimization and approximation, dynamical systems, mathematical economics and elsewhere

Characterization of Oil Flow within Radial Cooling Ducts of Disc Type Transformer Windings Using Particle Image Velocimetry

In this article, flow rate in radial cooling ducts in a disc type winding model is characterized and measured using Particle Image Velocimetry (PIV). Good seeding density and reliable image calibration are essential to obtained accurate PIV velocity measurements. PIV measurement captured the existence of oil reverse flow and oil recirculation at duct entrances. A 3D flow was observed in radial cooling ducts under some OD cooling conditions

Experimental Study of the Influence of Different Liquids on the Transformer Cooling Performance

Mineral oil is traditionally used in liquid immersed transformers to act as a coolant, an information carrier, and as an electrical insulator. Emerging alternative transformer liquids provide advantages, such as improved fire safety and better biodegradability, which transformer operators would like to utilize. In this paper, an experimental study is conducted to compare the thermal performance of a mineral hydrocarbon transformer oil, a gas-to-liquid hydrocarbon transformer oil, and a synthetic ester transformer liquid as coolants in a zig-zag disc type winding model. Comparisons are made under liquid directed cooling modes and under liquid natural cooling modes. It was found that under both cooling modes, the mineral based transformer oil and the gas-to-liquid based transformer oil behaved almost with comparable liquid flow and temperature distributions. Under liquid directed cooling modes, the synthetic ester gave more uniform flow distribution and delayed the occurrence of liquid reverse flow compared to the other oils. Under liquid natural cooling modes and using the zig-zag disc type winding model, synthetic ester, due to its higher viscosity which caused lower inlet flow rate to develop under the specific tested retrofilling conditions, caused less uniform oil flow distribution within the pass and higher hot spot temperature