Abstract
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.
Highlights
Transformer utilization in terms of balancing lifetime with both long and short term loading depends critically on an accurate calculation of the hottest spot temperature under a range of load and ambient temperature conditions
This power loss corresponds to the resistive power loss with a current density of 4.85 A/mm2 flowing through a copper plate of the winding model plate size at 75 °C
The liquid flow and temperature distribution trends are found to be mainly controlled by the Reynolds number (Re) at the pass inlet
Summary
Transformer utilization in terms of balancing lifetime with both long and short term loading depends critically on an accurate calculation of the hottest spot temperature under a range of load and ambient temperature conditions. The ageing rate of the cellulose based insulation paper increases exponentially with temperature, and this ageing is generally taken to determine the useful lifetime of the transformer [1]. There is a temperature at which bubbles will form in the liquid-solid insulation system, which could trigger a transformer failure. Liquid immersed power transformers represent the most common type in power networks with the liquid serving both as a dielectric and a coolant. The cooling relies on thermal convection from either a natural liquid circulation between the winding and the cooler/radiator by thermosiphon action or on forced liquid circulation using pumps.
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