[Thesis]. Manchester, UK: The University of Manchester; 2017.
Reliable thermal modelling approaches are crucial to transformer thermal design and
operation. The highest temperature in the winding, usually referred to as the hot-spot
temperature, is of the greatest interest because the insulation paper at the hot-spot
undergoes the severest thermal ageing, and determines the life expectancy of the transformer
insulation. Therefore, the primary objective of transformer thermal design is to control
the hot-spot temperature rise over the ambient temperature within certain limit.
For liquid-immersed power transformers, the hot-spot temperature rise over the ambient
temperature is controlled by the winding geometry, power loss distribution, liquid
flow rate and liquid properties. In order to obtain universally applicable thermal
modelling results, dimensional analysis is adopted in this PhD thesis to guide computational
fluid dynamics (CFD) simulations for disc-type transformer windings in steady state
and their experimental verification. The modelling work is split into two parts on
oil forced and directed (OD) cooling modes and oil natural (ON) cooling modes. COMSOL
software is used for the CFD simulation work
For OD cooling modes, volumetric oil flow proportion in each horizontal cooling duct
(Pfi) and pressure drop coefficient over the winding (Cpd) are found mainly controlled
by the Reynolds number at the winding pass inlet (Re) and the ratio of horizontal
duct height to vertical duct width. The correlations for Pfi and Cpd with the dimensionless
controlling parameters are derived from CFD parametric sweeps and verified by experimental
tests. The effects of different liquid types on the flow distribution and pressure
drop are investigated using the correlations derived. Reverse flows at the bottom
part of winding passes are shown by both CFD simulations and experimental measurements.
The hot-spot factor, H, is interpreted as a dimensionless temperature at the hot-spot
and the effects of operational conditions e.g. ambient temperature and loading level
on H are analysed.
For ON cooling modes, the flow is driven by buoyancy forces and hot-streak dynamics
play a vital role in determining fluid flow and temperature distributions. The dimensionless
liquid flow and temperature distributions and H are all found to be controlled by
Re, Pr and Gr/Re2. An optimal design and operational regime in terms of obtaining
the minimum H, is identified from CFD parametric sweeps, where the effects of buoyancy
forces are balanced by the effects of inertial forces. Reverse flows are found at
the top part of winding passes, opposite to the OD results. The total liquid flow
rates of different liquids for the same winding geometry with the same power loss
distribution in an ON cooling mode are determined and with these determined total
liquid flow rates, the effects of different liquids on fluid flow and temperature
distributions are investigated by CFD simulations.
The CFD modelling work on disc-type transformer windings in steady state present in
this PhD thesis is based on the dimensional analyses on the fluid flow and heat transfer
in the windings. Therefore, the results obtained are universally applicable and of
the simplest form as well. In addition, the dimensional analyses have provided insight
into how the flow and temperature distribution patterns are controlled by the dimensionless
controlling parameters, regardless of the transformer operational conditions and the
coolant liquid types used.