Numerical investigation of influences of liquid types on flow distribution and temperature distribution in disc type ON cooled transformers

Abstract

In this paper, dimensional analysis is adopted to study fluid flow distribution and temperature distribution in disc-type ON (oil natural) cooled transformer windings. Theoretical analysis on the thermal driving force and the pressure drop in the liquid circulation loop is also performed to determine the total liquid flow rate in the transformer. The thermal performance of three different liquids, a hydrocarbon mineral oil, a hydrocarbon gas-to-liquid (GTL) oil and a synthetic ester are investigated using computational fluid dynamics (CFD) simulations, based on the understanding from dimensional analysis and the flow rates determined from the theoretical analysis. It is found in the investigated conditions that the hydrocarbon mineral and GTL oil share similar thermal performance; the synthetic ester has a lower total flow rate due to its higher viscosity and therefore requires additional effort to improve the thermal performance.

Investigation of the Total Flow Rates in Oil Natural Transformer Retrofilling Scenarios

“…Under the assumption of linear oil temperature variation in the radiator and linear variation of oil density with temperature, the thermal driving force in (1) can be approximated to be [4,7]:…”

“…Thermohydraulic behaviour of different liquids in disctype windings for OD and ON cooling modes are presented in [3][4][5][6] for given total flow rates in the winding. For OD cooling modes, the total flow rate can be adjusted by choosing different pumps.…”

“…The flow and temperature distributions are found to be mainly controlled by the Reynolds number (Re) at the winding pass inlet [3,5]. For ON cooling modes, flow and temperature distributions in the winding are found mainly controlled by Re and the Richardson number (Ri) [4,6]. These controlling parameters are closely linked to the total liquid flow rate in the winding, which is dictated by the thermosiphon force and the pressure drop in the whole cooling loop.…”

“…The total liquid flow rate is found to be approximately proportional to the square root of the total power loss in the winding and the square root of the thermal head, which is the distance between the centre point of the radiator and the centre point of the winding. The flow rate ratios for different liquids are determined in [4,6] with assumptions of linear temperature distribution along the radiator panels and negligible "minor losses" arising from the change of flow directions in the cooling loop.…”

Comparisons of transformer thermal behaviours between conventional disc type and S disc type windings

“…Flow distributions in pump-driven oil directed (OD) cooling modes are quite different from those of thermosiphon-driven oil nature (ON) cooling modes [7]. OD flow distributions feature higher flow rates in the top part of the pass and lower flow rates in the bottom part of the pass [7][8][9][10], whereas ON flow distributions demonstrate the opposite trend [11][12][13][14]. The pitfall of conventional disc-type winding thermal design for OD cooling modes is pumping the oil too hard or having a winding geometry with too high ratio of radial duct height to axial duct width and therefore causing severely uneven flow distribution or even reverse flows at the bottom of the pass [8,10].…”