Hot-spot temperatures in dry-type transformer windings

Abstract

Synopsis: With the increasing use of drytype transformers, attention has been focused upon the fact that the difference be tween the average temperature rise by resistance and the hottest-spot temperature rise is much greater than in liquid-immersed transformers.In this paper there are given the results of an extended series of thermal tests on a variety of dry-type transformer coils. Data are presented in regard to the effect of dif ferent heights of coil stacks, different con figurations, and, in addition, some essential precautions in making temperature meas urements are pointed out. Hot-spot tem peratures were investigated in coils cooled by natural draft and by forced-air circula tion at different velocities for both directed flow and random flow.Sufficient data are given to determine the ratio between the average temperature rise of the windings and the hottest-spot tem perature rise. Based upon this relationship, the need is shown for review and modifica tion of the conventional hot-spot allowances for dry-type transformers.

Air temperature effect on thermal models for ventilated dry-type transformers

“…The measured surface temperatures and calculated surface temperatures at constant and varying air temperatures in the fourth air duct at 78% of the height for the different loads are shown in Table 2. The largest difference between measured and calculated temperature for varying [8]. Their data were collected with transformer coils under uniform heat generation condition.…”

“…In the former a constant air temperature inside the vertical air ducts of 25 • C was assumed. In the varying air temperature analysis the temperature of the air inside the ducts was assumed 90% of the average winding temperature rise as derived from the temperature test data by Stewart and Whitman [8]. It was assumed to vary linearly from the inlet to the outlet.…”

“…In other words, the fluid (air in this case) temperature inside the vertical ducts varies along the axial length, and thus varying air temperature should be used in solving the heat transfer equations for obtaining accurate temperature distribution. However, since Stewart and Whitman [8] reported an experimentally obtained air temperature differences between inlet and outlet for different winding height and coil type in 1944, the air temperature variations in the vertical ducts have not received much attention. In particular, previous research of thermal models for dry-type transformers using the heat diffusion equation did not detail the surrounding air temperature conditions even though the operating temperatures of the dry-type transformers is largely affected by the variations of the air temperature.…”

“…The rates of variations of the air temperatures in the vertical ducts were calculated based on the experimental data presented by Stewart and Whitman [8]. By employing an electromagnetic-thermal coupled model, non-uniform heat generation caused by the eddy current in the foil winding was taken into account when computing the temperatures.…”

Thermal analysis of a dry-type distribution power transformer using FEA

“…The importance of finding the hot-spot in dry-type transformers can be tracked back in 1944, when Stewart and Whitman [6] performed a series of experimental thermal tests in different transformer ratings with natural and forced air-circulation.…”