First-Hand:Heat Losses in Isolated-Phase Bus Enclosures

Submitted by R. H. Rehder, LSM, Peterborough, Ontario, Canada

In the 1960’s large power generating stations were increasing in size and rating and 1500 MW steam turbine units were going into service in some of the nuclear powered plants. The full load current of these units could be as high as 30,000 Amperes depending on the generator operating voltage. The design of a self cooled isolated phase bus at these high current ratings requires very large physical space and expensive material costs. As a result manufacturing companies decided to force cool the existing self cooled designs that had approximately half the material and space requirement. The forced air cooled rating of a self cooled bus would be approximately twice the current in amperes depending on the amount of air moved through the bus. The design used by most companies passed air down through the centre phase enclosure and back through the two outside phase enclosures to the heat exchanger. The heat losses in the centre phase are higher than the losses in the outside phases. The Utility companies liked the reduced space requirement and the lower first cost but they objected to higher losses that would cost money over the 35 year life span of the unit. The request for quotations began to have a clause stipulating that losses would be evaluated at say $1500 per KW and the amount of the losses had to be within a guaranteed value.

The losses in the conductor can be readily calculated and verified by physical measurement. However, the path and magnitude of the induced circulating currents in the enclosures is complex to calculate and in 1960 had not been accurately measured. At that time each of the major manufacturing companies had their own private formulae for calculating and plotting enclosure current magnitudes and paths leading to an estimate of electrical losses in the enclosures. The companies reviewing the bids on new installations began to question the loss values because it was obvious that there were discrepancies in calculating these losses. A working group was established in IEEE to develop a standard for calculating losses in isolated phase bus including both the non-continuous and continuous enclosure designs.

The Canadian General Electric Company Limited in Peterborough, Ont., Canada had recently entered the isolated phase bus duct business under the guidance of the General Electric Company, Philadelphia, Pa. The design group in Peterborough included an engineering laboratory with Dr. E. C. Elgar as the leader in heat transfer technology. Dr. Elgar made direct enclosure loss measurements using a calorimetric method at 12 points around the enclosure on production assemblies. The method involved placing a thermistor at a test point and this was connected to a strip chart recorder to record temperature. The bus was connected to a high current power supply adjusted to 5000A and on command the full current was turned on for about 40 seconds and then turned off. The recorder was active for 40 seconds before the power was switched on and then turned off when the power supply was switched off. The recorded curve was a line showing the cooling from a previous test and then a transient curve upwards when the power was applied. This was an exponential type heating curve due to the losses in the enclosure. Dr. Elgar then determined the initial slope of the heating curve by marking three points on the heating curve at a distance of x, 2x and 3x from the initial point and then using a slide rule calculated the initial slope using a formula derived from a numerical analysis of a simple exponential curve. The losses are proportional to the sum of the tangents of the heating and cooling angles. For a detailed description of the procedure refer to the discussions on “Heat Losses in Isolated Phase Bus Enclosures” by A. Conangla, IEEE Transactions PAS, Number 66, June 1963, 63-5, page314.

It was important that the thermistor used in the measurements be pressed with a constant pressure against the aluminum enclosure. Also the thermistor needed to be protected against casual drafts. The technologist applying the thermistor used a 6 inch cube shaped corrugated cardboard box with one face open. He then used a child’s balloon and blew it up and stuffed it in the box with the air pressure such that the balloon just bulged out of the box by about 2 inches. The balloon and box were then pressed against the thermistor and held in place with a belt that extended around the enclosure. The balloon provided the necessary pressure and protection against drafts. Dr. Elgar had difficulty in explaining the cost of the balloons to the accountants examining his expense accounts.

Canadian General Electric made specific measurements of losses as reported in the discussions on the reference paper and the results were used to establish the formulae to be used in the IEEE standard for enclosure losses. The IEEE standard is now used in evaluating cost of losses in contracts.