Discussion on “relative costs and operating efficiencies of polyphase and single-phase generating and transmitting systems.” (Hobart) Boston, Mass., February 21, 1912. (see proceedings for February, 1912)

W. C. Smith: Mr. Hobart has given us some very interesting figures on a phase of the railway problem which has been very little touched upon. I will add a few comments from the transformer standpoint. Let us take first the step-up transformers. We have, as Mr. Hobart has shown, 48,000 kw. to transform. The development of the transformer has kept pace with that of the generator and turbine, so that there would be no difficulty in providing three-phase units to transform the 48,000 kw. in 8,000, 10,000 or 12,000-kw. sizes. These transformers would be water cooled, no doubt, and would cost, as Mr. Hobart has said, about $2.50 per kv-a. We might take 8000 kv-a. for the capacity of each unit, one transformer corresponding to each turbine, so we would have six 8000-kv-a. threephase units. Now in the single-phase station we should probably not employ over 4,000 to 5,000 kv-a. transformers. It is possible to secure bids today up to 6000 kv-a. but I think we can take 4000 for the units which would be chosen for this case. Since 75 per cent power factor has been set, we would have sixteen 4000-kv-a. units, which would probably also be water cooled. The high-tension side of the transformers would be 35,000-volt on the single-phase system and 30,000 — volt on the three-phase, as Mr. Hobart has pointed out. He has also mentioned a few places where the single-phase situation has been aided by neglecting certain points. I might state here another point. If we have sixteen 4000-kv-a. units it is going to take a much greater space, and the piping arrangements, etc., will be mtick more in amount than for the three-phase, and altogether the station would have to be enlarged to some considerable extent over that for three phase. On the other end of the line on the three-phase side, the bulk of the transformers and converters are stated in one amount of $20 per kv-a. Probably the transformers would cost in the neighborhood of $3.25 or $3.50 per kv-a., depending on the size. Wou- d we not need more substations on the d-c. line than on the singlephase line? Not being a railway man, it would appear to me that the single-phase would not need as many substations as the d-c. line. However, assuming the same number of stations, we can state that the units would be three-phase and perhaps air blast, or water cooled, depending on the local conditions; The transformers would today probably be called for with inherent reactance to take care of compounding the converters. Up to a few years ago it was necessary to supply external reactances for compounding converters, so that perhaps the cost could be cut down somewhat now, due to the fact that we have our compounding reactance as an inherent part of the transformer. Going to the single-phase substations, unquestionably these should be without attendants, which means self-cooled units. Self-cooled units within the last few years have been developed so that there would be no trouble in providing transformers of 1500 to 3000 kv-a., and I think we could take about 2000 kv-a. as the size that would be ordered under present conditions. With 64,000 kv-a. to be transformed, we would have 32 units. The cost of these was taken at $4 per kv-a. I at first considered this too high. However, on second thought, considering that the transformers are self-cooled and that it has only been by very recent developments that we have gone up to such sizes, it is probable that $4 as compared with the $2.50 is an approximate figure. Mr. Hobart pointed out the fact that three-phase units are cheaper than single phase. That may be taken wrongly, and I wish to say that a three-phase unit of the same capacity will cost some 15 per cent more than a single-phase, but three single-phase units having the same aggregate capacity as one three-phase unit will probably cost 10 to 15 per cent more than one three-phase unit. Then there is the question of voltage. On the three-phase and d-c. system, we have 30,000 for the high tension line.