Abridgment of voltage standardization of A-C. systems from the viewpoint of the electrical manufacturer

Abstract

In outlining the history of voltage standardization, it is observed that there has been separate standardization of voltages of various types of apparatus, rather than standardization of a complete operating system. The results of a questionnaire answered by 22 operating companies are analyzed and the conclusion is drawn that the use of the old standard transformer voltages involved, in many cases, over-exciting the transformers or generators in order to maintain satisfactory voltage at the consumers' terminals. The reason is that the existing transformer voltage standards do not compensate for the line drop in transmission lines and feeders. The voltage standards of the International Electrotechnical Commission are set forth, indicating a partial agreement as to maximum system voltages with the proposed standards, although arrived at by a different method. The I. E. C. standardization, however, is not so complete as the proposed system. In describing the proposed standards, certain basic principles are laid down as the conditions which must be fulfilled. The proposed system of voltage standards starts with already standardized utilization voltages at the low end of the scale and suggests tranformer voltage ratings and ratios which will allow proper voltage to be supplied to the consumers without over-exciting any of the transformers or generators in the system, and ties in the transformer and apparatus voltages with system voltages, based on the A. I. E. E. definition of rated circuit voltages. The proposed standards thus cover the whole field of voltages of a-c. apparatus of all kinds and harmonize them with system voltages in such a way that all reasonable operating requirements may be met. The salient features of the proposed standardization are as follows: The system voltage is the same as the highest rated voltage of transformers supplying the system; it thus corresponds to the A. I.E. E. rated circuit voltage and fixes test voltage on all apparatus used on the system. Step-down transformer secondary voltages from 115 volts up to 69,000 volts will be multiples of 11.5, excepting transformers supplying 2400-volt systems, which will be rated 2400 volts. Thus, typical step-down transformers will deliver 460 volts 6,900 volts 23,000 volts 69,000 volts For higher voltages, step-down transformers will have secondary voltages in multiples of 11, thus: 88,000 volts 110,000 volts 132,000 volts 154,000 volts complying with well established practise. In order to enable the step-down transformers to deliver these voltages, their primaries will be rated in multiples of 11, thus: 6,600, 22,000, etc., up to 66,000 and above that multiples of 10 1/2, thus: 105,000 126,000 210,000, etc. Step-up transformers, excepting the 2400-volt class, will have their high-tension windings rated in multiples of 11 1/2 up to 69,000, and multiples of 11 above that, whereas their low-tension winding will be rated 5 per cent lower than the system voltages or generator voltages. Thus, step-up and step-down transformers will not be interchangeable, but each will have the proper ratio for its purpose. To make them interchangeable' would require 25 per cent range, which would involve too great an expense if applied to all transformers. Transformers of 25 per cent range may be required in many cases where power flows in either direction but such transformers should be of special design and this extra cost should not involve the whole line of transformers. The tabulation of voltages gives also the present manufacturers, standards for apparatus voltages, such as oil circuit breakers, disconnecting switches, etc. These standards for 88,000 volts and above correspond to the system, voltages of multiples of 11. Below 88,000 volts they are somewhat higher than the recognized system voltages given in the tabulation in order to meet existing conditions in these lower voltages. The tabulation also gives motor voltages in multiples of 11, while the generator voltages, in order to allow for line drop, are multiples of 12 up to 2400 volts, and multiples of 11 1/2 from 6900 volts up. The last section of the paper gives a discussion of the economic advantages of voltage standardization, indicates the magnitude of the investments involved, and gives a general idea of the savings which may be made by standardization.