1、(E ST*?! AIEE No. 65 Nov. 1956 .r MR VNS Rwove /V/Msedf Tesf Procedure for THERMAL EVALUATION of VENTILATED DRY-TYPE POWER and DISTRIBUTION TRANSFORMERS (Published for trial use) 10 Published by AMERICAN INSTITUTE OF ELECTRICAL ENGINEERS 33 West Thirty-ninth Street, New York 18, N. Y. PREFACE For ma
2、ny years, temperature limits for electric appa ratus have been founded upon classification of insulating materials by broad general definitions based upon chemi cal composition1. Recently, it has become apparent that this method no longer meets the needs of the industry and that a new approach is ne
3、eded, based upon functional tests.w66 The latest revision of AIEE No. I1 recognizes this need by the following statement: The electrical insulation of equipment is made up of many different components, selected to withstand the widely different electric, mechanical and thermal stresses occurring in
4、different parts of the structure. To assure satisfactory service life, therefore, insulation specifica tions need to be supported by service experience or life tests, on the complete insulation system. Accelerated life tests are increasingly important as a means of evaluating the many new synthetic
5、insulating materials that are available, thus shortening the period of service experience required before they can be used with confi dence. It is desirable that life tests on materials be supplemented by tests on complete insulation systems, representative of the different types of equipment. It is
6、 contemplated that, in the course of time, Insti tute Test Codes will be developed, giving procedures for such accelerated life tests, and other methods of evaluating insulating materials and systems. Accordingly, this test procedure is intended to establish methods for evaluating the insulating sys
7、tems of venti lated dry-type power and distribution transformers. It is expected that test procedures covering other classes of transformers will be prepared from time to time as the lengthy but necessary study and trials are completed. During the preparation of this test procedure, members of the s
8、ubgroup built and tested more than fifty trans former models in accordance with the recommended methods. The results demonstrated that the methods are workable, even though there were a few details on which further work will be necessary before final decisions can be made. The test procedure, theref
9、ore, is offered for trial use in order to obtain wider distribution and to encourage discussion, criticism, and trial by others. ACKNOWLEDGEMENT Acknowledgment and thanks are extended to those who have so freely given their time and knowledge and have conducted experimental work on which this test c
10、ode is based. The major work of preparing the code and trying out its procedures was done by the Subgroup on Functional Tests for Dry-Type Transformers of the Working Group on Life of Materials, with personnel as follows: Paul Narbut, Chairman T. R. Walters M. L. Manning H. G. Zambell Personnel of t
11、he Working Group on Life of Materials: J. L. Cantwell (chairman) W. C. Farneth W. W. Satterlee C. E. Arntzen W. E. Harrison, Jr. F. J. Vogel L. H. Cardwell F. T. Hausle T. R. Walters F. M. Clark M. L. Manning A. J. Wiltzer T. W. Dakin A. S. Mickley C. P. Xenis J. F. Dexter G. A. Monito H. G. Zambell
12、 2 Proposed Test Procedure of the American Institute of Electrical Engineers THERMAL EVALUATION OF VENTILATED DRY-TYPE POWER AND DISTRffiUTION TRANSFORMERS The object of this Test Procedure is to establish a uniform method for determining operating temperature limits for ventilated dry-type power an
13、d distribution transformers, to give a reasonable life expectancy under normal operating conditions. The subjects covered by this Test Procedure are sub divided according to the following plan: Sections 1 to 7 Basic considerations Sections 10 to 12 Specifications for test samples Sections 20 to 27 T
14、he test procedure Section 30 Reporting Section 1. The test covered by this Test Procedure is intended to give a direct evaluation of the composite insulation system of a dry-type transformer, tested as an integral unit. Section 2. It is the intent of this Test Procedure to have each component of the
15、 composite structure operate and perform during the test under conditions which are as nearly as possible the same as its actual operating conditions in the transformer. Thus, it is intended that each of the com ponents is evaluated in accordance with its actual function. Section 3. The primary agin
16、g factors shall be temperature and time. Section 4. Each test shall be carried out simultaneously on a group consisting of a number of identical samples, sufficient to give the desired statistical accuracy. Section 5. The life of a sample, aged at one temperature, shall be equal to the duration of t
17、emperature exposure during one test cycle, multiplied by the number of cycles to failure, less one half of one cycle. The word “failure“, above, shall be construed to mean any form of dielectric breakdown of insulation or a dielec tric flashover, which may occur on any of the specified electrical te
18、sts. The type of failure shall be reported. Section 6. If a transformer is operated continuously at the per missible maximum hottest-spot temperature, its life ex pectancy will be less than that obtained under usual serv ice conditions. Consequently, and because of the severity of the test condition
19、s specified in this test procedure, the minimum acceptable value of extrapolated life of the test sample shall be taken as 4 years (35,000 hours). Ex perience and experimental evidence indicate that attain ment of this nominal life under test results in a service life comparable to that of dry-type
20、transformers now in use. Section 7. Tests specified in this Test Procedure are of an ac celerated nature. Hence, extrapolation of the life obtained at the test temperature is required in order to obtain the temperature limit for normal life expectancy. Furthermore, a certain amount of variation in t
21、he life of individual samples tested at the same temperature is to be expected. In order to insure valid results obtained from such data, free of bias, and suitable for comparative studies, the test data shall be reduced statistically, according to procedure given in Appendix II, and the results rep
22、orted according to Section 30. Tests may be carried out in accordance with Method 2, described below, when ap plicable ; otherwise, Method 1 shall be used. Extrapolations indicated in Section 30 shall be applied only for failures occurring in the same part of the insulat ing system. Should failures
23、be occurring in more than one part of the system, data for each mode of failure shall be treated separately from others. Likewise, the temperature class shall be determined by extrapolation separately for each mode of failure, and the lowest value obtained shall be used as representing the temperatu
24、re class for the complete transformer. a. Method 1. This method shall be used when incomplete informa tion is available regarding the life of the insulating materials involved, or the life-temperature dependence. At least three groups of samples shall be tested at three (or more) different temperatu
25、res, differing, pref erably, by not less than 10 percent of the maximum test temperature in degrees centigrade. The results shall be reduced as directed in Appendix II, and re ported as directed in Section 30. b. Method 2. This method is applicable when the life (L)-tem-3 perature (T) dependence is
26、known to be expressible by the equation: L = A (2.718)b/T in which A and b are constants, and the value of b for the type of system being tested is known from previous tests. In this case only one group of samples (n-samples) needs to be tested, at one temperature value Ta (degrees Kelvin). Extrapol
27、ation may be carried out by equation (1). b Tl=ln(WL2) + (VT2)(1) in which: Tt = temperature limit, degrees Kelvin (absolute),* to give life expectancy equal to or greater than Li, as provided in Section 6, with 95 percent confidence level. K = C + 273 w =T2 1.65 VS (L,-L,)Vn Li = Mean value of life
28、 obtained at test temperature, T2(K), expressed in the same units as Li. b = rate factor in the life-temperature dependence of the required properties for the materials in volved. In = natural logarithm to the base e s= 2.718. For many phenolic base resins, varnishes, and similar materials, value of
29、 constant b = 11,500 may be used with good approximation. When the value of 11,500 is known to be applicable, Table I, which is based on this value and on Li = 4 years (1460 days), may be used as a guide. 1 TABLE I Aging Temp. ! C 188 202 216 230 246 263 5 1 110 120 130 140 1 150 Accumulated Days of
30、 Aging (Life of the Samples) 10 20 40 80 110 120 130 140 150 160 120 130 140 150 160 170 130 140 150 160 170 180 140 150 160 170 180 190 8 1 *- Figure 2. Suggested arrangement for thermocouple. A. Thin, flexible copper strap B. Fibreglass tape packing C. Thermocouple lead D. Thermocouple junction E.
31、 High-voltage barrier cylinder (3) Section 21b provides for certain requirements in the temperature distribution within the sam ple. It was found in the tests on the 15-kv models that this requirement is not satisfied if the models are heated by electric current, in ambient air. Specifically, under
32、these conditions the required barrier temperature is not attained. It was found that enclosing the models indi vidually in light aluminum foil enclosures rem edies this situation. These aluminum enclosures serve a further purpose of thermally isolating the several models from each other, thus pre ve
33、nting the models at the ends of a row run ning cooler than those in the middle of a row. Preferred form of such an enclosure is a bell cover, made on a rectangular block form, open at the bottom but closed at the top, and ex tending down far enough to cover the flange of bottom end plate. It should
34、be of identical size and shape for all samples under test. It should cover the samples in a reasonably identi cal manner. (4) For the control of temperature of the samples during the heating cycle the use of an accurate electronic temperature controller is preferred. A reasonably accurate control ma
35、y be obtained by the use of a sensitive thermostat (e.g., a Fen wall thermostat), heated by a coil connected in series with the test samples. It is to be understood that such control will not be necessary if an accurately controlled volt age is available as the source of the heating power and if the
36、 room temperature is main tained constant. (5) It was found during the tests that an accurate control of humidity during the humidity expos-sure is essential. It was, furthermore found that to control the humidity using wet and dry bulb thermometers is quite difficult because, for the humidity speci
37、fied in the code, the temperature difference between the dry and wet bulbs is only about 1 degree C. For this reason, humidity control using a suitable salt solution in a vapor-tight enclosure is recommended. C. Section 7 gives two methods of determining the permissible operating temperature of the
38、transform ers, based on the test data. Section 7, Method 2 contains Table I, the use of which is suggested when the insulation consists of phenolic base resins. It was deemed desirable to formulate a similar table for silicone type resins. Such a table is given below, as Table II. It is based on a c
39、onservative value of b in equation (1), as indicated by the available aging data for silicones. TABLE II 1 C Aging Temp. 278 293 309 325 Accumulated Days of Aging (Life of Samples) 10 20 40 80 1 1 210 220 210 220 230 210 220 230 220 230 Hottest-Spot f Operating / Temperature Limit, C 9 APPENDIX II P
40、rocedure for Statistical Reduction of Test Data of Life vs. Temperature for Electrical Insulation (Linear Regression Analysis) Procedure described herein consists of a method of calculation of the best relationship between the life and temperature, based on the method of least squares, using acceler
41、ated test data. The results of the calculations are the best estimate of such a relationship in tabular form, as well as the confidence limits, which permit extrapola tion of the test data with a desired degree of confidence. A basic assumption of this procedure is that the rela tionship between the
42、 logarithm of life and the reciprocal of the absolute temperature is linear. A test of the ex istence of a significant level of such linearity is included in the calculations. The method of calculation is presented in a step-by-step manner, and can be conveniently carried out using the forms, which
43、are a part of this procedure. Two methods of calculations are given, the choice of one or the other depending on facilities available, and the manner in which the data were obtained. The basis for the choice is outlined in the description of the two methods. Method 1 This method of calculation is pa
44、rticularly adapted to the use of a slide rule, when a machine calculator is not available. Furthermore, it applies to a case when a num ber of samples were tested at any one temperature, and when the test temperatures were few (e.g. three or four test temperatures, with 10 samples, or more, at each
45、temperature. The following steps outline the calculation procedure. A. Using Form No. 1 calculate x“ and A: 1. Tabulate the test temperatures, 0, in C. (col umn 1). 2. Tabulate the number of samples, n, for which the life was determined at each temperature. 3. Tabulate the absolute temperature, Tt=t
46、f + 273. 4. Tabulate the reciprocal of the absolute tempera ture, x t= 1/T. 5. Tabulate the product, nx, in each line. 6. Add the values of nx, to obtain the sum 2 nx. 7. Add the values of n, to obtain 2 n. 8. Divide 2nx by 2n, to obtain the mean value of X = X. 9. Subtract x from x in each line, an
47、d tabulate the difference (x x). 10. Square the difference (x x), (multiply by itself, to obtain (x x)2. 11. Multiply (x x)a by n, in each line, to obtain n(x x)a. 12. Add n(x x)a, to obtain sum, 2n(x x)2 = A. B. Using one Form No. 2 for each group of test samples, run at the same temperature: 13. F
48、ill in values of “Line No.“, 0, (x x), and n, obtained from one line of Form No. 1, at the top of this form. 14. Tabulate the test values of life, L, for individual samples in this group. 15. Tabulate log L t= y. 16. Select arbitrarily a two-digit number y0, in vicin ity of average value of y in the
49、 table (or one just smaller than the smallest y). 17. Tabulate the difference (y yo) = u. (Note: if y0 is numerically greater than y, the difference will be negative). 18. Add positive values of u. 19. Add negative values of u. 20. Subtract the negative sum from the positive sum to obtain the sum, 2u. (Note: by choosing y0 to be smaller than the smallest y, negative val ues of u may be avoided). 21. Tabulate the square of u = u2 (all values of u2 will be positive). 22. Add u2, to obtain the sum, 2u2. 23. Calculate 2y, using schedule (a), Form N
