Document2

1090 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 30, NO. 4, JULY / AUGUST 1994

Thermal Considerations in Specifying Dry-Type Transformers

Linden W. Pierce, Member, IEEE

Absh-act-In 1944, the hottest spot temperature allowance for ventilated dry-type transformers was established as 30°C for 80°C average winding temperature rise and IEEE standards use a constant 30°C hottest spot temperature allowance for all insulation temperature classes and all transformer ratings. Thermal tests under different loading conditions were performed on a prototype 2500 kVA ventilated dry-type transformer and six full-size test coils with imbedded thermocouples in the windings. The test data indicated that the hottest spot temperature allowance used in IEEE and IEC standards is too low for ventilated dry-type transformers above 500 kVA. It is impossible to design ventilated dry-type transformers above 500 kVA with an average temperature rise of 150°C exceeding the permissible hottest spot temperature rise of 180°C due to large thermal gradients. The average temperature rise for ventilated dry-type transformers above 500 kVA with 220°C insulation temperature class should be 120°C. IEEE standards should require measurement of hottest spot temperature rise on prototype transformers or windings as a design test to qualify a design family and the manufacturer‘s mathematical models. This is especially important for ventilated dry-type transformers rated for nonsinusoidal load currents. The specification suggested in the paper should be used until IEEE standards are revised.

I. INTRODUCTION

A. History of Dry- Type Transformers T H E first transformers invented [ll in the 1880s were ~~

1 dry type. To meet requirements for larger ratings, forced air blast was used to limit temperatures to acceptable values [2]. The use of oil for insulating induction apparatus was patented by David Brooks of Philadelphia in 1878. The use of oil in transformers was introduced by Elihu Thomson and commercial introduction was by Westinghouse in 1886 [l], [3]. Higher-voltage transformers were impossible without the use of oil as a dielectric and coolant. Air-blast transformers continued to be produced into the early 1900s. In a paper and discussion comparing the relative fire risk of oil and air-blast transformers, Rice [41 reported that these early dry-type transformers con- tained insulating materials of cloth, paper, and wood impregnated with oil or varnish.

The open dry transformer with class B insulation was introduced in the 1930s along with askarel liquid to satisfy

Paper ICPSD 93-15, approved by the Power Systems Engineering Committee for presentation at the 1993 Industrial and Commercial Power Systems Department of Technology Conference, St. Petersburg, FL. The tests described in this paper were financed by the General Electric Company for its Dry-Type Transformer Development Project. Manuscript released for publication January 25, 1994.

The author is with the General Electric Company, 1935 Redmond Circle, Rome, CA 30165-1319.

IEEE Log Number 9402617.

the requirement for fire-resistant indoor transformers [5] . During World War I1 silicone materials were developed which were suitable for operation at higher temperatures than the class B insulations. Class H insulation consisting of inorganic materials such as glass, porcelain, mica, and asbestos bonded or impregnated by silicone resins were introduced in the 1950s and initially used in sealed nitro- gen gas-filled [6], [71, [SI and later in ventilated units. In the 1960s sealed gas units using fluorocarbon gases were introduced [5]. These gases permitted dry-type transformers to be designed with dielectric performance equivalent to askarel units due to the higher dielectric strength and improved heat transfer properties compared with nitro- gen. In the 1980s asbestos insulations were replaced with alternate materials.

In the 1990s, ventilated dry-type transformers are re- placing liquid-filled transformers in many industrial and commercial installations. Many of these installations require designs rated for nonsinusoidal load currents which increase the stray and eddy losses in the transformer windings and structural parts. The increased stray and eddy losses must be considered in the design to limit temperature rises to acceptable values. Occasionally units rated for nonsinusoidal load currents are also specified with lower than standard average temperature rises. This approach results in larger and expensive designs with higher sound levels.

B. Definitions References to “temperature rise” mean winding tem-

perature rise over ambient air temperature. The two winding temperature rises to be considered are the “average” and “hottest spot.” “Average rise” is the average winding temperature rise by resistance, but test methods may include the transformer busbars in the resistance measurement. Frequently the word “temperature” is omitted and the terminology “average winding rise” is used. “Hottest spot rise” refers to the highest temperature rise in the winding. Frequently “ h ~ t ” is used for “hottest.” The hottest spot temperature rise usually cannot be measured by tests on production transformers.

The “hottest spot allowance” is a number used in industry standards to establish the average temperature rise for rating purposes. The rated ambient temperature and hottest spot allowance are subtracted from the rated insulation temperature class to determine the average temperature rise. Common but incorrect practice in the industry has been to add the hottest spot allowance to the

0094-9994/94$04.00 0 1994 IEEE

Authorized licensed use limited to: The University of Toronto. Downloaded on July 24,2010 at 17:14:55 UTC from IEEE Xplore. Restrictions apply.

PIERCE: THERMAL CONSIDERATIONS IN SPECIFYING DRY-TYPE TRANSFORMERS 1091

tested average temperature rise to give the hottest spot temperature rise. Misconceptions are that the hottest spot allowance is an inherent characteristic of dry-type transformers and the hottest spot temperature rise cannot be calculated. The “hottest spot increment” is defined as the difference between tested hottest spot temperature rise and tested average temperature rise by resistance. The “hottest spot ratio” is defined as the ratio of winding hottest spot temperature rise divided by average winding temperature rise.

The term “K-factor” has become a commonly used term to describe nonsinusoidal load current requirements. The “K-factor” is a multiplier for increased stray and eddy losses due to nonsinusoidal load currents. It is not defined in current IEEE standards. The letter “ K ” ap- pears in many IEEE standards to describe many other parameters.

C. Winding Types Two winding types are used in dry-type transformers.

“Layer”-type windings with internal cooling ducts are used by many manufacturers of ventilated dry-type transformers for primary and secondary windings. A layer winding is a spiral-type winding in which each turn lies beside and touches the adjacent turn. A group of vertical turns constitute a winding layer. Insulation is wound between each layer and all cooling ducts are vertical. Sheet conductor layer windings have one turn per layer. “Disc” windings are also used as a primary winding with a layer- type secondary winding. A disc winding consists of many sections connected together with several turns per section. Each section is termed a “disc” and each disc is separated by a horizontal cooling duct with one vertical duct under the disc sections. Layer- and disc-type windings may be either round or rectangular.

D. Heat Transfer Iniiestigations Analytical and experimental investigations of the tem-

perature distributions in ventilated dry-type transformers are lacking in the technical literature, .especially recent literature. An excellent experimental study was conducted by Stewart and Whitman [91. In 1944, they reported laboratory test results of hottest spot and average temperature rises for large ventilated dry-type coils of various lengths. They concluded that for a given coil design the hottest spot ratio was constant and that no single hottest spot increment applied for all types and ratings of dry-type transformers. The hottest spot increment was approximately 30°C at 80°C average winding temperature rise but varied with winding length. The test results were reported again with additional data for disc windings by Whitman [lo] in 1956. Whitman [ll] also presented a paper on loading of ventilated dry-type transformers and suggested that the hottest spot temperature rise should be approximately 1.375 times the average temperature rise.

For very small transformers the hottest spot temperature increment is less than 30°C as indicated by the data of Antalis and Duncan [12l. They experimentally deter-

mined a hottest spot temperature increment of 6°C to 11°C at average temperature rises from 80°C to 138°C for single-phase units from 0.5 through 15 kVA.

Dormer [13] in 1973 reported test results on a 600 kVA ventilated dry-type unit. A hottest spot temperature rise of 153°C at an average temperature rise of 118°C for the high-voltage disc-type winding and a hottest spot temperature rise of 162°C at 113°C average temperature rise for the low-voltage winding was reported. The tested hottest spot temperature increments of 35°C and 49°C and the ratios of hottest spot temperature rise to average temperature rise appear consistent with the data of Stewart and Whitman.

The author [14] recently reported an investigation of the hottest spot temperatures in one ventilated 2000 kVA cast-resin winding transformer design. The ratio of hottest spot temperature rise to average temperature rise was also found to be approximately constant. The author [15] also recently reported test results on six ventilated dry-type layer windings. The ratio of hottest spot temperature rise to average temperature rise was also found to be constant and agreed with the data of Stewart and Whitman from 1944.

The only analytical paper treating heat transfer in large ventilated dry-type transformers is the paper by Halacsy [16]. The basic heat transfer mechanisms were considered in a solution of the average temperature rise. Core heating effects were not added and no calculation methods were developed for hottest spot temperature rise predic- tion.

11. INDUSTRY STANDARDS

A. Limits of Temperature Rise The hottest spot temperature allowance for ventilated

and cast-resin dry-type transformers has been a major unknown to IEEE Working Groups. ANSI/IEEE Stan- dard 1 [17] states that the value is arbitrary, difficult to determine, and depends on many factors, such as size and design of the equipment. Based on the 1944 experimental works of Stewart and Whitman [9] and Satterlee [18], standards used a hottest spot allowance of 30°C for 80°C average temperature rise. The conclusion of Stewart and Whitman from 1944 that the ratio of hottest spot temperature rise to average temperature rise was constant ap- pears to have been forgotten in present industry standards for ventilated dry-type transformers. The 30°C hottest spot temperature allowance established in 1944 for 80°C average temperature rise was approximately correct. At that time average kVA ratings were less than the present.

The 220°C insulation temperature class, 150°C average temperature rise was initially used in sealed units. For these units, the 30°C hottest spot temperature allowance was probably correct due to operation in the hotter inside gas. Thermal gradients from bottom to top of the coils are less. The 1959 Loading Guide [19] used rated load-limiting hottest spot temperatures of 150°C for ventilated units and 220°C for sealed units. The 220°C insulation tempera-


1092 IEEE TRANSACITONS ON INDUSTRY APPLICATlONS, VOL. 30, NO. 4, JULY / AUGUST 1994

ture class was extended to ventilated units initially in NEMA Standards [20] and later in IEEE standards [231 and the 30°C hottest spot allowance incorrectly retained. The 1989 Loading Guide [22] is based on 150°C average temperature rise as measured by resistance or a 220°C limiting temperature at rated load for ventilated dry-type units. The 1989 IEEE standard [23] used a constant 30°C hottest spot allowance for all insulation temperature classes and all size transformers. The IEC standard [241 uses a variable hottest spot allowance from 5°C to 30°C.

The insulation temperature classes and average temperature rises given in the 1989 standard are shown in Table I. This table should be compared with Table 4 of the 1979 standard [21] shown as Table 11. Table I1 (from 1979) gives a limit for both average temperature rise and hottest spot temperature rise. In the 1989 standard the table was expanded to include additional insulation temperature classes but a limit on hottest spot temperature rise was omitted. Frequently an average temperature rise below 150°C and a 220°C insulation temperature class is specified. The permissible hottest spot temperature rise for this specification is subject to interpretation.

B. Product Standards There are different product standards [25], [261 for

ventilated dry-type transformers 500 kVA and below and above 500 kVA. Responsibility for revising these standards has recently been transferred to the IEEE Trans- formers Committee. In both these standards the kVA ratings are based on 150°C average temperature rise as measured by resistance and a 220°C insulation temperature class. Stewart and Whitman's data indicated that the ratio of hottest spot temperature rise to average temperature rise decreases as the height of the coil decreases. Ventilated dry-type transformers 500 kVA and below will have shorter height windings than those above 500 kVA. The hottest spot temperature increments will be different.

C. Design Tesls Thermal tests are design tests per C57, 12.01-1989.

They are also performed as optional tests when specified by the purchaser on units above 500 kVA. The average temperature rise is measured by resistance in accordance with the IEEE Test code [27]. For three-phase transformers there is a difference between average temperature rises of the three legs with the center leg usually but not always the hottest. A question that arises is whether the permissible average temperature rise applies to individual legs or whether all three legs should be averaged together. The author has received reports that some manufacturers average the primary and secondary windings together. It has also been reported that some manufacturers measure the average temperature rise of the center phase only of three-phase units.

There is a standard for thermal evaluation of insulation systems for ventilated dry-type transformers [281 to establish the limiting hottest spot temperature rating of the transformer insulation system. Current industry standards

TABLE I

CONTINUOUSLY RATED DRY-TYPE TRANSFORMER WINDINGS (IEEE C57.12.01-1989 TABLE 4A)

LIMITS OF TEMPERATURE* AND TEMPERATURE RISE FOR

INSULATION SYSTEM AVERAGE WINDING TEMPERATURE, 'C TEMP. FUSE, ' C

~~~ ~~

130 150 185 200 220

60 80

115 130 150

*Maximum ambient of 40 'C.

TABLE I1

TRANSFORMERS (IEEE STD (37.12.01-1979 TABLE 4) LIMITS OF TEMPERATURE RISE FOR CONTINUOUSLY RATED

INSULATION AVERAGE HOTTEST SPOT SYSTEM WINDING WINDING

TEMPERATURE TEMP. RISE TEMP. RISE ('C) ('C) ('C)

150 80 1 85 115 220 150

110 145 180

require no substantiation that the hottest spot temperature is less than the rated insulation system temperature class at the nameplate rating.

D. Nonsinusoidal Load Currents IEEE standards [29] give a method of derating the kVA

of a standard transformer for nonsinusoidal load currents. Many manufacturers have chosen to develop designs rated for nonsinusoidal load currents. Some listing organizations require that the stray and eddy losses for rated sinusoidal current be determined by test and then multi- plied by the K-factor. A thermal test is then performed with additional sinusoidal current to generate load loss equivalent to that determined by the calculation, and average temperature rises are measured. Current listing requirements require no substantiation that the hottest spot temperature is less than the rated insulation system temperature class for K-factor rated designs. The effect of localized eddy losses on the hottest spot temperature is ignored [30].

111. SUMMARY OF TEST RESULTS

A. Test Windings For high-voltage windings, core loss is thought to have a

minor effect on the temperature distribution. Six full-size test windings were constructed with imbedded thermocouples and 133 thermal tests performed in the laboratory. The winding length over the turns was 28.5 inches and considered a nominal value used by manufacturers of



ventilated dry-type transformers rated above 500 kVA. The purpose of the test program was to obtain basic heat transfer data on natural convection in winding cooling ducts of different sizes. Information on this subject ap- pears in heat transfer papers but the papers are not well known and the subject is not covered in heat transfer textbooks. The test data was used to refine a mathematical model to calculate hottest spot temperatures in ventilated dry-type transformers. Core loss effects were incor- porated into the mathematical model. The test results and description of the test windings were reported in detail in a recent paper by the author [141.

The temperature distribution from the bottom to the top of the winding layer with the hottest spot is shown in Fig. 1 for three duct sizes. Most ventilated dry-type transformers above 500 kVA have 0.50-in cooling ducts within the coils. The data from the curve for the 0.50-in duct size are summarized in Table I11 to explain the reason for the incorrect 30°C hottest spot allowance in IEEE standards. A hottest spot temperature rise of 180°C at the top of the winding and an average temperature rise of 150°C assumed to be at the middle of the winding would require the temperature rise at the bottom of the winding to be 120°C. Actual test data indicate that with a hottest spot temperature rise of 180°C at the top of the winding, the temperature rise at the bottom of the winding is about 40°C and the average temperature rise is about 120°C. The actual hottest spot increment is approximately 60°C and not 30°C.

Heat transfer analysis provides further confirmation of the validity of the results. The large thermal gradient from the bottom to the top of the winding is due to the natural convection heat transfer phenomena. At the bottom of the winding the boundary layer in the duct has not formed and the heat transfer coefficient is high, giving a low temperature rise. At the top of the winding, the boundary layer is fully developed, the heat transfer coefficient is low, and the temperature rise of the air in the cooling duct is high, which gives a high temperature rise at the top of the winding.

The test coils had four groups of layers between cooling ducts and 13 total layers. The inner and outer groups were cooler than the inner groups due to radiation heat loss. The hottest spot temperature was located in the second group of layers in layer 5. This contributed to a low average temperature rise for the total coil. An optimum winding design would have identical hottest spot temperature rises in all groups. The average temperature rise of layer 5 was used to compute the hottest spot increment and ratio to compare with previous published data. The hottest spot ratio for this layer was considered more representative of actual windings and represents the minimum possible value for a winding of the length tested. The hottest spot temperature increment for three duct sizes is shown in Fig. 2. The hottest spot increment varies with hottest spot temperature rise and is not the constant 30°C indicated by IEEE standards. The hottest spot ratio for coil 2 compared with IEEE standards is shown in

375 DUCT S O 0 DUCT .625 DUCT ---- - . . . . . . . . . .

0 20 40 60 80 100

PER CENT HEIGHT

Fig. 1. Effect of duct size on temperature rise. Tests at same current.

.375 DUCT .500 DUCT .625 DUCT ---- - . . . . . . . . ..

f 2 0 l , . . . . , . . . , . , . . . , . . . . . 1 g 60 120 180 240 300

Fig. 2.

HOTTEST SPOT TEMPERATURE RISE C

Hottest spot increment for different duct sizes. Coils 1,2,3.

TABLE I11 COMPARISON OF IEEE STANDARDS WITH TEST

~ ~ _ _ _ _ _ ~ ~ ~

IEEE STDS. TEST (ASSUMED)

TOP, HOT SPOT RISE ('12) 180 1 80 MIDDLE, AVE. RISE ('C) 150 120 BOTTOM RISE ( ' C) 120 40

Fig. 3. IEEE standards assume a variable ratio. Test results indicate the ratio is constant for a given winding. A summary of the test results for all the test coils is shown in Table IV.

B. Disc winding Test Results Disc-type windings are also used for the high-voltage

windings of 15-kV class ventilated dry-type transformers. Data from Whitman [lo] for a 30-in-high disc winding were compared with the author's layer winding data as shown in Fig. 4. The data indicate that hottest spot temperature increments for disc windings are similar to those of layer windings of the same length for sinusoidal current. The duct air temperature rise data indicate that a



200 Ui

2

> w 1.75

K

s i -,r-Crc------r,------------

TABLE IV SUMMARY OF TESTED HOTTEST SPOT RATIOS

, DUCT LAYER HOT SPOT SIZE INS. IN RISE RANGE IN. DUCT MIN;-MAX.

C

0.375 NO 111.9-251.1 0.500 NO 85.3-271.2 0.625 NO 101.5-242.0 0.375 Yes 98.9-195.6 0.500 Yes 113.1-1Y0.5 0.500 Yes 102.3-218.8

RATIO RATIO HS RISE/ HS RISE/ COIL AVE. LAYER 5 i 1.585 1.409

1.688 1.536 1.583 1.457 1.549 1.436 1.717 1.530 1.601 1.458

WIND

LV

LV

large vertical temperature gradient due to the natural convection air flow occurs in disc windings also.

C. Sheet Conductor Layer Winding Data Core loss contributes significantly to the temperature

distribution in the low-voltage windings. Thermal tests must be performed on a complete transformer to obtain thermal data with core loss present. A three-phase prototype transformer rated 2500 kVA AA, 3333 kVA FA was constructed with 125 imbedded thermocouples in the center-phase low-voltage winding to obtain data on a full- length sheet conductor layer winding.

Production thermal tests are made by either the short- circuit method or the loading-back method in accordance with the ANSI/IEEE Test Code [27]. The short-circuit method requires two tests to be performed. A test with rated current is performed with the low-voltage windings shorted. During this test, core loss is negligible. The short is removed and an excitation thermal test is performed with rated voltage which gives core loss but no load on the unit. The average temperature rises are determined for the two tests and combined by means of an equation in the Test Code to determine the corrected average temperature rise with rated current and voltage. The loading- back test method produces rated current in the windings simultaneously with rated voltage to give core loss. The loading-back test method is more representative of actual loading but it requires a greater amount of testing facilities and becomes increasingly difficult to perform as the size of the transformer increases.

.

COOL. HS. AVE. RATIO AVE. RATIO HS RISE RISE HS/AVE RISE HS/AVE INC. C C F . CTR. 30LEGS 3 LEGS F.

C LEG C C

AA 158.6 112.2 1.41 107.5 1.48 46.4 178.2 128.8 1.38 120.8 1.48 49.4 213.0 153.4 1.39 141.3 1.51 59.6

FA 153.2 104.4 1.47 116.3 1.32 48.8

AUTHOR DISC IEEE - ---- .......... 0

..........................

$ IO[ , , , 5 6o

, , , , , , , , , , , , , , j 120 180 240 300

t o HOTTEST SPOT TEMPERATURE RISE C

Fig. 4. Hottest spot increments for 30-in disc windings, 3.04411 build, 0.25-in horiz. ducts.

TABLE V PROTOTYPE TEST RESULTS FOR LOADING-BACK TESTS

A prior test program of the author [13] indicated that the loading-back test method gave higher low-voltage winding hottest spot temperature rises than expected from the short-circuit test method. Core loss raises the temperature of the low-voltage winding nonuniformly. The top portion of the winding nearest the core increased the most, contributing to the higher hottest spot temperature. A similar effect could occur in the high-voltage winding although the effect is thought to be less.

The test program on the prototype included tests by the short-circuit and the loading-back test methods. Hot- and cold-resistance measurements were taken from the line terminals to the neutral terminal of the low-voltage winding to obtain individual coil average temperature rises. Equipment developed to accurately measure low resistance was used. A cooling curve of the hot resistance was extrapolated to the instant of shutdown in accordance with the IEEE Test Code. Test data for the loading-back thermal tests are given in Table V. For the self-cooled (AA) tests, the center-phase low-voltage winding was the hottest, but for the forced air (FA) test, the end coils were hotter.

D. Summay of Test Results Compared with Standards The test data indicated that the hottest spot tempera-

ture allowance used in IEEE standards for ventilated dry-type transformers above 500 kVA is too low. For



wire-wound layer and disc-type windings this is due to the large thermal gradient from the bottom to the top of the windings caused by natural convection air flow. Sheet conductors used in low-voltage windings have smaller thermal gradients from the bottom to the top of the winding than wire-wound layer windings but core loss causes nonuniform heating, giving large gradients in these windings also. The high current leads also influence the thermal gradients in sheet conductor windings. Due to the large thermal gradients inherent in ventilated dry-type transformers, it is impossible to design ventilated dry-type transformers above 500 kVA with an average temperature rise of 150°C without exceeding the 180°C hottest spot temperature rise limit. To meet 180°C hottest spot rise requires a reduction of the average temperature rise to approximately 120°C.

The test program showed that the ratio of hottest spot temperature rise to average temperature rise was constant. This confirmed the conclusions of Stewart and Whitman [9] from 1944. The conclusion of the test program compared with IEEE Standards is summarized in Table VI.

IV. MATHEMATICAL MODEL The test data were used to refine a mathematical model

[32] to calculate hottest spot temperature rise in ventilated dry-type transformers with layer-type windings. Comprison of the test results with calculations from the mathematical model showed that the model predicts hottest spot temperature rise with reasonable accuracy. The mathematical model was used to investigate the effect of various parameters on the relationship of hottest spot and average temperature rises. The number of conductor layers, insulation thickness, and conductor strand size were found to have only a minor effect on the ratio of hottest spot to average temperature rise. Each of these parameters affect the hottest spot and average temperature rise; however, the effect on the hottest spot ratio was minor. Winding height was found to be the main parame- ter influencing the hottest spot ratio. The ratio increases with increasing winding height. This is due to the thermal gradient from the bottom to the top of the winding due to the natural convection air flow. Increasing the cooling duct size decreases the hottest spot ratio slightly; however, a point is reached where the increase is of no further benefit.

V. RECOMMENDATIONS

A. Average Winding Temperature Rises A ratio of hottest spot temperature rise to average

temperature rise of 1.5 is proposed for IEEE standards for ventilated dry-type transformers above 500 kVA instead of a constant 30°C allowance. The ratio is depen- dent on the height of the windings. Shorter windings will have a lower ratio and longer windings will have a higher ratio. The hottest spot temperature allowance based on a

IEC IEEE PROPOSED 0 ---- . . . . . . . . . . -

~~ ~~ ~ 6 60 80 100 120 140 160 180 I

HOTTEST SPOT TEMPERATURE RISE C

Fig. 5. Proposed hottest spot temperature allowances. Ventilated dry- type transformers > 500 kVA.

TABLE VI COMPARISON OF IEEE STANDARDS WITH T E ~ [91, [lo], [14]

IEEE STDS. TEST

HOT SPOT RATIO VARIABLE CONSTANT

HOT SPOT CONSTANT VARIABLE ALLOWANCE 30 'C 30-6O'C

TABLE VI1 PROPOSED LIMITS OF TEMPERATURE RISE 220°C INSULATION

TEMPERATURE CLASS VENTILATED DRY-TYPE TRANSFORMERS > 500 kVA

~~

APPLICATION AVERAGE HOTTEST SPOT

STANDARD 120 'C 180 'C SPECIAL 80 'C 120 'C

1.5 ratio is shown in Fig. 5 and compared with current IEEE [231 and IEC [24] standards.

The 220°C insulation temperature class is a standard by most manufacturers of ventilated dry-type transformers even when average temperature rises below 150°C are specified. For this standard insulation temperature class the limits of temperature rise given in Table VI1 are proposed. For large units with tall windings, the average temperature rise should be reduced below 120°C by the transformer manufacturer to meet the limiting hottest spot temperature rise of 180°C. The 80°C average temperature rise with a 120°C hottest spot temperature rise is proposed as an option for additional overload capability or applications with nonsinusoidal load currents.

B. Average Temperature Rise Tests There are differences between the average temperature

rises of the individual legs of a three-phase unit. The IEEE Task Force on Dry-Type Transformer Thermal Tests has suggested that the average temperature rise be



calculated for each leg individually and the highest value used to determine if average temperature rise guarantees are met. For three-phase transformers the individual coil average temperature rises can be determined from the phase terminal measurements of the cold and hot resistance. The procedures and equations are included in a proposed revision [31] of the IEEE Test Code. These procedures are considered necessary to assure that the hottest spot temperature rise limit is not exceeded for any individual coil.

C. Hottest Spot Temperature Rise Tests Confirmation of hottest spot temperature rise is one

performance characteristic for which no standard test method exists. Consideration should be given to adding this measurement to the IEEE Standards as a design test requirement using prototype transformers or windings to qualify a design family and the manufacturer's mathematical model. For the high-voltage winding, test coils should be satisfactory. For the low-voltage winding, tests should be performed on a prototype transformer. A standard recommended test procedure should be developed. Certi- fication of hottest spot temperature rise on a production transformer design should be based on a hottest spot temperature allowance determined from the mathematical model and added to the average temperature rise of the hottest winding leg. Primary and secondary windings should be considered separately.

The loading-back test method is the preferred test method to determine hottest spot temperature rise in prototype transformers. The loading-back test method requires a greater amount of testing facilities and becomes increasingly difficult to perform as the size of the transformer increases. The author has developed a procedure to determine the hottest spot temperature rise using the short-circuit test method. The validity of the method was confirmed by comparing the loading-back and short- circuit test method results for the 2500-kVA prototype. The procedure gives conservative results when compared with the loading-back test method. This method should permit manufacturers without facilities for loading-back tests to substantiate hottest spot temperature rises in their prototype designs. Additional experience from other manufacturers would further substantiate the method. A summary of the procedure is as follows.

1) Install a sufficient number of thermocouples in a prototype transformer to ensure that the hottest spot temperature is measured. The coil geometry should be considered. Placement of thermocouples should be cir- cumferentially around the coil and should include the portion of the lead in contact with insulation within the coil.

2) Short the low-voltage winding and apply rated current until temperature rises are constant.

3) Remove the short and apply excitation voltage until temperature rises are constant.

4) Calculate hottest spot temperature rise for rated conditions by means of (1) and (2). It is necessary to

perform the calculations for many points in the coil since the core loss affects the temperature distribution nonuniformly. The hottest spot location for the current-only test may be different than for the excitation voltage test and thus different than for a loading-back test.

The equations for calculating hottest spot temperature rise for rated conditions are

where

0,

0;.

0,

0 HS

TA TAT Tk N

hottest spot temperature rise over ambient during current-only test corrected hottest spot temperature rise over ambient due to current only hottest spot temperature rise over ambient during excitation-voltage-only test hottest spot temperature rise over ambient for rated conditions rated ambient temperature, usually 30°C ambient temperature during current test 234.5"C for copper, 225°C for aluminum 0.8 for self-cooled (AA), 0.9 for forced air (FA).

D. Product Standards IEEE standards should distinguish between cast-resin,

sealed, and ventilated dry-type transformers with different hottest spot allowances applicable to each design. For sealed units the hottest spot allowance should be less since the windings operate in a hotter gas instead of ambient air. The thermal gradient from the bottom to the top of the winding is less. For cast-resin dry-type transformers there may be a different relationship between the vertical temperature gradient and the radial temperature gradients caused by the large amount of epoxy. Different hottest spot allowances should be used for ventilated dry-type units 500 kVA and below and above 500 kVA.

E. Nonsinusoidal Load Current Requirements Design of transformers for nonsinusoidal load currents

should include an analysis of the eddy loss distribution in the windings and calculation of the hottest spot temperature rise. Eddy losses due to the leakage flux distribution are concentrated in the ends of the winding [29]. For layer-type windings the extra heat due to increased eddy losses for nonsinusoidal load currents is conducted along the layer to the cooler region in the middle of the coil. For disc windings the upper disc sections are isolated from lower sections and thermal conduction to the lower sections does not occur. Layer-type windings appear more suitable for nonsinusoidal load current requirements.

The technical merits of the requirements of listing organizations for units rated for nonsinusoidal load cur-



rents are being debated in the industry. Consensus industry standards defining the performance and test requirements for dry-type transformers rated for nonsinusoidal load currents should be developed by an organization such as the IEEE. A more definitive term other than “K-factor” should be adopted since the letter “K” is used in many IEEE standards to define other parameters. A definition of the term is needed.

VI. SUGGESTED SPECIFICATION Present standards for ventilated dry-type transformers

require revision as noted in this paper. Specifiers can assist in developing future standards by requiring that manufacturers substantiate their performance claims with test data and signed test reports. Suggested specification statements for ventilated dry-type transformers above 500 kVA incorporating the thermal considerations stated in this paper are listed below.

1) The transformer shall be of ventilated dry-type con- struction manufactured in accordance with IEEE C57.12.01 and ANSI C57.12.51.

2) The transformer shall be rated - kVA, AA; primary voltage, volts (1) ; secondary voltage - volts, (1) ; - Hz, with two 2-1/2% full-capacity taps a b o v e x a l and two 2-1/2% full-capacity taps below normal in the primary.

3) Forced air cooling is (2) . When supplied, forced air cooling shall increase t h E m e p l a t e kVA by 33% of the self-cooled kVA.

4) The insulation temperature class of the windings shall be 220°C. The average temperature rise by resistance shall not exceed (3) “C and the hottest spot temperature rise shall not exceed (4) “C for either the primary or secondary winding whenoperated at 100% of nameplate kVA rating in a 40°C maximum, 30°C average ambient as defined by IEEE C57.12.01.

5 ) Hottest spot temperature rises shall be calculated using mathematical models verified by thermal tests on test windings and/or a prototype transformer representative of the design family. Tests shall have been conducted at conditions of full load or conditions simulating full load. A test report signed by a registered professional engineer shall be on file in the manufacturer’s plant and available for review.

6) Tests shall be conducted in accordance with ANSI/IEEE (257.12.91. Certified test reports shall be supplied summarizing the results of a1 tests. Calculated hottest spot temperature rises of the primary and secondary windings shall be shown on the certified test report.

7) A temperature rise test is (2) . When a temperature rise test is required, it shall b e r f o r m e d on one unit at the minimum and maximum kVA ratings. Average temperature rises of the individual coils shall be calculated from the thermal test data and used to calculate hottest spot temperature rises of the primary and secondary winding. Fill in blanks with alternatives as follows:

-

-

(1) delta, wye, or wye four wire, (2) required, not required, (3) 120, 80, (4) 180 if average is 120 in (3), 120 if average is 80 in

(3).

VII. CONCLUSIONS The hottest spot temperature allowance used in IEEE

and IEC standards is too low for ventilated dry-type transformers above 500 kVA. Test data indicate the hottest spot ratio is constant and the hottest spot allowance varies from 30°C to 60°C. Due to the large thermal gradients inherent in dry-type windings cooled by natural convection, it is impossible to design ventilated dry-type transformers above 500 kVA with an average temperature rise of 150°C without exceeding the permissible hottest spot temperature rise of 180°C. The average temperature rise for ventilated dry-type transformers above 500 kVA with 220°C insulation temperature class should be 120°C. This imposes an economic penalty for manufacturers who conduct development programs and calculate hottest spot temperature rises for their designs.

IEEE standards should require measurement of hottest spot temperature rise on prototype transformers or windings as a design test to qualify a design family and the manufacturer’s mathematical models. This is especially important for ventilated dry-type transformers rated for nonsinusoidal load currents. For high-voltage windings, a thermal test program using full-size test windings should be satisfactory since core loss has a minor effect. For low-voltage windings, core loss contributes significantly and thermal tests should be conducted on a prototype transformer.

IEEE standards should be revised as recommended in this paper. The validity of the recommended changes is confirmed by the author’s recent test data, prior data of others, and mathematical modeling of ventilated dry-type transformer heat transfer phenomena. The specification given in this paper should be used until IEEE standards are revised.

ACKNOWLEDGMENT Mike Mitelman, Dry-Type Transformer Development

Project Manager for the General Electric Company, se- cured the funding for this work and encouraged the author to conduct the research program. Jon Jaynes made 1000 thermocouples and assisted in the assembly of the test windings. Co-op students Richard Black, John Cauthran, and Chris Waddell assisted in manufacture and testing of the windings and prototype transformer under supervision of the author.

REFERENCES [l] A. A. Halacsy and G. H. Von Fuchs, “Transformer invented 75

years ago,” AIEE Trans. Power Appar. Syst., vol. 80, no. 54, pp. 121-125; disc. pp. 125-128, June 1961.

[2] W. L. R. Emmet, “Existing commercial applications of electrical power from Niagra Falls,” AIEE Trans., vol. XII, pp. 482-499, 1895.



[51

C. C. Chesney and C. F. Scott, “History of the AC system in America,” AIEE Trans., Elect. Eng., vol. 55, pp. 228-235, March 1936. E. W. Rice, Jr. “The relative fire risk of oil nd air-blast transformers,” AIEE Trans., vol. XXIII, pp. 171-173; disc. pp. 175-197, 236-238, 246-247, 1904. B. F. Allen and G. F. Simmons, “C,F, as a coolant and dielectric in high temperature sealed dry type transformers,” presented at AIEE Winter General Meeting, New York, Jan. 29-Feb. 3, 1961, AIEE Paper CP 61-36. W. M. Terry Jr. “Hermetically sealed transformers are here to stay,” Allis Chalmers Electr. Rev., vol. XV, no. 1, pp. 4-8, 1950. M. L. Manning, “The application of class H insulation to transformers,’’ AIEE Trans., vol. 70, pt. 11, pp. 1427-1434; disc. pp. 1434-1435, 1951. W. M. Terry, “Performance characteristics of 150 C rise dry-type transformers,” Allis Chalmers Electr. Rev., vol. XX, no. 1,

H. C. Stewart and L. C. Whitman, “Hot spot temperatures in dry-type transformer windings,” AIEE Trans., vol. 63, pp. 763-768,

L. C. Whitman, “Co-ordination of dry type transformer models with transformer geometry,” AIEE Trans., vol. 75, part 111, pp. 328-332, June 1956. L. C. Whitman, ‘‘hading of ventilated dry type transformers,” AIEE Trans., vol. 76, part 111, pp. 1077-1084, Dec. 1957. S. J. Antalis and G. I. Duncan, “Temperature distribution in insulation systems for dry type transformers and their effect on design,” presented at IEEE Winter Power Meeting, New York, Jan. 30-Feb. 4, 1966, IEEE Paper 31CP66-163. R. L. Dormer, “Designing class C dry type transformers for maximum efficiency, a review with guidance on predicting behavior under various conditions,” Electr. Rev., pp. 313-317, Sept. 1973. L. W. Pierce, “An investigation of the temperature distribution in cast-resin transformer windings,” IEEE Trans. Power Delivery, vol. 7, no. 2, pp. 920-926, April 1992. L. W. Pierce, “Hottest spot temperatures in ventilated dry type transformers,” presented at IEEE Power Engineering Society Winter Meeting, Columbus, OH, Jan. 31-Feb. 5, 1993, IEEE paper 93 WM 052-1 PWRD. A. A. Halacsy, “Temperature rise of dry type transformers,” AIEE Trans., vol. 77, part 111, pp. 456-462, Aug. 1958. IEEE Standard General Principles for Temperature Limits in the Rating of Electric Equipment and for the Evaluation of Electrical Insulation, ANSI/IEEE Standard 1-1986, p. 8. W. W. Satterlee, “Design and operating characteristics of modern dry-type air cooled transformers,” AIEE Trans., vol. 63,

Guide for Loading Dry Type Distribution and Power Transformers, ANSI Appendix C57.96, Nov. 1959. Commercial, Institutional and Industrial Dry-Type Transformers, NEMA Pub. No. TR27-1965, part 4, p. 2. IEEE Standard General Requirements for Dry Type Distribution and Power Transformers, IEEE Standard C57.12.01-1979, Table 4. IEEE Guide for Loading Dry-Type Disbibution and Power Trans- formers, ANSI/IEEE Standard C57.96-1989. IEEE Standard General Requirements for Dry Type Distribution and Power Transformers Including Those with Solid Cast and/or Resin

pp. 10-15, 1955.

1445-1448, Oct. 1944.

pp. 701-704, 1445-1448, Oct. 1944.

Encapsulated Windings, IEEE Standared C57.12.01-1989, Table 4A, p. 16; Table 7, p. 23.

[24] Dry Type Power Transformers, IEC Standard Publication 726, 1st ed., 1982, Table IV, p. 19, and Amendment 1, Feb. 1986.

[25] American National Standard Requirements for Ventilaied Dry-Type Distribution Transformers, I to 500 kVA, Single Phase, and 15 to 500 kVA, Three-phase, with High-Voltage 601 to 34500 Volts. Low-Volt- age 220 to 600 Volts, ANSI Standard C57.12.50-1981.

[26] American National Standard Requirements for Ventilaied Dry-Type Power Transformers, 501 kVA and Larger, Three-phase, with High- Voltage 601 to 34500 Volts, Low-Voltage 208Y/ 120 to 4160 Volts, ANSI Standard C57.12.51-1981.

[27] IEEE Standard Test Code for Dry Type Distribution and Power Transformers, ANSI/IEEE Standard C57.12.91-1979, Sec. 11.

[28] IEEE Standard Test Procedure for Thermal Evaluation of Insulation Systems for Ventilated Dry-Type Power and Distribution Transformers, ANSI/IEEE Standard C57.12.56-1986.

[29] IEEE Recommended Practice for Establishing Transformer Capabiliy When Supplying Nonsinusoidal Load Currents, ANSIjIEEE Stan- dard C57.110-1986.

[30] I. Kerszenbaum, A. Mazur, M. Mistry, and J. Frank. “Specifying dry-type distribution transformers for solid-state applications,” IEEE Trans. Industry Applicat., vol. 27, no. 1, p. 173-178, Jan./Feb. 1991.

[31] Draft IEEE Test Code for Dry-Type Distribution and Power Trans- formers, IEEE PC57.12.91 Draft 4, Feb. 15, 1992.

[321 L. W. Pierce, “Predicting hottest spot temperatures in ventilated dry type transformers,” presented at the IEEE Power Engineering Society Summer Meeting, Vancouver, July 18-23, 1993, IEEE paper 93 SM 392-1 PWRD.

Linden W. Pierce (M’70) was born in Athens, TX on January 4, 1941. He receibed the B.S. degree in mechanical engineering from the Uni- versity of Texas, Austin in 1963, completed the General Electric Advanced Course in Engineer- ing in 1966, and received the M.S. degree in mechanical engineering from the LJniversity of Tennessee, Knoxville in 1973.

In 1963 he joined the General Electric Com- pany and since 1965 has worked for the Trans- former DeDartment at Rome. GA with various

positions in transformer design, development, and progr‘im manage- ment. He is currently Senior Engineer, Product Technolog. He holds eight patents.

Mr. Pierce is a member of the IEEE Industry Applications, Power Engineering, Magnetics, and Dielectrics and Electrical Insulation Soci- eties, and CIGRE. He is a member of the IEEE Transformers Commit- tee, Chairman of the Working Group on Development of the Loading Guide for Cast-Resin Transformers, and Chairman of the Task Force on Revision of the Test Code for Performing Temperature Rise Tests on Dry Type Transformers. He is a Registered Professional Engineer in the State of Georgia.


Documents

Document2