LIFE EXPECTANCY VS ACTUAL LIFE OF THE MAIN TRANSFORMERS FOR THE 3.4 MW POWER SUPPLY SUMMARY

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1 SLACTN6820 C. A. Harris July 16, 1968 not to be abstracted, quoted oz further diaolosedwithout > LFE EXPECTANCY VS ACTUAL LFE OF THE MAN TRANSFORMERS FOR THE 3.4 MW POWER SUPPLY SUMMARY One of the two 2,000 kva rectifier transformers in the 3.4 MW power supply failed after only about 200 hours of operation at maximum temperatures. These transfo.mers had been energized for about 446 hours total. A measurement of the resistance change of the kv primary windings taken before the insulation failure had indicated that the winding temperatures were excessive. A study was underway at the time of failure to estimate the life expectancy of the insulation and to recommend some practical changes to extend the life. This study predicted a maximum temperature of 330 C and an expected life 1 of from 100 to 300 hours. l

2 NTRODUCTON The two 2,000 kva transformers under discussion are the main rectifier transformers for the 3.4 MW power supply (340 v at 10,000 amps. ) The kv primary winding insulation is t Nomextl (made by Du Pont) a dry type insulation with naturalconuectioncooling. The secondary windings have water cooled hollow conductors andare not of concern in this report. These transformers have a long history of overheating. There have been other winding failures, but we had not tried to find a correlation between temperature and life before the present investigation. Blowers had been installed to increase the cooling of the primary windings for the previous set of trans formers; these blowers were in operation at the time of this last coil failure. High Voltage Coil Configuration Figures 1 and 2 shows the type of primary winding connections used. Note that this is a 3 phase transformer with an extended delta primary. The transformer has a Wye and a delta secondary to produce a 12 phase rectifier output. Two such transformers were used with the extentions in opposite senses to produce a 24 phase rectifier output. Average Resistance Change Due to Heating Figure 3 shows the details of the construction of the primary windings. Table shows the approximate resistances of the windings in each coil and the final resistance expected because of the calculated temperature rises. The expected resistances between terminals H2 and H3, (see Fig. 2) would thus be 1.20 ohms when hot; this hot resistance corresponds to an average temperature 225OC. 2

3 TABLE coil identification (Figure 3) Ti Maximum coil temperatures 215 C Maximum verage coil 1 :mperatures 180 Cav Coil dentification (Figure 2) R3t Coil resistance 20 C ohms Coil resistance hot ohms 0.33 Tii 145Oc Tiv TV Tvi 215Oc 255 C 155Oc 208 Cav R6t Tix 220 C TX 330 c 2 70 Cav Rgt Txi 260 C H2 to H Measured Resistance Change of the Primary Windings The resistance from H2 to H3 was measured on April 4, 1968 after an extended operation at 100% rated current. This should represent the conditions used by ). Walz for his thermal calculations. Measurements of this type must be made shortly after turn off, but the measured data must be corrected because the coil is cooling rapidly at this time. The value of resistance at the time of turn off is determined by measuring the resistance vs time and extrapolating the data back to zero time. Figure 4 shows just such a cooling curve plotted as the actual resistance for two transformers in parallel with a scale showing the corresponding temperatures vs time. t shows the average temperature at turnoff to be about 23O C. 3

4 Note that the measured hot resistance for one transformer is then 0.63 X 2 = 1.26 ohms, and that this is close to what would be expected using the temperature data of D. Walz. The fact that the measured hot resistance is slightly above the calculated hot resistance can be explained because the average temperature of the cooling air at the surface of the coils will be higher than the input air to the power supply. This difference is due to the temperature rise of the air as it passes through the ventilation ducts. The close correspondence shown between the calculated average temperature rise and that determined empirically allows us to use the Hot spot temperature of 330 C shown in Table, as an accurate temperature, Life Expectancy The life of this insulation should be based upon 330 C and the operating voltage gradient of 75.5 volts ac peak per mil average. (No stress concentrations are considered. ) The major insulation is flnomexll a nylon fibrous material made by Du Pont and is described in their Bulletin N204 February Figure 5 is a copy of their Fig. 10 which shows the expected life of this insulation vs temperature. At 330 C, Nomex would have about a 200 hour life. Figure 6 shows a plot of life vs voltage gradients for several different materials. have dashed in a line showing what would expect for our operating conditions for ttnomexll. This line shows an expected life of only 100 hours for our operating conditions. The reader must keep in mind that these graphs can only be considered approximate averages. A considerable spread of data is to be expected. The Transformer Failure One of the pair of transformers (Type El5109 serial PT853) failed on May 17, 1968 after about 446 total hours of operation of which only about 200 hours ( an estimate) was at full operating temperature. The location of the failure seems to be in coil R7 of phase 1 which is one with the highest temperatures. The coil would have to be unwound to confirm the location of the fault (see Fig. 2). 4

5 The resulting short circuit currents (that were limited by #400E high voltage fuses) caused some mechanical damage tc coil R3. The transformer terminals had been protected from voltage transients with surge capacitors and lightning arresters; therefore believe that this hilure is due to the high temperatures associated with major insulation of the transformer. The power supply was put back into operation on June 27, 1968 with new oil filled transformers purchased for the purpose by SLAC. ACKNOWLEDGEMENTS Credit must be given to the many suggestions offered by M. Berndt, P. Edwards, and D. Walz during the course of this investigation. REFERENCES 1. D. Walz; Temperature Distribution in the Windings of the 3400kW P. S. SLAC TN Bulletin N204 Du Pont Co., February

6 S TO S5 = 315 S2 TO S4 = 315 volts* APPROX volts /turns*, 0 s3 A! SECONDARY \!, 55 H2 S A /a+ RATED 94 AMPS ; \ RATED 52.2 S4 / / / i SECONDARY #2 S6 *APPROXMATE NUMBERS H2,= 12,470 V H3,B = 11,420 V B C = 9,540V H3 C =, 880 V Fig. 1

7 = ul K

8 nner Winding # 8 Square 26.1 Amps/Cond. nsul. Film.0035 Coil Resistance R9 TX1 X TX lx Tx V \\\\)&is\\\\ \\\\\A\\ ~,\\\\\,\\\\\\\,,,,,,,,~ LV Winding Air JD f e Ft/Min 1 Air Duct h Material Press Board Air # 8 Square # 8 Square # 8 Square Thickness in inches T Center Winding V # 8 Square 26.1 Amps/Cond. T nsul. Film.OO35 V Coil Resistance R6 T V SOMCA Air # 8 Square # 8 Square # 8 Square Outer Winding # 9 Rectangular 47 Amps/C&d. nsul. Film.004 Coil Resistance R3 :klnnnn p ~::, ~; ::....:.r..:::..*.:.._... :.t...,_.:,.$i z..:.,..._c\_... i _..L.....,.. Air Up Natural Vent. SOMCA Air # 9 Square # 9 Square Aluminum Press Board FG. 3Schematic Cross Section Through HighVoltage Coil 1095A5

9 ZBE601 AjO ElNklfll ElMOd tl3ljv S3lCNW El tl L 9 C b C 2 0 j EE O lUdV ~N3~8RW 3oL 30 QyuW OO() l 02 SE 0 LE. O ot : 0 09 lb O 7'0 OS Et. si 7 o 001 9NllVtl % 001 Li ! 6, b O 6 i'0 Otl \ El E 091 \ \ E E; 0 \ 081 \ L 5 0 \ 6 \ \ =Jgkj&j3l\w XV R oocz J x

10 FGURE 10 EXTRACTED DUPONT BULLETN N204 FEB FROM Useful Life vs. Temperature Nylon Paper (lomil, Type 410 paper) O6 L Upper 95% Confidence Limit\,, l,,i n5 _ Confidence Limit 160 i TEMPERATURE, C. (Reciprocal AbsoluteTemperature Scale) Fig. 5

11 O year 100, kw P.S. TRANSFORMERS DESGN LEVEL ac Peak Average Stress POLYETHYLENE CABLE LFE Computed From AEE TRANS 5851 (ac Peak) MAX Stress TEST ON ONE SHEET 10 ML AT ROOM TEMPERATURE,+ By Du knt (ac Peak) (Average Stress) year 10, ML NYLON AEE 5470 ac Peak Room Temperature; Average Stress 2 ML MYLAR N AR AEE 5470 Room Temperature (4,800 v/mil Breakdown) (ac RMS) Average Stress month 1, Average \ \ \ \ \ \ \ \ \ \ \ \ \. Stress H, J /, H 3 x O Mils Nomex.0 CABLE (OL PAPER) ac Peak Reference Peek 1929 (Page 248) MAX Stress \ GRADENT n 3,400 kw P.S VOLTAGE GRADENT VOLTS PER ML (pk 1 Fig

Western Mining Electric Association San Antonio TX Layer vs. Disk Windings Discussion NOVEMBER 15, SPX Transformer Solutions, Inc.

Western Mining Electric Association San Antonio TX NOVEMBER 15, 2012 PRESENTED BY David L. Harris, PE Customer Technical Executive SPX Transformer Solutions, Inc. Office: 262-521-0166 Cell: 262-617-3039