E A T U R E A R T I C L E

ary h Evaluation o w-Backed Mic

igh Voltage, C ion usina Va ure

ey Words: Film-backed mica paper tape, glass-backed mica tape, epoxy impregnating resin, high voltage coils

by F. T. EMERY Generator Materials Group Westinghouse Power Generation

T h e evaluat ion test results for the KAP tape w h e n used with the Westinghouse epoxy resin and high voltage coil processing shows good dielectric and mechanical properties w h e n compared with GBMP tape tes t results.

NTRODUGTION tis of interest to find a mica paper tape with higher dielec- tric properties than commercially available tapes. This ar- ticle presents the results of a preliminary evaluation of

corona resistant (CR), KaptonB-backed mica paper tape. This film-backed mica paper tape was evaluated for possible use for high voltage, coil groundwall insulation using vac- uum pressure impregnation (VPI) with epoxy resins. CR Kapton mica paper tape was used to insulate three aluminum test bars and was impregnated with epoxy resin using a VPI process. The impregnating resin and process is used in the fabrication and processing of high voltage stator coils usually

insulated with glass-backed mica paper tape. An evaluation of the insulated test bars was done, and the results were com- pared with those of other commercially available, glass-backed mica paper taped test bars processed and evalu- ated under identical conditions. The CR Kapton tape was used to make up insulated test bars, which in turn were sub- ject to electrical, chemical, and mechanical testing. The eval- uation of CR Kapton tape shows good dielectric properties, which could make it suitable for use in high voltage coil insu- lation. Groundwall insulation systems for vacuum-pres- sure-impregnated (VPI), high voltage stator coils usually use an epoxy or polyester resin, glass-backed mica paper (GBMP) tape system. The coils are insulated with mica paper tape and then vacuum pressure impregnated with the resin. Groundwall tapes with improved dielectric properties allow a reduction in coil groundwall thickness, which improves the thermal performance, which in turn allows a stator wind- ing design of higher power density [l-31. With that in mind, a new CR Kapton-backed mica tape was evaluated. In this paper, the test bars taped and processed with the CR Kapton tape will use the KAP tape identification. The CR Kapton-backed mica paper tape is commercially available as Porofol CR 2578. Porofol CR 2578 is fabricated from a highly absorbent mica paper (160 gm/m2) based on uncal- cined muscovite and a Kapton, corona-resistant (CR) polyimide film, which serves as a carrier. The binder is a hardener free epoxy resin. The tape dimensions are 1-inch-wide with a nominal thickness of 0.0051 inches. No glass backing is used in the tape construction.

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EVALUATION CRITERIA In order to optimize the epoxy groundwall insulation

used in the design and fabrication of high voltage stator coils, mica tape assessment criteria were defined and are detailed in Table I. A number of custom and commercially available tapes were evaluated in conjunction with the KAP tape. The evaluation process included making and processing test bars and then subjecting the test bars to thermal cycling and elec- trical evaluation. Limited mechanical and chemical tests were also done as part of the evaluation process.

TAPE EVALUATION PROCEDURE The candidate CR Kapton tapes were used to insulate

%-inch (1.27 cm) by 2-inch by 48-inch-long (121.92 cm) aluminum test bars insulated to 13.8 kV using a double wall thickness (DWT) of 230 mils (5.84 mm). Groundwall tape was applied using an automatic taping machine. To prevent breakage of the CR Kapton tape, the applied tension had to be reduced from the normal ten pounds (4.54 kg) to eight pounds (3.63 kg). Typically, 13 half-lap layers were applied to each test bar. All test bars were vacuum pressure impreg- nated using an epoxy resin. The test bars were pressed to size and fully cured using the same cure cycle as would be used for full-size production coils using the same impregnating

I -0.9 1- -1 -1

AW CR V P KAP Tape Identification

resin. Ground electrodes and voltage grading electrodes were applied to each test bar. Normally, a ground electrode length of 12 inches (30.48 cm) is used with5-inch-long (12.7 cm) voltage grading electrodes on each end of the test bar. The test bars were then subject to both electrical and me- chanical testing and evaluation. Following the tests per- formed on the test bars, the data were compared to data obtained from similar testing performed on test bars insu- lated with commercially available, glass-backed mica paper tapes.

TEST RESULTS Electrical tests and limited mechanical tests were conducted

on the processed test bars. Since the tape met all the electrical test criteria, mechanical tests were performed. The experimen- tal test results are given for the tapes used in the evaluation.

Initial Power Factor The three test bars were power factor tested at room tem-

perature and the resulting data are shown plotted in Fig. 1. The (.8U-..2U) value of power factor tip-up is given on the plot. These values are very low (.04,-.24, and -.21%), which indicates good impregnation and a void-free insulation.The initial value of (.8U-.2U) power factor tip-up is compared

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Table I: Groundwall Mica Paper Tape Assessment Criteria ' Used for coil ratings from 13.8kVrms and higher

' Operating voltage stress capabilies-20% above present design

' Breakdown voltage (1 min)-z500 vpm

' Voltage endurance-IEEE 1043-1996 @ 120°C.

' power factortip-up.+ 1% @ 25°C. (.8U-.2U). Before and afterthermal cycling at

' Hot tan deIta-slO% @ 0.2 U@l55"C.

Operating temperature class-I 55°C.

155°C.

' Cured insulation tensile strength-->6000psi @I 00°C.

' Compatible with VPI resin and processing U = Coil Design Vobge

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Fig. 2 Comparison of the initial power factortip-up of KAP test bars with test bars insulated with GBMP tape

Fig. 1 Initial power factor test data for the three KAP test bars

Sep tember /Oc tober 1998 - Vol. 14, No. 5

Fig. 3 Power factor tip-up after 4th thermal cycle

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with test bars insulated with GBMP tape in Fig. 2. The IaP-insulated test bars compare favorably with different GBMP tape test bars identified as AW, CR, and VP [3] .

Thermal Cycling In order to determine if the insulation system delaminates

after exposure to elevated temperatures, a four-cycle thermal cycling test was conducted. Each test bar was heated at 155"C, unrestrained, for 48 hours and allowed to return to room temperature. After the test bars cooled to room temperature, power factor measurements were performed.

The (.8U-.2U) power factor tip-up was calculated for each test bar after the final temperature cycle. After the final ther- mal cycle, the average value of the power factor tip-up for the three IC4P bars was 0.11% and is shown plotted in Fig. 3, where it is compared with a number of GBMP tape test bar results. Low tip-up values indicate good groundwall consoli- dation was maintained following the thermal cycle.

At the conclusion of each of the four thermal cycles, the 155°C power factor was measured at a voltage level of 2.8kVrms. The values are shown plotted in Fig. 4. The hot power factor is an indication of the dielectric loss of the insu- lation system when operated at elevated temperature, and it is desirable that it be below 10% at the test temperature.

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Fig. 4 Hot tan delta measured at 155°C @ 2.8 kVrms

Tape Identification

Fig. 5 Voltage hold values in volts per mil for the tapes

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Voltage Breakdown Test bar voltage breakdown tests were conducted under

oil in order to obtain a true breakdown value and to reduce the possibility of voltage flashover. The plotted value, in Fig. 5, is the actual hold voltage; in units of volts per mil (VPM), the test bar will hold prior to a breakdown voltage. The hold value is less than or equal to the actual failure voltage of the test bar.

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Fig, 6 Tensile strength of test bar insulation

Tape Identification

Fig 7 Extract of uncured resin

Westinghouse Thermalastic Insulation Voltage Endurance Curve

O J 10 100 1000 10,000 100,000 1,000,00010,000,000

Fig 8 The plotted point is the test time for KAP Failure has not occurred after 5200 hours @ 8.7 kVrmsImm

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Tensile and Extractions Dog bone-shaped samples were cut from

each test bar. The groundwall tensile strength (see Fig. 6) and percent of uncured resin (see Fig. 7) were measured. The tensile strength is a measure of how well bonded the insulation system is and the percent extractable is a mea- sure of the remaining uncured resin in the insu- lation system after coil cure.

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Voltage Endurance One of the three test bars was placed on

voltage endurance at room temperature and at a voltage level of 24 kVrms. The test bar had

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Fig. 9 Thermal conductivity of Mi? A sample from each of the three test bars was measured not failed after 5,200 hours at a stress level of I 8.7 kVrms/mm. This point is shown plotted on the Westinghouse coil voltage endurance curve. The KAP test bar has passed the Westinghouse Thermalastic insulation voltage endurance test require- ments.

Thermal Conductivity A sample of groundwall insulation was removed from each

of the three test bars and the thermal conductivity was mea- sured per the ASTM D1530-93 test method. The measured values are given in Fig. 9. The measured values are about the same as for the Westinghouse Thermalastic insulation.

CONCLUSIONS The evaluation test results for the KAP tape when used

with the Westinghouse epoxy resin and high voltage coil pro- cessing shows good dielectric and good mechanical proper- ties, when compared with GBMP tape test results. Good resin impregnation resulted even though the KAP tape is im- pervious to the impregnating resin.

The positive points are: Low power factor tip-up Good thermal cycle performance (155OC) High voltage breakdown strength High tensile strength Able to impregnate with low viscosity epoxy resin Good cure (low extractable) Compatible with impregnating resin used No tape blocking during application No stringers (glass fibers) from taping Nomicaflalung Passed room temperature voltage endurance Thermal conductivity the same as Westinghouse Thermalastic groundwall

The negative points are: High hot tan delta Low taping tension required

As a final comment, a wider (1.5-inch) KAP tape was used to insulate six test bars that were subjected to the same pro- cessing. The purpose of trying a wider tape was to find out if

the dielectric strength could be increased through the use of a wider tape. However, impregnation of the insulation did not occur and the test bars did not pass the initial power fac- tor test. It was obvious from these test results that resin pene- tration of the insulation is between the tape layers and not through it, as is the case with GBMP tape.

The next step in the evaluation process is to use 1-inch-wide KAP tape in the insulation, processing, and test- ing of full-size generator coils, probably of the 13.8 kV volt- age class.

ACKNOWLEDGMENTS Klaus Engelhardt, International Consulting, and Richard

Theis, technical consultant for DuPont, are acknowledged for their help in the technical evaluation of the KAE Mark Williams of Electrolock, Inc. is acknowledged for his help in the procurement of the KAE

TIM EMERY received a B.S., M.S., and Ph.D. in electrical engineering from the University of Pittsburgh. He is a fellow engineer in the Gener- ator Materials Group with Westinghouse Power Generation, located in Orlando, Florida. He is responsible for the dielectric engineering associ- ated with electrical generators designed and manufactured by Westinghouse Power Genera-

tion. Tim is an active senior member of the IEEE and belongs to the DEIS and PES. He is a registered professional engineer is the states of Florida and Pennsylvania.

REFERENCES 1. ET. Emery, J.D.B. Smith, and K.F. Schoch, Jr., “Improved Groundwall Insulation for VPI’ed High Voltage Stator Coils,” 2997 ElCiEMCW Conference, Chicago, IL, Sept. 22-25,1997. pp. 589-592. 2. ET. Emery, R.F. Weddleton, “Latest Advances Associated with the Insulation Systems of High Voltage Stator Coils,” 2996 IEEE International Symposium on Electrical Insulation, Montreal, Quebec, Canada, June 16-29, 1996, pp. 226-229. 3. ET. Emery, E. Barrutia, “Latest Advances in Mica Paper Tapes for Application to VPI’ed High Voltage Generators,” 2998 IEEE International Symposzum on Electncal Insulation, Arlington, VA, June 7-10, 1998. pp. 266-269.

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