thermal and lifetime behavior of innovative insulation systems for rotating machines

8/8/2019 Thermal and Lifetime Behavior of Innovative Insulation Systems for Rotating Machines

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Proceedings of the 2008 International Conference on Electrical Machines

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III. APPLIED TESTS

Following tests were applied to the generator bars:

Measurement of dielectric properties and materialconsistency: The dielectric constant was determined bymeasurement and the resin content by burning out in afurnace.

Thermal observation of the warming of the voltagegrading end corona protection tape. This test was done by means of a wireless temperature measurement(Infra-red camera) at different rated voltages.

Lifetime of generator bars. The generator bars weretested with high voltage stress: 2.2 and 3.0 times ratedvoltage. The residual time until breakdown wasrecorded.

IV. DIELECTRIC CONSTANT MEASUREMENT AND RESINCONTENT DETERMINATION

For the determination of the dielectric behaviour an

unbalanced dissipation factor bridge was used [3]. The

capacitance of the model bars was measured at several

points on both sides and an average value of the dielectric

constant r was calculated. The test setup of the electrode

configuration is shown in Fig. 2.

Fig. 2: Test setup for capacitance and rmeasurement

The result of these measurements showed a variance of

the capacitances and r values of 5 %, which is caused by

deviations of the dimensions due to production of the bars.

The average

rof the VPI bars was between 4.0 and 4.5 andthe RR bars showed a r of 4.9 up to 5.2. A significant

difference between the standard material and the new

material could not be found out.

By the means of incinerating the resin content of the bars

was determined with a high-temperature furnace. The bars

were heated until the resin burned out and only mica and

glass was left in the insulation. By weighing with a

chemical balance the resin content could be calculated.

Typical values for the resin content are 27 % for VPI STD,

22 % for VPI FAB, 34 % for RR STD and 30 % for RRFAB insulations.

V. THERMAL BEHAVIOUR OBSERVATION

For the observation of the thermal behaviour of the

different model bars and the performance of the end corona

protection a test setup as shown in Fig. 3 was used.

The bars were clamped on a bracket and the temperature

was measured with an IR-camera [4].

Fig. 3: Test setup for observation of thermal behaviour

The generator bars were loaded with following electrical

stress ratings: nominal voltage UN, 2xUN+1kV, 3xUN. A

test cycle took a duration of 10 minutes and the starting

temperature for each cycle had to be the same (room

temperature). The special interest of these tests was the

temperature rise, the location of hotspots and the

determination of absolute temperatures. The results showed

a similar behaviour of bars with same production

technology and used materials. Hot spots could be observedat the point with the highest electrical field strength such as

at the corners and at the beginning of the stress grading

area. Fig. 4 is an example of an IR picture.


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3

0

10

20

30

40

50

60

70

80

90

100

110

120

1.0 2.0 3.0

Voltage U/UN

Temperature[C]

STD VPI after 1 min (3.0 kV/mm)STD VPI after 10 min (3.0 kV/mm)FAB VPI after 1 min (4.0 kV/mm)

FAB VPI after 10 min (4.0 kV/mm)STD RR after 1 min (2.75 kV/mm)STD RR after 10 min (2.75 kV/mm)FAB RR after 1 min (3.5 kV/mm)FAB RR after 10 min (3.5 kV/mm)

Fig. 4: Thermal performance of voltage grading tape

Hotspots at the points of highest electrical field strengths

can be easily recognized as well as at the edges of the bars.

Fig. 5: Warming-up of voltage grading tape

The results showed that VPI bars with new FAB

technology had the highest temperatures as to be expected

due to testing at very high field strength of 4.0 kV/mm.

The maximum temperature of the end corona protection

after 10 min at 3 times the nominal voltage reached almost

120 C

In this test the temperature of RR STD and FAB

generator bars was significantly lower compared to VPI

bars as they were operating at a lower electric stress.

VI. LIFE TIME TESTS

The life time behaviour of six different insulation

systems for generator bars was tested.

The superior electrical properties of the Resin Rich and

VPI FAB insulation system have been described in detail

earlier in [1], [2]. In this voltage endurance test the

influence of different types of mica, grain sizes of mica and

production technologies according to table II was

examined.

For each system a number of four similar bars wereproduced and two voltage levels (2.2 and 3.0 times nominal

voltage) were applied for the voltage endurance tests. The

tests were stopped after a maximum of 1000 h when no

breakdown of the insulation system has occurred.

The test setup as shown in Fig. 6 was designed to

simulate realistic conditions; according to this request a

conducting tape was applied to reproduce the grounded

stator slot.

Fig. 6: Test setup for voltage endurance tests

In addition a dissection of the generator bars and

microscopic inspection of the breakdown channel was

done. The bars were cut to thin slices that a microscope for

transmitted light could be used. Good results can be

achieved by polishing the surface of conductor and

insulation system. An example of a prepared bar can be

found in Fig. 7.


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4

11,5

8

25,9

1

0,1

0,

06

1100

15,

07

38,

19

2,

5

2,

04

488,

61

27,

41

27,

85

1260

258,

66

178,

07

1100

1100

515,

3

417,

96

1100

1100

71,

90

155,

57

114,

84

1,00

10,00

100,00

0,01 0,1 1 10 100 1000 10000

Lifetime [h]

ElectricFieldStrength[kV/mm]

LD VPI FAB

LD RR STD

LD VPI STD

Fig. 7: Preparation of bars for optical investigation

VII.LIFE TIME DIAGRAM

The test results of the voltage endurance tests were

evaluated by application of a life time diagram with

doubled logarithmic scale according to the inverse power

law for electrical ageing processes [5]. The statistical

parameters for this diagram were determined by the median

of the breakdown times. Normally a Weibull distribution

[6] is applied for life time investigations of electric

equipment, but due to a statistically low number of test

specimen its use was not appropriate.

The endurance points of the test specimens according to

table II are displayed in the following diagram.

Fig. 8: Life time points of VPI and RR bars producedwith different mica types

Uncalcined mica as well as muscovite with big grain size

show a tendency to have slightly increased life times.

To obtain statistically significant results further

evaluations with more specimens have to be performed.

VIII. SUMMARY

The dielectric, thermal and lifetime behaviour of

different motor and generator insulation systems were

investigated. The test objects were produced in RR and VPI

technology comparing a standard (STD) and innovative

(FAB) glass cloth carrier. Also different types of mica

paper and grain sizes were used.

The measurement of the dielectric constant showed a

deviation of 5 % due to production variances. The influence

due to the variation of the material was in the same range.

For this reason the resin content was determined by using a

high-temperature furnace to glow out the resin.

The observation of the thermal behaviour was done with

warm-up test. The evaluation of the results showed that the

highest temperatures were recorded at the VPI FAB

material having the highest electrical load.

The best results concerning voltage endurance was also

recorded at the VPI bars, the FAB bars showed better

lifetime behaviour with respect to the applied field strength.

ACKNOWLEDGMENT

The authors gratefully acknowledge the manufacturer of

the mica insulation system, Isovolta, System Development

High Voltage, for the support and production of the model

generator bars.

REFERENCES

[1] Ladsttter W., Marek P., Grubelnik W., Senn F.: New InsulationTechnology impacts Generator Design, Power-Gen InternationalConference 2006 Orlando

[2] Senn F., Ladsttter W., Grubelnik W., Marek P.: Improved Mica Insulation System for HV Rotating Machines, DISEE 2006,International Conference, Slovakia

[3] LDV-6, www.ldic.de[4] www.FLIR.com[5] Sumereder C., Weiers T., Significance of Defects Inside In-Service

Aged Winding Insulations, IEEE Trans. On Energy Conversion, Vol.23, No. 1, March 2008, p. 9-14

[6] Sumereder C., Statistical Lifetime of Hydro Generators and FailureAnalysis, IEEE Trans. DEI, Vol.15, No.3, June 2008

thermal and lifetime behavior of innovative insulation systems for rotating machines

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