Testing Solid Insulation of Electrical Equipment

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CONTENTS


Par.   Page
1.   Purpose   1
2.   Insulation life and deterioration   1
3.   Causes of insulation failures   1
4.   Insulation test methods   2
Insulation Resistance and Dielectric Absorption Tests
5.   Definitions   3
6.   Theory of insulation resistance and dielectric
absorption measurement   3
7.   Skill required in making insulation resistance
and dielectric absorption measurements   3
8.   Effect of previous charge   3
9.   Effect of temperature   3
10.   Effect of moisture   4
11.   Effect of age and curing   4
12.   Polarization index   5
13.   Test procedures and records   5
14.   Temperature correction factors for insulation resistance   6
Insulation Resistance Meter Tests
15.   General   7
16.   Description of test   7
17.   Test equipment   7
18.   Dielectric absorption curve   7
19.   Minimum values of machine insulation resistance   7
20.   Transformer insulation resistance   8
21.   Cable insulation resistance   8
High-Voltage DC Tests - Rotating Machines
22.   Application - rotating machines   9
23.   Correlation between dc and ac tests   9
(FIST 3-1 12/91)   i


CONTENTS - Continued


Par. No.   Page

High-Voltage DC Tests - Stepped Voltage Method

24.   Description of test   10

25.   Current versus voltage curve   10

26.   Resistance versus voltage curve   10

27.   Log-log plots and discharge curves   10

28.   Data sheet and voltage/time schedules   10

29.   Test technique   11

30.   Polarization index   11

31.   Reason for voltage/time schedules   11

32.   Applicability of test schedules   11

33.   Maximum voltage for test   12

34.   Importance of regulated power supply   12

35.   Discharge of winding after test   12

36.   Minimum period of grounding after test before

returning to service   12

37.   Effects of temperature and humidity   12

38.   Interpretation of curves   13

39.   Progressive condition of insulation and

frequency of tests   13


High-Voltage DC Tests - Ramped Voltage Method


40.   General   14

41.   Description of test   14

42.   Ramped dc test schedule   14

43.   Ramped voltage test results   15

44.   Grounding   15

45.   Discharging the winding   15





(FIST 3-1 12/91)   ii


CONTENTS-Continued


Par.   Page
High-Voltage DC Tests - Cables
46.   Cable testing    16
47.   Advantages of dc high-potential test    16
48.   Test technique    16
49.   Interpretation of test curve    16
Power-Factor Tests
50.   Application    17
51.   Theory of power-factor measurement    17
52.   Significance of dielectric losses    17
53.   Generator windings    17
54.   Transformer windings    17
55.   Cables    18
56.   Test equipment    18
Ionization Tests
57.   Application    18
58.   Test methods for slot discharge    19
High-Potential AC Proof Tests
59.   Application    19
60.   Induced-potential and impulse tests    20
61.   Voltage values for ac tests    20
High-Potential DC Proof Tests
62.   General    21
63.   Voltages for high-potential dc tests on machines    21
64.   Cables    21







iii   (FIST 3-1 12/91)


CONTENTS-Continued
Par   Page Turn-to-Turn Testing
65.   General   21
Bibliography   22
TABLES
Table   Page
I   5
II   6
A   Maximum dc test voltages for machines   27
B   High-voltage dc insulation test record   28
C   Voltage/time schedules for high-voltage dc
tests of stator insulation   30
D   Cable dc test voltage guide   31
E   Temperature correction factors for the power
factor of power transfer windings   45
F   ANSI standard test voltages for 60-hertz,
1-minute, high potential tests   46
FIGURES
Figure   P.a.g.e.
I   Continuous operating temperature vs. insulation life   53
2   Dielectric absorption curves before and after initial dry out for Grand Coulee unit L-6 108,000 kV, 120-r/min, 13.8 kV,
60-Hz generator   54
3   Insulation resistance and temperature correction
factor vs. temperature for figure 2   55
4   Temperature correction factors for insulation resistance   56
5   One- and ten-minute resistance values during dry out of
37,500 kV.A, 13.8-kV hydrogenerator   57
6   DC ramped-voltage test with unit buswork connected -
Yellowtail unit 2, 0A, February 26, 1979   58
(FIST 3-1 12/91)   iv


CONTENTS-Continued
Figure   Page    

7   DC ramped-voltage test with unit buswork not connected -
Yellowtail unit 2, 0A, March 13, 1979   58

8   Arrangement for plotting insulation test data   59

9   Log-log plots of dielectric absorption curves and discharge curves   60

10   Template analysis   61

11   Comparison of test curves between 30- and 10-minute schedules,
OB, Grand Coulee, L-8, June 28, 1960   62

12   Comparison of tests to show progress of deterioration
of winding insulation   63

13   Characteristics of very dry winding before and after varnishing   63

14   Typical stator insulation ramped-voltage test results -
new epoxy-mica insulation   64

15   Typical stator insulation ramped-voltage test results -
new asphalt-mica insulation   64

16   Typical stator insulation ramped-voltage test results -
asphalt-mica insulation with a localized weak spot   65

17   Typical stator insulation ramped voltage test results -
very old asphalt-mica insulation   65

18   High-voltage, dc test on a 1,500-kcmil varnished cambric,
250-mil (6.34-mm), lead-sheathed cable   66

19   Approximate charging kilovolt amperes of generator stators for
determining capacity of ac high- potential set required   67