book

Dielectric Phenomena in High Voltage Engineering (1915) — part 5 of 12

1 January 1915

12.0

1.0

0.394

22.0

1.5

0.591

31.5

2.0

0.787

41.0

3.0

1.181

59.0

4.0

1.575

76.0

75.0

5.0

1.969

91.0

89.0

6.0

2.362

105.0

102.0

7.0

2.756

118.0

112.0

8,0

3.150

130.0

120.0

9.0

3.543

141.0

128.0 .

10.0

3.957

151.0

135.0

12.0

4.72

167.0

147.0

15.0

5.91

188.0

160.0

17.5

6.88

201.0

168.0

20.0

7.87

213.0

174.0

DIELECTRIC PHENOMENA

correction is quite simple. Fig. 94 gives typical sphere-gap curves for both spheres insulated and for one sphere grounded. Tables XXI to XXV give spark-over curves for 6.25, 12.5, 25, 50 and 100 cm. spheres at sea level (25 deg. C. — 76 cm. barometer).

5 = 1.

Table XXIII. — Sphere Gap Spark-over Voltages

26-cm. Spheres

Spacing

Kilovolta eflPeotive

Cm.

In.

Non-grounded

Grounded

0.5 ,

0.197

1.0

0.394

1.5

0.591

2.0

0.787

2.5

0.983

3.0

1.181

4.0

1.575

5.0

1.919

6.0

2.362

112

110

7.5

2.953

135

132

10.0

3.937

171

166

12.5

4.92

203

196

15.0

5.91

230

220

17.5

6.88

255

238

20.0

7.87

278

254

22.5

8.85

297

268

25.0

9.83

314

280

30.0

11.81

339

300

40.0

15.75

385

325

Calculation of Curves, — The gradient or stress on the air at the sphere surface, where it is greatest, is found mathematically

g = ^f kv./cm.

(13)

Where e is the applied voltage in kilovolts, X is the spacing in centimeters, / is a function of X/R and R is the radius of the sphere in centimeters.

SPARK-OVER

Table XXIV. — Sphere Gap Spabk-ovbb Voltages

50-cm. Spheres

Spacing

Eilovolta effective

Cm.

In.

Non-grounded

Grounded

0.787

40.0'

1.575

76.5

2.362

115.5

112

3.150

149.0

145

3.937

189.0

185

4.72

224.3

220 '

5.51

255.5

250

6.30

285.0

275«

7.87

335.0

320

9.83

393.0

377

11.81

445.0

420

13.80

493.0

456

15.75

537.0

489

17.72

573.0

516

19.19

605.0

541

21.65

633.0

561

23.62

660.0

579

25.60

684.0

594

27.56

705.0

608

29.55

725.0

619

^ These values are calculated.

' Spacings above 16 cm. are calculated.

Then

9* = ^f kv/cm.

where e» is the spark-K)ver voltage and g» is the apparent strength of air.

/ is found mathematically and tabulated on page 27, for the non-grounded and grounded cases. For the non-grounded case we have found experimentally that ^„ the apparent surface gradi- ent at spark-over, increases with decreasing radius of sphere, as 0, for corona on wires increases for decreasing radius of wire.

DIELECTRIC PHENOMENA

Table XXV. — Sphere Gap Spark-oveb Voltages

100-cm. Spheres These values are calculated

Spacing

Kilovolta effective

Cm.

In.

Non-grounded

Grounded

1.0

0.394

3.0

1.181

5.0

1.969

100

10.0

3.937

195

15.0

5.91

283

280

20.0

7.87

364

360

30.0

11.81

520

505

40.0

15.75

650

615

50.0

19.69

770

730

60.0

23.62

870

810

70.0

27.56

956

895

80.0

31.50

1044

956

00.0

35.43

1107

1010

100.0

39.37

1182

1057

110.0

43.35

1238

1090

120.0

47.20

1290

1133

130.0

51.20

1335

1160

140.0

55.70

1378

1189

150.0

59.10

1412

1212

For a given size of sphere, g, is practically constant, independent of spacing, between the limits of X = 0.54/fi and X = 2/2. The average gradient between these limits of separation is

g»

(a)

= 27.2/ 1 H ^Jkv./cm. max.

(0 54\ 1 H — yrr ikv./cm. efif. sine wave.* (6)

The maximum variation from the average between the limits may be 2 per cent. When X is less than 0.64Vfi> g» increases very rapidly because the spacing is then less than the ''rupturing energy

1 F. W. Peek, Jr., "Law of Corona III," A.I.E.E., June, 1913.

SPARK-OVER 93

distance."^ Above X = 3/2, g, apparently gradually increases. This increase seems only apparent and due to the shanks, sur- rounding objects, etc., better distributing the flux or lessening the flux density. When both spheres are insulated and of practical size, the change is not great within the prescribed limits. In this case the neutral of the transformer should be grounded so that spheres are at equal and opposite potential. When one sphere is grounded, however, this apparent increase of gradient is very great if the mathematical /, which does not take account of the effect of surrounding objects, is used. For this reason / was found experimentally, assuming g, constant within the limits, as it is in the non-grounded case, and finding values of fo correspond- ing to the different values of X/R. Any given value of the ratio X/R should require a constant /a to keep g, constant independent of R, This was found to check.' The curves may be approximately calculated thus:

X (non-grounded) ,. « >

^' " ^* / effective sine wave. ^^^^^

e :=a — (grounded) ..«,.

• ^* fo effective sine wave. ^^^J

Where g» is calculated from the equation (6), and / or fo are found from the table on page 27 for the given X/R. These equations have been given for theoretical rather than practical reasons. Curves should be calculaied only when standard measured curves can- not be obtained. Measured curves are given here. The average error, however, for curves calculated from the above equations, for 2-cm. diameter spheres and over, should not be greater then 2 per cent. The accuracy of calculations is not as great as in the case of the starting point of corona on wires.

The Effect of Air Density or Altitude and Temperature: Cor^ rectum Factor. Practical Application, — We have found that the average gradient for various air densities may be expressed

g, = 27.25(1 H — ^)kv./cm. max.

i^-m

g, = 19.35(1 H ^F=)kv./cm. effective.

where 5 is the relative air density. (See page 61.)

IF. W. Peek, Jr., "Law of Corona III," A.I.E.E., June, 1913.

*fo was determined with the grounded sphere 4 to 5 diameters above ground. In practice, this may vary from 4 to 10 diameters without great error. See Table XXXIV. Voltage values in tables correspond to 4 to 5 diameters above the ground for this case.

DIELECTRIC PHENOMENA

The standard curve may be made to apply to any given altitude by multiplying the standard curve voltage at different spacings by the correction factor thus

ei = e

= ey/~b

19.35( 1 +

{

0.54 \

l.3(

19.3( 1 +

0.54 \

y/Rl

Vag + 0.54

y/R +0.54

= ea

Table XXVI

, Approximate corresponding altitude

Barometer

Values of a at

25 deg.

C. for standard spheres of the

Cm., Hg

In.. Hg

following diameter, cm.

Ft.

6.25

12.5

25.0

37.5

50.0

75.0

100.0

76.00

29.92

1.000

500

74.58

29.36

0.981

0.980

0.979

1,000

73.14

28.79

0.964

0.963

0.962

0.961

0.960

1,500

71.77

28.25

0.948

0.946

0.945

0.944

0.943

0.942

2,000

70.42

27.72

0.932

0.929

0.927

0.926

0.925

0.924

2,500

60.09

27.20

0.916

0.913

0.911

0.909

0.908

0.907

3,000

67.74

26.67

0.902

0.899

0.897

0.895

0.893

0.892

0.891

3,500

66.51

26.18

0.887

0.884

0.882

0.880

0.878

0.876

0.875

4,000

65.25

25.69

0.873

0.870

0.867

0.865

0.863

0.861

0.860

4,500

64.01

25.23

0.859

0.855

0.852

0.850

0.848

0.846

0.845

5,000

62.79

24.72

0.845

0.841

0.838

0.835

0.833

0.831

0.830

6,000

60.45

23.80

0.817

0.812

0.808

0.805

0.803

0.801

0.800

7,000

58.22

22.93

0.791

0.786

0.782

0.779

0.776

0.774

0.772

8,000

56.03

22.05

0.765

0.759

0.754

0.750

0.748

0.746

0.744

9,000

53.84

21.20

0.739

0.733

0.728

0.724

0.721

0.719

0.717

10,000

51.85

20.41

0.716

0.709

0.703

0.698

0.694

0.692

0.690

12,000

48.09

18.93

0.669

0.661

0.656

0.651

0.647

0.644

0.642

15,000

42.88

16.88

0.606

0.596

0.589

0.585

0.580

0.577

0.675

In order to avoid the trouble of calculating in practice, the factor is tabulated in Tables XXVI and XXVII. This correc- tion is very accurate. Table XXVI gives the correction factor for different sizes of spheres at different barometric pressure and at constant temperature. When the voltage strikes across a

SPARK-OVER

given gap the voltage, e, corresponding to the gap is found from the standard curve and multiplied by the correction factor a, or a curve may be plotted corresponding to a given barometric pressure.

Thus ei = ea

Table XXVII gives the correction factor for various values of 5. d may be calculated for the given temperature and barometric pressure and correction factor then found from the table. Fig. 95 gives the standard curve for the 25-cm. sphere (non-grounded) (25 deg. C, 76 cm. bar. pressure) and curves calculated therefrom for 25 deg. C. and various barometric pressures.

Table XXVll. — Calculated Values of a for Diffe

RENT VaL

UES OF 6

VdR + 0.54

Vr +0-54

Relative

Values ol

air density

Diameter of standard

spheres in i

pm.

6.25

12.5

25.0

37.5

50.0

75.0

100.0

0.50

0.547

0.535

0.527

0.522

0.519

0.517

0.516

0.55

0.594

0.583

0.575

0.570

0.567

0.565

0.564

0.60

0.640

0.630

0.623

0.618

0.615

0.613

0.612

0.65

0.686

0.677

0.670

0.665

0.663

0.661

0.660

0.70

0.732

0.724

0.718

0.714

0.711

0.709

0.708

0.75

0.777

0.771

0.766

0.762

0.759

0.757

0.756

0.80

0.821

0.816

0.812

0.809

0.807

0.805

0.804

0.85

0.866

0.862

0.859

0.857

0.855

0.854

0.853

0.90

0.910

0.908

0.906

0.905

0.904

0.903

0.902

0.95

! 0.956

0.955

0.954

0.953

0.952

0.951

1.00

1.000

1.05

1.044

1.045

1.046

1.047

1.048

1.049

1.10

1.092

1.094

1.095

1.096

1.097

1.098

Experimental Determination of the Effect of Air Density, — The equation for the air density correction factor was determined by an extensive investigation of the spark-over of spheres in a large wooden cask arranged for exhaustion of air. This cask was built of paraflSned wood and was 2.1 meters high by 1.8 meters in diameter inside. (See Fig. 96.)

DIELECTRIC PHENOMENA

Tests were made by setting a given size of sphere at a given spacing, gradually exhausting ttie cask, and reading spark-over

->.•

//.

'''rT'

K'^^Z'

[//

'/^

1 1

4 IE U to

voltage at intervals as the air pressure was changed. (Tem- perature was always read, but varied only Iwtwcen 16 deg. and

Fig. 96.^Cask for the study of the vnriatiriH of spark-uver and corona vohages with air pressure.

21 deg. C.) This was repeated for various spacings on spheres ranging in diameter from 2 cm. to 25 cm. At the start, the

SPARK-OVER

possible effect of spark-overs on the succeeding ones in the cask was investigated and found to be nil or negligible. A resistance of 1 to 4 ohms per volt was used in series with the spheres. Wave shape was measured and corrected for. Voltage was read on a voltmeter coil, by step-down transformer and by ratio. Pre- cautions were taken as noted in other chapters.

In order to illustrate the method of recording data, etc., a small part of the data for various spheres and spacings is given in Tables XXVIII to XXXII. Considerable data are plotted in curves, Figs. 97 to 99. The points are measured values. The drawn lines are calculated by multiplying the voltage values from the standard curves at 5 = 1, by the correction factor.

Tabus XXVIII. — Sphere Gap Spark-over Voltages and Gradient

2.54-cm. Spheres. Non-grounded Barometer 75.5

Spacing

Temp.

ProM.f cm. of Hg

Relative

air density

Kv.

g» measured

Cm.

In.

Eff.

Max.

E£f.

Max.

0.635

0.25

15^

75.5 72.6 70.5

68.4 65.8 62.6

60.0 56.7

54.8

52.6 50.5 47.6

45.4 42.4 40.0 36.8

1.028 0.990 0.957

0.930 0.895 0.852

0.818 0.776 0.745

0.717 0.686 0.649

0.618 0.576 0.544 0.500

15.7 15.3 14.9

14.4 14.0 13.6

13.2 12.5 12.1

11.8 11.3 10.7

10.4 9.8 9.3 8.7

22.2 21.6 21.1

20.4 19.8 19.2

18.7 17.2 17.1

16.7 16.0 15.2

14.7 13.8 13.1 12.2

29.2 28.5 27.7

26.8 26.0 25.3

24.6 23.2 22.5

22.0 21.0 20.0

19.3 18.2 17.2 16.1

41.3 40.3

39.2

»•••••

38.0

36.8

35.7

34.8

32.0

31.8

31.1

29.8

28.3

27.4

25.8

24.4

22.8

DIELECTRIC PHENOMENA

Table XXIX. — Sphere Gap Spark-over Voltagbb and Gradients

5.08 Spheres. Non-grounded

SiHtoiiig

Relative

air density

Kv.

Qt measured

Cm.

In.

Effective

Maximum

Effective

Maximum

5.08

1.018

78.4

111.0

27.4

38.9

5.08

0.980

76.6

107.0

26.6

37.6

6.08

0.944

73.1

103.6

25.6

36.2

6.08

0.903

70.7

100.0

24.8

36.0

6.08

0.872

68.7

97.2

24.0

34.0

6.08

0.836

64.0

90.6

22.4

31.7

6.08

0.798

63.6

90.0

22.3

31.5

6.08

0.764

61.1

86.4

21.4

30.2

5.08

0.726

68.9

83.3

20.6

29.2

6.08

0.682

56.1

80.5

19.6

28.2

5.08

0.664

54.0

7er.4

18.9

26.8

6.08

0.618

51.5

73.0

18.0

25.6

6.08

0.678

49.0

69.3

17.2

24.3

5.08

0.644

45.8

64.7

16.0

22.6

5.08

0.610

43.3

61.2

15.2

21.4,

Table XXX. — Sphere Gap Spark-over Voltages and Gradients

12.5-cm. Spheres. Non-grounded

Spacing

Relative

air density

Kv.

09 measured

Cm.

In.

Effective

Maximum

Effective

Maximum

12.7

0.982

163.0

230.0

23.1

32.7

12.7

0.951

166.0

221.0

22.2

31.4

12.7

0.917

150.0

212.6

21.3

30.2

12.7

0.880

147.0

208.0

20.9

29.5

12.7

0.846

143.6

203.0

20.4

29.8

12.7

0.807

139.6

197.6

19.8

28.0

12.7

0.780

134.6

190.0

19.1

27.0

12.7

0.736

131.0

185.5

18.6

26.3

12.7

0.699

125.0

177.0

17.7

25.1

12.7

0.666

120.6

170.0

17.1

24.2

12.7

0.637

116.5

163.0

16.4

23.1

12.7

0.698

109.6

155.0

16.5

22.0

12.7

0.561

104.6

147.5

14.8

21.0

12.7

0.541

101.0

142.5

14.3

20.2

SPARKjOVER

a .2 .S A .5 .6 .7 .8 .9 1.0

Selative Air Duiltj 6

Fig. 07. — Spark-over voltages at different air densities. (12 . 5 cm. spheres non-grounded. Figures on curves denote spacing.)

Table XXXI. — Sphere Gap Spabk-oveb Voi/rAQEs and Gradients

12.5-cir

L. Spheres.

Grounded

Spacing

Relative

air density

Kv.

qm measured

Cm.

In.

Effective

Maximum

Effective

Maximum

6.35

2.5

0.908

95.5

135.0

21.2

30.0

6.35

2,5

0.869

92.5

130.5

20.5

29.0

6.35

2.5

0.828

88.6

125.0

19.7

27.8

6.35

2.5

0.796

85.8

121.0

19.0

26.9

6.35

2.5

0.758

81.1

114.5

18.0

25.4

6.35

2.5

0.723

78.2

110.5

17.3

24.5

6.35

2.5

0.690

73.2

103.5

16.2

23.0

6.35

2.5

0.653

71.5

101.0

15.9

22.4

6.35

2.5

0.620

68.2

96.3

15.1

21.4

6.35

2.5

0.582

64.3

90.9

14.2

20.2

6.35

2.5

0.539

60.7

85.8

13.6

19.0

6.35

2.5

0.439

55.6

78.5

12.3

17.4

100

DIELECTRIC PHENOMENA

88 26 24

£16

• 12 8 10

^ 8 6

Dim

koeoo

Dbi.

ma.

ttaob Mi.

so 26

<518 Ol6

«12

■^

■—

—

(DL7 06

—

♦

—

■^

-__

«^

^^^*

—

M • 1A

"•

^ 6

0.10.20^)0.4 0.6 0.6 0.7 0.8 04»U0 2 4 6 81012 14 161820222426

Belatlre Air Deuaity 5 Dtameter of Bpliei«a cm.

Fia. 98. Fig. 99.

Spark-K>ver gradients at different air densities for several sizes of spheres.

(Points measured. Curves calculated from g^ = 19.3a (l H — ^=j j

Table XXXII. — Spherb Gap Spark-over Voltaobs and Gradients

25-cm. Spheres. Non-grounded

Spacing

Relative

air density

Kv.

Ot measured

Cm.

In.

Effective

Maximum

Effective

Maximum

7.62

1.018

139.0

196.5

22.2

31.6

7.62

0.978

133.6

189.0

21.4

30.3

7.62

0.942

129.5

183.0

20.8

29.3

7.62

0.906

126.0

178.0

20.2

28.5

7.62

0.888

121.5

172.0

19.5

27.6

7.62

0.839

115.0

163.0

18.4

26.1

7.62

0.796

111.0

157.0

17.8

25.2

7.62

0.752

105.5

149.0

16.9

23.9

7.62

0.718

101.5

142.0

16.3

22.7

7.62

0.685

96.2

136.0

15.4

21.8

7.62

0.646

91.5

129.5

14.6

20.7

7.62

0.608

87.0

123.0

13.9

19.7

7.62

0.570

81.3

115.0

13.0

18.4

7.62

0.627

74.7

105.5

12.0

16.9

7.62

0.491

70.8

100.0

11.3

16.0

SPARK-OVER

101

The calculated values check the measured values closely.

The equation for the correction factor was deduced from meas- ured values as follows:

From a former investigation it was found that at 5 = 1 the average gradient

From this investigation it was found that the average gradient at various values of i is

,/, . 0.54 \

Table XXXIII. — Average Effecttivb Rupturing Gradient for Spheres of Several Diameters and Varying Air Densities

Diameter of Spheres, cm. Surface Gradients

^/ 0.54 \ Columns marked "Calc." are from, ^« = 19.3d| 1 H 7= |

•

5.(

Meas.

Calc.

Meaa.

Calc.

Meaa.

Calc.

Meaa.

Calc.

1.00

28.7

28.5

25.8

24.0

23.4

21.9

22.2

0.90

26.2

26.1

23.7

23.3

21.8

21.2

19.9

20.1

0.80

24.0

23.7

21.3

19.6

19.1

17.9

18.0

0.70

21.3

21.2

19.0

17.4

16.9

15.7

15.9

0.60

18.7

16.7

16.6

15.2

14.7

13.6

13.8

0.50

16.1

14.6

14.3

13.0

12.5

11.6

11.7

The average measured gradients for various values of 5 are given in Table XXXIII, the calculated values from the equation are also given. The check is quite close. It should be remembered, however, that these are average values over this range of spacing and that there is a small variation at different spacings as already explained. (See Figs. 98 and 99.)

Precautions against Oscillations in Testing. — A non-inductive resistance of 1 or 4 ohms per volt should always be placed directly in series with the gap. For the non-grounded gap, one- half should be placed on each side. When one gap is grounded all of the resistance should be placed on the insulated side. One

F. W. Peek, Jr., "Law of Corona III,*' A.I.E.E., June, 1913. F. W. Peek, Jr., Discussion, A.I.E.E., Feb., 1913. 7

' • • •

•102

• • •. ; • • • • •

'DlELSCTRIC PHENOMENA

object of the resistance is to prevent oscillations from the test piece, as a partial arc-over on a line insulator, reaching the gap. Another object is to limit the current discharge. This resist- ance is of special importance when tests are being made on appa- ratus containing inductance and capacity. If there is no resist- ance, when the gap sparks over, oscillations will be produced which will cause a very high local voltage rise over parts of the winding. If sufficient resistance is used these oscillations will be damped out. This is illustrated in Fig. 100, which shows results of a test on a high-voltage transformer.

UiUUL)

Test Trana.

■AAAA^ ^ 1' cm*. ,^_

(mMM5

ismsim.

■♦n cms. ♦ •-il cmiT (US-cmi.—

jmsmsmsi)

4432 cms-* «-01 cmir-*

r^im 0.13 Ohms per 7olt I (d ■ 0.25 OhiDB per Volt

l« -Very8m«n V fa; • 1 Ohm pw Volt

FlQ. 100.

Referring to Fig. 100, the high- voltage winding of the trans- former under test is short circuited and connected to one terminal of the testing transformer. The other side of the testing trans- former is grounded. The low- voltage winding of the transformer under test is short circuited, connected to the case and ground. Voltage is gradually applied to the transformer under test until the "measuring gap" sparks over. Insulated taps, 1, 2, 3, 4, 5, are brought out at equally spaced points on the high-tension winding of the transformer under test. Auxiliary needle gaps

SPARK-OVER 103

are placed between 1 and 2, 2 and 3, and 1 and 3, to measure the voltage which appears across these sections of the winding when the main measuring gap discharges. The numbers between 1-2, 2-3, and 1-3 represent the sparking distances of the local voltages caused by a discharge of the measuring gap. Four cases are given with different values of resistance (a in the main gap. When a> = 1 ohm per volt, the local oscillations are completely damped out.

With small resistance in the gap, a 19-cm. spark-over causes a voltage to build up between coils 1 and 3 (which sparks over a 150-cm. gap), although the total applied voltage across the transformer is only equivalent to a 19-cm. gap. The apparatus may thus be subjected to strains far beyond reason, and either broken down or very much weakened. Water-tube resistance is the most reliable. A metallic resistance, if non-inductive and of small capacity, may be used. Carbon or graphite rods should be avoided as, although they may measure up to a very high re- sistance at low voltage, the resistance may become very low at high voltage by "coherer" action.

When the tested apparatus is such that there is considerable incipient arcing before spark-over, it is better to use the sphere to determine the "equivalent" ratio of the transformer at a point in voltage below the voltage at which this arcing occurs. The sphere gap should then be widened out, the spark-over voltage measured on the low-voltage side of the transformer or in the voltmeter coil, and multiplied by this equivalent ratio. It must also be remembered that resistances do not dampen out low frequency surges resulting from a short circuit, etc.

Miscellaneous Precmdions. — In making tests it is desirable to observe the following precautions:

The shanks should not be greater in diameter than one-fifth the sphere diameter. Metal collars, etc., through which the shanks extend should be as small as practicable, and not come closer to the sphere than the gap distance at maximum opening. The effect of a large plate or plates on the shanks is given in Table XXXIV. The sphere diameter should not vary more than 0.1 per cent., and the curvature, measured by a spherometer, should not vary more than 1 per cent, from that of a true sphere of the required diameter. The spheres should be at least twice the gap setting from surroundings. This is especially important if the objects are large conducting, or semi-conducting masses,

104

DIELECTRIC PHENOMENA

walls, floor, etc. Care must also be taken to so place the spheres that external fields are not superposed upon the sphere gap. This is likely to result, especially in the nongrounded case, from a large mass of resistance units or connecting leads, etc., in back of and in electrical connection with either sphere. The error may be plus or minus as indicated for the small plates in Table XXXIV. With the water tube resistance this condi- tion is not likely to obtain as the tube may be brought directly to the sphere as an extension of the shank. Great precautions

Tablb XXXIV. — Effect of Metal Plates on the Shanks. Distance TO Ground on Arc-over — op 6.25-gm. Spheres

Per Cent. Change of Voltage

Sphere

Non-grounded 6 om. diameter platee. 6.25 cm. Back of

Grounded 5 diameter platee. 6.25 cm. Back of

g&Pt oni.

Both spheres

One sphere

Insulated sphere

Grounded sphere

1.5 3.0 6.0

0.0

1.0 +2.0

0.0

-1.0 -2.0

-0.7 -1.5 -2.0

Approximate Effct of Distance above Ground When One Sphere Is

Grounded

Diameters of grounded sphere

Percentage variation from standard curves 1

;or different spacings

R "2

above ground

2-2/2

x" R

10.0
5.5

0.0

2.5
10.0

0.0

When both spheres are insulated, with the transformer neutral at the mid point, there is practically no variation in voltage for different distances above groimd.

SPARK-OVER 105

are necessary at very high voltages to prevent leakage over stands, supports, etc., and to prevent corona and brush dis- charges. Unless such precautions are taken errors will result.

Rain and Water on Sphere Surface. — figs. 101 and 102 show the effect of rain and water on points and spheres. The ratio

Fig. 102. — Spark-over voltages between 25 cm. spheres. 1, dry; 2, wet surface; 3, rain (0.25" per min.).

of dry to rain (0.2 in. per min.) spark-over voItE^e for a given spacing will averse about 2.5 for 6.25 to 50-cm. spheres.

High Frequency, Oscillations, Impulses. — At the present time a great deal is said of the effects of "high frequency" on insula-

106

DIELECTRIC PHENOMENA

tion without differentiating between continuous sinie wave high frequency, oscillations, and steep wave front impulses. This has caused considerable confusion, as the effects may be quite dif- ferent, and are all attributed to the same cause.

High frequency from an alternator, or a series of oscillations, may cause high-insulation loss, heating, and the resulting weak- ening of the insulation. Single trains of oscillations, or single impulses, may not produce heating. Energy, and therefore definite finite time, is required to rupture insulation. For a single impulse, or oscillation, where the time is limited, a greater vol- tage should be required to break down a given insulation than for continuous high frequency or for low frequency. Experiments

80 28

26 24 22

5» |l8 «16

3 10

8 6

4 2

*■

. u

.1 .2 .8 .4 .5 .6 .7 .8 .9 1.01.1 Spacing cm.

Fig. 103. — Sphere gap spark- over voltages at 60 cycles and 1000 cycles. 5,08 cm. spheres.

.1 .2 .8 .4 .6 .6 .7 .8 .9 1.0 1.1

Spaclof Mn.

Fig. 104, — Sphere gap spark- over voltages at 60 cycles and 1000 cycles. 12 . 5 cm. spheres.

(Drawn curves 60 cycles. Points 1000 cycles.)

bear this out. High local voltages may result from high fre- quency, oscillations, etc. The flux may lag behind the voltage in non-homogeneous insulations. (This does not apply to air.) For a given thickness of a homogeneous insulation, and when heating does not result, a greater oscillatory or impulse voltage of short duration is generally necessary to cause puncture than at 60 cycles.

Some of the effects will now be discussed:

Frequency, — Over the commercial range there is no variation in sphere-gap voltages due to frequency. Figs. 103 and 104 show spark-over curves up to 25 kv. at 1000-cycle sine wave from an alternator. The voltage was measured by a static voltmeter

i

SPARK-OVER

107

calibrated at 60 cycles. The drawn curve is the 60-cycle curve, the points are measured values. Fig. 105 gives a 60-cycle curve, and also a 40,000-cycle curve from a sine wave alternator. The voltage in this case was measured by a static voltmeter. No special care was taken to polish the sphere surfaces. At low frequencies, at rough places on the electrode surface, there is local overstress; but even if the air is broken down, the loss at these places is very small and the streamers inappreciable. At continuous high frequency, say 40,000 cycles, a local breakdown at a rough point probably takes place at very nearly the same gradient as at 60 cycles, but the energy loss after the breakdown at this point oc- curs may be 1000 times as great. This forms a needle- like streamer which increases the stress and local loss. Spark-over then takes place from the "electric needle" at a lower voltage than the true sphere-gap voltage; thus it seems that the air at high fre- quency of the above order is only apparently of less strength. These " electric needles " when once formed may be blown to different parts of the sphere surface. The corona starting point appears to take place at a lower voltage at high frequency, because the local loss at rough points, which occurs before the true critical voltage is reached, is very high at high frequency and distorts the field and masks the true starting voltage. The loss at rough points starts at the same voltage at low frequency but is inappreciable and cannot change conditions. If the sphere surfaces are very highly polished it seems that the high-frequency spark-over voltage should check closely with the 60-cycle voltage. This should also apply for corona on polished wires. The following limitation, however, applies to both cases. At continuous high frequency when the rate of energy or power is great, frequency may enter into the energy distance equation thus

A^\ 4> (J)

80 28

3wl

24 .22 |20

5 10 "^14

a 12

10 8 6 4 2

■i

r *

/ "

0.1 0.2 04)0.4 0.5 0.6 0.7 0.80.9 1.0 la 1.2 L8 Spacinf cm.

Fia. 106. — Sphere gap spark-over volt- ages at 60 cycles and 40,000 cycles.

108 DIELECTRIC PHENOMENA

and spark-overs take place at lower voltages at very high fre- quency. This is more fully discussed in Chapter VIII.

Destruction of insulation by high frequency, when heating does not result, is due to local overvoltage. For instance, low high-frequency voltage may be applied to a piece of apparatus containing inductance and capacity, as a transformer. On ac- count of the capacity and inductance, very high overvoltage may be built up and breakdown result due to overvoltage. The petti- coat of an insulator may be broken down by an "electric needle," forming as described above, bringing the total stress on the thin petticoat. The sphere curves check very closely for oscillatory voltage of short duration and voltages of steep wave front, even under ordinary conditions of surface. With needle gaps the results are quite different at high and low frequency. At continuous high frequency the needle point becomes hot, due to the loss, and spark over takes place at low voltage. For impulses, the opposite is true. The sphere gap is thus the most reliable means of measuring oscillatory voltages, and voltages of steep wave front. When used for continuous high frequency the surfaces must be kept highly polished.

Dielectric Spark Lag, Steep Wave Front, and Oscillations. — A given voltage is required to spark over a given gap when the time is not limited. The time necessary to supply the rupturing energy to the gap or surface depends upon the shape and length of the gap or surface, and the initial condition — as somewhat on^ initial ionization, rate of application of voltage, etc. There is a given minimum time at which the rupturing energy may be supplied at any voltage. If the time is limited, as by steep wave front, a higher voltage is required to accomplish the same results.

If the voltage required to spark over a given surface or gap is measured at 60 cycles and at steep wave front, it is found that the latter voltage is higher. The ratio of impulse spark-over voltage to the 60-cycle spark-over voltage is here termed the "impulse ratio." Spheres measure very closely the actual volt- age, even at steep impulses. Test results made by the author showing the increase in voltage for the needle gap are given in Table XXXV. The impulse used was not steep enough to appreciably affect the sphere. The impulse voltage was in- creased until spark-over took place on the test piece. The voltage was then measured by the sphere (not in multiple with test piece). Note the very high impulse voltage required to

^ See page 198.

SPARK-OVER

Tablb XXXV. — Impulse Ratio of Needle Gap

109

Spacing of needles

Voltage required to cause needle gap to spark over

Ratio impulse to

(om.)

80 cycles (max.)

Impulse measured by

25-cm. spheres (dia.)

(max.)

60 cycles, voltage

4.3

8.0

12.0

25.0

43.1

64.8

85.0

134.0

62.0

96.1

140.0

260.0

1.45 1.49 1.65 1.94

Table XXXVI. — Impulse Ratio of 6.25-cm. Sphere (Compared to 25-cm. Sphere)

6.26-cm. diameter

Voltage required to cause 8.25-cm. sphere to spark ovor at gaps greater than the diameter

Ratio impulse to

sphere gap, om.

60 cycles (max.)

Impulse measured by

25-cm. sphere (dia.)

(max.)

60 cycles, voltage

127

134

1.05

144

152

1.05

154

160

1.04

156

168

1.06

168

180

1.08

180

198

1.10

spark over a needle gap compared to the 60-cyole spark-over voltage. This indicates that the needle gap is very slow. A spark-over test for oil is also given at 60 cycles and for impulse. (See Table LXIX.) The impulse voltages show that oil requires greater energy than air to cause rupture.

Care must be taken when measuring the spark voltage on one gap by another gap in parallel ; for instance, if a needle gap is set so as to just spark over when voltages of steep wave front or high frequency oscillation of a given value are applied^ and a sphere gap is similarly set, and these two gaps are then placed in parallel , and the same impulse voltage applied, apparent discrepancy re- sults. Spark-over will take place across one gap and not the other, even when the spacing on the non-sparking gap is decreased. This will be noticed in all cases where electrodes of different shapes are placed in parallel. When the impulse voltage is applied across the "fast" gap and the "slow" gap in multiple, the "fast"

110

DIELECTRIC PHENOMENA

gap will spark over, relieving the stress, before the ''slow" gap has time to act, even though the slow gap is set for a much lower voltage. The difference would not be apparent at commercial frequencies.

Table XXXVII. — Impui^b Ratio of Nbbdlb Gap (Impulse Circuit, Fig. 106)

Voltace required to cause needle gap to spark ovor

spacing of needle (cm.)

60 cycles (max.)

Impulse measured

by 2ft-cm. spheres

(max.)

Impulse calculated from circuit con- stants (max.)

Ratio impulse to 80 cycles, voltage

2.30 3.19 3.95 4.44 5.34 10.00

27 35 41 45 50 76

32.5 45.5 55.0 65.0 73.0 133.0

32.5 46.0 56.0 64.0 74.0 135.0

1.20 1.30 1.35 1.40 1.45 1.75

An example of the impulse used by the author to obtain the data in Table XXX Vll is shown in Fig. 106. ^ It reaches its

maximum in 5 X 10^^ seconds and approximates a single half cycle of a 5(K),000-cycle wave. The calculated values of these voltages check approximately with those measured by the spheres. It thus appears that the sphere voltages are very little affected, even at this steep impulse.

The needle requires the maxi- mum time of any gap as con- siderable air must be " ionized " before spark-over can result. The spheres are very little affected because corona does not precede spark-over, and the discharge is small and confined to a short path. The line insulator is generally designed so that at commercial

^ Impulse takes place at I due to discharge of condenser C, through R, L, and power arc at A. (See page 162.) ' One micro-second ^one millionth of a second.

0.2 0.4 0.6 0.8 Time-Mlcro-Seconds

Fio. 106. — Impulse voltage and cir- cuit producing it.*

SPARK^VER 111

frequencies the spark-over voltage is lower than the puncture voltage. By applying a sufficient number of impulses at vol- tages higher than the 60-cycle puncture voltage, such an insulator may be punctured. During the time that the air is breaking down, this overstress produces small cracks in the porcelain. The effect is cumulative. The cracks gradually extend and puncture results. This subject is, hence, of great importance. In making spark measurements it is necessary to consider the spark lag, as otherwise totally erroneous conclusions may be reached. It is therefore necessary in making spark-over measure- ments on certain apparatus to guard against this. For instance, on line insulators there is generally considerable corona discharge long before spark-over. This discharge may cause oscillation which will affect the measuring gap and cause a lower voltage to arc over given distances than normal, or cause the gap to cause higher voltages than really exist. This is eliminated in practice by the use of high resistance in the measuring gap.

Another investigation of impulse voltages, indicating that time and energy are necessary to rupture insulation,^ shows that "the disruptive discharge through a dielectric requires not merely a sufficiently high voltage, but requires a definite minimum amount of energy.

''The disruptive discharge does not occur instantly with the application, but a finite, though usually very small, time elapses after the application of the voltage before the discharge occurs. During this time the disruptive energy issupplied to the dielectric."

The disruptive energy of oil seems to be about 30 times greater than it is for air. This is further discussed in Chapter VI.

Effect of Altitude on the Spark-over Voltages of Bushings, Leads, and Insulators. — For non-uniform fields, as those around wires, spheres, insulators, etc., the spark-over voltage decreases at a lesser rate than the air density. The theoretical reasons for this have been given, as well as the laws for regular symmetrical electrodes, for cylinders and spheres.

It is, however, not possible to give an exact law covering all types of leads, insulators, etc., as every part of the surface has its effect. The following curves and tables give the actual test results on leads, insulators and bushings of the standard types. The correction factor for any other lead or insulator of the same

^ Hayden and Steinmetz, A.I.E.E., June, 1910.

112

DIELECTRIC PHENOMENA

type may be estimated with sufficient accuracy. When there is doubt h may be taken as the maximum correction. It will gen- erally be advisable to take b because the local corona point on leads and insulators will vary directly with 5. This is so because the corona must always start on an insulator in a field which is locally more or less uniform.

The tests were made by placing the leads or insulators in the large wooden cask, already referred to, exhausting the air to approximately 5 = 0.5, gradually admitting air and taking the spark-over voltage at various densities as the air pressure in- creased. The temperature was always read and varied between 16 and 25 deg. C.

Provenance

Shelf: Reference library
Book: Dielectric Phenomena in High Voltage Engineering (1915)

Author: F.W. Peek Jr.
Rights: Published in 1915, before 1929, and therefore in the public domain in the United States.
Collected By: StanBot reference library