where 6 = the voltage to

neutral.

8 = distance between conductor centers. •

r = the radius.

When r and s are in centimeters and e is in kilovolts, g is ex- pressed in kilovolts per centimeter. If eo, which has been called the disruptive critical voUage, is taken for e,

r log, s/r

Qo then is the stress at the conductor surface corresponding to eo, and will be called the disruptive gradient, to distinguish it from the visual gradient gv. Values of go for wires and cables taken under a variety of conditions on the outdoor line are given in Tables LVIII and LIX. These values are corrected to standard tem- perature and pressure by dividing by 6.

• F. W. Peek, Jr., High Voltage Engineering Journal, Franklin Institute, Dec, 1913.

138

DIELECTRIC PHENOMENA

Table LVIII. — Disruptivb Crii^cal Voltage Gradient por Wises (Values Corrected to 76 cm. Barometer and 25 deg. C, Outdoor Line)

Teat No.

Spacing, cm.

Radius, cm.

g. kv./cm. max.

Per cent.

variation

from mean

Per cent.

variation

max. to min.

91

152.0 229.0 410.0 550.0

122.0 183.0 244.0 366.0 488.0

91.4 183.0 275.0 397.0

91.4

183.0

• 214.0

275.0

0.084

31.3 31.6 29.1 36.5

94

92

95

0.164

5.8

134

Avg. =30.9

28.8 27.1 29.0 25.7 25.3

7.9

135

136

137

138

0.259

7.0

125

Avg. =27.2

.28.7 26.5 26.0 26.2

12.7

126

127

128

0.463

6.7

122

Avg. = 26.9

28.7 30.4 30.5 31.0

9.4

123

120

124

4.8

Avg. =30.1

7.6

Total Avg. 29 .

If the values of Co for standard line A (1.8 cm. seven-strand cable) are examined it is found that the average value of go is 25.8 kv. per centimeter maximum. For cables between 0.583 cm. and 1.18 cm. in diameter and various spacings, the average value of go is 25.6 kv. per centimeter maximum, or, in other words, go is constant for commercial sizes of cables, at practical spacings, and is 25.6 kv. per centimeter maximum. In determining the value go for seven-strand cables r was taken for convenience as the outside radius. Hence the above go is not the actual go as ob- tained for wires, but is an apparent go. The actual go may be

CORONA LOSS

139

obtained by taking some mean radius ft between the outside radius r and the radius to the point of contact of the outside strands ri. rt approaches r in value as the number of strands is increased.

Table LIX. — Disruptive Critical Voltage Gradient por Cables (Values Corrected to 76 cm. Bar. and 25 deg. C, Outdoor Line)

Test No.

Spacing, cm.

RadiiiB, ezn.

g. kv./cm. max.

Per cent.

variation

from mean

Per cent.

variation

max. to min.

73

91.4 152.0 244.0 310.0 432.0

91.4

91.4

183.0

275.0

366.0

310.0 310.0 310.0 310.0 310.0 310.0 310.0 310.0 310.0 310.0 310.0

0.292

26.5 24.0 23.9 23.9 25.0

80

1

79

77

82

0.476

7,3

100

Avg. =24.7

25.5 26.2 26.0 26.4 28.1

9.8

115

116

,

117

•

118

Line A 0.590

6.4

18

Avg. = 26.4 — 1. 2. 3. 4 25.5 26.0 25.3 25.8 26.5 26.1 25.7 26.0 25.8 25.1 26.0

9.3

36

•

37

84

t

101

103

104

105

109

119

119

Total

2.7 5.5

Avg. =25.8 Avg. =25.7

5.3

8.1

The values of Qo for wires varying in diameter from 0.168 cm. to 0.928 cm. and for spacings from 90 to 600 cm. are constant within the limits of experimental error. This more than covers the commercial range. The mean max. value is:

00 = 29 kv. per cm. at 25 deg. C, and 76 cm. barometric pressure.

140

DIELECTRIC PHENOMENA

Considerable variation should be expected in Qo values obtained on a long outdoor line, due to

1. Necessarily imperfect conductors, kinks, etc., in an outdoor line of this length.

2. Progressive change in the value of successive points on a given curve due to slight changes of wave shape, etc., as the vol- tages are increased, and the apparent shift of Co,

The close agreement of Qo for wires is for the above reasons remarkable.

Discrepancies due to progressive change are not to be expected for standard line A to any great extent as the conductor spacing was always the same, and test conditions were kept as nearly constant as possible.

M

8

ou 40 90

V

Om

2:8.8

Av»

X

X

ft

\vir

ao

10

9*

«.»

o ■

^-1

'

,

1

I

( 1

i

4

.

S

1

s

f

7

Badlua la em9

FlG. 128.— . 90 for wires and cables. (Data from Tables LVIII and LIX.)

The above data shows that go may be considered constant for diameters of conductors and spacings within the practical range. This immediately suggests that Qo is the actual rupturing gra- dient of air or identical with Qo of the expression for visual corona. This was further investigated over a greater range on the indoor line.

Qo cables

• — ,- - = rrio

Qo wires

where mo is a fraction which approaches unity as the irregularity of the surface is reduced, or number of strands increased. Then

Co = dm„gor log* s/r (35)

(See curve. Fig. 128.) The loss equation may now be written

P =

241

(/ + 25)\A7s(e - Coy 10-'^ kw./km. (34)

CORONA LOSS

141

where p expresses the loss above the visual critical voltage e«, and Co is given in (35).

Investigations on the indoor line over a range covering small conductors show that the disruptive gradient may be considered for all practical purposes, constant when the conductor radius is greater than 0.20 cm. As the size of the conductor is decreased the disruptive gradient increases very rapidly. If the disruptive gradient for any size of conductor is called Qd we have found that this law may be expressed:

Qd

= Mr

0.30

^/^r

1

(1 + 230 r

^)

Bd = Qd r log. -

r

(36) (35a)

160 140 180 120 110 BlOO S.90

2 70 9^60

**«) 40 80 80

,

t

%

is

r'^sfe:

S^^^~~~—

Oo^^^ _.-

.1

.2 .3 .4 .6 .6 .7 .8 .9 LO

Badlui in cm.

Fig. 129. — Ck)mparison of g^, gdi and Qo. {Qd values from Table LX-)

Thus, when r is large (above 0.2 cm.) practically Qd = Qo, and where r = o, (7d == gv.

This apparent increase in disruptive gradient for small con- ductors gives the impression that the residual from the very highly ionized air resulting at each cycle in effect increases the size of the conductor. Thus, the Qd formula is the same as the g« formula with the energy distance term apparently modified or lessened by the residual ionization at each cycle. Qo results for large conduct- ors. Measured values of Qd for different conductors on the indoor line are given in Table LX. Calculated and measured values are plotted in Fig. 129.

The corona loss equation over a greater range of conductor diameter and spacing may thus be written

142

DIELECTRIC PHENOMENA

• • • 0.04 p = 241(/ + 26)/ (c - a) 10-»kw./km. (34a)

where e<j is obtained from equation (35a).

Tablb LX

Test No.

r cm.

Od maximum measured

Qd maximum calculated

12 B

0.032

67.5

70.7

13 B

0.032

70.3

70.7

14 B

0.032

71.0

70.7

18 B

0.032

71.0

70.7

15 B

0.032

71.0

70.7

16 B

0.032

71.0

70.7

19 B

0.032

69.6

70.7

17 B

0.032

71.2

70.7

20B

0.032

69.5

70.7

21 B

0.032

69.3

70.7

Avg.-70.0

27 B

0.057

51.7

•51.3

26 B

0.057

51.3

51.3

25B

0.057

51.7

51.3

24 B

0.057

51.3

51.3

Avg. =51 .6

«

SOB

0.071

45.4

45.4

33 B

0.0914

39.4

39.4

32 B

0.0914

39.4

39.4

42 B

0.105

42.2

37.6

41 B

0.105

42.2

37.6

37 B

0.105

42.5

37.6

Avg. =42.3

45 B

0.164

36.5

33.0

44B

0.164

33.6

33.0

Avg. =35.1

49 B

0.256

30.8

31.1

48 B

0.256

32.1

31.1

47 B

0.256

33.2

31.1

Avg. =32.0

56 B

0.464

31.8

30.2

55B

0.464

31.8

30.2

54B

0.464

31.5

30.2

Avg. =31.7

CORONA LOSS

143

It may be interesting to note that equation (34) may be written in terms of the gradient, thus

p = A'VTTs (/ + 25) (log ^) * Cr») (g - Qo)^ X lO"'

Fig. 130 shows measured curves plotted between g and p for a given wire at three different spacings. These curves all intersect the axis at go max. = 30, at 5 = 1.0.

10 2080406060708090 100 110 120 ISO 140 160 160 170180

Appftrent Sarfaca Oradiauk

Fig. 130. — Relation between power loss and apparent surface gradient.

Losses Near the Disruptive Critical Voltage — Co- — If the con- ductors could be made perfect no appreciable loss would occur below the visual critical voltage. However, near the starting point of corona two effects occur, which cause a deviation of the loss from the quadratic law, equation (34), and which affect the loss in opposite directions:

(a) With perfect conductors loss of power does not begin at the voltage Co, at which the disruptive gradient is reached at the conductor surface, but only after the disruptive strength of air has been exceeded over a finite and appreciable distance a from the conductor; that is, at a higher voltage e». Since the con- vergency of the lines of dielectric force is great at the surface of small conductors, with such conductors, a considerable increase of the voltage is required to extend the disruptive gradient to some distance from the conductor, and Cv is considerably higher than Co. With such conductors there would be no loss until e, were reached. The loss would then suddenly take nearly the definite value calculated for this applied voltage from equation (34). Even with polished conductors, in practice the decrease of loss below that given by equation 34 is appreciable with small conductors within the range between e© and Cv, as seen in Fig. 131.

144

DIELECTRIC PHENOMENA

2.2

2.0

1.8 1.6

1.4 0,1.2

I

§1.0

• .8 .6

.4

.2

/

r

/

/

I

7

/

1

';

1

1

7

Lsarec

Losi

J

u

1

Quad .00221

ratic

/

1

t.

y.

4

f

60 20

60

80

100 120

Between Linei

80 40 60 60

KUovolU Effective ^^ Neutral

With large conductors, however, the less convergency of the

lines of dielectric force at the conductor surface requires a less voltage increase beyond Bo to extend the disruptive gradient to some distance from the conductor; eo and e, are therefore closer together, and this decrease of the loss below the theoretical value given by equation (34) is not appreciable.

(6) As the conductor sur- face can never be perfect, some loss of power occurs at and below the disruptive criti- cal voltage at isolated points of the conductor, where irreg- ularities of the surface,

Fig 131.-Ck>rona loss for small con- scratches, spots of mud or ductors near the cntical voltage. . *• i_

No. 8 copper. Diameter, . 328 cm. dirt, etc., give a higher poten-

Length, 29,060 cm. Spacing, 122 cm. tial gradient than that corre-

Temp. 1.5. Bar., 76.6. Line B. spending to the curvature of

the conductor surface. With small conductors, this loss is rarely appreciable, since the curvature of the con- ductor surface is of the same magnitude as that of its irregu- larities. It becomes appreciable, however, for larger conductors, as seen in Fig. 132. This excess of the loss beyond that given by the quadratic law equation essentially depends on the con- ductor surface and is ^^^' ^^2* — Corona loss for large conductors near ^, , ,, ' , the critical voltage. (Data same as Fig. 115.)

the larger the rougher

or dirtier the surface is. It is a maximum at the disruptive critical

1.6

L4

1.2

1.0

I

o

.8

.6

.4

.2

/

/

1

V,'

^

uare(

ILosi

i

//

'Qum

Iratic p -.

»116

-0

*. / d

^Bxc<

'o)«

__J— <

*-^

'J^

f ^.

100 60

110

120 60

130 140 160 160 170 180 190 200

Between Llnei 70 80 90 100

KilovoltB Effeotive To Neutml

CORONA LOSS

145

voltage Cof and decreases above and below 6«, and is with fair accuracy represented by the probability curve:

p = ge

(37)

where 9 is a coefficient depending on the number of spots, and h is a coefficient depending upon the size of spots.

Snow, sleet and rain losses seem to be of the same nature, but frequently of far greater magnitude.

Equation (37) is probably of no practical importance as the loss is small, and q and h naturally cover a wide range of values, depending upon the condition of the conductor surface. Experi- mental values near Co are taken from Table XLIV and tabulated in Table LXI, together with, values calculated by the quadratic law. Corresponding experimental and calculated values are subtracted and also tabulated.

Table LXI.— Line A 1-2-3-4 from Table XLIV, Test 36

c« = 0.0115 Co = 72.1

KOovolts between conductors e'

Kw. exp. Po

Kw. p - 0.01 15(e - ea)'

1

Excess loss Pi - (Po - P)

log. Pi

1

(«• - e)»

120.4

0.11 0.15 0.22 0.40 0.79 1.42

0.11

0.15 0.22 0.30 0.16 0.08

2.40 2.71 3.09 3.40 2.77 2.10

146.5

130.2

49.0

139.2

6.2

150.0 150.0 165.8

0.10 0.63 1.34

8.5 54.7 11.7

Fig. 132 is plotted to a large scale to show the excess loss near 60. As this is for large conductors, Co and e^ are near together, and the effect of (Jb) predominates. In order to see if equation (37) holds, write

log. pi = log g - ft(6o- e)*

Then the curve between log pi and (Co — ey should be a straight line. This is shown in Fig. 133. Values of q and h are of the following order for Line A :

Test No.

q per cm. total conductor

h

30 105 103

3 . 19 X 10-« 2.47 X 10-« 2.74 X 10-«

-0.0220 -0.0208 -0.0304

146

DIELECTRIC PHENOMENA

Fig. 131 is plotted from values for a small smooth conductor. Here Co and 6« are far apart, and as the curvature of the conductor surface is of the same magnitude as its irregularities they do not

greatly influence the loss. The (a) effect here predomi- nates — that is, the loss near 6c is lower than that shown by the quadratic law.^

The loss between eo and 6« is unstable as it depends upon surface conditions. In aU cases the quadratic law is closely followed above c„, and on large practical sizes of con-

M

O l4 .

^

^

*^

^

«

2

5

6

7

6

1

W

125

160

Fio. 133. — Determination of equa- tion (37).

ductors with sufficient accuracy over the whole range.

Temperature and Barometric Pressure. — Values of the dis- ruptive critical voltage e© covering a considerable temperature range are tabulated in Table LXIL Correction is made to a

Table LXII. — Temperatube and Disruptive Critical Voltage e«

(Standard Line A 1-2-3-4)

Test

Temperature

Bar. cm.

e'o ky. bet. lines

e'o corr. to 76 cm.

1/e'o

Weather

No.

Wot

Dry

18

16.0

18.5

75.5

138.5

140.0

0.00715

Bright sun

15

20.0

22.0

75.2

140.0

142.0

0.00705

Cloudy sun

37

10.0

13.0

75.7

141.0

142.0

0.00705

Bright sun

36

10.0

12.0

75.0

144.2

146.5

0.00683

Cloudy

100

-3.0

-2.0

73.9

148.8

150.8

0.00663

Hazy sun

84

1.0

3.0

75.2

149.0

151.0

0.00662

Cloudy

101

-1.0

-1.0

74.7

153.8

156.8

0.00638

Cloudy

103

-4.9

-4.5

75.7

155.3

156.7

0.00638

Sun

104

-9.5

-9.5

76.2

157.0

157.0

0.00637

Sun

119

-6.5

-6.0

76.5

156.6

156.1

0.00642

Sun

105

-13.0

;-13.0

1

76.2

161.0

161.0

0.00622

: Sun

1

barometric pressure of 76 cm. on the assumption that e'o varies directly with the pressure. In Fig. 134 l/e'o is plotted with temperature. The straight line through these points cuts the temperature axis at —273 deg. C. or absolute zero. Temperature was always measured in the shade. The points that do not fall

^ This condition is likely to obtain to a greater extent at high altitudes as Co and e« are farther apart.

CORONA LOSS

147

well on the curve are the Bummer sunny day points. This is what would be expected, as the conductors were at a higher tem- perature than the temperature read.

Fig. 134 shows that the disruptive critical voltage or the dis- ruptive gradient varies inversely as the air density; The data range, however, is not great.

The density of air at 25 deg. C. and 76 cm. barometrie pressure is taken as the standard.

Humidity, Smoke, Wind. - Humidity. — ^Line A was kept ® as a standard throughout the "'' tests. A careful study of the disruptive critical volt^^ and

c* shows no appreciable e£Eect '

of either humidity or "vapor products."' (See Table LXIII.) Visual tests, made on two short parallel wires indoors over a great humidity range, also bear this out.

Table LXIII.— Stand a hd Line A.' Condijctor8 1-2-3-4

TMtta.

T™p.»tur,

bamidity

V.por

g, mduoed to

25 do.. C.. 78

om, bar.

Wet

Dry

15

20.0

22.0

0.84

0.56

18.8

IS

16.0

18.5

0.75

0.35

18,6

36

10.0

12.0

0.78

0.25

18,8

37

10.0

13.0

0.67

0.21

18.3

84

1.0

3.0

0.69

0.11

18.7

101

-1.0

-1,0

1.00

0-17

19.2

103

-4,9

-4.9

1,00

0,13

18.0

104

-9.6

-9.5

1,00

0.08

18.6

105

-13.0

-13.0

1,00

0.06

18,8

109

-2.0

-2,0

1.00

0,12

18.7

Theoretically there should be an appreciable effect of humidity

' Merahon, High Voltage Measurementa at Niagara, A.I.E.E., June 30, 1908.

• Line A was kept at constaot test conditions for use as a standard in the Btudy of varying atmospheric coDditioos, etc.

148 DIELECTRIC PHENOMENA

on Co, since even if the water vapor, which may be considered as a gas dissolved in air, has a different disruptive gradient than air, the percentage of the gas in the mixture should be too small to cause any appreciable change. It has been suggested that "vapor products" is a measure of ionization and, in that way, the critical voltage varies with vapor products. This does not seem likely, because with all ordinary atmospheric air the per- centage of ionization is so small that it would not be expected to produce any effect. To test this, the visual critical point was determined on two parallel wires. The room was then closed and the wires were run at a point very much above this critical point for about an hour, or until a very intense odor of ozone filled the room. The voltage was then removed and the surface of the wires cleaned in order to remove oxidization. The critical point was then redetermined and found to be the same, although the amount of ionized air was many times that which could be expected in free atmospheric air. Of course if the percentage of ionization were great enough, as for instance in an ozone machine, a change in the disruptive strength would then be expected.

It has been claimed that ultraviolet light reduces the sparking point. This is not borne out in tests, where any quantity of continuously applied power is involved. Though ultraviolet light, ionized air, and various radiations cause the small energy in condensers to discharge when applied over comparatively great time; in case of large energy discharge in a very short time, as spark discharge, and corona due to continuously applied voltages, no appreciable effect should be expected, or can be observed, since the discharge which takes place by ionized air is not of the same order of magnitude as that produced by the spark discharge. The time required for a given voltage to pro- duce ionic saturation or discharge, should, however, be less with high initial ionization than with low initial ionization. Hence for impulse voltages of steep wave front and short duration, the spark voltage may be affected by initial ionization. This is further discussed in Chapters IV and VIII (page 198).

It is quite probable that humidity has some slight effect on the loss after corona has once formed due to the change of the gas into vapor and the agglomeration of the water particles by the ions. This is noticed in the spark-over between needle points at very high voltage. In this case there is a very heavy brush discharge before spark-over. The sparking voltage increases

CORONA LOSS 149

with increasing humidity, due to the fog formed, when there is not a mixture of two gases, air and water vapor, as in the case where humidity is concerned, but actual water particles are in the air. Steinmetz has observed that fog actually raises the striking distance between needle points. The effect of humidity on the spark discharge observed by the author is discussed in Chapter IV. Greater loss should be expected in corona measurements during fog due to charge and discharge of the water particles. This causes loss at lower voltages and has the effect of decreasing the critical point.

While humidity has no effect on the starting point of corona, an effect might be expected on the loss after the discharge had already started. The reason that this is inappreciable is because the corona discharge from wires covers very little space.

Smoke, — It was difficult to get measurements to show the effect of smoke, as the prevailing winds were over the fields toward the city. At one time, however, during a change in the wind thick smoke was blown over the line from smoke stacks of a fac- tory, and the loss was increased. This, however, will probably not be a serious consideration in practice.

Wind. — ^Losses measured during very heavy winds show no variation from losses measured during calm weather. That the losses do increase with increasing air velocity has been shown in laboratory apparatus. There is, however, no appreciable effect due to the comparatively low air velocity in practice.

Moisture, Frost, Fog, Sleet, Rain and Snow. — During some of the first tests it was noted that the losses were sometimes greater on the "going-up curve" than on the "coming-down curve," especially in the early mornings after heavy dew. The losses became less after the line had been operated for a while at high' voltage. Fig. 135 shows this well for a conductor with a coating of frost. This excess loss was thought at first to be due to leakage through moisture on the insulators. Insulators were put up without line wires, but measurements showed a very small insulation loss even during storms. It was then concluded to be due to moisture on the conductors themselves. Visual tests made on short lengths of wet and dry cables showed this in a very striking manner. Two parallel dry cables were brought up to the critical point. Water was then thrown on the cables. What had been a glow on the surface of the dry cables now became, at the wet spots, a discharge extending as much as 5 to 8 cm.

10

150 , DIELECTRIC PHENOMENA

from the cable surface. This discharge reminded one of an il- luminated atomizer. Illustrations, Figs. 67 and 6S show this, but a greater part of the effect is lost in reproduction. The wires became quite dry and down to normal discharge after runnii^ at high volt^e for a I very few minutes. Biiov-iu ».eti.. ,j,jjg curves, Fig. 136,

Fig. 135. — Corona loss with frosty and dry . , j„_:„„ j.i,„ j„„

conductors. taken during the fog

(Conductor length, 109,500 cm. Spacing, also show the combined 310 cm Diameter. 1.18cm. 3/0,7;*trand effect of condensed cable. Line A. Temp.,-2°C, B»r..74cm.)

moisture on the cables,

and free water particles in the ear. The moisture particles on

/

/

/

1^

^

/

s

I.

1>

r

^

_1

f.-y

_

_

_

_

w

i; - - J

u L

i I-

u ^i

I ? U i ^

1 TT-Pf

, ' • 9

J 2 *'

; , -i>

I ' w

! z li

/ .1

\ ^ -J/-

'ssagag 31 sssia

(Conductor length, 109,500 ci Diameter, 1.18 cm. Spacing, 310ci

the conductor become charges and are repelled. The particles

CORONA LOSS

151

in the air also become charged and discharged thus increasing the loss very greatly above that for dry conductors.

The losses during snow and rain storms are much greater than fair weather losses at the same temperature and barometric

r

^tu.

--- -#/

100 110 120 uo uo uo uo m i£D 1)0 a» 110 m no

Fi<3. 138. — Corona Jobs during sleet storm.

(Conductor length, 100,500 cm. Diameter, I.IS cm. Spacing, 310 cm.

3/0,7-fltrand cable. Line A. Temp., - 1 .0. Bar., 75.4.)

pressure. In Fig. 137 the actual measured loss is plotted, and also a corresponding calculated fair weather loss. The difference between the two curves shows the excess loss due to snow. The effect of snow is greater than that of any other storm condition. This is because the particles are larger and a greater num- ^ bar strike the line, or come lo near the line. ^

The sleet curves are of s^ special interest. Sleet had ^o already started to form on the a ' conductors, and was still falling , when the tests were started. s Fig. 138 shows the loss curves. * After the curves were taken the line was kept at 200,000 volts for over an hour with no apparent diminution of sleet. This seems to show that sleet

will form on high-voltage trans- Temp. 10.0. Bar, 76.) misaion lines.

The day after these tests were made was bright and clear and the conductors were still coated with sleet. A set of readings was taken, and it is interesting to note that the excess loss here is as great as when sleet was falling. (See Fig. 139.)

(Conductor length, 106,500 cm. Diameter, 1.18 cm. Spacing, 310 3/0,7-strand cable. Line A.

152

DIELECTRIC PHENOMENA

The excess loss for sleet, rain or snow storms (over the fair weather loss) seems with increasing voltage to approach a maxi- mum and then to decrease again (the latter at a value very far above the disruptive critical voltage), and the curves of loss seem to have the general shape of the probability curve, as is to be expected theoretically.

The above readings show the im- portance of taking weather conditions into account in the design of high- voltage transmission lines.

Very High Frequency. — Corona losses at very high frequency are diffi- cult to measure. The curve in Fig. 140 is interesting. The drawn curve is calculated from formula 34(a); the points are measured values. Power was supplied from an Alexanderson 100,000-cycle alternator. The power ^'''' SlSueSc^^^^^ ** WM measured by adjusting reactance (Two parallel wires, and capacity until unity power factor Spacing, 67 cm. Radius^ ^g-g obtained. The watts input was wire, 0.127 cm. / = 100,000 ^, ^, j x r ix j

~. Total length, 200 cm.) then the product of volts and amperes Tests by Alexanderson. g^g measured by hot wire meters.

This good check seems to show that the formula applies over a large range, but complete conclusions cannot be drawn from this small amount of data.

600

1

/

Ourre OftlcaUted — , Points Heaaored X 1

f

400

/

,800

1

' 1

1

/

aoo

(■

j

100 60

/

y

f

%

/

14 16 1820a24M2880

Kilo-Volts SffectiTe,S«tw«eo Linos

CHAPTER VI

CORONA Aim SPARK-OVER IN OIL AND LIQUID INSULATIONS

The most common liquid insulation is transformer oil obtained by fractional distillation of petroleum. This oil has various characteristics, as flashing point, freezing point, viscosity, etc., depending upon the specific use to which it is to be put. The average characteristics of transformer oil are as follows:^

Medium

Light

Flashing temperature

Burning temperature

Freesing point

Specific gravity at 13.5 deg. C

ViscoBity at40deg.C. (Saybolt test) Acid, alkali, sulphur, moisture

180*'-190° C. 206''-216«' C. -10** — IS*' 0.865-0.870 100-1 10 sec. None

130^-140*' C. 140'»-150'» C. -15* — 20* 0.845-0.850 40-50 sec. None

Various other oils are insulators, as gasoline and cylinder oil; animal oils, as fish oil; vegetable oils, as linseed oil, nut oil, china wood oil, etc. AH of these when pure have the same order of dielectric strength.

The so-called compounds made by dissolving solid gums in oil to increase viscosity are generally unreliable, unless used dry as varnish. Under the dielectric field the dielectrics of different permittivities tend to separate, and there is considerable loss. As in air there is very little loss in pure oil until local rupture, that is, brush discharge or corona, occurs.

The dielectric strengths of oils are usually compared by noting the spark-over voltage between two parallel brass discs 1.25 cm. in diameter, and 0.5 cm. separation. The spark-over voltage for good oils in the above gap should be between 50 and 70 kv. maximum, and hence, if the voltage wave is a sine, from 35 to 50 kv. eflfective.*

» Tobey, Dielectric Strength of Oil, A.I.E.E., June, 1910.

• The breakdown voltage of oil like that of air depends upon the maximum point of the wave.

153

154

DIELECTRIC PHENOMENA

Different Electrodes. — In Fig. 141 are plotted spark-over curves for^ifferent electrodes in good transformer oil. The character- istics are very much the same as for air excepting the apparent strength is very much higher.

Effect of Moisture. — The slightest trace of moisture in oil greatly reduces its dielectric strength. The effect of moisture is shown in Fig. 142(a) for the standard disc gap. Water is held in suspension in oil in minute drops. When voltage is applied

aoo

280 260 240 220 200

/

/»

em.

8pbe

ret ( »».)

•

/

/

/

<,

10 I Co

:m. I i«c« . Dlftf on 1

d(ei

/

y

/

1

A

/

/

y

/-

•y

cm.

8ph<

rea

y

/

3

CiflO

/

/

^

/^

,/^

:<•

BdlM

Sl40 e

Bl20

)

'

/

A

/

y

r —

y

/

I

f

J

/

100 80 60 40 20

f

/

/

A

/

'/

/

(Tei

ip.8

'o)

•

1284 6678910 SpAciog cm.

Fia. 141. — Spark-over of various shaped electrodes in oil.

these drops are attracted by the dielectric field. Thus they are attracted to the denser portions of the field and may form larger drops by collision. When attracted to, and after touching a metal part, and thus having the same potential, they are imme- diately repelled. If the field is uniform the drops form in conduct- ing chains along the lines of force. It can be seen that the effect of moisture should vary greatly with the shape of the electrode, and with some shapes the moisture may even be removed from the space between the electrodes by the action of the field, in which case its presence would not be detected by low-voltage

CORONA AND SPARK-OVER IN INSULATIONS

155

breakdowns. In transformers moisture will generally be attracted to points under greatest stresa. The most effective way of removing moisture is by filtration through blotting paper. Dirt in oil may have an effect very similar to moisture and the small conducting particles be made to bridge between electrodes by the dielectric field.

Temperature. — Temperature over the operating range has very little bifluence on the strength of oil. The strength increases at the freezing point. The curve is shown in F^. 142(6). The insulation resistance is also shown. The increase in strength

(o)

(b)

"V

"-

'

with temperature is only apparent and due partly to the decreas- ing insulation resistance which allows more current to flow through the oil, which tends to even up the stress, but mostly to the drying out of moisture particles by the high temperature. The increase at freezing should be expected, as an actual improve- ment in dielectric properties results. For a perfectly dry oil the strength actually decreases with increasing temperature or den- sity, as in the case of air. The specific resistance of transil oil ia approximately: Temp., C Ohms, cm. cube.

100

250 X 10" 100 X 10'» 15 X 10"

The permittivity is approximately 2.6 times its specific gravity and thus decreases with increasing temperature.

Spark-over and Corona in Oil. — A phenomenon similar to corona in gases also takes place in liquid insulations, as oil, due to a tearing apart of the molecules. Corona in oil is not as steady or definite as in air. It appears to start quite suddenly and extend

156

DIELECTRIC PHENOMENA

much farther out from the electrode than corona in air. It is much more difficult to detiBct the starting point, and unless the conductors are very small or far apart (s/r large) corona does not appear before spark-over. For instance, in the Table LXVII when the outer cylinder is 3.81 cm. radius and the inner 0.0127cm. radius (R/r = 300), the corona and spark-over voltages are prac- tically the same (see condition for spark-over and corona, Chapter II). The absence of corona, or rather the simultaneous appear- ance of corona and spark-over, unless the wires are very small or far apart, seems to mean that the mechanism of breakdown in oil is very similar to that in air but the energy distance in oil is much greater. Thus, as the voltage is increased, corona rupture occurs out to the energy distance; this increases r to the condition for spark-over.

R = ^<t

(r + energy distance broken down) Vi

and spark follows. Therefore, the spark-over voltages and

Table LXIV. — Dielectric Strength op No. 6 Transil Oil — Spark-over

BETWEEN Spheres

Radius of spheres, em.

Needles

Spacing, cm.

• 0.169

0.555

1.27

3.12

6.25

Kv. max.

Gra- dient max. kv./cm.

Kv. max.

Gra- dient max. kv./cm.

Kv.

max.

Gra- dient max. kv./cm.

Kv. max.

Gra- dient max. kv./cm.

Kv. max.

Gra- dient max. kv./cm.

Kv.

max.

0.129 198

44.6 50.5 59.2 65.0

71.1 81.0

449 360 365 360

364 368

48.0

394

47.1

364

1 1

0.264 322

73.9

333

73.5

310

74.3

288

74.8

289

0.378 508

90.0

295

•

99.0

222

105.0 220

107.0" 214

650

41.6

0.766

1.010 1.270 1.780 2.540

3.810 5.080 7 620

89.9

97.6 104.0 111.0 124.0

145.0 168.0

353

358 361 384 416

470 542

106.0

112.0 116.0 131.0 149.0

172.0 191.0

225

192 176 171 174

184 192

117.0

187

128.0

146.0 171.0 203.0 240.0

266.0

182

166 158 137 122

103

132.0

159.0 177.0 214.0 245.0

280. 0|

180

166 147 130 110 '

90

157.0 165.0 185.0

206.0 231.0

158 143 130

131 133

67.0

84.0

108.0 124.0

166.0

10 150

203.0

Oil between standard discs 0.5 cm. apart tested 68.5 kv. max. 25 deg. C.

CORONA AND SPARK-OVER IN INSULATIONS

157

Table LXV. — Spark-over between Parallel Plates in No. 6 Tranbil

Oil

ekv. max.

X

spacing cm.

kv./cm. (max.)

Remarks

34.6

0.254 0.508 0.762

1.015 1.270 1.525

1.270 1.525 1.778

2.03 2.29

2.54 2.79

3.05 3.30

3.56 3.81

1.27 2.54

5.08 7.62

136.3

102.8

93.0

m

104.8 89.6 82.8

98.2 90.0 85.0

82.2 81.8

77.2 76.6

69.5 73.2

72.2 63.0

88.8 61.0

41.7 35.0

52.3

71.0

106.0 114.0 126.5

•

hn=.rfT'

^

l^'lliV

125.2

Distance 0.476 en

between 1.

small and large

discs "

137.5

158.0

167.7

187.3

196.5

Same as above, except distance small and large discs = 1.58 cm.

between

214.5

212.7

242.0

257.5

241.0

113.2 155.5

212.0 269.0

4-in. flat discs, 0.5-cm. radius edge.

Table LXVI. — Corona in Oil, Wire and Plate (Distance of wire from plate = 16.5 cm.)

Ky. efif.

Radius wire, cm.

0» kv./cm. eff.

0*

kv./cm. max.

50

0.025

278

393

60

0.050

185

262

80

0.0635

201

284

100

0.1520

122

173

55

0.00508

615

870

158

DIELECTRIC PHENOMENA

corona voltages up to fairly high ratios of R/r are the same, and may be used in determining the strength of oil.

Table LXVII.— Spabk-ovbb Voltages for No. 6 Tranbil Oil Con- centric Cylinders

R om.

r cm. 1

Kv. eff.

Kv. max.

Oa max. kv./cm.

1

R

r

RemarlcB

3.81

0.032

45.3

64.0

420.0

5.61

120\00

Tests made in

3.81

0.238

60.0

84.8

127.7

2.05

16.00

long cylinders

3.81

0.317

60.5

85.5

108.1

1.77

12.00

with belled

3.81

0.635

69.5

98.3

86.3

1.26

6.00

ends. Oil be-

3.81 3.81 3.81 3.81

0.794 0.952 1.111 1.270

75.0 73.0 76.7 76.0

106.1 103.2 108.5 107.5

85.5 78.1 79.4 77.0

1.12 1.02 0.95 0.89

4.80 4.00

3.00

tween standard discs 0.5 cm. apart tested 58 kv. (max.) 25 deg. C.

3.81

1.587

73.7

104.3

75.1

0.79

2.40

3.81

1.905

66.3

93.7

70.7

0.72

2.05

3.81

2.540

45.5

64.3

62.4

0.63

1.57

6.67

5.560

29.2

41.3

40.6

Tests made on

6.67

5.080

43.7

61.8

44.8

short cylinders

6.67

3.970

70.1

99.2

48.1

where field is

6.67

3.240

84.4

119.5

51.0

somewhat dis-

6.67

2.230

105.8

149.7

61.2

torted.

6.67

1.955

105.0

148.5

61.9

6.67

1.615

103.5

146.5

64.0

6.67

1.270

101.3

143.2

67.5

6.67

0.953

98.3

139.0

75.0

6.67

0.635

94.0

133.0

89.2

6.67

0.477

91.5

129.5

102.8

6.67

0.318

85.3

120.7

121.0

6.67

0.159

65.2

92.2

155.5

6.67

0.079

62.2

88.0

251.4

11.42

3.930

144.0

203.5

48.3

11.42

2.280

163.0

230.4

63.0

11.42

1.910

158.0

223.2

64.5

11.42

1.270

160.0

226.0

81.5

11.42

0.880

148.0

209.0

93.0

11.42

0.520

90.0

127.3

79.5

The strength of oil for different sizes of wire from Table LXVII is plotted in Fig. 143. The curve is similar to that for corona in air. Fig. 144 shows that a straight line relation holds approxi-

CORONA AND SPARK-OVER IN INSULATIONS

159

mately between —7= and g^. Values are not used when R/r > 3.5. Thus, as in the case of air:^

g'v = g'ol

1 +

Vr)

max.

g^ = 36(1 + --^jkv./cm.

Qv = 25.5( 1 + —V I kv./cm. effective si

sine wave.

200 180 160 IM

00

40

ao

?— —

. — i^HBM .«^aa ^M^M- ^^.^^ ^

.4 .8 U U 2.0 2.4 2.8 8.2

Badltti-em.

FiQ. 143.— Strength of oil. (Concentric cylinders — gv at surface of inner cylinder.)

aoo

180

.160

8

-^40 Sl20

100

80 60

40

20

— ^

8

L2

1.6 ^ 2.0

2.4 2.8 8.2 8.6

Fia. 144. — Strength of oil. (Method of reducing values given in Fig. 143 to equation.)

The energy distance is 1.2/r or almost four times that of air, indicating that a greater amount of energy is required to rupture oil, or a greater number of collisions are necessary before ionic

^ F. W. Peek, Jr., High Voltage Engineering, Journal Franklin Institute, Dec., 1913.

160

DIELECTRIC PHENOMENA

saturation is reached. Qo and a vary to a considerable extent in oil. The strength of oil, or the disruptive gradient, or the gradi- ent required to bring the ions up to collision velocity, seems fairly low. Oil should, therefore, have low strength in bulk, but high

TOO 600

8400

5800 •800

100

v

1

V

"V

^

SJ

■-.

h.

^

.i

«

-1

K)

1

1 1

1

1

4

BpacRiff em.

Fia. 145. — Showing increase of strength of oil at small spacings.

(Spheres, R » 3.33 cm.)

WO

04A

840

S8U

260

140 QooA

ssso

^aoo

H IDU

\

JJ 140

C lOA

\

^

o i«o

4AA

V

N^

100

QA

"^

AA

"^

^^

-.

00

40

OA

sso

A

L 1

I \

K <

i 1

5 (

J -i

' 8

; i

} 1

.0 1

1 1

2 18

Badlai*em.

Fia. 146. — Strength of oil between spheres. (Data from constant part of the curve.)

apparent strength when sub-divided or confined to make use of the large energy distance necessary to rupture.

Spark-over voltages and gradients are given for various spheres

CORONA AND SPARK-OVER IN INSULATIONS

161

at various spacings in Table LXIV. The characteristics of the curves between gradient and spacing are the same as those for air (see Fig. 42) as shown in Fig. 145. When the spacing is so small that it interferes with the energy distance the apparent strength of oil increases. At spacings above this the gradient is constant until the separation is so great that corona forms before spark-over.

Table LXVIII. — Spheres in Oil