book

Dielectric Phenomena in High Voltage Engineering (1915) — part 8 of 12

1 January 1915

Gradient at constant part of the curve

Data from Table LXIV

Radius, em.

Gradient, kv./om. max.

Radius, em.

Gradient, kv./om. max.

1 Vr

0.159 0.237 0.355 0.555

348 260 222 169

2.51 2.05 1.68 1.35

1.27

3.12

6.25

12.50

120 98 82 56

0.89 0.56 0.40 0.28

The rupturing gradient at the constant part of the curve for various spheres is given in Table LXVIII, and is shown in Fig. 146. This may be written approximately

g. = 28.3 f

1 +

Vr)

kv./cm. max.

The energy distance is approximately 2/B.

The spark-over voltages for concentric cylinders, where R/r< about 5, and for spheres above 2/A spacing on the constant part of the curve, may be approximated by substituting g, in the voltage formula

R e = g^r log, — cylinders

e = gt ^/spheres K

In oil, the apparent strength can be greatly improved by limiting the free energy distance by barriers, etc. This can be seen in Tables LXIV and LXV, where e/X is given for parallel planes, and gv for spheres. For the small spacings, where the free energy distance is limited, the apparent rupturing gradient is very high.

162

DIELECTRIC PHENOMENA

just as in the case of air. Strengths as high as 700 kv./cm. have been reached. Insulation barriers give an added eflfect by pre- venting moisture particles from lining up.

Care must be taken, however, that the barriers are not so placed as to increase the stress on the oil by the high permit- tivity of the solid insulation.

Transient Voltages. — Transient voltages or impulse voltages of short duration greatly in excess of the low frequency rupturing voltages may be applied to insulations without rupture. In other words, the rupture of insulation requires not only a suf-

(o)

A.C. Bnpply

I 1

■^AMA^

■^WSAAA-

Above circuit used by author. A is sparked-over by transformer voltaae. C then discharges through L, arc at At and R. Impulse appears across A, at I. Arc at A is non-osoillatory and, in effeott a switch.

Fia. 147. — "Impulse" circuits.

ficiently high voltage, but also a definite minimum amount of energy. This means also that a definite but very small time elapses between the application of voltage and breakdown. This is sometimes called the "dielectric spark lag" and has already been discussed for air in Chapter IV. The rupturing energy seems to be much greater for oil than for air, as indicated by the large energy distance in the gradient equation above. At low frequency a given definite voltage is required to cause

CORONA AND SPARK-OVER IN INSULATIONS

163

rupture during thfe comparatively unlimited time of application. This voltage is constant until the application is limited to a definite minimum time, when a higher voltage is required to accomplish the same results in limited time. Such transient voltages of short duration, and impulses of steep wave front, must not be confused with continuously applied high frequency where breakdown will generally take place at lower voltages, due to loss, etc.

In Table LXIX the relative breakdown voltages of gaps in oil, at 60 cycles, and for impulse voltages of steep wave front, are given. An impulse voltage much higher than the 60-cycle vol- tage is required to break down a given gap. If similar air and oil gaps are set to rupture at the same low frequency voltage, a much higher transient voltage will be required to rupture the oil gap than the air gap, indicating greater time. An air gap may thus protect an oil gap, but not vice versa. See page 108.

Table LXIX.-

—Comparison of 60-ctclb and Impulsb

Spark-over in

Oil and in Air

Air

Gap

spacing, cm.

Kv. 60

cy., max.

Kv. im- pulse, max.

Gap

Spacing, cm.

Kv. 60

cy., max.

Kv. im- pulse, max.

Stand, disc

0.6

66.6

170.0

2/0 needles...

1.0

60.2

103.3

2/0

6.1

60.2

67.8

2.0

68.8

167.0

needles

17.5

108.0

198.0

3.0

89.2

233.3

4.0

108.0

321.0

0.26

37.2

117.3

2.64-cm.

0.51

62.8

199.4

spheres.

0.77 1.02

87.6 111.2

279.0 337.0

6.26-cm.

0.26

47.6

162.6

spheres.

0.61 0.70 1.02

68.3

88.3

115.8

244.6 267.0 284.0

The above voltages are measured by sphere gaps at low fre- quency and for impulse. The diflFerence between the 60 cycles and impulse voltages increases as the steepness of the im- pulse increases. Fig. 147 shows impulse circuits. Fig. 147(6),

164

DIELECTRIC PHENOMENA

8.4 8.0 7.6 7.2 6.8 6.4 6.0 M

6.2 4.8

«4.0 8.6

8JB 2.8 2.4 2.0 L6 L2 0.8 0.4

^00

^9}

— '

—

.60

■ "

-ORri

•tu

10 2080 40 6060708090 100110 Ello.Yolts

Fia. 148. — Needles in air.

2.1

2.0

1.9 1.8 U L6 1.6 U L8 U 1.1 §1.0 0.9 0.8 0.7 0.6 0.6 0.4 0.3 0.2 0.1

'lo

8U>

.<"\

2.0

/ /

■

_i.

■^

—

.60

.28

(Transient voltages. Limited energy.)

10 2080 40 6060708090 100 110 Kilo-Yolts

Fia. 149. — Spheres in air.

10 20 80 40 60 60 70 80 90 100 Kllo.Yolts

Pia. 150. — Needles in oil.

10 2080406060708090 100 Kilo-Yolto

Fia. 151. — spheres in oiL

(Transient voltages. Limited energy.)

CORONA AND SPARK-OVER IN INSULATIONS

166

however, does not give a circuit for which the constants can be definitely calculated. Oil is an excellent insulation in combina- tion with barriers. On soUd insulations the effect of transient voltages is cumulative. A partial break occurs which is en- larged by each succeeding impulse, until finally dynamic follows. With oil, such "cracks'' are closed up by new oil immediately.

One investigation shows that when the energy is limited a voltage is reached where the spark distance becomes a function of the energy and practically independent of the voltage.^ Figs. 148, 149, 150, 151 taken from this investigation show the spark- over curves for oil and air. The figures on the curves represent the current allowed to enter in the impulse, and thus indirectly indicate, to a certain extent, the energy. The impulse was produced by suddenly applying a continuous voltage to the low side of a transformer (Fig. 1476). The curve marked «> is for infinite energy supply (60 cycles). If sphere gaps are set in air and in oil for 30 kv. at 60 cycles the spacings are 0.95 cm. and 0.16 cm. respectively. If a transient voltage of 100 kv. is applied, the gap in oil will not spark over if the energy applied to the circuit is less than 1.04 joules (4 amp.). For air the minimum energy is 0.46 joule (1.6 amp.).

At continuously applied high frequency oil breaks down at lower voltages than at 60 cycles. The following comparison made by the author is of interest.

Tbansil Oil — ^between Flat Terminals — Square Edge — 2.5-cm. Di- ameter — 0.25-CM. Space — Breakdown Voltage Gradients

60 cycles

Kv./nim. (max.)

High frequency

alternator

00|000 cycles

Kv./nim. (max.)

Single impulse, sine

shape, corresponding to

200,000 cycles

Kv./mm. (max.)

6.7

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

CHAPTER VII

SOLID INSULATIONS

General. — Some of the principal solid insulations are varnished cambric, oiled and varnished pressboard, built up pressboard, treated wood, mica, micanite, soft and hard rubber, synthetic resins, glass, and porcelain.

With gaseous and liquid insulations as air and pure oil there is very little loss up to the breakdown gradient. A gradient just under the breakdown gradient may be applied and held and the loss is so small that no appreciable heating results. Thus, the loss in air and oil is essentially a phenomenon above the electric elastic limit. This loss generally exists in some locally broken down part of the insulation, as the corona on the surface of a wire. The break does not extend through the whole insulation, and when the stress is removed, new air takes the place of the broken, down air and all evidence of overstress is removed; in other words, in -air and oil a local breakdown is ''self healing.''

Almost all insulations are partially conducting or have a very high resistance which is spoken of as insulation resistance. The actual resistance of the iosulation itself, which is very high, apparently has no direct connection with the dielectric strength, which is measured by the gradient or flux density or stress re- quired to electrically strain the dielectric above the "electrical elastic limit" so that actual rupture or breakdown occurs. For instance, in a condenser made of two metal plates with a solid dielectric between them, when a.c. potential is applied, energy is stored in the dielectric by electric displacement at increasing potential and delivered back to the circuit at decreasing potential, as long as the potential does not stress the insulation above the elastic limit. If the dielectric were perfect a wattmeter in the circuit would indicate no loss. In all practical insulations the wattmeter does read a loss due to the PR loss in the insulation, and the dielectric loss, sometimes called dielectric hysteresis.* If the voltage is sufficiently increased to exceed the elastic limit actual rupture or breakdown occurs; along this discharge path the insulation is destroyed. Air has a very high insulation

^ In what follows the total loss will be called the dielectric loss. See pages 36, 37.

166

SOLID INSULATIONS 167

resistance, but not a very high dielectric strength. When the insulation resistance, however, of a given solid insulation becomes very low, as caused by moisture, etc., it is an indica- tion of large loss and low' breakdown voltage. Thus, the term ''insulation resistance" generally takes into account the resist- ance of the occluded moisture.

Insulation Resistance. — The actual resistance of the insulating material itself is generally very high. Practically all solid and liquid insulations absorb moisture to a greater or less extent. The capillary tubes and microscopic interstices, etc., in the struc- ture become filled with moisture and gases. In the non-homo- geneous structure this makes a complicated arrangement of ca-

(a)

(b)

U <^»

Fig. 152. — Diagrammatic representation of resistance arrangement in

imperfect insulations.

pacities and resistances in series and in multiple. A simplified diagrammatic illustration of the distribution is shown in Fig. 152.

Let 152(a) represent a magnified section of insulation between two terminals, and 152(&) a diagrammatic representation. The resistance of the insulation itself, which is very high, may be represented by u. 1 may be a moist fiber which extends only partially through the insulation and is thus in series with a capacity. It may be represented by Va* 3 and 4 may be wet fibers which extend all the way across, and are represented by r». There may also be leakage resistance over the surface.

Direct current is used to measure insulation resistance, as the charging current for a.c. is very large and masks the resistance current. Watt measurements are necessary, as well as volts and amperes, to determine the effective a.c. resistance. The a.c. and

168

DIELECTRIC PHENOMENA

d.c. resistance should, however, be quite diflferent. When d.c. is used sufficient time must elapse after the application of voltage to allow for absorption (see Chapter II, page 36).

With d.c. the only path for the current is through r< and r^ in multiple, which is, therefore, the resistance measured. The resistance varies with the applied potential, decreasing with in- creasing potential. The conducting particles are caused to line up, cohere, occluded gases break-down, etc., as the potential is increased.

When a.c. voltage is applied to insulation the capacity current must pass through Va) in shunt with this is the circuit through r< and r». The loss in To must increase with increasing frequency, while the loss through r» and fm must remain constant at a given voltage, independent of the frequency. The greater loss will generally occur in Ta. The d.c. insulation resistance cannot be used in approximating the JV a.c. loss. If the a.c. loss is meas- ured, as well as the voltage and current, the eflfective resistance may be calculated. This resistance loss, however, must be greater than that due to Ta as other losses are included.

In Table LXX some d.c. resistances are given for different materials. Note the effect of moisture absorbed from the air even for varnished materials.

Table LXX. — Insulation Resistance Resistance Megohms per Cm. Cube (Data obtained by Evershed, J.I.E.E., Dec. 15, 1913)

Material

Cotton, dry

Cotton, not dried

Micanite, exposed to air Micanite, dried

Cylinder oil, trace of moisture Cylinder oil, dry

350.0

2.8

220.0

Varnished cloth, 12 hr. after

baking. Varnished cloth, 9 days after

baking.

17,000

D.c. volts

100

275.0 2.5

175.0

22,000 36,000

35,000

14,000

200

200.0 2.2

160.0 300,000

22,000 36,000

35,000

11,000

500

140

140 200,000

22,000 36,000

35,000

9,000

SOLID INSULATIONS

169

With air and oil an appreciable loss begins only when a definite gradient, sufficiently large to cause local rupture as brush or corona, somewhere results; loss occurs after the elastic limit has been exceeded. The dielectric loss in solid dielectrics is essen- tially a loss below the elastic limit. In solid dielectrics a stress may be applied lower than the elastic limit, which after a short time — on account of the heating and hence weakening of the insulation — will cause rupture.

Rupturing Gradients. — Apply voltage lower than the puncture voltage between concentric cylinders with dry insulation between surfaces, and gradually increase to the rupturing voltage within a short time so that there can be no appreciable rise in temperature. The mechanism of rupture will be quite diflferent for oil or air, glass or porcelain, and cambric. For oil and air corona results near the surface of the inner cylinder, the breakdown is local, and disappears

4»

—

(

10]

tflx

iat4

Fig. 153. — Insulation puncture voltage vs. time.

(Oiled pressboard, 2.5 mm. thick. The curve does not cut the kv axes as

it appears to on account of the time scale.)

when the stress is removed; or, if the stress is further increased spark-over finally results. For glass or porcelain, as soon as over- stress is reached locally, a crack results at the surface of the inner cylinder and complete breakdown follows. With cambric there is local rupture and local charring of the insulation, which forms carbon "needles.'' The break is progressively increased; this continues until the breakdown is complete. A wet thread or gas bubble, as (1) in Fig. 152(a), may, in effect, act as a needle and thus cause breakdown. The breakdown gradients of solid insulations are thus variable and not as definite as with air and oil. The puncture tests on solid insulations vary greatly between different samples of the same material, shape and area of the electrodes, time of application of voltage, etc. Insulations should be thoroughly dried before tests are made.

170

DIELECTRIC PHENOMENA

In comparing solid insulations it is generally best to make some arbitrary time tests to include the effect of dielectric loss, and thus, heating on the breakdown voltage. The effect of loss is cumulative; the insulation becomes warm and while the loss increases with the temperature, the dielectric strength generally decreases with increasing temperature. The ultimate strength naturally, then, depends on the rate at which this heat is con- ducted away. This is illustrated in Fig. 153. With the electrode used, if the voltage is ''rapidly applied" before heating occurs, 80 kv. are required to cause rupture.^ If 60 kv. are applied rupture does not occur until 4 minutes have elapsed, while 30 kv. may be applied indefinitely without rupture if the room tem- perature is not increased. The curve does not cut the voltage axes as appears in the cut, on account of the time scale, but is an asymptote to it. The effect of temperature is also illustrated in Table LXXI, where, in one case, the electrode is of brass giving good heat conduction, and in the other case of wood coated with tin-foil giving poor heat conduction. As the applied voltage approaches the "rapidly applied" rupturing voltage there is not sufficient time for the insulation to heat to a great extent and the effect is about the same for either brass or the coated wood electrode.

Table LXXI. — Effect of Heat CoNDUcnNa Properties of Terminals on Time of Breakdown

Two Thicknesses of No. 12 Oiled Cloth (From Rayner, Journal I.E.E., Feb. 8, 1912)

Volts

Time of breakdown

Brass terminalB

Wood terminals*

9,000

570.0 sec.

50 . sec.

10,000

48.0 sec.

19.0 sec.

11,000

16.5 sec.

10.0 sec.

12,000

10.2 sec.

6.2 sec.

14,000

5 . 2 sec.

4 . 5 sec.

^ Rapidly applied voltage as used above means voltage applied within a fairly short time, a few seconds, and not impulse voltage or voltage of very steep wave front. '* Instantaneous'' is commonly used to designate this test; this term is confusing. The test itself is not wholly satisfactory as it is indefinite. It offers, however, a means of making a preliminary com- parison of insulations.

Coated with tin-foil.

SOLID INSULATIONS

171

The effect of applying a high voltage, allowing different periods of rest for cooling, and then applying voltage until rupture, is shown in Table LXXII. The injurious effects of applying high voltage for different lengths of time is shown in Table LXXIII. This is probably due to the effect of heat and injury to the sur- face by corona. y

Table LXXII. — Recovery in Vabtinq PERiobs op Rest afteb Appli- cation OF 9000 Volts for 1 Minute*

Two Thicknesses of No. 12 Oiled Cloth (From Raynor, Journal I.E.E., Feb. 8, 1912)

Period of rest

Time to break at 11,000 volts

Period of rest

Time to break at 11.000 volts

2.6 sec.

9 . 5 sec. 11.9 sec. 12.0 sec.

o sec. 15 sec. 60 sec. Fresh material

2 . 5 sec.

1 min

4 . 4 sec.

2 min

9.0 sec.

Fresh material

9.8 sec. 20.5 sec.

Table LXXIII (From Raynor, Journal I.E.E., Feb. 8, 1912)

Time of treatment at

Time of breakdown Geft to cool over night)

6000 volts

At 6500 volto

At 7000 volto

22.0 min. 6 . 5 min. 3 . sec.

8.5 min.

Ihr

8.3 sec.

2 hr

Table LXXIV. — Effect of Air Gap (Corona)

Two Thicknesses of No. 12 Oiled Cloth 10,000 Volts

(Raynor, Journal I.E.E., Feb. 8, 1912)

Air gap, ram.

Time to puncture, seconds

0.00 0.30 0.50

0.75

.1.05^

42.0 34.0 27.5 24.0 120 (irregular)

1 Time to break at 9000 volts, 1-^^ to 2 minutes.

172

DIELECTRIC PHENOMENA

The action of corona or breakdown of the air at the surface of the insulation is shown in Table LXXIV.

The arbitrary practical tests for comparing insulations are the Rapidly Applied (Instantaneous) Test, the One-minute Test, and the Endurance Test. Rapidly applied breakdown voltage is found by applying a fairly low voltage and rapidly increasing until breakdown occurf . Voltage should be increased at about 5 kv. per sec.

The Minute Test is made by applying 40 per cent, of the rapidly applied voltage, and increasing this voltage 10 per cent, at 1- minute periods until puncture occurs. (Total time about 5 min.)

The Endurance Test is made by applying 40 per cent, of the

minute test voltage and increasing the voltage 10 per cent, every

hour or half hour until puncture occurs. These tests may be

made at any given temperature. The electrode should be of a

given size and weight. Ten-centimeter diameter electrodes,

slightly rounded at the edges, will be found convenient. Table

LXXV gives an example of such tests.

Table LXXV

Muslin with Three Coats Varnish — Total Thickness 2 mm.

Rapidly Applied (Instantaneous) Breakdown and Resistance

Temp., deg.

Resistance in megohms for samples

cent.

100

73,400 650 100

73,400

390

97,900

310

97,900

270

73,400

170

Breakdowns in kilovolts

100

40.6

36.5

Average breakdown 40.6 kilovolts

Apply 40 Per

One-minute Test • Cent, of Rapidly Applied Breakdown Voltage for nth 10 Per Cent. Increase in Voltage Each Minute

1 Minute,

Temp., deg.

Resistance in megohms

cent.

100

73,400

320

73,400

333

73,400

461

97,900

274

97,900

330

SOLID INSULATIONS

173

Time under stress

Potential applied

Start

1 min

2 min

3 min

4 min

5 min

5 min. 5 sec

5 min. 46 sec. . . .

6 min

6 min. 10 sec

6 min. 35 sec. . . .

7 min

7 min. 58 sec

16,000 17,500 19,000 20,500 22,000 23,500 23,500 23,500 25,000 25,000

26,500

Temperature for samples

100 101 102

105 111 121

100 101 102 104 108 112

101 103 104 106 108 111

146 punctured 118

103 104 105 108 111 117

100 102 103 105 107 108

Air

100

100 100 100 100 100

115 punctured

114 129

135 punctured

118 punctured 135 140 punctured

100

Endurance Test

Apply 40 Per Cent, of 1-minute Period Endurance Voltage, for 30-minute Periods Endurance Test, with 10 Per Cent. Raise in Voltage Each Period

Temp., deg.

Resistance in megohms

cent.

100

74,300

390

74,400

420

74,400

230

99,100

290

49500

110

Time under stress

Start

0.5 hr

34 min

54 min

Ihr

1.5 hr

2hr

2 hr. 23 min.

2 hr. 25 min

2.5 hr

3hr

3 hr. 2 min. .

Potential applied

10,000

11,000 11,000 11,000 12,000 13,000 14,000 14,000 14,000 15,000 16,000

99 103

104 106 106

136 143

100 107

108 111 118

Temperature

100 133

101

108

99 115

182 punctured

Air

100 100

203 punctured

112 114 122 215 punctured

198 punctured 100 100 100

145 punctured

174

DIELECTRIC PHENOMENA

A considerable amount of data is given here for the l-minute time test. It must be remembered that in design only a fraction of the maximum gradient corresponding to this voltage is per- missible for continuous operation, the particular per cent, depend- ing upon the design, the insulation, the rapidity at which heat may be radiated, or conducted away, etc. It is generally not more than 10 per cent.; it is often as low as 5 per cent., sometimes as high as 30 per cent.

Insulation tests are generally made for convenience on sheets between flat terminals. The gradient at the edges, even when these are rounded, is generally higher than the average gradient

e/x. This edge effect is differ- ent with different thicknesses of insulation. The puncture voltage per centimeter thick- ness is always greater for thin sheets of insulation than for thick ones. This is partly due to the edge effect which cannot be corrected for and becomes relatively greater as the thick- ness of insulation is increased. (At small spacings e/x is very nearly the true gradient; at large spacings e/x is not the true maximum gradient.) ^ It ,02 j04 .06 416^002 .14 .16.18.20.22.24.26^ is also greatly due, in the time

Fig. 154.-lMukU^^ vs. ^ution and dissipation in the

thin sheets and partly due to the fact that energy is necessary for disruption, so that when the rupturing distance is limited, as in the case of thin sheets, the apparent strength increases. Fig. 154 shows apparent variation in strength with thickness. Between parallel plates the apparent strength is approximately:

"i'^vr)

where g and a are constants and t is thickness.

When shielded edges are used, it is generally best to make in- sulation tests under oil unless it is desired to make a study of the

^Special flat terminals are sometimes used with "shielded" edges so that e/x is the true gradient.

SOLID INSULATIONS

175

effect of corona on the insulation. With pointed electrodes the in- stantaneous puncture voltage of a given insulation will be less in oil than in air. This is not because the oil weakens the insulation but because corona forms on the point in air and spreads over the surface giving the effect of a flat plate electrode.

Solid and Laminated Insulation. — The structure of most insu- lations is not homogeneous. If a given insulation is tested with terminals of varying area it is found that the average puncture voltage becomes lower as the area is increased, and thus the chance of it covering a weak spot is increased. As would be expected this approximately follows the probability law as shown in Fig. 155.

S 20

babi

Uy(

lorT(

eriix

enu

iOni

.^^^

▼8

1 2 S 4

Badlaa oi iOat IfttmlaaU-Om.

Fig. 165. — Insulation strength V8. area Fig. 166. — Variation in di- of terminal. electric strength at different

parts of a piece of insulation.

The reason that the experimental curve bends down is that the gradient is fairly constant until the terminal becomes small when the gradient increases, due to flux concentration with the smaller terminal. If the curves were plotted with actual gradients instead of voltages, the experimental curve would follow more closely the probability curve. The experimental gradient curve would, however, for the smaller sizes bend up faster than the probability curve, due to greater apparent strength for smaller terminals, as for small spheres in the case of air. The curve between e/x and diameter for flat terminals of the larger sizes when the concentration at the edges is about constant should follow the probability curve.

For example, suppose Fig. 156 represents a piece of solid insula-

176

DIELECTRIC PHENOMENA

tion 0.026 mm. thick and suflSciently large to contain every condition of "weak spot." Divide this into six equal squares each of area a. The strength is marked on the various areas. Assume that an electrode is used giving no edge effect. With electrode of area a six tests are required to go over the whole piece. With electrodes of area 2a three tests are required, with area 3a two tests, and 6a one test. The following results may be obtained:

Area

No. of punctures

Total area covered

Volts per mm.

electrode

Maximum

Minimum

Aver ace

6 3 2 1

6a 6a 6a 6a

20 18 12 10

10 10 10 10

The results are somewhat similar to the lower points of the curve in Fig. 155.

On account of these characteristics alone an insulation built up of laminations is much better than a solid insulation as the weak spots in the laminations are not likely to line up. It is also much easier to make better and more uniform insulation in thin sheets. Another probable important reason for greater strength of laminated insulation is that the energy distances are interrupted by the discontinuous surfaces.

Tests are useless for comparing insulation strengths unless made upon some standard basis. Great caution is necessary in the use of tabulated values of insulation strength in design. On account of the variable quality of solid insulation, tests must be continually made to see that the product does not change. Vacuum treatment is necessary before use to remove moisture. Even when all of the test conditions are known, experience is necessary to judge the proper factor of safety. Aside from this, stress concentrations due to the shapes and spacings of the con- ductors must always be considered and allowed for. It is gener- ally not possible to do this with mathematical exactitude, but approximation must be made with all factors in mind. Care must be taken that the solid insulation is below the rupturing gradient at any local point. If such a point is broken down locally the flux becomes still further concentrated. The puncture

SOLID INSULATIONS 177

voltage will decrease with frequency, even over the commercial range, due to increasing loss with increasing frequency.

Impulse Voltages and High Frequency. — It takes energy and therefore time to rupture insulation. For a given potential a given number of cycles of very high frequency voltages, where heating does not result, are therefore much less injurious than the same number of cycles at low frequency. This also applies to impulse voltages of steep wave front. Continuously applied high frequency is, however, generally very injurious for two dis- tinct reasons:

(1) On account of the very great loss at high frequency the insulation may be literally burned up in a very short time even at low voltages. This condition does not result in practice from surges, etc., on low frequency lines, but in high frequency genera- tors, and transformers, etc. In such apparatus it is important to use very smooth electrodes to prevent local concentration of stress and charring of insulation. This is especially so where contact is made with the air. If a local brush starts, on account of the great loss, it becomes very hot and extends out a consider- able distance.

(2) In certain apparatus containing inductance and capacity very high local potential differences may be produced by reso- nance and thus cause rupture by overpotential. The high fre- quency thus does not cause the rupture directly but makes it possible by causing overpotential. Local concentration of stress may also result in non-homogeneous insulation, as across the condenser and resistance combinations in Fig. 152.

The term "high frequency" is generally used in such a way that no distinction is made between sinusoidal high frequency from an alternator, undamped oscillations, damped oscillations, impulses of steep wave front, etc. Naturally the effect of con- tinuously applied undamped oscillations is quite different from a single high-voltage impulse of extremely short duration. As the effects are attributed to the same cause — "high frequency'* — apparent discrepancies must result. (See comparative tests, Table LXXVI, page 184.)

If the time of application is limited below a definite value, higher voltages are necessary to produce the same results in the limited time. Impulse voltages of steep wave front many times in excess of the rupturing voltage may be applied to insula- tions without rupture if the application is very short — measured

178 DIELECTRIC PHENOMENA

in microseconds. They may be caused in practice by lightning, switching, etc. If such voltages are sufficiently high, complete rupture may result at once. In any case if these voltages are higher than the 60-cycle puncture voltages the insulation will be damaged. As an example, an impulse voltage equal to three times the 60-cycle puncture voltage may be applied to a line insulator. During the very small time between the application of the voltage and the arc-over through the air, the insulator is under great stress. It may be that up to the ninth applica- tion of such a voltage there is no evidence of any injury, while on the tenth application failure results. Each stroke has con- tributed toward puncture. It is probable that each application adds to or extends local cracks.

Cumulative Efifect of Overvoltages of Steep Wave Front. — Voltages greatly in excess of the *' rapidly applied" 60-cycle puncture voltage may be applied to insulation without rupture if the time of application is sufficiently short. All such over- voltages injure the insulation, probably by mechanical tearing, and the effect is cumulative. A sufficient number will cause breakdown. For example: A piece of oiled pressboard 3.2 mm. thick has a rapidly applied breakdown at 60 cycles of 100 kv. maximum. If sinusoidal impulses reaching their maximum in 2.5 microseconds are applied, the number of impulses to cause break-down is as follows:

Kv. maximum of Number to cause

impulse applied breakdown

100

140 100

150 16

155 2

165 1

If the impulses are of still shorter duration, a greater number are required to cause breakdown at a given voltage. Insulations, and line insulators, are often injured and gradually destroyed in this way by lightning.

Strength vs. Time of Application. — It was stated above that the strength varies with the time of application. A curve is given in Fig. 153. The range of time shown in this curve is from a few seconds ("instantaneous") to an indefinitely long time. Over the greater part of the plotted curve heating is a factor and the great decrease is principally due to beating. Where the

SOLID INSULATIONS 179

time of application is much less than ''instantaneous" value, and heating can have no appreciable effect, the strength still increases very rapidly as the time of application is decreased. The increased strength at this part of the curve is due to limited energy, as explained on page 177. Some values from an actually measured curve are:

(14 Layers Impregnated Paper between Concentric Cylinders)

= 0.67 cm.

= 0.365

Kilovolta to puncture

Time, sec.

(maximum)

32.6

60.0

37.5

1.0

49.3

o.r

61.0

0.01

85.0

0.001

113.0

0.0001

196.0

0.00001

^ _ 1

e E 1 1

^ 0-6,

... J

550.0 (calculated)

w^m wk w*«%«k mm

The equation of the strength-time curve over the complete range obtained from an examination of a number of curves is of the form

^ = fi'- (l + 1^)

when T is the time of application in seconds, and g« is the gradient in kv./m.m. for indefinite time.

Both a and g« vary with the thickness of the insulation, tem- perature, etc.

In order that the strength may be high for indefinite time, the loss should be low.

Pennittivity of Insulating Material. — In design, a knowledge of the permittivity of insulating materials is as important as the dielectric strength. For solid insulations the permittivity in- creases with the specific gravities of the material in almost a direct ratio. The various properties of some of the common insulations are given in Table LXXVI.

180

DIELECTRIC PHENOMENA

Table LXXVI. — Dielectric Strength op Solid Insulations

Variation of Dielectric Strength with Thickness and Number of Layers. (One-minute Tests — 60-cycle — 10-cm. Terminals in Oil. Values

Effective Sine Wave)

Pressboard

No. of layers

Thickness per layer, ram.

Total thick- nesst mm.

Kv./mm

— temperature 25 deg. C.

Varnished, ky./mm.

Oiled transil, kv./mm.

liinseed oil, kv./mm.

0.178 0.254 0.508 0.787 1.575 2.390 3.170

0.178 0.254 0.508 0.787 1.575

0.178 0.254 0.508 0.787 1.575 2.390 3.170

0.356 0.508 1.016 1.574 3.150 4.790 6.340

0.712 1.016 2.032 3.148 6.300 9.560 12.680

1.068 1.524 3.048 4.702 9.450

25.3 26.3 16.1 19.1 15.5 11.1 9.5

22.5 17.7 12.8 13.7 10.6 7.54 6.3

26.7 22.6 16.8 16.5 11.6 9.4 6.6

25.5 20.6 15.4 14.9

25.3

39.3

23.6 17.7

28.0 29.2 23.0

21.1

•

23.5 19.0 15.1 14.2

21.0

33.5

19.7 17.2

2 2 2 2

22.9 19.7 16.1 14.7

20.2 15.3 12.1 11.0

16.6

4 4

29.8

15.3 15.8

4 4

20.6

13.65

11.5

19.7 12.7

15.0

6 6

27.5

13.1 14.4

6 6

20.0 11.3

17.8

Treated Wood Temperature 25 deg. C.

Across grain

kv./mm.

With grain

kv./mm.

6.42

2.47

4.53

1.57

3.85

1.27

3.02

120

1.12

SOLID INSULATIONS

181

Papeb

(Untreated)

No. of layers

Thiokneas por layer, mm.

Total thickness, mm*

25* C, kv./mm.

100* C, kv./mm.

0.064

9.3

0.127

8.7

7.9

0.254

7.9

7.3

0.064

0.258

8.7

8.3

0.127

0.508

7.5

6.7

0.254

1.016

6.6

6.2

0.064

0.516

8.7

8.1

0.127

1.016

7.4

6.6

0.254

2.032

6.3

6.0

Varnished Cloth

0.305

0.30

26.2

23.6

0.305

0.61

20.5

19.7

0.305

0.91

18.5

17.0

0.305

1.22

16.8

14.9

0.305

1.52

15.5

13.1

0.305

1.83

14.6

11.5

0.305

2.13

14.0

10.3

0.305

2.44

13.3

9.2

0.305

2.74

12.8

8.3

0.305

3.05

12.3

7.5

Hard Rubber

No. ol layers

ThioknesB per layer, mm.

Total thioknesa/ mm.

Puncture voltage

25° C, kv./mm.

100° C, kv./mm.

0.0193

0.0223

0.0312

2.0

3.0

4.0

5.0

6.0

59.7 55.6 48.7 17.3 14.2 12.7 11.8 11.2

2.0 3.0 4.0 5.0 6.0

■

182

DIELECTRIC PHENOMENA

Mica

0.0508 0.1016 0.1524 0.2032 0.5080

Total thickness 6.0

Glass

Kv./miD.

The strength of glass decreases rapidly with thickness.

Porcelain

Total thickness, mm.

Kv./mm.

0.5

16.0

1.0

14.5

2.0

12.2

5.0

11.0

10.0

9.6

15.0

9.2

Variation op Insulation Strength with Time op Applied Voltage

TUI^tMri».\

Thickness,

Time to

25»C.

100*»C.

iTXBvvriai

mm.

puncture, min.

kv./mm.

kr./mm.

Oil impregnated paper,

1.90

"Inst."

39.4

32.0

30 layers, 60Kjy., 10-

33.1

27.3

cm. diameter discs,

31.0

25.7

round edges.

29.2

24.5

28.2

23.6

26.8

22.7

25.5

21.6

23.6

20.5

22.7

19.7

22.1

19.3

100

21.6

19.0

Note that for a given thickness the strengths of materials do not vaiy greatly as might be expected.

SOLID INSULATIONS

183

Puncture voltage,

26° C.

ThioknesB, mm.

Time to puncture,

•

MaterUI

1 layer,

5 layers.

10 layers,

0.30 mm.

1.50 mm.

3.0 mm.

kv./mm.

Varnished cloth, 0.30mm.,

0.30

"Inst."

52.5

36.1

27.2

60-cy., 10-cm. diameter

0.1

37.7

27.5

22.3

in air, round edges

0.2

34.5

25.8

19.7

0.5

32.8

22.9

16.4

1.0

32.5

21.0

14.3

2.0

32.1

20.3

13.0

3.0

31.8

19.9

12.6

5.0

31.5

19.8

12.5

10.0

31.1

19.7

12.3

20.0

30.2

19.6

9.9

Permittiyity of Insulating Matebiaia

Permittivity

Asphalt

Bakelite

Cambric (varnished)

Fiber (horn) dry — Fiber (horn) oil

Glass (crown)

Glass (heavy flint) . . Gutta percha

Lead stearate

Lead palmitate

Lead oleate

Mica

Oil Qinseed)

Oil (transU)

Paper (dry)

Paper (paraffined)

Paper (oiled)

Paraffine

2-^4 4-W

4-^i to 5-H

2-J4 4-H to 5

3-Hto4

5.2

5to7

Specific gravity

1.15

0.7 to 1 0.9to 1.5

3to3.5 4.5

3-w

0.95

2 to 2-H

0.8to0.9

2.6

1.00

3->i

1.25

2 to 2.3

0.9

184

DIELECTRIC PHENOMENA

Peemittivity op Insulating Materials. — Continued

Permittivity

Specific gravity

Pressboard (dry) . Pressboard (oiled) Porcelain

Rubber (hard)

Rubber (vulcanized)

Shellac . Sulphur

Wood (treated)

3 4to6

4-jrito5

3 4

3 to 3-\i

1.25 1.40 2.4

0.8 to 0.0

Comparative Insulation Strength for High Frequency, Impulse,

Oscillation and 60-cycle Voltages

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