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Behavior of insulators under dew and fog
Behavior of Insulators under Dew and Fog &KARL
BERGER
and PRITINDRA
Forschungskommission
ABSTRACT:
The
des SEV und VSE j&r Hochspannungsfragen
sparkover
characteristics
50 Hz voltage under simulated with different duration front
materials
CHOWDHURI*
conditions
are discussed.
of tests is proposed.
of insukztors
A test procedure
It consists of the superposition
on the rated 50 Hz voltage applied
subjected
from
of dew and fog are described. for
the purpose
20 to 80 min Results
to
obtained
of reducing
the
of a Burge voltage of slow-rising
to the insulator.
1. Introduction
The reduction in the breakdown strength of contaminated insulators has long been a serious problem (l-9). However, recently a resurgence of interest in the behavior of contaminated insulators has been caused by the need for extra high voltage power transmission (1040). It is generally assumed that a reduction in the breakdown strength of insulators becomes critical in heavily polluted areas. However, drastic reduction of the breakdown strength of insulators has been observed in countries such as Switzerland where the atmosphere in many sections is reasonably unpolluted. For example, during spring and autumn in Central Europe, a warm humid wind-the socalled “Fohn’-occasionally blows from the Alps to the valleys. This wind precipitates moisture on cold insulator surfaces, sometimes causing flashover at rated 50 Hz voltages. This phenomenon is particularly noticeable during the early morning hours (4). Even indoor-type insulators which are fairly uncontaminated are affected by these incidents. Such flashovers are initiated by partial evaporation of the insulatorsurface moisture as a result of increased leakage current over the surface. The moisture evaporation produced by the leakage current is counteracted by a continual condensation of moisture from the surrounding air. In this way, the surface becomes coated with irregular patches of water layer. This layer may increase the surface electrical stresses sufficiently to cause flashover at the system voltage. The addition of contamination in the water layer would reduce the flashover strength even further. The purpose of this study is to investigate the flashover characteristics of insulators under various conditions of moisture in the laboratory, and to determine a suitable test procedure in order to evaluate the performance of different types of insulators. Both indoor and outdoor types of insulators were tested. For insulator materials, porcelain, phenolic laminate and a * Now with General Electric
Company,
Erie, Pa., U.S.A.
385
Karl Berger and Pritindra Chowdhuri resinous epoxy compound known as Araldite were chosen. Figure 1 shows the different types of insulators investigated. As contamination on insulator surfaces adds another not always reproducible factor, all insulators were cleaned before testing. To account for manufacturing tolerances, at least two insulators of each type were tested. ZZ. Test Methods
Three types of tests were performed. During the first type, dew was simulated and the insulators were tested with long-duration 50 Hz voltages near the dry flashover value. The second type was similar to the first, except slow-front impulse voltage waves were superimposed on the rated 50 Hz voltage. During the third type, fog was simulated and the insulators were tested with long-duration 50 Hz voltages. Unlike the first type of test, the 50 Hz test voltage levels were near the rated voltage of the insulator. First Test Method The insulator was air cooled to 0°C. Then 90 per cent of its dry flashover voltage was applied to it, and warm humid air was blown over its surface (Pig. 2). Surface leakage current was measured by a recording milliammeter,
FIG. 2. Schematic diagram of principle of dew generation: (A) test insulator, (B) flap doors to control air flow, (C) heater, (D) boiling water, (E) fan.
and humidity as well as the temperature of the test chamber and the insulator surface were monitored continuously. After 10 min, the flow of humid air was cut off and the test was continued for another 10 min. If the insulator flashed over, 85 per cent of the dry sparkover voltage was impressed on its terminals for another 20 min and the same procedure was repeated until no further flashover occurred at a certain lower voltage level. Tests were repeated three times at this voltage level to make sure that the insulator did not spark over. The surface temperature of the insulator under test could not be measured directly when voltage was applied. Therefore, the surface temperature was measured on an identical but de-energized insulator having the same current of air passing over its surface. At the same time, precautions were taken that the second insulator did not have the proximity effect on the sparkover voltage of the first. All surface and air temperatures were measured by copper-constantan thermocouples.
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Journal of The FranklinInstitute
Behavior of Insulators Under Dew and Fog Measurement of humidity posed a problem. The change of humidity caused by the blowing of warm air was so rapid that conventional hygrometers were useless. Instead, humidities were measured by a special humidity recorder (51). This instrument is essentially a Wheatstone bridge with its four arms made of platinum wires enclosed in four separate chambers. These four platinum wires were heated from a direct current source. The humid air was passed through two chambers and then after being dried by Silica gel the same air was passed through the other two chambers. The difference in radiation coefficients of dry and humid air produced a resistance unbalance. The unbalance current was read by a milliammeter which was directly calibrated in grams of water in cubic meter of air at atmospheric pressure. The continuous voltages applied to the insulators were much higher than their steady-state rating (Tables I and II). It was suspected that the leakage current due to these higher terminal voltages might have dried the surface so much that the optimum proportion of dry and wet patches was not produced to cause a complete flashover. To check this possibility a second series of tests was made. Xecond Test Method The ambient conditions were the same in this test as in the first test procedure. Electrically, only the rated 50 Hz voltage was impressed on the test piece simultaneously with the blowing of warm, humid air on its cold surface. The insulation strength was measured by superimposing, at 30 set intervals, a surge voltage wave of the same time to crest as the 50 Hz voltage. The surge wave, being applied momentarily to the insulator, did not disturb the drying processes under normal voltage condition, and since this surge had the same time to crest as the continuous applied voltage, the insulation strength under power-frequency condition could be obtained. The schematic diagram of the test setup is given in Fig. 3. Humidity and temperatures were measured in the same way as in the first test method.
FIG. 3. Schematic diagram of test circuit for superimposing long-front surge wave on power-frequency voltage : (A) phase-shifting network, (B) auxiliary impulse generator, (C) impulse generator, (D) quench gap, (E) environmental tank, (F) high-voltage transformer, (G) harmonic filters, (H) voltage regulator.
Vol. 294, No. 6, December
1972
387
3 & B m p i B B
Fi
White
Brown
Brown
D4 D5
D7 D8
Ll L2
The conductivity
White
C6 c7
120 117
178 186
180 182
69 69
t;
138
148
126 136 145
122
(kv) (rms)
Dry 50 Hz flashover voltage
(First test method)
Dry flashover voltage
Flashover voltage under dew
39.0
60.0
68.5
25.5
per element
15.0
36.5
29.0
Creepage distance
TABLE I insulators
P:, =
130.0 126.5
197.5 208.0
201.5 204.0
76.6 76.2
101.2 106.0
153.0
164.5
141.0 149.5 161.0
137.0
(kv) (rms)
Corrected dry flashover voltage
$/cm.
(Third test method)
Dry flashover voltage
Flashover voltage under fog
737/9*0/4.9 737/9.0/4.9
727/- 0*2/4.6 730/0.1/4.5
728/-O-314.1 730/- 0.514.4
725/0.0/4.4 725/0.4/4,7
72310.414.7 724/0*8/4.5
728/0.5/4*7
72810.314.4
730/0.0/4.6 720/1.0/4.2 72810.614.5
731/0.0/4.1
Atmospheric conditions of air (mm Hg/“C/glm3)
of reaults with porcelain
of water generating dew and fog was between 328-358
PO =
26.2
52.0
50.0
17.5
28.0
25.5
Brown
Brown
Cl
36.5
c4 c5
Brown
A3 B2 B3
21.5
skirt dia.
Brown
A2
Arcing distance
Length (cm)
c2
Color of material
Designation of insulator
Summary
76.5 71.5
64,4 57.2
72.0 77.0
82.0 81.5
72.4
65.0
71.6 72.7 72.0
71.5
(G
61.5 63.5
35.4 33.6
27.3 27.0
36.5 34.0
59.5 52.0
43.5
39.5
42.5 46.8 43.5
40.1
P: (%)
Stab insulator
Indoor type
Indoor type
Indoor type
type Bushing
suspension
Three-element
insulators Bell-type wideskirt insulators
Motor
Remarks
? Y cr.
2
z
& 3
; rr’ cr.
f
3 Y
Phenolic -laminate yellow
Araldite reddish brown
J5 J6
K4 K5
Length
voltage
> 81.0 > 81.0
162.0 161.0
voltage
under dew
voltage Dry flashover
Flashover 328-358 @/cm.
P:, =
722/2.2/5*1 724/1*7/4.8
727/0*4/4,5 727/0.9/4*5
195.5 182.0
> 82.5 > 82.5
140.0 140.0
723/1,7/4.9 722/l-3/4.9
72.0 72.0
196.5 198.0
730/0.8/4.7 730/1*0/4.5
voltage
under fog
73.5 73.0
81.0 72.0 > 72.7
177.0 167.5 171.0
30.7 33.0
46.5 46.5
25.4
32.8 32.4
82.0 81.7
729/0*9/4*5 726/0.6/4.2 722/0*9/4.4
37.8 35.8 36.3
73.8 82.0 > 82.5
27.2 27.0
72.0 67.5
198.5 201.0
158.5 153.2 151.5
202 204
P:, (%)
(B)
insulators
Corrected dry flashover voltage (kV) (rms)
laminate
721/15/4,5 7.26/0*9/4.8
722/2.0/5*2 719/0.6/4*6 719/1*4/4*7
727/0.1/4+2 726/1.0/4.2
Atmospheric conditions of air (mm Hg/‘C/g/m3)
dew and fog was between
Dry flashover
Flashover
180 186
146 145
55.0
50.0
128 128
52.0
177 178
159 151 155
63.0
146 140 139
182 184
Dry 50 Hz flashover voltage (kV) (rms)
181 182
62.5
TABLE II
of results with araldite and Phenolic
52.5
40.0
59.5
Creepage distance
(cm)
generating
PO =
48.0
42.0
36.0
44.5
44.5
52.5
40.0
52.0
Arcing distance
of water
Phenolic laminate yellow
53 54
The conductivity
Phenolic laminate
grey Phenolic laminate red
Araldite
Hl H2
E F9
F4 F5
red
Araldite
Fl
E
Araldite dark brown
Dl D2
grey
Color of material
Designation of insulator
Sumrnaq
type
type
type
type
type
Indoor type material burnt and charred during fog test
Indoor
Indoor type material fractured during fog test
Indoor type-H1 burnt and charred during fog test
Indoor
Indoor
Indoor
Indoor
Remarks
Karl Berger and P&n&a
Chowdhuri
The advantage in this second test method was that it greatly reduced the time of testing. In the first test method, the flashover test sometimes lasted 8 hr, whereas the time required by the second test method was only 30 min1 hr. Unfortunately, the slow front of the surge wave reduced the efficiency of the surge generator with the consequence that the available crest of the surge was much reduced. Because of this, only the insulators of the lower voltage classes could be tested. Third Test Method This third test method is similar to the first. It was devised to determine the effects of fog on insulator surfaces. Figure 4 shows the general principle l-7
I
FIG. 4. Schematic diagram of principle of fog generation : (A) water reservoir, (B) valve, (C) apparatus for measuring water conductivity, (D) valve, (E) valve, (F) manometer, (G) pump, (H) water tank, (I) valve, (J) valve, (K) water atomizer.
of fog generation. Either rain or fog could be artificially made by opening the valve D or E. The insulator which was precooled to 0°C was placed in a tank where fog was being generated at 20°C. After about 10 min the rated voltage was applied to the insulator for 30 min. If the insulator did not sparkover during this time, the voltage level was increased by 5 kV, and the whole procedure repeated until a voltage level was reached at which the insulator finally flashed over. The voltage level was then lowered by 5 kV and the test repeated three times to make sure that it did not sparkover at any time at this lower level of voltage. Although normally the duration of a test was 30 min, tests were extended up to a maximum duration of 80 min in doubtful cases. The duration of a test was set by the leakage current characteristics of the insulators.
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Journal
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Behavior of Insulators Under Dew and Fog The limitation of the crest value of the long-front surge wave was the deciding factor in not applying it during these tests under fog in order to be able to determine the flashover characteristics of all the insulators available. ZZZ. Results
and Discussion
The summary of the results obtained under dew (first test method) and fog (third test method) tests is shown in Tables I and II. Because of the limited capacity of the impulse generator, the second test method was applied only to the insulators C, and C,. The flashover voltages of C, and C, under dew by the first and the second test methods were the same. The lowest flashover voltage under warm, humid air (dew) was 57 per cent of the dry flashover voltage for the insulators tested. Under fog, the lowest flashover voltage was only 25 per cent of the dry flashover voltage. None of the insulators tested flashed over when voltage was applied. Under fog, the longest time required to flashover was about 35 min. The reduction of insulation strength of phenolic laminate and Araldite insulators under warm, humid air was less than that of porcelain insulators. However, phenolic laminate insulators were either swollen or charred under fog. On one occasion, a phenolic laminate insulator caught fire from the discharges. One Araldite insulator was also charred under fog. The curve of insulator leakage current vs. time under warm, humid air at 101kV and the leakage current curve of the same type of insulator under fog at 50 kV are shown in Figs. 5(a) and (b) respectively. Figure 6 shows the temperature and humidity variations corresponding to Fig. 5(a). Two components of the leakage current can be distinguished from Fig. 5, the a.c. component which depends upon the surface resistance and the superimposed current pulses. These current pulses are produced by discharges across the dry patches of the insulator surfaces. These dry patches are created by irregular heating of the insulator surface by the leakage current; the current pulses also depend on the insulator material. That the origin of these current pulses is the irregular drying of the insulator surface is shown by the fact that these current pulses appear when the leakage current starts to decrease. The current pulses superimposed on the leakage current were observed earlier by Forrest (1).He concluded that the voltage withstand capability of an insulator is dependent on the magnitude and the repetition rate of these current pulses. Similar conclusion can also be drawn from our study. However, we have found that the characteristic of the current pulses is strongly influenced by the material of the insulator. For instance, two insulators of different materials having the same withstand capability in wet conditions have different leakage current characteristics. In general, the current pulses on porcelain are larger than that on either Araldite or phenolic laminate. Some interesting discharge phenomena were observed and photographed (Fig. 7) during the fog tests. At the beginning of a test under constant voltage, glow discharges appeared. These glow discharges were subsequently
Vol. 294, No. 6, December 1972
391
Karl Berger and Pritindra Chowdhuri replaced by violet-colored brush discharges which partially covered the insulator surface [Fig. 7(b)]. The brush discharges increased in length and number as the test continued at no increase in voltage [Fig. 7(c)] and finally started revolving around the insulator surface [Fig. 7(d)] causing the small current pulses in the leakage current. From these revolving discharges,
a
2.0
E
0
5
IO
15
Time,
min
20
25
a
E
2.0
‘; 2
2 0 g
I.0
s x -I 0
0
IO
20
30 Time,
FIG. 5. Lmkage
40
50
60
min
current on the surface of insulator: (a) under dew at 101 kV of applied voltage, (b) under fog at 50 kV of applied voltage.
yellow burning points sometimes appeared which moved downwards. Burning yellow arcs appeared along their discharge path [Fig. 7(e)]. These burning arcs caused conspicuously large current pulses in the leakage current which afterwards disappeared. The different stages of the abovementioned discharge phenomena were repeated with time, but they never stayed in any particular stage more than a few seconds. If the voltage
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Journalof The FranklinInstitute
Behavior of Insulators Under Dew and Fog stress was too high, a very intense bluish-white discharge having a crackling sound appeared immediately after the yellow arc [Fig. 7(f)]. Generally, these bluish-white discharges resulted in insulator flashover. In some cases, the insulators recovered again and the pattern of the discharge phenomena was repeated.
0 0
2
4
6
8
IO
I2
Time,
L
'0
“1
2
1
4
6
8
F
18
20
22
24
12
14
16
18
20
22
24
16
I8
20
22
24
min
4 2 '0
2
4
6
8
IO Time,
FIG. 6. Humidity
16
”
1’
IO Time,
iT z k 3
14 min
and temperature
I2
14 min
variations of air and insulator surface during test under dew.
The pattern of the discharge phenomena on the K4 insulator is shown in Fig. 8. After tests with 60 kV voltage it was found that its surface was charred and damaged by creepage paths. Figures 9(a) and (b) show the surface before and after cleaning with alcohol. The dry flashover voltages before and after cleaning were 7.7 and l-9 per cent lower than the earlier dry flashover
Vol.294,No.6,December1972
393
Karl Berger and P&&a
Chowdhuri
voltage. The test on the Hl insulator at 50 kV was discontinued when it caught fire. The insulation material of the J3 and 54 insulators was swollen after the fog test. A few tests were performed without cooling the surface of the insulator to 0°C; in other words, the insulator surface and the fog had the same temperature. It was found that the initial leakage current for the precooled insulator was very high. However, the insulator surface dried fast because of the higher leakage current, so that eventually the leakage current became identical to that of the insulator which was not precooled. The small current pulses were frequent, however, with the cooled insulator. The difference between the small and the large current pulses should be borne in mind. The small pulses were produced by the revolving discharges, whereas the large pulses were produced by the burning arc. As these burning arcs are more important than the revolving discharges, it seems that the precooling of the insulator does not play a very important role in the withstand capability of the insulator. In an additional test, an insulator was wetted first by fog, and then before applying the voltage, the fog was discontinued. For comparison, the same insulator was tested under the same conditions except that the fog was not shut off during the period the voltage was applied. By comparing the leakage current curves under these two conditions, it was found that the leakage current curve for the continued fog condition had large current pulses, thus signifying the presence of the burning arc. Therefore, under this condition the withstand capability would be lower. The D-type insulators, and the insulators F4 and F5 have about the same dry flashover voltage and equal insulation length. Besides, the D-type insulators have the same configuration of electrodes. Of these four types, the brown insulators D7 and D8 have the highest withstand capability under dew and fog. It is possible that the brown insulators warm up faster because of the better heat absorption capacity of the color, thus preventing further precipitation on the insulator surface. The performance of the stab insulator was better than that of the others. It was observed during the precipitation by fog that water precipitated mostly on the skirts. During the test under voltage, the discharges appeared mostly along the vertical portions (Fig. 10) because these parts dried up quicker. In such cases where the total discharges are divided up by smaller discharges, higher voltages are necessary to cause flashover. IV.
Conclusions
In this research, the following conclusions have been reached. (1) The effect of fog is much more harmful than that of dew. (2) A long-duration test is more severe to an insulator under fog or dew than a flashover test where the power-frequency voltage is increased rapidly. (3) Phenolic laminate and epoxy compound may not be good insulator materials for fog or a damp atmosphere. (4) In an isolated neutral system, the tested indoor-type
394
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Behavior of Insulators Under Dew and Fog insulators may flashover, even when clean, if a ground fault occurs during a sudden rise of temperature and formation of heavy fog. (5) Under polluted conditions, the results would be catastrophic even at the normal system voltage during a sudden rise of temperature and formation of heavy fog. References S. Forrest, “The electrical characteristics of 132 kV line insulators under various weather conditions”, J. IEE, Vol. 79, pp. 401-423, 1936. (2) M. J. CuilhB, “Recherches effect&es au Laboratoire Central d%lectricit6 sur les conditions de d&p&s de roske sur les isolateurs des lignes ahriennes”, Rev. Gkn. de E’l&ec., Vol. 46, pp. 427-428, 1939. (3) J. S. Forrest, “The characteristics and performance in service of high voltage porcelain insulators”, J. IEE, Vol. 89, pp. 60-92, 1942. (4) P. Schuepp, “Contribution it 1’8tude des ‘incidents du matin’ sur les lignes de transport d’bnergie”, Rev. Ge’n. de I’.l&ec., Vol. 56, pp. 103-124, 1947. als Fremdschicht(5) W. Estorff and H. v. Cron, “Der Hochspannungsisolator Problem”, Elektrotechnische Zeitschrift, Ausgabe A, Vol. 73, pp. 57-62, 1952. (6) H. v. Cron, W. Estorff and H. Lapple, “The insulating ability of high-tension insulators under various surface conditions”, CIGRg report No. 218, 1954. (7) G. Reverey, “Der Fremdschicht-uberschlag an Isolatoren bei Betriebspannung”, Elektrotechnische Zeitschrift, Ausgabe A, Vol. 76, pp. 36-42, 1955. (8) H. v. Cron and H. Dorsch, “Proportioning transmission system insulation to service frequency overvoltages and switching surges, with due consideration for loss of insulation strength through foreign body surface layers”, CIGRZ paper No. 402, 1958. (9) K. Yamazaki, N. Mita, J. Tomiyama and Y. Miyoshi, “Countermeasures against salt pollution on insulators used for extra high voltage transmission system located near and along sea coast”, CIGRZ paper No. 412, 1958. (10)J. S. Forrest, P. J. Lambeth and D. F. Oakeshott, “Research on the performance of high-voltage insulators in polluted atmospheres”, Proc. IEE, Vol. 107, pp. 172-196, 1960. (11) E. Nasser, “Zum Problem des Fremdschichtiiberschlages an Isolatoren”, Elektrotechnische Zeitschrijt, Ausgabe A, Vol. 83, pp. 356-365, 1962. (12) N. Takasu, “Development of a new durable salt-dust-fog insulator”, Elec. Eng. Jap., Vol. 83, pp. 63-75, 1963. (13) G. Sala and L. Squintani, “A note on the behavior of electrical insulators near the sea”, RC(65), Riun. Assoc. Elettrotec. Ital., (Palermo), paper 11, 1964. (14) M. Covino, “The use of silicone for improving the performance of insulators in a polluted atmosphere”, ibid., paper 21. (15) B. F. Hampton, “Flashover mechanism of polluted insulation”, Proc. IEE, Vol. 111, pp. 985-990, 1964. (16) C.H. A. Ely and P. J. Lambeth, “Artificial-pollution test for high voltage outdoor insulators”, ibid., pp. 991-998. (17) C. Gregoire, “Behavior of line insulators and equipment under conditions of natural and artificial pollution: Belgian experiments”, CIGR& paper No. 211, 1964. (1) J.
(18) H. Baatz, G. Boll, 0. Brennecke, G. Niehage, G. Reverey and T. Vogelsang, “New field experience with outdoor insulators in pollution areas and methods of assessing the performance of insulation under condition of pollution”, CIGR$ paper No. 212, 1964. (19) J. Kopeliowitch, “Operating results of the h.v. network of Israel from the point of view of insulator performance under pollution”, CIGRg paper No. 228, 1964.
Vol.
294,
No.
6, December
1972
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Karl Berger and Pritindra Chowdhuri (20) D. Mark, “Design and operating problems for electrical insulators in polluted areas”, Electrotechnica (Rumania), Vol. 13, pp. 320-326, 1965. (21) D. J. Parr and R. M. Scarisbrick, “Performance of synthetic insulating materials under polluted conditions”, Proc. IEE, Vol. 112, pp. 1625-1632, 1965. (22) J. E. Toms and A. B. Suttie, “Insulator surface treatments”, Elec. Rev., Vol. 177, pp. 412-415, Sept. 1965. (23) C. H. W. Clark, “Wet tests on high voltage insulators”, ibid., pp. 754-756, Nov. (24) R. J. Tetreault, “The effects of weather on epoxy insulators for high voltage transmission lines”, Insulation, Vol. 11, pp. 37-41, 1965. (25) A. Tominaga, “Moisture absorption and leakage resistance on contaminated surfaces”, Elec. Eng. Jap., Vol. 85, pp. 33-42, 1965. (26) G. I. Lysakovskii, “Combating the fouling of insulation”, Elekt. Stan&i (USSR), Vol. 3, pp. 85-86, 1966. (27) M. P. Fedotov, “Some methods of combating the fouling of high voltage insulation”, ibid., pp. 88-89. (28) G. Reverey, “Die Isolierung bei Verschmutzung und Regen”, Elektrotechnische Zeitechrift, Ausgabe A, Vol. 87, pp. 46-52, 1966. (29) H. Nacke, “Stabilitgt der Fremdschichtentledungen und Theorie des Fremdschichtiiberschlags”, ibid., pp. 577-585. (30) F. H. Last, T. H. Pegg, N. Sellers, A. Stalewski and E. B. Whittaker, “Live washing of h.v. insulators in polluted areas”, Proc. IEE, Vol. 113, pp. 847-860, 1966. (31) P. J. Lrtmbeth, J. S. T. Looms, A. Stalewski and W. 0. Todd, “Surface coatings for h.v. insulators in polluted aress”, ibid., pp. 861-869. (32) S. D. Merkhalev and E. A. Solomonik, “How the capacity of the test circuit affects the flashover characteristics of fouled insulators with alternating voltage”, Elektrichestvo (USSR), 9, pp. 43-46, 1966. (33) Z. Pohl, “The use of hydrophobic paste as a protection to high voltage insulation in polluted atmosphere”, BiuZ Inst. Energetyki (Poland), Vol. 22, pp. 301-305, 1966. (34) A. Annestrand and A. Schei, “A test procedure for artificial pollution test on direct voltage”, Direct Current, Vol. 12, pp. l-8, 1967. (35) L. Causse, “Et& des experiences sur la tenue Pllectrique des chaines d’isolateurs sous pollution naturelle-stations d’essais automatiques in situ”, Rev. G&n. de Z’EZectricitd, Vol. 76, pp. 172-185, 1967. (36) S. Hesketh, “General criterion for the prediction of pollution flashover”, Proc. IEE, Vol. 114, pp. 531-532, 1967. (37) M. M. Khalifa and R. M. Morris, “Performance of line insulators under rime ice”, IEEE Trans. Power Apparatus and Systems, Vol. PAS-86, pp. 693-698, June 1967. (38) J. D. M. Phelphs, J. B. Owens and A. Foti, “Testing EHV station insulation for performance in contaminated conditions” , ibid., Vol. PAS-87, pp. 448-454, Feb. 1968. (39) S. Fujitake, T. Kawamaru, S. Tsurumi, H. Kondo, T. Seta and M. Yamamoto. “Japanese method of artificial pollution tests on insulators”, ibid., pp. 729-735, March 1968. (40) E. Nasser, “Some physical properties of electrical discharges on contaminated surfaces”, ibid., pp. 957-963, April 1968. (41) J. Johnson, R. T. Henderson, W. S. Price, D. E. Hedman and F. J. Turner, “Field and laboratory tests of contaminated insulators for the design of the State Electricity Commission of Victoria’s 500-kV system”, ibid., pp. 1216-1239, May 1968. (42) S. Zoledziowski, “Time-to-flashover characteristics of polluted insulation”, ibid., pp. 1397-1404, June 1968.
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of The Franklin
Institute
Behavior of Insulators Under Dew and Fog (43) ‘M. Kawai and D. M. Milone, “Tests on salt-contaminated insulators in artificial and natural wet conditions”, ibid., Vol. PAS-S& pp. 13941399, Sept. 1969. (44) B. Macchiaroli and F. J. Turner, “A new contamination test method”, ibid., pp. 1400-1411. (45) B. Macchiaroli and F. J. Turner, “Comparison of insulator types by the wet contaminant and clear fog test method” , ibid., Vol. PAS-89, pp. 190-191, Feb. 1970. (46) M. Kawai and D. M. Milone, “Flashover tests at project UHV on saltcontaminated insulators”, ibid., pp. 755-761, May/June 1970. (47) B. Macchiaroli and F. J. Turner, “A study of some variables affecting contamination testing using the wet contaminant method”, ibid., pp. 761-770, 1969. (48) A. J. McElroy, W. J. Lyon, J. D. M. Phelps and H. H. Woodson, “Insulators with contaminated surfaces: Part I. Field conditions and their laboratory simulation”, ibid., pp. 1848-1858, Nov./Dee. 1970. (49) H. H. Woodson and A. J. McElroy, “Insulators with contaminated surfaces : Part II. Modeling of discharge mechanisms”, ibid., pp. 1858-1867, 1970. (50) H. H. Woodson and A. J. McElroy, “Insulators with contaminated surfaces: Part III. Modeling of dry zone formation”, ibid., pp. 1868-1876, 1970. (51) E. Kobel, “Elektrischer Feuchtigkeitsmesser” , Schweizer Arch& fiir Ange. Wk. u. Tech., Vol. 11, pp. 238-241, 1945.
Vol. 294,
No.
6, December
1972
397
BERGER
and PRITINDRA
Forschungskommission
ABSTRACT:
The
des SEV und VSE j&r Hochspannungsfragen
sparkover
characteristics
50 Hz voltage under simulated with different duration front
materials
CHOWDHURI*
conditions
are discussed.
of tests is proposed.
of insukztors
A test procedure
It consists of the superposition
on the rated 50 Hz voltage applied
subjected
from
of dew and fog are described. for
the purpose
20 to 80 min Results
to
obtained
of reducing
the
of a Burge voltage of slow-rising
to the insulator.
1. Introduction
The reduction in the breakdown strength of contaminated insulators has long been a serious problem (l-9). However, recently a resurgence of interest in the behavior of contaminated insulators has been caused by the need for extra high voltage power transmission (1040). It is generally assumed that a reduction in the breakdown strength of insulators becomes critical in heavily polluted areas. However, drastic reduction of the breakdown strength of insulators has been observed in countries such as Switzerland where the atmosphere in many sections is reasonably unpolluted. For example, during spring and autumn in Central Europe, a warm humid wind-the socalled “Fohn’-occasionally blows from the Alps to the valleys. This wind precipitates moisture on cold insulator surfaces, sometimes causing flashover at rated 50 Hz voltages. This phenomenon is particularly noticeable during the early morning hours (4). Even indoor-type insulators which are fairly uncontaminated are affected by these incidents. Such flashovers are initiated by partial evaporation of the insulatorsurface moisture as a result of increased leakage current over the surface. The moisture evaporation produced by the leakage current is counteracted by a continual condensation of moisture from the surrounding air. In this way, the surface becomes coated with irregular patches of water layer. This layer may increase the surface electrical stresses sufficiently to cause flashover at the system voltage. The addition of contamination in the water layer would reduce the flashover strength even further. The purpose of this study is to investigate the flashover characteristics of insulators under various conditions of moisture in the laboratory, and to determine a suitable test procedure in order to evaluate the performance of different types of insulators. Both indoor and outdoor types of insulators were tested. For insulator materials, porcelain, phenolic laminate and a * Now with General Electric
Company,
Erie, Pa., U.S.A.
385
Karl Berger and Pritindra Chowdhuri resinous epoxy compound known as Araldite were chosen. Figure 1 shows the different types of insulators investigated. As contamination on insulator surfaces adds another not always reproducible factor, all insulators were cleaned before testing. To account for manufacturing tolerances, at least two insulators of each type were tested. ZZ. Test Methods
Three types of tests were performed. During the first type, dew was simulated and the insulators were tested with long-duration 50 Hz voltages near the dry flashover value. The second type was similar to the first, except slow-front impulse voltage waves were superimposed on the rated 50 Hz voltage. During the third type, fog was simulated and the insulators were tested with long-duration 50 Hz voltages. Unlike the first type of test, the 50 Hz test voltage levels were near the rated voltage of the insulator. First Test Method The insulator was air cooled to 0°C. Then 90 per cent of its dry flashover voltage was applied to it, and warm humid air was blown over its surface (Pig. 2). Surface leakage current was measured by a recording milliammeter,
FIG. 2. Schematic diagram of principle of dew generation: (A) test insulator, (B) flap doors to control air flow, (C) heater, (D) boiling water, (E) fan.
and humidity as well as the temperature of the test chamber and the insulator surface were monitored continuously. After 10 min, the flow of humid air was cut off and the test was continued for another 10 min. If the insulator flashed over, 85 per cent of the dry sparkover voltage was impressed on its terminals for another 20 min and the same procedure was repeated until no further flashover occurred at a certain lower voltage level. Tests were repeated three times at this voltage level to make sure that the insulator did not spark over. The surface temperature of the insulator under test could not be measured directly when voltage was applied. Therefore, the surface temperature was measured on an identical but de-energized insulator having the same current of air passing over its surface. At the same time, precautions were taken that the second insulator did not have the proximity effect on the sparkover voltage of the first. All surface and air temperatures were measured by copper-constantan thermocouples.
386
Journal of The FranklinInstitute
Behavior of Insulators Under Dew and Fog Measurement of humidity posed a problem. The change of humidity caused by the blowing of warm air was so rapid that conventional hygrometers were useless. Instead, humidities were measured by a special humidity recorder (51). This instrument is essentially a Wheatstone bridge with its four arms made of platinum wires enclosed in four separate chambers. These four platinum wires were heated from a direct current source. The humid air was passed through two chambers and then after being dried by Silica gel the same air was passed through the other two chambers. The difference in radiation coefficients of dry and humid air produced a resistance unbalance. The unbalance current was read by a milliammeter which was directly calibrated in grams of water in cubic meter of air at atmospheric pressure. The continuous voltages applied to the insulators were much higher than their steady-state rating (Tables I and II). It was suspected that the leakage current due to these higher terminal voltages might have dried the surface so much that the optimum proportion of dry and wet patches was not produced to cause a complete flashover. To check this possibility a second series of tests was made. Xecond Test Method The ambient conditions were the same in this test as in the first test procedure. Electrically, only the rated 50 Hz voltage was impressed on the test piece simultaneously with the blowing of warm, humid air on its cold surface. The insulation strength was measured by superimposing, at 30 set intervals, a surge voltage wave of the same time to crest as the 50 Hz voltage. The surge wave, being applied momentarily to the insulator, did not disturb the drying processes under normal voltage condition, and since this surge had the same time to crest as the continuous applied voltage, the insulation strength under power-frequency condition could be obtained. The schematic diagram of the test setup is given in Fig. 3. Humidity and temperatures were measured in the same way as in the first test method.
FIG. 3. Schematic diagram of test circuit for superimposing long-front surge wave on power-frequency voltage : (A) phase-shifting network, (B) auxiliary impulse generator, (C) impulse generator, (D) quench gap, (E) environmental tank, (F) high-voltage transformer, (G) harmonic filters, (H) voltage regulator.
Vol. 294, No. 6, December
1972
387
3 & B m p i B B
Fi
White
Brown
Brown
D4 D5
D7 D8
Ll L2
The conductivity
White
C6 c7
120 117
178 186
180 182
69 69
t;
138
148
126 136 145
122
(kv) (rms)
Dry 50 Hz flashover voltage
(First test method)
Dry flashover voltage
Flashover voltage under dew
39.0
60.0
68.5
25.5
per element
15.0
36.5
29.0
Creepage distance
TABLE I insulators
P:, =
130.0 126.5
197.5 208.0
201.5 204.0
76.6 76.2
101.2 106.0
153.0
164.5
141.0 149.5 161.0
137.0
(kv) (rms)
Corrected dry flashover voltage
$/cm.
(Third test method)
Dry flashover voltage
Flashover voltage under fog
737/9*0/4.9 737/9.0/4.9
727/- 0*2/4.6 730/0.1/4.5
728/-O-314.1 730/- 0.514.4
725/0.0/4.4 725/0.4/4,7
72310.414.7 724/0*8/4.5
728/0.5/4*7
72810.314.4
730/0.0/4.6 720/1.0/4.2 72810.614.5
731/0.0/4.1
Atmospheric conditions of air (mm Hg/“C/glm3)
of reaults with porcelain
of water generating dew and fog was between 328-358
PO =
26.2
52.0
50.0
17.5
28.0
25.5
Brown
Brown
Cl
36.5
c4 c5
Brown
A3 B2 B3
21.5
skirt dia.
Brown
A2
Arcing distance
Length (cm)
c2
Color of material
Designation of insulator
Summary
76.5 71.5
64,4 57.2
72.0 77.0
82.0 81.5
72.4
65.0
71.6 72.7 72.0
71.5
(G
61.5 63.5
35.4 33.6
27.3 27.0
36.5 34.0
59.5 52.0
43.5
39.5
42.5 46.8 43.5
40.1
P: (%)
Stab insulator
Indoor type
Indoor type
Indoor type
type Bushing
suspension
Three-element
insulators Bell-type wideskirt insulators
Motor
Remarks
? Y cr.
2
z
& 3
; rr’ cr.
f
3 Y
Phenolic -laminate yellow
Araldite reddish brown
J5 J6
K4 K5
Length
voltage
> 81.0 > 81.0
162.0 161.0
voltage
under dew
voltage Dry flashover
Flashover 328-358 @/cm.
P:, =
722/2.2/5*1 724/1*7/4.8
727/0*4/4,5 727/0.9/4*5
195.5 182.0
> 82.5 > 82.5
140.0 140.0
723/1,7/4.9 722/l-3/4.9
72.0 72.0
196.5 198.0
730/0.8/4.7 730/1*0/4.5
voltage
under fog
73.5 73.0
81.0 72.0 > 72.7
177.0 167.5 171.0
30.7 33.0
46.5 46.5
25.4
32.8 32.4
82.0 81.7
729/0*9/4*5 726/0.6/4.2 722/0*9/4.4
37.8 35.8 36.3
73.8 82.0 > 82.5
27.2 27.0
72.0 67.5
198.5 201.0
158.5 153.2 151.5
202 204
P:, (%)
(B)
insulators
Corrected dry flashover voltage (kV) (rms)
laminate
721/15/4,5 7.26/0*9/4.8
722/2.0/5*2 719/0.6/4*6 719/1*4/4*7
727/0.1/4+2 726/1.0/4.2
Atmospheric conditions of air (mm Hg/‘C/g/m3)
dew and fog was between
Dry flashover
Flashover
180 186
146 145
55.0
50.0
128 128
52.0
177 178
159 151 155
63.0
146 140 139
182 184
Dry 50 Hz flashover voltage (kV) (rms)
181 182
62.5
TABLE II
of results with araldite and Phenolic
52.5
40.0
59.5
Creepage distance
(cm)
generating
PO =
48.0
42.0
36.0
44.5
44.5
52.5
40.0
52.0
Arcing distance
of water
Phenolic laminate yellow
53 54
The conductivity
Phenolic laminate
grey Phenolic laminate red
Araldite
Hl H2
E F9
F4 F5
red
Araldite
Fl
E
Araldite dark brown
Dl D2
grey
Color of material
Designation of insulator
Sumrnaq
type
type
type
type
type
Indoor type material burnt and charred during fog test
Indoor
Indoor type material fractured during fog test
Indoor type-H1 burnt and charred during fog test
Indoor
Indoor
Indoor
Indoor
Remarks
Karl Berger and P&n&a
Chowdhuri
The advantage in this second test method was that it greatly reduced the time of testing. In the first test method, the flashover test sometimes lasted 8 hr, whereas the time required by the second test method was only 30 min1 hr. Unfortunately, the slow front of the surge wave reduced the efficiency of the surge generator with the consequence that the available crest of the surge was much reduced. Because of this, only the insulators of the lower voltage classes could be tested. Third Test Method This third test method is similar to the first. It was devised to determine the effects of fog on insulator surfaces. Figure 4 shows the general principle l-7
I
FIG. 4. Schematic diagram of principle of fog generation : (A) water reservoir, (B) valve, (C) apparatus for measuring water conductivity, (D) valve, (E) valve, (F) manometer, (G) pump, (H) water tank, (I) valve, (J) valve, (K) water atomizer.
of fog generation. Either rain or fog could be artificially made by opening the valve D or E. The insulator which was precooled to 0°C was placed in a tank where fog was being generated at 20°C. After about 10 min the rated voltage was applied to the insulator for 30 min. If the insulator did not sparkover during this time, the voltage level was increased by 5 kV, and the whole procedure repeated until a voltage level was reached at which the insulator finally flashed over. The voltage level was then lowered by 5 kV and the test repeated three times to make sure that it did not sparkover at any time at this lower level of voltage. Although normally the duration of a test was 30 min, tests were extended up to a maximum duration of 80 min in doubtful cases. The duration of a test was set by the leakage current characteristics of the insulators.
390
Journal
of The Franklin
Institute
Behavior of Insulators Under Dew and Fog The limitation of the crest value of the long-front surge wave was the deciding factor in not applying it during these tests under fog in order to be able to determine the flashover characteristics of all the insulators available. ZZZ. Results
and Discussion
The summary of the results obtained under dew (first test method) and fog (third test method) tests is shown in Tables I and II. Because of the limited capacity of the impulse generator, the second test method was applied only to the insulators C, and C,. The flashover voltages of C, and C, under dew by the first and the second test methods were the same. The lowest flashover voltage under warm, humid air (dew) was 57 per cent of the dry flashover voltage for the insulators tested. Under fog, the lowest flashover voltage was only 25 per cent of the dry flashover voltage. None of the insulators tested flashed over when voltage was applied. Under fog, the longest time required to flashover was about 35 min. The reduction of insulation strength of phenolic laminate and Araldite insulators under warm, humid air was less than that of porcelain insulators. However, phenolic laminate insulators were either swollen or charred under fog. On one occasion, a phenolic laminate insulator caught fire from the discharges. One Araldite insulator was also charred under fog. The curve of insulator leakage current vs. time under warm, humid air at 101kV and the leakage current curve of the same type of insulator under fog at 50 kV are shown in Figs. 5(a) and (b) respectively. Figure 6 shows the temperature and humidity variations corresponding to Fig. 5(a). Two components of the leakage current can be distinguished from Fig. 5, the a.c. component which depends upon the surface resistance and the superimposed current pulses. These current pulses are produced by discharges across the dry patches of the insulator surfaces. These dry patches are created by irregular heating of the insulator surface by the leakage current; the current pulses also depend on the insulator material. That the origin of these current pulses is the irregular drying of the insulator surface is shown by the fact that these current pulses appear when the leakage current starts to decrease. The current pulses superimposed on the leakage current were observed earlier by Forrest (1).He concluded that the voltage withstand capability of an insulator is dependent on the magnitude and the repetition rate of these current pulses. Similar conclusion can also be drawn from our study. However, we have found that the characteristic of the current pulses is strongly influenced by the material of the insulator. For instance, two insulators of different materials having the same withstand capability in wet conditions have different leakage current characteristics. In general, the current pulses on porcelain are larger than that on either Araldite or phenolic laminate. Some interesting discharge phenomena were observed and photographed (Fig. 7) during the fog tests. At the beginning of a test under constant voltage, glow discharges appeared. These glow discharges were subsequently
Vol. 294, No. 6, December 1972
391
Karl Berger and Pritindra Chowdhuri replaced by violet-colored brush discharges which partially covered the insulator surface [Fig. 7(b)]. The brush discharges increased in length and number as the test continued at no increase in voltage [Fig. 7(c)] and finally started revolving around the insulator surface [Fig. 7(d)] causing the small current pulses in the leakage current. From these revolving discharges,
a
2.0
E
0
5
IO
15
Time,
min
20
25
a
E
2.0
‘; 2
2 0 g
I.0
s x -I 0
0
IO
20
30 Time,
FIG. 5. Lmkage
40
50
60
min
current on the surface of insulator: (a) under dew at 101 kV of applied voltage, (b) under fog at 50 kV of applied voltage.
yellow burning points sometimes appeared which moved downwards. Burning yellow arcs appeared along their discharge path [Fig. 7(e)]. These burning arcs caused conspicuously large current pulses in the leakage current which afterwards disappeared. The different stages of the abovementioned discharge phenomena were repeated with time, but they never stayed in any particular stage more than a few seconds. If the voltage
392
Journalof The FranklinInstitute
Behavior of Insulators Under Dew and Fog stress was too high, a very intense bluish-white discharge having a crackling sound appeared immediately after the yellow arc [Fig. 7(f)]. Generally, these bluish-white discharges resulted in insulator flashover. In some cases, the insulators recovered again and the pattern of the discharge phenomena was repeated.
0 0
2
4
6
8
IO
I2
Time,
L
'0
“1
2
1
4
6
8
F
18
20
22
24
12
14
16
18
20
22
24
16
I8
20
22
24
min
4 2 '0
2
4
6
8
IO Time,
FIG. 6. Humidity
16
”
1’
IO Time,
iT z k 3
14 min
and temperature
I2
14 min
variations of air and insulator surface during test under dew.
The pattern of the discharge phenomena on the K4 insulator is shown in Fig. 8. After tests with 60 kV voltage it was found that its surface was charred and damaged by creepage paths. Figures 9(a) and (b) show the surface before and after cleaning with alcohol. The dry flashover voltages before and after cleaning were 7.7 and l-9 per cent lower than the earlier dry flashover
Vol.294,No.6,December1972
393
Karl Berger and P&&a
Chowdhuri
voltage. The test on the Hl insulator at 50 kV was discontinued when it caught fire. The insulation material of the J3 and 54 insulators was swollen after the fog test. A few tests were performed without cooling the surface of the insulator to 0°C; in other words, the insulator surface and the fog had the same temperature. It was found that the initial leakage current for the precooled insulator was very high. However, the insulator surface dried fast because of the higher leakage current, so that eventually the leakage current became identical to that of the insulator which was not precooled. The small current pulses were frequent, however, with the cooled insulator. The difference between the small and the large current pulses should be borne in mind. The small pulses were produced by the revolving discharges, whereas the large pulses were produced by the burning arc. As these burning arcs are more important than the revolving discharges, it seems that the precooling of the insulator does not play a very important role in the withstand capability of the insulator. In an additional test, an insulator was wetted first by fog, and then before applying the voltage, the fog was discontinued. For comparison, the same insulator was tested under the same conditions except that the fog was not shut off during the period the voltage was applied. By comparing the leakage current curves under these two conditions, it was found that the leakage current curve for the continued fog condition had large current pulses, thus signifying the presence of the burning arc. Therefore, under this condition the withstand capability would be lower. The D-type insulators, and the insulators F4 and F5 have about the same dry flashover voltage and equal insulation length. Besides, the D-type insulators have the same configuration of electrodes. Of these four types, the brown insulators D7 and D8 have the highest withstand capability under dew and fog. It is possible that the brown insulators warm up faster because of the better heat absorption capacity of the color, thus preventing further precipitation on the insulator surface. The performance of the stab insulator was better than that of the others. It was observed during the precipitation by fog that water precipitated mostly on the skirts. During the test under voltage, the discharges appeared mostly along the vertical portions (Fig. 10) because these parts dried up quicker. In such cases where the total discharges are divided up by smaller discharges, higher voltages are necessary to cause flashover. IV.
Conclusions
In this research, the following conclusions have been reached. (1) The effect of fog is much more harmful than that of dew. (2) A long-duration test is more severe to an insulator under fog or dew than a flashover test where the power-frequency voltage is increased rapidly. (3) Phenolic laminate and epoxy compound may not be good insulator materials for fog or a damp atmosphere. (4) In an isolated neutral system, the tested indoor-type
394
Joumal
of The Franklin
Institute
Behavior of Insulators Under Dew and Fog insulators may flashover, even when clean, if a ground fault occurs during a sudden rise of temperature and formation of heavy fog. (5) Under polluted conditions, the results would be catastrophic even at the normal system voltage during a sudden rise of temperature and formation of heavy fog. References S. Forrest, “The electrical characteristics of 132 kV line insulators under various weather conditions”, J. IEE, Vol. 79, pp. 401-423, 1936. (2) M. J. CuilhB, “Recherches effect&es au Laboratoire Central d%lectricit6 sur les conditions de d&p&s de roske sur les isolateurs des lignes ahriennes”, Rev. Gkn. de E’l&ec., Vol. 46, pp. 427-428, 1939. (3) J. S. Forrest, “The characteristics and performance in service of high voltage porcelain insulators”, J. IEE, Vol. 89, pp. 60-92, 1942. (4) P. Schuepp, “Contribution it 1’8tude des ‘incidents du matin’ sur les lignes de transport d’bnergie”, Rev. Ge’n. de I’.l&ec., Vol. 56, pp. 103-124, 1947. als Fremdschicht(5) W. Estorff and H. v. Cron, “Der Hochspannungsisolator Problem”, Elektrotechnische Zeitschrift, Ausgabe A, Vol. 73, pp. 57-62, 1952. (6) H. v. Cron, W. Estorff and H. Lapple, “The insulating ability of high-tension insulators under various surface conditions”, CIGRg report No. 218, 1954. (7) G. Reverey, “Der Fremdschicht-uberschlag an Isolatoren bei Betriebspannung”, Elektrotechnische Zeitschrift, Ausgabe A, Vol. 76, pp. 36-42, 1955. (8) H. v. Cron and H. Dorsch, “Proportioning transmission system insulation to service frequency overvoltages and switching surges, with due consideration for loss of insulation strength through foreign body surface layers”, CIGRZ paper No. 402, 1958. (9) K. Yamazaki, N. Mita, J. Tomiyama and Y. Miyoshi, “Countermeasures against salt pollution on insulators used for extra high voltage transmission system located near and along sea coast”, CIGRZ paper No. 412, 1958. (10)J. S. Forrest, P. J. Lambeth and D. F. Oakeshott, “Research on the performance of high-voltage insulators in polluted atmospheres”, Proc. IEE, Vol. 107, pp. 172-196, 1960. (11) E. Nasser, “Zum Problem des Fremdschichtiiberschlages an Isolatoren”, Elektrotechnische Zeitschrijt, Ausgabe A, Vol. 83, pp. 356-365, 1962. (12) N. Takasu, “Development of a new durable salt-dust-fog insulator”, Elec. Eng. Jap., Vol. 83, pp. 63-75, 1963. (13) G. Sala and L. Squintani, “A note on the behavior of electrical insulators near the sea”, RC(65), Riun. Assoc. Elettrotec. Ital., (Palermo), paper 11, 1964. (14) M. Covino, “The use of silicone for improving the performance of insulators in a polluted atmosphere”, ibid., paper 21. (15) B. F. Hampton, “Flashover mechanism of polluted insulation”, Proc. IEE, Vol. 111, pp. 985-990, 1964. (16) C.H. A. Ely and P. J. Lambeth, “Artificial-pollution test for high voltage outdoor insulators”, ibid., pp. 991-998. (17) C. Gregoire, “Behavior of line insulators and equipment under conditions of natural and artificial pollution: Belgian experiments”, CIGR& paper No. 211, 1964. (1) J.
(18) H. Baatz, G. Boll, 0. Brennecke, G. Niehage, G. Reverey and T. Vogelsang, “New field experience with outdoor insulators in pollution areas and methods of assessing the performance of insulation under condition of pollution”, CIGR$ paper No. 212, 1964. (19) J. Kopeliowitch, “Operating results of the h.v. network of Israel from the point of view of insulator performance under pollution”, CIGRg paper No. 228, 1964.
Vol.
294,
No.
6, December
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Karl Berger and Pritindra Chowdhuri (20) D. Mark, “Design and operating problems for electrical insulators in polluted areas”, Electrotechnica (Rumania), Vol. 13, pp. 320-326, 1965. (21) D. J. Parr and R. M. Scarisbrick, “Performance of synthetic insulating materials under polluted conditions”, Proc. IEE, Vol. 112, pp. 1625-1632, 1965. (22) J. E. Toms and A. B. Suttie, “Insulator surface treatments”, Elec. Rev., Vol. 177, pp. 412-415, Sept. 1965. (23) C. H. W. Clark, “Wet tests on high voltage insulators”, ibid., pp. 754-756, Nov. (24) R. J. Tetreault, “The effects of weather on epoxy insulators for high voltage transmission lines”, Insulation, Vol. 11, pp. 37-41, 1965. (25) A. Tominaga, “Moisture absorption and leakage resistance on contaminated surfaces”, Elec. Eng. Jap., Vol. 85, pp. 33-42, 1965. (26) G. I. Lysakovskii, “Combating the fouling of insulation”, Elekt. Stan&i (USSR), Vol. 3, pp. 85-86, 1966. (27) M. P. Fedotov, “Some methods of combating the fouling of high voltage insulation”, ibid., pp. 88-89. (28) G. Reverey, “Die Isolierung bei Verschmutzung und Regen”, Elektrotechnische Zeitechrift, Ausgabe A, Vol. 87, pp. 46-52, 1966. (29) H. Nacke, “Stabilitgt der Fremdschichtentledungen und Theorie des Fremdschichtiiberschlags”, ibid., pp. 577-585. (30) F. H. Last, T. H. Pegg, N. Sellers, A. Stalewski and E. B. Whittaker, “Live washing of h.v. insulators in polluted areas”, Proc. IEE, Vol. 113, pp. 847-860, 1966. (31) P. J. Lrtmbeth, J. S. T. Looms, A. Stalewski and W. 0. Todd, “Surface coatings for h.v. insulators in polluted aress”, ibid., pp. 861-869. (32) S. D. Merkhalev and E. A. Solomonik, “How the capacity of the test circuit affects the flashover characteristics of fouled insulators with alternating voltage”, Elektrichestvo (USSR), 9, pp. 43-46, 1966. (33) Z. Pohl, “The use of hydrophobic paste as a protection to high voltage insulation in polluted atmosphere”, BiuZ Inst. Energetyki (Poland), Vol. 22, pp. 301-305, 1966. (34) A. Annestrand and A. Schei, “A test procedure for artificial pollution test on direct voltage”, Direct Current, Vol. 12, pp. l-8, 1967. (35) L. Causse, “Et& des experiences sur la tenue Pllectrique des chaines d’isolateurs sous pollution naturelle-stations d’essais automatiques in situ”, Rev. G&n. de Z’EZectricitd, Vol. 76, pp. 172-185, 1967. (36) S. Hesketh, “General criterion for the prediction of pollution flashover”, Proc. IEE, Vol. 114, pp. 531-532, 1967. (37) M. M. Khalifa and R. M. Morris, “Performance of line insulators under rime ice”, IEEE Trans. Power Apparatus and Systems, Vol. PAS-86, pp. 693-698, June 1967. (38) J. D. M. Phelphs, J. B. Owens and A. Foti, “Testing EHV station insulation for performance in contaminated conditions” , ibid., Vol. PAS-87, pp. 448-454, Feb. 1968. (39) S. Fujitake, T. Kawamaru, S. Tsurumi, H. Kondo, T. Seta and M. Yamamoto. “Japanese method of artificial pollution tests on insulators”, ibid., pp. 729-735, March 1968. (40) E. Nasser, “Some physical properties of electrical discharges on contaminated surfaces”, ibid., pp. 957-963, April 1968. (41) J. Johnson, R. T. Henderson, W. S. Price, D. E. Hedman and F. J. Turner, “Field and laboratory tests of contaminated insulators for the design of the State Electricity Commission of Victoria’s 500-kV system”, ibid., pp. 1216-1239, May 1968. (42) S. Zoledziowski, “Time-to-flashover characteristics of polluted insulation”, ibid., pp. 1397-1404, June 1968.
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Behavior of Insulators Under Dew and Fog (43) ‘M. Kawai and D. M. Milone, “Tests on salt-contaminated insulators in artificial and natural wet conditions”, ibid., Vol. PAS-S& pp. 13941399, Sept. 1969. (44) B. Macchiaroli and F. J. Turner, “A new contamination test method”, ibid., pp. 1400-1411. (45) B. Macchiaroli and F. J. Turner, “Comparison of insulator types by the wet contaminant and clear fog test method” , ibid., Vol. PAS-89, pp. 190-191, Feb. 1970. (46) M. Kawai and D. M. Milone, “Flashover tests at project UHV on saltcontaminated insulators”, ibid., pp. 755-761, May/June 1970. (47) B. Macchiaroli and F. J. Turner, “A study of some variables affecting contamination testing using the wet contaminant method”, ibid., pp. 761-770, 1969. (48) A. J. McElroy, W. J. Lyon, J. D. M. Phelps and H. H. Woodson, “Insulators with contaminated surfaces: Part I. Field conditions and their laboratory simulation”, ibid., pp. 1848-1858, Nov./Dee. 1970. (49) H. H. Woodson and A. J. McElroy, “Insulators with contaminated surfaces : Part II. Modeling of discharge mechanisms”, ibid., pp. 1858-1867, 1970. (50) H. H. Woodson and A. J. McElroy, “Insulators with contaminated surfaces: Part III. Modeling of dry zone formation”, ibid., pp. 1868-1876, 1970. (51) E. Kobel, “Elektrischer Feuchtigkeitsmesser” , Schweizer Arch& fiir Ange. Wk. u. Tech., Vol. 11, pp. 238-241, 1945.
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