36WG11/Mannheim/14 1999, Apr. 27-28
HOME PROGRAMME (MODIFIED at Mannheim)
1- Validity of the specific creepage distance concept In our opinion the “creepage distance” concept should still be considered a valid approach for the insulator design. It may be also utilised in setting-up maintenance criteria of insulators installed many years ago or localised in areas subjected to pollution increases. Our experience (principally based on the salt-fog laboratory test) suggests that this concept should be applied especially to insulators having a high profile efficiency.
2- ESDD values for table I On this matter more than one suggestion have been advanced in the past (see: Electra n°64 page. 131- Transmission Line Reference Book page 323 - IEEE PES Summer meeting July 1977: Report n° F77 639-8, page 4). The following table I shows the ranges of the ESDD values suggested by the authors for the relevant pollution levels. The right column of the table shows the ranges utilised for drawing the Italian pollution map (paper 3.20, CIRED, Brussels, May 1995). In this work the ESDD was measured on more than 4000 insulators removed from overhead lines. Less than 1% of all the values was higher than 0,5 mg/cm2, and therefore this value was assumed as the top value for the 4 pollution levels listed in the IEC 815. The range of each pollution level has been defined adopting a ratio equal to 2,5 between the minimum and maximum value. It should be taken into account that the ENEL specifications require a minimum Withstand Salinity of 14 kg/m3 for insulator sets installed in lightly polluted areas. Greasing and/or washing are generally put into practice on the insulators installed in areas characterised by the higher pollution level (very heavy). Table I- Ranges of the ESSD values suggested by various authors for the listed pollution levels. Electra n°64 (WG04SC33) Pollution level No pollution Very light Light Medium Heavy Very heavy Exceptional
Trans. Line Refer. Book
IEEE (In the paper the
values are given as an example)
ENEL proposal
ESDD (mg/cm2) 0.0075÷0,015 0,015÷0,03 0,03÷0,06 0,06÷0,12 0,12÷0,24 0,24÷0,48 >0,48
-
-
0,02÷0,025 0,04÷0,05 0,07÷0,1 0,2÷0,5 -
0÷0,03 0,03÷0,06 0,06÷0,10 0,1 (Up)
< 0.03 0,03÷0.08 0,08÷0.20 0,2÷0.50 >0.50
36WG11/Mannheim/14 1999, Apr. 27-28
3- Additional definitions for table I Not all the mountainous areas may be retained lightly polluted. For instance in valley-beds localised far from the see, a concentration of human activities can be found, especially if the valley-bed is large (glacial origin). The experience acquired in drawing the aforementioned pollution map suggests that these areas may be characterised by a mean pollution level. In a few cases exceptional pollution degree have been found on insulators installed in the proximity of particular human activities (mining, production and/or transformation industries). Very high values have also been found on MV (≡ 7-8 m above the ground) insulators localised near to the beaches, especially on the lower surfaces of the insulators having antifog profile. We suggest the addition of further pollution level, named exceptional; that could comply with the following description: - Areas of particularly moderate extent, subjected to heavy deposits of industrial or marine origin or a combination of both. - Areas characterised by low rainfall concentrated in a limited period of the year, but subjected to heavy wetting by condensation.
4- ESDD measurement (field experience) 12 strings of 9 standard type insulators and 12 strings of 9 anti-fog insulator were exposed in a lightly polluted side and supplied at 84 kV. At monthly intervals, two strings (one for each insulator type) is removed and measurements are made of the ESDD, separately on the two surface of each unit. After these measurements the two cleaned strings are again espoused, to be re-tested 12 months later. A similar experience was repeated in a heavily polluted site on two groups of 6 strings and the measurements were carried out every two months. The overall results are given in fig. 1 and 2. Comments. - Upper surfaces: - for both types of string the ESDD values show not clear effect of the position of the unit in the string. In lightly polluted side the ESDD seasonal variation seems to be more important than that of the rainfall rate. When the site pollution is higher the influence of the rainfall rate seems to increase. - Lower surfaces: -for both type of string the ESDD value relating to the first unit on line side is markedly higher than that of the remaining units. This may be attributed to the high electric field affecting this unit or to the fact that its lower surface is not protected by anything below, or to both reasons. To point out the possible effects of arcing devices the two strings washed at the end of February 76 were again exposed equipped with arcing rings. The results of measurements carried out on these strings one year after show ESDD values quite lower than the expected values for the strings without arcing device (see the ringed points of fig.1). For both types of strings the ESDD values reach a stabilised condition in about 1 year. A shorter exposition period is sufficient to reach a value higher than 80% of that relevant to stabilised condition.
36WG11/Mannheim/14 1999, Apr. 27-28
5- Opinion/knowledge on the key points of insulator geometry 5.1-Horizontal porcelain masts for insulated cross-arms AXIAL
TOTAL THE SET TEST OBJECT (m) N*
RATIO
A A* *
4,05
3
3,52 2+
B
4,05
3
C
4,05
3
C’
3,00.
2
A’
3,00
2
F
3,00
2
D
2,72
2
E
2,72
2
SPECIFIC CREEPAGE WITHSTAND DISTANCE SALINITY (mm/KV) (kg/m3)
HIGHEST RATIO l/d)
VOLTAGE
1,11
3,8
60
47,1
40
2,68
1,11
3,8
69
38,9
14
2,38
1,33
3,5
60
39,7
20
2,76
1,17
4,5
60
45,9
56
a+i a a+i
2,82
1,17
4,5
81
34,9
7
2,83
1,11
3,8
81
36
14-10* * *
2,52
1,63
3,9
81
31,1
7
s s
3.35
0,57
4,7
89
37,4
5
2.43
0,81
3,6
89
27,2
3,5
LENGTH OF
CREEPAGE FACTOR
s/p
a a a a+i
2,83
SHED SHAPE
(1)
* Number of insulators of the set (1) Shed shape: a = alternating sheds
STRESS
(kV/m)
**Length partially short circuited i = inclined s = standard shape
***Repeated test
The table gives the results of salt-fog tests carried out at the same test voltage (243 kV) on porcelain masts arranged in a lightly inclined position (6° versus the horizontal plane). The maximum shed diameter of all the masts ranges from 214 mm to 315mm. The WS values are also plotted in fig. 3 as a function of the specific creepage distance. Both the insulators C and C’ are characterised by a l/d ratio close to the limit (
