Role of Non-soluble Contaminants on HV Porcelain Insulator Flashover Voltage

IEEE Transactions on Dielectrics and Electrical Insulation Vol.
3 No. 1, February 1996 113
Role of Non-soluble Contaminants on the
Flashover Voltage of Porcelain Insulators
R. Sundararajan and I?. S. Gorur
Department of Electrical Engineering, Arizona State University, Tempe, AZ
ABSTRACT
The influence of non-soluble or inert contaminants on the flashover voltage of HV
outdoor porcelain insulators is examined. A dynamic arc model, presented earlier
for predicting the flashover voltage of porcelain insulators covered with water sol-
uble contaminants, has been modified suitably. The extended model has been used
to predict the flashover voltage of suspension and station post insulators
of various
shapes for both ac and dc voltages. Good correlation with experimental data has been
shown. The model can thus be used as a computer aided tool for design and mainte-
nance
of HV porcelain insulators.
1. INTRODUCTION
ELECTION of insulators for HV transmission (> 230 kV) is
S based primarily on their electrical performance under con-
taminated conditions. Surface contamination on insulators is
composed of both water soluble and non-soluble or inert mate-
rials. The soluble component, expressed in terms of the equiva-
lent salt deposit density (ESDD), has been used widely in artificial
contamination tests to characterize the outdoor pollution severi-
ty. Characterization of pollution severity by ESDD is appropriate
for ceramic and glass insulators in a majority of outdoor locations.
However, there are certain locations in close proximity to indus-
trial zones, such as cement and paper plants, heavy machinery
and major roadways, where the contaminants are not soluble in
water, thereby giving low ESDD values. But the deposition of this
contamination can be large enough to promote a thick water film
and large leakage current that can result in a flashover. The non-
soluble part of the pollution is denoted in the literature as NSDD
(non-soluble deposit density). Because the NSDD influences the
flashover voltage of the insulator under contaminated conditions,
it would be desirable to consider this during insulator selection. It
has been observed that for the same ESDD, the flashover voltage
(FOV) reduces with increasing NSDD, mainly due to the thicker
water film supported by the inert material
111. Both ESDD and
NSDD are expressed in terms of mg/cma of the insulator surface
area.
The clean fog method for laboratory testing of porcelain insula-
tors under contaminated conditions has been well established [2].
The insulator is contaminated using a slurry containing sodium
chloride (soluble part) and an inert binder, kaolin. The standard
slurry contains
40 g of kaolin per liter of water, and an appropri-
ate amount
of NaCl to give the desired ESDD 121. The NSDD of
this standard solution for porcelain insulators is
N 0.1 mg/cm2.
However, from field measurements, NSDD levels as high as 1.6
mg/cm2 have been recorded, and the ratio of NSDD to ESDD
varies considerably
(1 to 14) [3]. Thus there is a need for deter-
mining the flashover performance for a wide range of NSDD val-
ues.
Experimental determination of the FOV for different insulator
shapes covering a wide range of outdoor conditions is extremely
time consuming, and a tedious process. It is desirable to have a
theoretical model, which along with limited experiments can be
used for insulator design and maintenance. Presented in this pa-
per is the extension of a computational model published recently
[4]. The modified model is shown to handle both the soluble and
the non-soluble components of the contamination for predicting
flashover voltage. The model is capable of handling ac as well as
dc applied voltage for porcelain suspension and station post in-
sulators of various shapes
[5]. The model predictions have been
compared with published experimental data for several insulator
shapes, and good correlation has been established.
2. EXPERIMENTS
There is sufficient information regarding the variation of ES
DD and flashover voltage as a function of the soluble salt con-
tent. But similar information regarding the inert material is limit-
ed. To understand this better, experiments were performed with
glass slabs of size 12.5x5x0.5 cm3.
As the purpose was to deter-
mine the flashover voltage (FOV) with varying amount of inert
material relative to the FOV at the standard quantity
(40 g/l), the
use of glass slabs was quick and convenient. Different amounts
of kaolin ranging from
20 to 100 g per liter of water, keeping the
NaCl content the same as in a standard sIurry (15 g/l) was used
for artificially contaminating the test object.
FOV experiments were conducted using clean fog with the
standard steam input rate
of 0.05 kg/m3/h [2], using ac voltage.
Table
1 gives the kaolin content, normalized ESDD, relative model
and experimental FOV with respect to the FOV at
40 g/lof kaolin,
1070-9878/96/ $3.00 @ 1996 IEEE

114
Kaolin
Sundarajan et al.: Role of Non-soluble Contaminants on the Flashover Voltage of Porcelain
Norm Norm RelFOV RelFOV
NSDD ESDD (Expt) (Model [41)
D
mm
254
254
320
320
320
254
36/65 (Proj)
95 (Proj)
1.2
100 2.3
H
mm
146
146
165
180
197
146
95
27.5/42.5
per unit
1.14
1.00
0.98
0.96
Table 2. Dimensions of insulators [51.
IEEE
SA
-
__
z
280
430
512
545
457
305
366
240
mm
__
~
__
for various kaolin contents, along with the normalized NSDD val-
ues. The experimental results were compared with the predic-
tions
of the model published earlier which considers ESDD alone
141. It can be seen that the kaolin content has a significant effect on
the experimentally determined FOV, especially for larger NSDD
values. This justifies the need to incorporate the inert component
in the model.
3. MODELING OF NON-SOLUBLE
DEPOSIT FOR PORCELAIN
I N S U LATO RS
3.1. INPUT PARAMETER AS NSDD
It is known that the standard quantity of 40 g/1 kaolin used in
the standard slurry corresponds to an NSDD n = 0.1 mg/cm2
11/61. The equivalent ESDD, denoted by zneu, that reflects the
influence of NSDD compared to the ESDD corresponding to n =
0.1 mg/cm2, denoted as zo{d, is derived as follows:
=1 n = 0.1
%{;; Sold n > 0.1 (1 )
n < 0.1
Thus
zneW = kzold[l - (0.1 - n)]
(2)
K = 1 satisfies the first constraint (Equation (1)).
Using the above expression, the model results were compared
with the experimental data
111. Figure 1 shows the insulator pro-
files considered in this study (IEEE insulator has the same shape
as type
A insulator, but with different leakage distance). Table 2
gives the important dimensions of the insulators.
Figure 2 shows a comparison
of the model and experimental
results obtained for tvDe
A insulator using. dc voltaee 111. It can
A R c D
E SA SD
Figure 1. Suspension (A-E) and station post (SA and SD) insulator
geometries considered (Nomenclature as in previous paper [5j).
l.lL,l,l ,',,,,,,'
1-05 ~ Expt RFOV E 0.846 H-'."
Model RFOV = 0.846 n*0'07
>
0
LL1
t oj
NSDD, n mg/cm2
Figure 2. Comparisonof dc model and experimental [l] results for
type A insulator.
0 0.1 0.2 0.3 0.4 0.5 0.6
0.71' I ' '
I ' ' ' I ' ' ' ' " ' I ' I' ' I ' I ' I
NSDD n mg/cmz
Figure 3. Comparison of dc model and experimental [6] results for
type C insulator.
be seen that there
is good agreement between the model and the
experimental data. The relative
FOV (RFOV) T, obtained from the
model and experiment can be expressed as,
r = lcn-0.07, where
k = 0.846. Thus the model is capable of simulating the experi-
mentd data
with good accuracy. Similar results were obtained al-
so for the type C insulator [6], as shown in Figure 3. It can be seen
that the relative FOV with respect to NSDD is almost the same for
many profiles. This is in agreement with the experimental results
obtained elsewhere [61. Figure 4 shows the model results on effect
of NSDD for various types of insulators with dc voltage in terms
of mm/kV at flashover. This shows that with an increase in NS-
,I U " -1 DD, a higher mm/kV value is required for satisfactory operation

IEEE Transactions on Dielectrics and Electrical Insulation
Vol. 3 No. 1, February 1996
115
for a given value of ESDD. Figure 5 shows similar results obtained
using model for a standard IEEE insulator for ac voltage.
NSDD, mg/cms
Figure 4. Variation of mm/kV vs. NSDD for insulator types A, B,
D, and E at 0.1 ESDD for dc voltage.
1.1
2 0.9
0.8
0.7
0 0.2 0.4 0.6 0.8 1 1.2
Figure 5. Model results for standard IEEE insulator - ac voltage.
NSDD n mg/cn?
3.2. INPUT PARAMETER AS AMOUNT OF
KAOLIN
Measurement of NSDD is an elaborate process. It is more con-
venient to express inert component in an easily obtainable form.
In the case of artificial contamination tests performed in the lab-
oratory, the amount
c of kaolin content in g/1 used in the slurry
prepared is known directly, and could be used as an alternative
to NSDD, provided the accuracy of modeling is not significantly
affected. Accordingly, an attempt has been made to use the kaolin
content in the model as follows in g/1
=1 c = 40
%{:; XoZd c > 40 (3)
c < 40
znew zoid[l - (1 - (~/40)~)] (4)
Xnew = kzoZd [I - (1 - (c/40) b)]
or
(5)
where c is the kaolin content in g/l. Here also, K = 1 to satisfy
the first constraint, while
0 < b < 1.
Table 3. Comparison of dc Model (b = 1) and Experimental Re-
sults
[ll at 0.1 ESDD
40
80
Relative FOV
Expt
per unit
1.13
1 .oo
0.97
1.19
1.00
0.82
1.17
1
.oo
0.94
Model
per unit
1.22
1 .oo
0.81
1.24
1 .oo
0.82
1.23
1.00
0.81
-
Error
%
7.96
16.49
4.20
0.00
5.13
13.83
-
-
-
-
-
-
Table 4. Comparison of dc model (b = 0.5) and experimental re-
sults
[l] at 0.1 ESDD.
'T
Kaolin
20
40
80
20
40
80
dl
Relative FOV
Expt
per unit
1.13
1.00
0.97
1.17
1.00
0.94
1 .oo
For a first approximation, taking b = 1, the model results were
compared with the corresponding experimental data for types
B,
D and E insulators using dc voltage [l]. Table 3 shows a com-
parison of model and experimental data. It can be seen that for
type D insulator, the agreement is very good. It is not
so good for
types
B and E. An observation of the experimental results shows
that while type
B and E insulators have 14 and 17% increase in
FOV with 20 g/l kaolin compared to the FOV with 40 g/1 kaolin,
the corresponding change in type D insulator was 24%. Similarly,
with 80 g/1 kaolin, the relative FOV are 93 and 97% respective-
ly for types
B and E insulators, while it is 82% for type D insula-
tors. This demonstrates that the relative variation between vari-
ous profiles of insulators can be different while expressing NSDD
in terms of kaolin content in g/l. Considering a nonlinear rela-
tion, with
b = 0.5, Table 4 compares the model and experimental
results for type
B and E insulators. It can be seen that the agree-
ment between the model and the experimental data in this case is
quite good.
The larger variation in relative FOV between profiles for dif-
ferent kaolin content as compared with NSDD is to be expected
because the contamination accumulated is dependent on insula-
tor shape and surface condition and also on the properties of the
kaolin itself. But the variation is within tolerable limits, thus en-
abling the use of kaolin content for FOV calculation. Figure
6 is
a reproduction
of the experimental data for a standard IEEE in-
sulator, reported using ac voltage 171. It shows both the effect of
content and type of kaolin on the FOV for a given ESDD. With

116
Sundarajan et al.: Role of Non-soluble Contaminants on fhe Flashover Voltage ofl'orcelain
,
I I
I
I
ii FK~, 40 Qlt
I I K2. 40n
K3, 40 gl1
I
K1, 60~11
\
I I
I
I I !
13, :
i i
I, , j , [ !,/
0.5 1.0
05
001 002 0.05 01 02
EOUIVALENT SALT DEPOSIT DENSlTY - mg I cm
Figure 6. Effect of amount and type of Kaolin on ac FOX repro-
duced from
[7]. K1, K2 and K3 are different types of kaolin: A-11
is the standard IEEE insulator: VEE - type of string.
respect to the
FOV at standard kaolin content of 40 g/l, the FOV
increase for kaolin type K1 is 25% with kaolin content of 20 g/l,
while the FOV decrease for kaolin content of 80 g/l kaolin was
N 25% (comparable to type D insulator above). This varies for
other
kaolin types, K2 and K3, For example, the relative FOV with
respect to kaolin type K1, with 40 g/1 kaolin is, N 0.9 for type K2
and 0.8 for type K3 kaolin types. Similar results have also been
obtained by Naito
et al. for kaolin and TonokoTM [81.
Y
10 20 30 40 60 60 70 80 90
Kaolin c dl
Figure 7. Comparison of ac model and experimental results 171 €or
standard
lEEE insulator at 0.05 ESDD using b = 1.
Figure 7 shows a comparison of model and experimental val-
ues using
B = 1 for IEEE standard insulator using the ac model at
0.05 ESDD, which is the same for other ESDD E71. It can be seen
that there
is excellent agreement between the two. This demon-
strates that index
b has to be a variable that reflects influence of
s and type of kaolin on the FOV. However, in order to use
del for practical applications, It is desirable
to have a gener-
ic value, that
is applicable reasonably well to a number of pro-
fifes for a given non-soluble contaminant. Accordingly, an aver-
value
of b = 0.75 has been considered and Figures 8 to 11
illustrate the results obtained for types B, D, E and standard IEEE
10 20 30 40 50 60 70 80 90
Kaolin fl
Figure 8. Comparison of dc model and experimental results el1 for
type B insulator using b = 0.75,
OQ7 0.6 0 L 20 40 60 80 100
Kaolin ~gil
Figure 9. Comparison of dc model and experimental results 111 for
type D insulator using b = 0.75.
1.3 E
--cg
Model
i
insulators.
Figures
8 to 10 show the
perimental results for type
age. Figure
11 shows corre
sulator
using ac voltage.
ment between
the model
of insulators considered
mum discrepancy
is N 12% for fype B i

Vol. 3 No. I, February 1996
Table 5. Relative FOV vs. non-soluble contaminant.
117
IEEE Transactions on Dielectrics and Electrical Insulation
1.3 ~r, i t 1 i t 18 I i t I i I
i
'1 i i IT I i i i 1 I i 1 r 1 11 TTTJ
Voltage
type
dc
ac
NSDD Kaolin
-
g/l
-
20
40
60
80
100
20
40
60
80
100
- -
i5xmv-
1.02
1 .oo
0.90
0.82
0.76
1.03
1.00
0.87
0.77
0.70
mg/cm2
0.05
0.10
0.50
1.00
1.60
0.05
0.10
0.50
1 .oo
1.60
Rel. FOV
b = 0.75
1.24
1 .oo
0.88
0.81
0.75
1.32
1 .oo
0.85
0.76
0.69
10 20 30 40 50 60 70 80 90
Kaolin c gfl
Figure 11. Comparison of ac model and experimental results [7]
for standard IEEE insulator using b = 0.75.
$ 0.95
I
3 0.85
0.8
0.71 0.75
0 0.2 0.4 0.6 0.8 1 1.2
NSDD n mgfcm'
0 0.5 1 1.5 2
NSDD n mg/crr?
Figure 13. Model results for station post type SA for ac/dc volt-
ages
at 0.03 ESDD.
* 1.2
0.9
0.8
10 20 30 40 50 60 70 80 90
Kaolin c g/l
Figure 12. Comparison of model ac and dc relative FOV for IEEE
insulator
at 0.03 ESDD (a) using NSDD in mg/cm2 and (b) using
kaolin in g/l.
Otherwise, the error is
< 10%. In general, the relative FOV fol-
lows the trend as given in Table 5. Figures 12(a) and
(b) show a
comparison of relative FOV of ac and dc voltages for IEEE insu-
lator using model results,
vs. n in mg/cm2 and kaolin content c
in g/1 respectively. It can be seen that the percentage change with
ac is slightly more with dc, especially at lower kaolin content.
Extending further the study
to other types of insulators, Fig-
ure
13 shows the model results for both ac and dc for station post
insulator type
A. Here, n has been used to represent the non-
soluble deposit. It can be seen that the results are comparable to
0 20 40 60 80 100 120
Kaolin c g/l
Figure 14. Model results for station post type SD for ac/dc volt-
ages at
0.03 ESDD.
suspension type insulators. Presented in Figure 14
is the corre-
sponding results for station post type
D insulator, but for kaolin
content
c.