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Silicone Rubber RTV Coating of HV Substations and Over Head Lines

Authors:
  • Iran Water and Power Resources Development Company IWPCO
TECHNICAL & ECONOMICAL EVALUATION OF USING SILICONE RUBBER RTV COATING
FOR H.V. SUBSTATION IN POLLUTED AREA
M. A. Talebi 1 A. Gholami 1 M. R. Shariati 2 M. Hasanzadeh 1
1. Iran University of Science and Technology 2. Niroo Research Institute
m_a_taleby@yahoo.com
Abstract:
High voltage insulators have to be used as insulating
supporters in various environments in polluted area. The
electrical performance of insulators will be degraded. The
polluted insulators will have over withstand voltage, flashover
can occur easily when the air becomes wet by humidity or
fog, which affects on the reliability of the power systems.
In order to define effective counter measures against pollution
flashovers there are conventional methods such as
periodically water washing and also using silicone grease for
insulation. Water washing of insulators which is normally
recommended so as to remove the pollution from the insulator
surface needs to be done more frequently than other
maintenance methods. Though involves some limitation in
practice from the technical and economical point of view.
Applying of silicon grease on insulators with high cost of
performance also is not recommended for use in an
environment where high level of NSDD pollution is present.
In this paper room temperature vulcanizing (RTV) silicone
rubber coating have been presented as a novel approach for
improvement of outdoor substation insulators maintenance in
polluted area. This method can be used in areas characterized
with “Instantaneous pollution” and or with “High or Low
level of NSDD” with long life expectancy. Financial analysis
shows that this method is also economic and optimal choice.
1-Introduction
HV insulation separates voltage levels. Wet polluted
deposition forms conductive layer. This layer extends
gradually and causes electrical breakage as well as outage.
Therefore, appropriate insulation design plays important
roll in system reliability improvement and faults reduction
in polluted regions. According to utility records,
inappropriate insulation constitutes 70 percents of HV lines
faults. Respecting to international standards, coastal regions
of Persian Gulf and Oman Sea are classified in "very
heavy" pollution degree and their hot and humid climate,
sea adjacency and salty water provide specific condition.
Present insulation design of HV device-lines design in the
regions is based on "very heavy" pollution degree
standards, but the insulation performance is inefficient.
Insulation weaken is the main effect of pollution. Different
methods, e.g. periodical washing, are used to reduce these
effects in southern regions of Iran.
In this paper, traditional maintenance methods in HV
substation as well as RTV silicon rubber coating, as a
replacement, with their technical-economical
considerations are presented and calculations for a typical
substation performed.
2-Typical maintenance methods in polluted regions
Different maintenance methods based on sweeping pollution
or changing surface characteristics are applied in polluted
zones. They are commonly as follows:
-Periodical washing
-silicon grease
2-1-Perodical washing
In this method the HV insulators are washed periodically
with distilled water. Times and intervals of washing are
determined according to site pollution degree, atmospheric
conditions and insulators shape. Insulators should be
washed before critical pollution achievement. This critical
value is estimated based on ESDD value of pollution
gauges (if valid), environmental conditions and operation
experience. Washing is applied manually or with spray (hot
or cold). Finally, insulators should be dried to avoid
leakage. Manual washing is one of the most effective
pollution sweeping methods, but it is time consuming and
difficult. Also, it is useless for instant pollution. Instant
pollution is a high conductivity pollution that deposit
quickly on insulation, change the clean surface to spark
situation less than one hour and return to base state after
electrical breakage. This type of pollution is found in
coastal region with salty water or conductive fog deposition
and sea salty water, SO2 (produced by factories) and road
salt scattering are the resources.
Furthermore, in these methods, intervals are shorter than the
others and the appropriate time determination are difficult. If
substation deenergize is unfeasible, spray can be used. This
method is faster and easier than manual one, but its electricity
consumption necessitate special devices as well as lower
conductivity distilled water. Electrical breakage threatens
during application. This method is used in southern regions of
Iran and times and costs are very high for "very high"
pollution degree in these regions.
According to IEEE 957, washing material in this method is
distilled water. The required cost for hot washing is
presented here. Low conductivity distilled water with 2725
to 6900 kPa pressure scatter on insulator surface. The other
requirements are available in [5]. The main costs are as
follows:
-Distilled water preparation
-Operational personnel
- Washing devices wear and tear
- Water and devices transportation
-Peripherals cost
Washing is periodic and more than one time a year. The
intervals are determined by pollution degree.
2-2-Silicon grease
Silicon grease coating is used more than 25 years as
protective layer in porcelain and glass insulators. All kinds
of available grease have water repellency and low surface
energy. The grease layer converts the insulator surface to
water repeller (figure 1). The deposit pollution surrounds
by the grease and avoids conductive layer formation. The
grease surface should be inspected for erosion; oxidation or
tracking. Grease oxidation is shown in figure 2. Grease can
be applied manually or by spray. If it is applied correctly,
lasts for one year. In spite of usefulness in instant pollution
(at the contrary of washing), it is useless in high NSDD
regions. Also, intense wind and rain remove grease layer.
High application cost, time consumption and difficulty are
the main restrictions. However, second greasing
necessitates removing previous film.
Greasing is applied manually or by spray, but spraying is
preferable and performs in cold state. The followings
should be considered:
-Technical conformation
- Minimum thickness (determined by pollution degree)
-Dimensions and clearance distances
- Previous layer careful removal
The main costs are as follows:
-Surface preparation
-Grease preparation
-Substation deenergizing
-Application cost
Insulators surface should be determined for cost estimation
and application is according to standards. Calculation
process is presented in appendix B.
2-3-RTV silicon rubber coating
These coatings are applied increasingly for porcelain and
glass insulators or bushings. This method is an efficient
replacement for the above. The operation mechanism is the
same as greasing except NSDD affectivity.
There are RTV coatings with different performances. The
key point in their performance is their ingredient. The
coatings that lost their water repellency by environmental
factors have short life and inefficient performance in
breakage elimination.
Figure1: insulator surface hydrophobicity after greasing
Convenient application is another advantage that affect
significantly on cost. Insulator surface should be cleaned
before coating. It is recommended to wash surface by high
pressure water and sweep with isopropyl alcohol. If grease
previous coating available, it should be solved with proper
solvent (e.g. Nafta).
Having surface prepared, coating should be provided. Film
characteristics (i.e. maximum thickness that deposit by one
time spraying) play the main roll in operation time and cost.
The layer thickness is influenced by viscosity, deposit
characteristic and surface material. Drying time
consumption is determined by solvent. 1, 1, 1-
tericholoroethan is the most common solvent. This solvent
reduces dry time by 30% compare to Nafta. Lower time,
lower the cost. The other effective parameters are studied in
next part.
RTV silicon rubber coatings provide water repellency for
insulators. Coating life depends to site pollution degree,
insulator dimension and coating application. According to
IEC 60815, additives can increase life and creepage
distance. More ten years life is achieved in this method.
RTV is the only way to defeat high pollution degrees for
irreplaceable insulators and provide long life and applied
hot or cold.
As well as greasing, the surface should be cleaned.
Application cost in hot state is calculated and classified as
follows:
-Insulator preparation
-Material cost
-Application cost
Material cost estimation requires thickness as well as
surface area. The desirable thickness is between 0.3 to 0.5
mm. At the contrary of greasing, the thickness is
independent of environment pollution degree.
3-Technical-economical assessment of different
maintenance methods in Queshm substation [4, 8, 13]
Technical and economic assessment of different
maintenance methods is applied on a 230kV substation in
Queshm. This substation contain 20, 63,230 kV voltage
levels in addition to a 230kV and six 63kV feeders. The
single line diagram is presented in appendix A.
Figure2: Grease oxidation
NSDD and ESDD index of this substation is very high. The
measured indexes are shown in table 1.
According to the results and IEC 60815, this region is
classified as “very heavy”. This level is approved by
operational records. In this section, a comparison of different
methods presented.
Table 1: Queshm pollution measurements
3-1-Periodical washing
Nowadays, washing is applied as maintenance method in this
region. Annual periods are proportional with pollution degree.
According to pollution index and Hormozgan utility reports,
this substation is washed 20 times a year. Each time cost is
presented in table 2. Therefore the annual cost is 162000000
Rails.
Table 2: one time washing cost in a typical substation
3-2-Silicon grease
Silicon grease covers insulators surfaces. This method is
inefficient in high NSDD regions. Therefore, in Queshm,
washing is not an appropriate alternative. Nevertheless, the
calculation performed. As mentioned in previous sections,
having cleaned the surfaces, the grease applied. The procedure
increases application time and cost.
Grease preparation is another cost effective part. Film
thickness and total surfaces should be calculated. In this
method, total surfaces of insulators are covered. H.V devices
list are available. Side surfaces of insulators are calculated
according to appendix B. It should be note that the suspension
insulators in the substation are silicon type and greasing is not
required. There are different types of greasing, so the applied
covering should be chosen respect to regional condition.
The required thickness and grease is based on [4]. According
to site pollution severity, chosen grease and table 3 the
required thickness is 3 mm. Third parts is application. This
part consists of operational personnel cost as well as devices.
This method is applied deenergiezedly. So, substation
deenergization, device wearing, undistributed energy,
decrement of reliability and network security is cost effective.
Total cost is presented in table 4.
If the covering applied correctly, it remains for a year. After a
previous film). Therefore the recovering cost is 15730000
Rials.
Table4: typical substation greasing cost
3-3-RTV
RTV coating is applied respect to IEEE 1523.The economic
calculation and initial cost is based on standard and [4],
respectively. The covered devices are the same as 2-2. Note
that, suspension insulators, surge arresters, 20 kV CTs and PTs
are silicon type and coating is unnecessary. Coated surfaces
are calculated according to 2-3 and appendix B. Having
calculated the surfaces, the thickness is determined respect to
[4,6]. The required material for 1 mm² insulator surface is
presented in table 5.Accoring to these data and total surface,
the initial material is calculated. Total cost presented in 6. If
the covering applied correctly, it remains for ten years. After
ten years, the previous firm should remove completely for
recoating.
A thirty years maintenance period is considered for
comparison purpose (table 7). According to the table, 1-3 is
plotted. It can be seen silicon grease cost is twice the
periodical washing .RTV is very economical respect to its ten
year life.
Table 6: RTV coating cost in atypical substation
Table 7: comparison between maintenance methods cost
4-Conclusion
In this paper, RTV covering presented as an effective and
economic maintenance method. In addition, this method
compares with alternatives and economic calculation is
presented for 230kV substation in Queshm.
In present day, maintenance method of the substation is 20
times annual washing (respect to pollution degree). Washing is
limited by technical constraints and is not economic. Also, it is
not effective in instant pollution and high NSDD regions (e.g.
Queshm and Jask).
The RTV method has not above limitations and the initial cost
obtained in the first 40 month.
Station ESDD(mg/ 2
cm ) NSDD(mg/ 2
cm )
Gheshm, HR-72-12 1.338 6.8716
Cost (Rial)
Distilled water preparation 2,600,000
Operational personnel 1,000,000
Water and devices transportation 4,000,000
Personnel transportation 300,000
Peripherals cost 200,000
Total 8,100,000
Cost (Rial)
Insulator surface preparation 12,100,000
Grease cost 275,000,000
Devices and personnel transportation 10,000,000
Application cost 25,000,000
Peripherals cost 500,000
Total 322,600,000
Table 3: grease weigh and thickness proportional with pollution degree
Pollution Level Grease Thickness
(mm)
Grease weight
(kg/ 2
m)
Medium 1.5 3.0
Heavy 2.25 4.0
Very Heavy 3.0 5.0
Cost (Rial)
Insulator surface preparation 12,500,000
Material cost 474,000,000
Devices and personnel transportation 9,000,000
Application cost 30,000,000
Peripherals cost 800,000
Total 526,3 00,000
Table 5: RTV weigh and thickness
Pollution Level Thickness (mm) Weight (kg/ 2
m)
Medium, Heavy and
Very Heavy 0.38 0.77
Years Water Washing
Million Rials
Grease Coating
Million Rials
RTV Coating
Million Rials
1 160.2 322.6 526.3
5 810 1613 526.3
10 1620 3226 1052.6
15 2430 4939 1052.6
20 3240 6452 1578.9
25 4050 8065 1578.9
30 4698 9355 1578.9
Therefore, HVIC is the only appropriate method for high
NSDD and “very heavy” pollution degree regions and
recommended.
Figure7: comparison between maintenance methods cost
5- References
1-James L. Goudie, “Silicone for Outdoor Insulator
Maintenance”, International Symposium on Electrical
Insulation, Boston, IEEE, 2002.
2-IEEE Std1523, “Guide for the Application, Maintenance,
and Evaluation of Room Temperature Vulcanizing (RTV),
Silicone Rubber Coating for Outdoor Ceramic Insulator”,
2002.
3-Hiroya Homma and Christophor L. Mirley, “Field and
Laboratory Aging of RTV Silicone Insulator Coating”, IEEE
Transaction on Power Delivery, Vol. 15, No. 4, October 2000.
4-K. T. Eldridge, “ Evaluating Silicone High Voltage Insulator
Coating”, Midsun group, 2003.
5-IEEE Std957, “IEEE Guide for Cleaning Insulators”, 1995.
6-Eskom Technology Group, “Practical Maintenance Guide
for Polluted Outdoor High Voltage Insulators”, Eskom
Insulator Study Committee, 1999.
7-Torbjorn Soqvist, “Long-term Field Experience with RTV
Coated Porcelain Insulators”, International Symposium on
Electrical Insulation, Boston, IEEE, 2000.
8-Midsun group, “A Technical and Economic Comparison
Between Resistive Glaze Insulator and Midsun 570 High
Voltage Insulator coating (HVIC)”, Midsun Group, 2002.
9-Guam Zhiecheng and Jia Zhidong, “The Developments of
Room Temperature Vulcanizing Silicone Rubber Coating and
Its Application in China”, IEEE, 2002.
10-Devendranath D. and Channakeshava, “Leakage Current
and Charge in RTV Coated Insulators Under Pollution
Conditions”, International Symposium on Electrical
Insulation, Boston, IEEE, 2002.
11- R. Omranipour, L. H. Meyer, “ Tracking and Erosion
Resistance of RTV Silicone Rubber: Effect of Filler Particle
Size and Loading”, Annual Report Conference on Electrical
Insulation and Dielectric Phenomena, 2002.
12-Jia Zhidong and Guam Zhiecheng, “Loss and Recovery of
Hydrophobicity of RTV Silicone Rubber Coating”, Annual
Report Conference on Electrical Insulation and Dielectric
Phenomena, 2002.
13-Midsun Group, “A Comparison Between Silicone Rubber
Composite Insulators and RTV Coated Glass Insulators”,
Midsun Group, 2003.
14-Nie Chung Wang, Chin Fu Chi, “Leakage Current Test and
Study of Room Temperature Vulcanizing Silicone Rubber
Coating for Outdoor high voltage Porcelain Insulators”, IEEE,
2000.
15-Zhidong Jia and Zhicheng Guam, “Discharge Along
Hydrophobic and Hydrophilic Surfaces”, IEEE, 2002.
Appendix A
Figure A-1: Single line diagram of Queshm 230kV substation
Appendix B
Type and characteristics of HV substation should be known
for surface calculation. According to figure c-1 and substation
data, device are listed in table 4-3.Depend to the insulator
type, external diameter and creep age distance is required for
the calculation. Side surface is achieved approximately by eq
A-1.
As= π.d1.
3
)( 1
DDD sc +
+
(A-1)
As: Insulator side surface (m2)
d1: Creepage Distance (m)
Ds: Diameter of smallest shed (m)
Dc: Diameter of core (m)
Dl: Diameter of largest shed (m)
For example, side surface of a 63 kV current transformer
achieved 1.80 m2 by eq. A-1.Necessary data is shown in A-
2.Now,considering surface, table 3 and5, the required
material grease is obtained.
Figure A-2: 63kV Current transformer
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The Developments of Room Temperature Vulcanizing Silicone Rubber Coating and Its Application in China
  • Guam Zhiecheng
  • Jia Zhidong
  • L H Omranipour
  • Meyer
Guam Zhiecheng and Jia Zhidong, "The Developments of Room Temperature Vulcanizing Silicone Rubber Coating and Its Application in China", IEEE, 2002. 10-Devendranath D. and Channakeshava, "Leakage Current and Charge in RTV Coated Insulators Under Pollution Conditions", International Symposium on Electrical Insulation, Boston, IEEE, 2002. 11-R. Omranipour, L. H. Meyer, " Tracking and Erosion Resistance of RTV Silicone Rubber: Effect of Filler Particle Size and Loading", Annual Report Conference on Electrical Insulation and Dielectric Phenomena, 2002.
A Technical and Economic Comparison Between Resistive Glaze Insulator and Midsun 570 High Voltage Insulator coating (HVIC)
  • Torbjorn Soqvist
Torbjorn Soqvist, "Long-term Field Experience with RTV Coated Porcelain Insulators", International Symposium on Electrical Insulation, Boston, IEEE, 2000. 8-Midsun group, "A Technical and Economic Comparison Between Resistive Glaze Insulator and Midsun 570 High Voltage Insulator coating (HVIC)", Midsun Group, 2002.
A Comparison Between Silicone Rubber Composite Insulators and RTV Coated Glass Insulators
  • Midsun Group
Midsun Group, "A Comparison Between Silicone Rubber Composite Insulators and RTV Coated Glass Insulators", Midsun Group, 2003.
Leakage Current Test and Study of Room Temperature Vulcanizing Silicone Rubber Coating for Outdoor high voltage Porcelain Insulators
  • Chung Nie
  • Chin Wang
  • Fu Chi
Nie Chung Wang, Chin Fu Chi, "Leakage Current Test and Study of Room Temperature Vulcanizing Silicone Rubber Coating for Outdoor high voltage Porcelain Insulators", IEEE, 2000. 15-Zhidong Jia and Zhicheng Guam, "Discharge Along Hydrophobic and Hydrophilic Surfaces", IEEE, 2002.
Evaluating Silicone High Voltage Insulator Coating
  • K T Eldridge
K. T. Eldridge, " Evaluating Silicone High Voltage Insulator Coating", Midsun group, 2003.
Long-term Field Experience with RTV Coated Porcelain Insulators
  • Torbjorn Soqvist
Torbjorn Soqvist, "Long-term Field Experience with RTV Coated Porcelain Insulators", International Symposium on Electrical Insulation, Boston, IEEE, 2000.
A Technical and Economic Comparison Between Resistive Glaze Insulator and Midsun 570 High Voltage Insulator coating (HVIC)
  • Midsun Group
-Midsun group, "A Technical and Economic Comparison Between Resistive Glaze Insulator and Midsun 570 High Voltage Insulator coating (HVIC)", Midsun Group, 2002.