Maintaining RTV Coated Glass Insulators in Zones of High Salt Contamination

Polymeric insulators were installed in Peru between 1996 and 2004 with the goal of reducing need for frequent insulator washing on transmission lines in coastal areas with heavy salt pollution, high humidity and scarce rainfall. However, after only a few years service, 17 failures occurred due to fracture of the insulator core. This experience forced a rethink of the strategy for insulator selection and maintenance by the country’s TSO, Red de Energia del Peru (REP).  

Starting 2009, RTV coated glass insulators were installed to replace polymeric types on lines in problematic coastal areas. Although research on RTV coatings suggested good results in increasing pollution withstand, reports from the field have also described cases where performance fell below expectations due to rapid loss of hydrophobicity, weakened ability to suppress partial discharge activity and coating pulverization.  

Advertisement
GIG Advertisement  Maintaining RTV Coated Glass Insulators in Zones of High Salt Contamination Coating Global Insulator Group copy

This edited contribution to INMR by Samuel Asto Soto of REP and D.A. Da Silva of Brazil’s Military Engineering Institute describes experience with RTV coated glass insulators on transmission lines near Peru’s coastline and the path taken toward optimizing their maintenance in this severe service environment.


Listen to Online Lecture on RTV-Coated Toughened Glass Insulators in Zones of High Contamination by Samuel Arturo Asto Soto.

The first transmission lines constructed by REP in coastal areas were limited to 220 kV and utilized porcelain and glass insulators. Coastal service areas received special attention when it came to insulator selection due to their unique characteristics that include high temperature and humidity, dense fog and a short rainy season that does not allow for natural washing. As shown in Fig. 1, the long Peruvian coastline is affected year round by a wind component coming from south. These winds blow salt particles inland that deposit on surfaces of electrical components and porcelain and glass insulators installed in these areas therefore had to be frequently cleaned. However, high costs made such a maintenance strategy no longer economically feasible.

Maintaining RTV Coated Glass Insulators in Zones of High Salt Contamination Bimonthly mean of 10 m wind speed m s   1 on coastline of Peru
Fig. 1: Bimonthly mean of 10 m wind speed (m s−1) on coastline of Peru.
CLICK TO ENLARGE

Porcelain and glass insulators were therefore replaced by polymeric types starting in the 1990s with the expectation that, based on manufacturer claims, these would not require any maintenance for up to 15 years. However, the polymeric insulators did not perform as expected and in fact experienced several brittle fracture failures. Based on this experience, application of RTV silicone coated glass insulators was implemented starting 2009 to replace polymeric types in coastal areas with high rates of corrosion and pollution accumulation.

Maintaining RTV Coated Glass Insulators in Zones of High Salt Contamination Failure of 220 kV polymeric insulator in Peru 7 years after installation
Fig. 2: Failure of 220 kV polymeric insulator in Peru 7 years after installation.
CLICK TO ENLARGE

Initial Maintenance Strategy for RTV Coated Glass Insulators

There were no past applications of RTV coated glass insulators installed on overhead lines in coastal areas of Peru exposed to severe environmental conditions. A succession of maintenance strategies therefore evolved as field experience increased.

Advertisement
Maintaining RTV Coated Glass Insulators in Zones of High Salt Contamination Zibo ad 14

One of the methods to monitor insulators exposed to saline and industrial contamination is by visual inspection using level of observed effluvia (discharge phenomena) to estimate degree of surface pollution. Such an evaluation is best carried out using a night vision camera that relies on absence of background daylight to most easily characterize effluvia (see Table 1).

Maintaining RTV Coated Glass Insulators in Zones of High Salt Contamination Insulator Pollution per Effluvium Level
Table 1: Insulator Pollution per Effluvium Level.
CLICK TO ENLARGE

The initial maintenance plan at REP was based on recommendations provided by the coating manufacturer and contemplated replacement of insulators in the 10th year after commissioning. Moreover, no preventative maintenance was expected over the time interval between installation and replacement, except for periodic visual inspection and monitoring of effluvia. However, events triggered by the special service conditions along South America’s Pacific coastline soon forced re-evaluation of this strategy.

Advertisement
Maintaining RTV Coated Glass Insulators in Zones of High Salt Contamination CSL compressed

RTV silicone coated glass insulation was found to offer excellent performance in coastal areas. In fact, there are areas where these insulators have been in service for as long as 10 years with no events or failures due to the heavy marine pollution. However, starting in the 5th year of commissioning, i.e. between 2015 and 2017, some 220 kV and 500 kV transmission lines in areas less than 5 km from the coast and exposed to high maritime pollution experienced failures or discharges due to contamination. Fig. 3 shows an example of one such affected insulator string installed on a 220 kV line in a central coastal area between Huanza and Caraballo. Fig. 4 shows an insulator that experienced pollution flashover on a 500 kV transmission line.

Maintaining RTV Coated Glass Insulators in Zones of High Salt Contamination Discharge on 220 kV coated glass insulator due to pollution
Fig. 3: Discharge on 220 kV coated glass insulator due to pollution.
CLICK TO ENLARGE
Maintaining RTV Coated Glass Insulators in Zones of High Salt Contamination Discharge due to pollution on 500 kV RTV coated glass insulator
Fig. 4: Discharge due to pollution on 500 kV RTV coated glass insulator.
CLICK TO ENLARGE

The 500 kV line from Chilca to Carapongo runs through a highly polluted coastal area and recorded 7 faults due to contamination on coated glass insulators from 4 to 6 years after their installation. Subsequent investigation revealed:

1. Presence of a salt pollution layer on insulators operating in marine areas, with evidence of electric discharges;

2. Existence of specific fault zones, i.e. areas with high humidity, high contamination and almost no rain;

3. While international standards consider extreme contamination as NSDD levels of 0.3 mg/cm2, analysis conducted by the insulator manufacturer found local NSDD levels of as high as 1.2 mg/cm2;

4. Increased humidity during autumn and winter.

5. Usual maintenance guidelines, based on a strategy of Reliability Centered Maintenance, would require light to thorough inspection to detect any anomalies on this line.

Advertisement
Maintaining RTV Coated Glass Insulators in Zones of High Salt Contamination INTEGRATED Engineering Software Mar 11

The main conclusion from the root cause analysis for these faults pointed to the need to review the initial maintenance strategy that had been applied for coated glass insulators operating in highly polluted areas. The result was a proposed revision in maintenance strategy to include manual cleaning of RTV silicone coated insulators with a wet cloth. Moreover, further analysis as well as a 2017 cost-risk assessment for performance of the 500 kV Chilca-Carapongo Line established the following:

1. In the case of the line’s RTV coated glass insulators, the activity of ‘cleaning these insulators with a wet cloth’ would be conducted every 4 years in areas of high contamination and humidity (i.e. the line section between towers T-111 to T-173) and every 6 years for areas of medium pollution and high humidity (i.e. the section between towers T-1 to T-110);

2. Concession contracts would be reviewed with the Peruvian Government to consider the scope and obligations to be fulfilled and avoid payment of compensation, penalties and fines;

3. The scope of this evaluation applied only to manual cleaning using a wet cloth.

Similarly, it was defined that this same criterion should also be applied to other 220 kV and 500 kV lines equipped with RTV silicone coated glass insulators. Fig. 5 shows typical manual cleaning of coated insulators.

Maintaining RTV Coated Glass Insulators in Zones of High Salt Contamination Manual cleaning of 500 kV RTV coated glass insulator at REP
Fig. 5: Manual cleaning of 500 kV RTV coated glass insulator at REP.
CLICK TO ENLARGE

Listen to Other Online Lectures on Insulators

Current Maintenance Strategy for Critical Lines

As described above, the proposed new maintenance strategy was initially applied to all lines equipped with coated glass insulators that operate in areas of very high pollution. However, due to budget limitations combined with some uncertainty about the real benefits of this approach, a decision was made in 2018 to initially apply this strategy only to the 500 kV line connecting Chilca and Carapongo. Insulator maintenance on other lines would then be carried out based on necessity.

Advertisement
Desma Advertisement  Maintaining RTV Coated Glass Insulators in Zones of High Salt Contamination DESMA INMR OnlineAd

However, only 12 months after insulator cleaning on the 500 kV Fenix – Chilca and Olleros – Chilca Lines that also operate in areas of very high maritime pollution, critical levels of effluvia were again observed on insulators (see Fig. 6). This was unexpected since the premise offered by the manufacturer was that RTV silicone insulation would recover its hydrophobic properties after cleaning. As such, it should not be necessary to clean again over the short term.

Based on this latest experience, it was felt necessary to revise the maintenance strategy for manually cleaning insulators. In this regard, a cost, risk and performance evaluation was made to adjust cleaning frequency on critical 500 kV lines (i.e. Fenix – Chilca and Olleros – Chilca) as well as on the 220 kV line connecting Zapallal and Huacho, all of which pass through areas of high pollution.

Maintaining RTV Coated Glass Insulators in Zones of High Salt Contamination Grade 3 effluvium on 500 kV line equipped with RTV coated glass insulators
Fig. 6: Grade 3 effluvium on 500 kV line equipped with RTV coated glass insulators (Feb. 2017).
CLICK TO ENLARGE

500 kV Fenix-Chilca Line (L5011)

This transmission line connects the 580 MW Fenix Generation Substation to Chilca Substation. Despite being only 7.5 km long, the line is deemed critical because the area in which it operates is characterized by heavy marine pollution with high humidity, especially in winter. Moreover, any failure on this line that results in service unavailability results in a high compensation penalty imposed by Peru’s quality standard. Over the period of 28 months since commissioning, presence of effluvia was detected and this resulted in cleaning the insulators due to increased risk of failure. But after only 18 months, a high level of effluvia was again detected. With the goal of better defining optimal frequency for cleaning RTV coated insulators based on cost, risk and performance, four different scenarios were investigated. Table 2 shows the savings involved with each scenario.

Maintaining RTV Coated Glass Insulators in Zones of High Salt Contamination Optimal Maintenance Interval for RTV Coated Glass Insulators
Table 2: Optimal Maintenance Interval for RTV Coated Glass Insulators (L5011).
CLICK TO ENLARGE

Maintenance frequencies of 6 or 18 months would mean that tasks would have to be carried out during different months. This would present a difficulty for maintenance planning given that the generation plant can undergo service only once a year during its annual maintenance shutdown. For this reason, the decision was made to choose a 12-month maintenance interval. Such an interval would also prove the best choice to allow carrying out nighttime inspection to diagnose level of effluvium on insulators during March 2019. This inspection revealed that 50% of insulator chains that had been cleaned only 12 months prior still showed effluvia that ranged from Grades 2 to 3 (see Fig. 7), meaning that they again needed cleaning. Moreover, considering this inspection had been done in a season with only medium humidity, it was inferred that effluvia during winter would have reached much higher levels, given the higher humidity. This presented a real risk of insulator failure.

Maintaining RTV Coated Glass Insulators in Zones of High Salt Contamination Night inspection of T 20 of L5011
Fig. 7: Night inspection of T-20 of L5011 (March 2019, 12 months after cleaning).
CLICK TO ENLARGE

500 kV Olleros-Chilca Line (L5013)            

Line 5013, with length of about 1.76 km at 500 kV and 700 MVA capability, connects Santo Domingo de Olleros Substation to SE Chilca Substation. Considering the same maintenance costs, penalties for unavailability and repair costs for failure as for Line 5011, 7 different scenarios were analyzed with a time interval of 6 months between each. Table 3 presents the main results. In this particular case, a frequency of every 36 months was the maintenance interval selected due to lowest overall cost.

Maintaining RTV Coated Glass Insulators in Zones of High Salt Contamination Optimal Maintenance Interval for RTV Coated Glass Insulators
Table 3: Optimal Maintenance Interval for RTV Coated Glass Insulators (L5013)*.
CLICK TO ENLARGE

Conclusions

1. 10 years following installation of RTV silicone coated glass insulators on ISA-REP transmission lines, these insulator have offered superior performance versus uncoated glass and also polymeric insulators. Failure rate and maintenance costs have both been reduced.

Advertisement
Maintaining RTV Coated Glass Insulators in Zones of High Salt Contamination OFIL banner 300X125 July1

2. The volume of field experience achieved during this period has allowed understanding the real performance of this type of insulator on transmission lines in coastal areas of South America, with their unique climate and pollution challenges.

3. It is recommended that current standards in regard to classification and design of insulators be reviewed and eventually revised to better cover the levels of contamination that occur in South America.

4. Design and testing of polymeric insulators for application in coastal areas should be reviewed with the goal of working to offer an alternative that equals or exceeds the current performance of coated glass insulators.

5. Due to performance of RTV coated glass insulators in coastal zones of Peru, a technical-economic evaluation, as well as analysis of cost, risk and performance, have determined that an appropriate maintenance strategy for this type of insulation in areas of heavy contamination is to carry out cleaning every 4 to 6 years.

Advertisement
Maintaining RTV Coated Glass Insulators in Zones of High Salt Contamination BG Big Box banner 300x250

Bibliography

[1] E.A. Cherney, “RTV Silicone – A high tech solution for a dirty insulator problem”, IEEE Electr. Insul. Mag., Vol. 11, No. 6, pp 8-14, November-December 1995.
[2] Lambeth PJ, Looms JST, Stalewski A, Todd WG,
Surface coatings for H.V. insulators in polluted areas, Proc. IEE, Vol. 113, No. 5, May, 1966, pp. 861-869.
[3] IEEE Committee S-32-3, Protective coatings for improving contamination performance of outdoor high voltage ceramic insulators, IEEE/PES Winter Meeting, Paper No. 94 WM 096-8 PWRD, 1994.
4 Z. Jia and S. Fang, “Development of RTV Silicone Coatings in China”, IEEE Electr. Insul. Mag., Vol. 24, No. 2, pp.28-41, 2008.
[5] E. A. Cherney and R. S. Gorur, “RTV silicone rubber coatings for outdoor insulators”, IEEE Trans. Dielectr. Electr. Insul., Vol. 6, no. 5, pp.605–611, 1999.
[6] CIGRE Taskforce 33.04.01, Polluted insulators: A review of current knowledge, CIGRE brochure No 158, 2000.
[7] IEC/TS 60815, Selection and dimensioning of high-voltage insulators intended for use in polluted conditions, 2008
[8] CIGRE WG C4.303, Outdoor insulation in polluted conditions: Guidelines for selection and dimensioning Part 1: General principles and the a.c. case, CIGRE Technical Brochure No 361, 2008.
[9] K. Siderakis, D. Pylarinos, E. Thalassinakis, E. Pyrgioti, I. Vitellas, “Pollution maintenance techniques in coastal high voltage installations”, Eng. Technol. Appl. Sci. Res, Vol. 1, No. 1, pp. 1-7, February 2011
[10] IEEE Guide for the Application, Maintenance, and Evaluation of Room Temperature Vulcanizing (RTV) Silicone Rubber Coatings for Outdoor Ceramic Insulators, IEEE Standard 1523, 2002.
[11] T. G. Gustavsson and S. M. Gubanski, “Aging of Silicone Rubber under ac or dc Voltages in a Coastal Environment”, IEEE Trans. Dielectr. Electr. Insul.,Vol. 8, pp. 1029-1039, 2001.
[12] R.E. Carberry and H. M. Schneider, “Evaluation of RTV Coating for Insulators Subjected to Coastal Contaminant”, IEEE Trans. Power Delivery, Vol. 4, pp, 577-585, 1989.
[13] R. G. Houigateand and D. A. Swiit, “Composite Rod lnsulators for ac Transmission lines: Electrical Performance of Various Designs at a Coastal Testing Station”, IEEE Trans. Power Delivery, Vol. 5, pp, 1944 -1955, 1990.
[14] S. Kumagai and N. Yoshimura, “Influence of Single and Multiple Environmental Stresses on Tracking and Erosion of RTV Silicone Rubber “, IEEE Trans. Dielectr. Electr. Insul., Vol. 6, No. 2, pp. 211-225, 1999.
[15] S. Kumagai and S. Yoshimura, “Hydrophobicity Transfer of RTV Silicone Rubber aged in Single and Multiple Environmental Stresses and the Behavior of LMW Silicone Fluid”, IEEE Trans. Power Delivery, Vol. 18, pp. 506-516, 2003.
[16] H. F. Gao, Z. D. Jia, Z. C. Guan, and K. N. Zhu, “Investigation on field-aged RTV-coated insulators used in heavily contaminated areas,” IEEE Trans. Power Delivery, Vol. 22, pp. 1117–1124, 2007.
[17] J. Chang and R. S. Gorur, “Surface recovery of silicone Rubbers used for outdoor insulation”, IEEE Trans. Dielectr. Electr. Insul., Vol. 1, pp.1039-1046,
1994.
[18] D. A. Rahna and R. D. Garreauda, “A synoptic climatology of the near-surface wind along the west coast of South America”, Int. J. Climatol., vol. 34, no. 3. Pp. 780 – 792, 2014.
[19] IEC TS 60815-1:2008 – Selection and dimensioning of high-voltage insulators intended for use in polluted conditions – Part 1: Definitions, information and general principles.
[20] Sediver technical report (PE17-00003.A). “Silicone coated glass insulators (FS160PR) returned from ISA PERU: Line 500kV Chilca – Carabayllo”, 2017.
[21] Norma Técnica de Calidad de los Servicios Eléctricos. DECRETO SUPREMO Nº 020-97-EM, Perú.