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.
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.
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.
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.
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.
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).
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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