As the power transmission system in India expands to meet ambitious targets for incorporation of renewable energy sources, challenges are being imposed high pollution levels and future climate uncertainty. This edited recent contribution to INMR by Neelesh Arora of Epsilon Asia Group provides an overview of Indian experience with different types of insulators as well as considerations to combat the effects of climate change and elevated pollution levels on outdoor insulation.
Ceramic and silicone rubber insulators (SRIs) have shown mixed performance in India. For example, uncoated ceramic types with older designs and some designs of SRIs did not always meet reliability expectations under pollution and had to be replaced with more reliable options.
While ceramic cap & pin insulators today have come a long way in terms of improved geometry, construction and MTBF values, the quality of SRIs in India has been adversely affected by intense domestic competition.
With climate change – notably acid rain from coal-fired stations and other sources – failure rates of SRIs are anticipated to be impacted by issues such as de-polymerization, brittle fracture, and accelerated loss of hydrophobicity leading to flashovers as well as downed conductors. The trend is therefore toward using RTV coated ceramic insulators to eliminate flashovers and mitigate risks of line drops.
Climate, Pollution & Impact on Silicone HTM
India faces the brunt of climate change and, according to a Global Climate Risk Index Report (2021), finds itself ranked 7th in terms of countries most affected by the devastating impact. Indeed, 65 of the 100 most polluted cities in the world are in India. As a result, incidence of acid rain due to higher ambient SOx-NOx levels will play an increasing role in performance of HTM type insulators in the years to come.
The inorganic filler, alumina trihydrate (ATH), in silicone rubber is believed to dissolve under acid rain and the increased heat from dry band arcing can lead to irreversible ATH breakdown. Hydrophobic groups of hydrocarbon branch (methyl in silicone rubber) also degrade under acid rain and consequently, the surface of aged HTM roughens and loses hydrophobicity. Surface discharge current increases and insulator life decreases.
Indian Power Grid
The power transmission system in India is one of the largest synchronous grids in the world spanning a nation with varied climatic conditions including a 7,500 km long coastline, salt deserts, sand deserts, mountain ranges, dense winter fog and some of the wettest places on earth. Large scale urbanization and industrialization and the need to cater to fast emerging RE sources continues to push the national grid – whose doubling is foreseen. The Central Transmission Utility (CTU) – Power Grid Corporation of India Limited (PGCIL) operates at 99.86% system availability (as stated) and carries 45% of India’s transmission capacity. The rest of the grid is operated by private operators and state utilities – some of whom face challenges from shortage of funds and right-of-way challenges.
Overview of Insulators in the Grid Transmission Lines
Pre-2008
Prior to 2008, the grid was served mainly by ceramic insulators. Several grid-crippling pollution flashovers highlighted the problem set which was identified as a combination of:
a. Unevolved insulator geometry;
b. Cases of relatively low design creepage (16 mm/kV);
c. Poor pollution performance;
d. Occasional de-capping of insulators.
An alternate solution was needed and the “Insulator rethink of 2008” followed.
2008-2023
The ‘2008-Insulator rethink’ looked at experience with SRIs among European and North American utilities and led to the widespread adoption of this technology in all service conditions. Within 5 years, it was not uncommon to have 20+ bidders vie for a single tender. Meanwhile, several ceramic insulator manufacturers began to close production.
Initial SRI experience was favorable but started to diminish in following years due to evidence that service life was less than originally anticipated. Increasing pollution levels, UV radiation, acid rain, and the extreme difficulty in maintaining desired quality levels in a hypercompetitive market often resulted in premature SRI failures that necessitated the ‘Insulator rethink of 2021’. While the decision in 2008 to replace most ceramic discs with SRIs was correct considering the circumstances and available information, the time to rethink this aspect of the grid has come once again.
Substations
Pre-2010
Most substations and switchyards employed ceramic insulators that were at times inadvertently ‘under-dimensioned’ due to misgauged pollution levels. Cases of flashovers (PFOs) became rampant.
Post-2010
Newly constructed private sector coal power stations running on imported coal were largely established in coastal areas near ports. Saline winds caused several flashovers and RTV silicone coatings which started out as a palliative measure began to be considered at the design stage itself.
Several legacy switchyards still struggle with flashovers and opt for the temporary relief offered by live-line insulator washing and, in some instances, application of silicone grease.
The Insulator Basket
Silicone Rubber Insulators (SRIs)
SRI designs continue to evolve in the quest to improve reliability. In general, performance has been very good and has served utilities well, especially under light to medium pollution conditions.
The first lots of SRIs supplied in India – several of which were installed in the North Indian plains and far from the coast – have received more positive feedback compared to those purchased in the years to follow.
A ‘one size fits all’ approach mandating the use of SRIs in all conditions that did not work in developed economies where Grade-A SRIs have often not met life expectations. Therefore, SRIs purchased in India at much lower costs from a wide supplier base and operating in much higher levels of UV, pollution and voltage levels, tended to exhibit significantly higher failure levels that are expected to further deteriorate. Raw material compounding remains largely opaque to the utilities and QC enforcement remains a challenge.
A key concern pertaining to SRI performance in pollution conditions (ref: CIGRÉ TB 837) is: “There are concerns about their performance in very harsh pollution environments, in particular with equivalent SDD higher than 1 mg/cm2, where severe ageing was reported.”
A total of 160 premature SRI failures have been recorded between 2016 and 2022 at 0.15% (i.e. approx. 4.3x the global average). It is pertinent to note that failure statistics are across the entire PGCIL network including in L-M pollution zones and inland, where most lines traverse. Failure probability if considering only coastal and/or H-VH areas could prove significantly higher.
As on 11 Dec. 2022, a total of 1898 SRIs were replaced well before originally anticipated life on a preemptively based on thermal vision camera images showing hotspots and abnormal heating. On 12 Dec 2022, the replacement of several hundred more insulators was decided upon by the utility. Total number of hours and cost of forced downtime for replacement activities is yet to be determined.
Assessment of Failed SRIs
• Several FRP rods had degraded severely, often resembling decayed wood;
• Degradation of interface between housing and FRP rod;
• Chalking and separation between glass fiber and epoxy resin matrix;
• Hotspots overheating before fracture;
• Degradation in axial direction;
• Punctures of housing;
• Punctures pointing to possible moisture ingress;
• Flash-under type of failure;
• Tracking across insulator surfaces;
• Darkening of sheds towards conductor side.
Private operators in general maintain a focus on quality and reliability. Nonetheless, the life of SRIs sourced from India and, in some cases, also from experienced European manufacturers has been less than originally anticipated. Failures have been attributed to reasons including sheds being damaged by birds and possible shortcomings in materials and workmanship.
For example, one operator of a 400 kV line faced a series of crippling SRI failures within 5 years of installation, leading to extended periods of line outages resulting in severe losses and penalties. Downtime losses, legal expenses, and penalties could mount further as a decision was made to replace approx. 20,000 SRIs.
RTV Silicone Rubber Coating on Ceramic Insulators: (Substation & Overhead Lines)
RTV Silicone Rubber coatings have proven to be an extremely effective measure in bringing pollution flashovers to near-zero levels even in highly contaminated service conditions globally. Considering the operating principle is based on creepage and not voltage, RTV coatings can be used on Insulators of all geometries, types, and voltage classes. RTV Coatings are also used on Silicone Rubber sheds of post insulators that may have started to lose hydrophobicity. Today, RTV Coatings are in use at over 200 switchyards in India of voltages up to 1200 kV HVAC and 800 kV HVDC with near-zero failure levels.
Silicone Surface Ceramic Insulators – or SSCIs (ceramic insulators with an HTM surface imparted by means of a silicone RTV coating) have gained substantial traction globally due to high levels of reliability and near-zero failures even under the harshest of service environments. As per CIGRÉ, the number of RTV-coated cap and pin insulators is expected to reach 100 million globally by 2030.
Given the success of RTV coatings in substations in India and SSCIs in coastal Europe, the Middle East and North America, utilities here have started to evaluate application of this technology on transmission lines.
Further confidence in the technology is drawn from the fact that since 2018, RTV Insulator coatings have been the subject of Guidelines and Standardization under IEEE 1523: 2018, CIGRÉ TB 837, Bureau of Indian Standards (IS/IEEE 1523: 2018) and currently being developed under IEC TC 36/535/PT IEC 63432 as well as success at substations.
A case study follows.
Tata Power CGPL 400 kV Yard: RTV Coating Overview
The benchmark case for the performance of HVICs is the Tata Power Coastal Gujarat Power Limited, (CGPL) 400 kV switchyard coated with Epsilon G4 brand of RTV coating in 2012/13.
Conditions Prior to Coating
• VH-pollution (<3 km from the coast, salt desert, several cooling towers)
• Being a salt desert, negligable rainfall
• Insulators dimensioned to exceed IEC recommendations.
• Pollution flashovers commenced in Yr-1
• All remedial measures including daily live-line insulator washing all failed.
• Flashover frequency kept rising.
• In 2012-13, RTV coating applied on all 400 kV insulators (bushings, BPIs, CBs, CVTs, PTs, surge arresters, etc.)
Findings Within One Year of Coating
The owner, Tata Power, presented a paper at the 2013 International Conference on High Voltage Engineering & Tech stating:
• Significant reduction in corona and leakage current was observed.
• Gradually, insulator washing eliminated.
• No monitoring or maintenance has been required.
Findings Ten Years Later (Sept. 2023)
• No pollution flashovers occurred in 11 years;
• No washing was required or carried out;
• RTV coating found to exhibit between HC 2 to HC 5 hydrophobicity levels, depending on location, as a reflection of unequal but severe stresses in different parts of the switchyard.
Glass Cap & Pin Insulators
Glass insulators have several advantages but are not widely prevalent in the Indian grid and therefore domestic performance data was not available.
Porcelain Cap & Pin Insulators
After initial failures, innovative insulator geometry, improved manufacturing and QC processes have made the risk of mechanical failure more remote. Though mechanical failure of disc insulators has become rare, some utilities have reported pollution flashovers on uncoated ceramic insulators.
As regards pollution performance, CIGRÉ TB 837 refers to stating “Insulators with hydrophobic (HTM) surfaces exhibit far higher performance in terms of withstand voltage under pollution conditions than uncoated ceramic insulators”. Therefore, it is with the HTM silicone surface imparted through RTV coating, pollution performance of modern ceramic disc insulators meets the highest benchmarks set by IEC 62217 and stringent cyclical multiple stress tests, as mandated by some European utilities.
A string of pin-side only RTV-coated porcelain discs passes Artificial Pollution Tests at 40% higher values than the requisite value as also set for SRIs as well. In the unlikely event an RTV coating was to completely fail on each insulator across the string, there would still be the safety of ceramic insulators based on evolved geometry that would itself meet pollution performance requirements.
Life of HTM Insulators & Coatings in Polluted Conditions
A common concern pertains to the life of HTM silicone insulators and silicone coatings. The chemical by-products of corona – nitrogen together with moisture – result in serious material degradation. Under the effect of corona, the pH value of the insulator surface can go from 7 to 3.4 within about 15 minutes.
Some grades of silicone rubber are more prone to deterioration when exposed to nitric acid. In a market scenario where QC is notoriously hard to enforce, poor raw material compounding and quality pose a greatly elevated risk in the medium term – especially at higher voltages where e-field levels are significantly greater and HTM life becomes shorter.
As for HTM coatings, CIGRÉ TB 837 states the life of an RTV coating could well exceed 20 years assuming the coating specifications, coating process and underlying insulator are correctly followed and dimensioned. In the case of partially coated SSCIs that are proving another viable option, only the pin-side of the discs need be coated. This provides natural shielding from UV that is expected to help extend life, possibly into the 30+ year realm. Further research and field experience collection is underway.
Pertinent Points:
a. End of life of coated discs needs to be considered across the insulator string and not from a partial section of one or more coated discs. Individual disc insulators may be eroded or degraded from a hydrophobic standpoint, but the string will still perform without any real risk of flashover.
b. While considering end of life of an HTM, failure of silicone of an RTV coating is far less serious than the silicone of a Silicone Rubber Insulator where the housing and consequent structural integrity would be compromised. Even if tab RTV fails, a sound ceramic insulator would still lie underneath.
Therefore, the silicone surface ceramic insulator (SSCI) with only pin-side coated shows promise as the most robust, reliable and versatile choice that would meet performance requirements for all conditions and any future changes in climate or pollution levels – and so could be considered “Climate-Change-Resilient”. The life of such an SSCI is believed to be nearing the life of the transmission line which would result in lowest cost of maintenance, least number of replacements and while also allowing for highest uptime.
Other advantages of partially coated insulators include:
1. Comparable pollution-performance versus fully coated insulators;
2. Mechanical strength that nearly eliminates risk of catastrophic failure;
3. Expected longer life (bottom coated insulators are partially UV shielded);
4. Possibility of in-situ live-line maintenance;
5. Cost advantages due to
a. Minimal modification required for packaging, transportation, handling, and installation.
b. Material saving.
Insulator selection criteria include operational safety, reliability, maintenance costs. The frequency of replacing several million insulators as they reach end-of-life is a monumental task that should therefore be brought as close to zero as possible by making correct insulator choices – especially at higher voltage levels.
Reliability Improvement Programs
As premature insulator ageing and performance have started to pose questions of reliability once again, the Ministry of Power and transmission utilities are in the early stage of preparing reliability strategies focusing on the following points:
1. Improved detailing of specifications and establishing tighter controls for the manufacturing, storage and field handling of SRIs.
2. Being a product comprising of polymeric materials, the life of an HTM insulator must be considered as finite. Therefore, utilities are devising methods for conducting Residual Life Assessment (RLA) studies of HTM insulators in service.
3. As the life of SRIs has been found to be less than the assumed life of a transmission line (35-years), the number of times insulators must be changed before reaching end-of-life can be as high as 4 to 6 times.
4. Strategies are to be devised for (a) when and (b) how to replace all insulators reaching end-of-life at the required frequency, (c) a modus operandi to do so, (d) arriving at a cost and, (e) allocating O&M funds for the same.
5. Understanding that if the life of a certain type of insulator is determined to be ‘X years’, all the insulators of that type or batch must be replaced before the completion of ‘Year X’. Therefore, utilities are working to establish trigger points for when to commence replacement activities that could be determined by, e.g.:
a. Means of RLA;
b. Upon surpassing failure threshold levels in terms of incidents of line drop;
c. By number of incidents of hot spots or abnormal surface activity;
d. Visual assessment;
e. Statistical failure modeling;
6. Conducting studies at a national level and utility level to arrive at the TCO of different insulator types while factoring in material, service, maintenance, monitoring, replacement cost (including the cost of downtime required for replacement).
7. Rising awareness of the above-listed action plans at a national and state level by policy makers who are required to implement policies best suited for the long-term health of the national power network.
8. If it is deemed not viable to shut down certain lines to replace tens of thousands of insulators as they approach end of life, contingency planning at a government policy level may be required.
Conclusions
SRIs failure rates in India are assumed at some 4.28 times the global average. These could be substantially higher if considering only the failures within H-VH pollution zones, where this number could be higher. As an alternative, pin-side RTV-coated silicone surface ceramic insulators (SSCIs) with their mechanical robust structure provide a reliable alternative – the more so because of the natural shielding from acid rain and UV. This could be considered as a ‘climate change resilient insulator’ based on ability to operate under all conditions for decades.
An insulator basket approach is suggested based on global experience as well as conditions in India, where different insulator types are suggested based on different service environment. Line safety and long-term grid health take precedence over all other factors.
References
[1] Powergrid vendor meet on “Failure Analysis of Composite Long Rod Insulators” held on 12 DEC 2022
[2] Contamination Performance of Silicone Rubber Insulators subjected to Acid Rain, X. Wang, S. Kumagai and N. Yoshimura, IEEE Transactions Dielectrics and Electrical Insulation, Vol 5, No. 6 Dec 1998
[3] Paper entitled “Application of RTV Gen-4 Silicone Rubber Insulator Coating in 400KV Switchyard at 4000MW CGPL, UMPP, Mundra
[4] Comparative tests on RTV Silicone Rubber Coated Porcelain Suspension Insulators in a Salt-Fog Chamber, S. Ilha, E A Cherney, IEEE Transactions on Dielectrics and Electrical Insulation Vol. 25, No. 3; June 2018
[5] “Learning from Service Experience with Composite Line Insulators” – INMR 2013, by Andrew Phillips and Chris Engelbrecht of EPRI
