Case Study for Using & Monitoring Composite Insulators on Overhead Lines under Natural Pollution

Berrêdo, A. C. S.; Coutinho, F. S.; Silva, S.A. and Sousa, A. R. Transmissora Aliança de Energia Elétrica S.A. (TAESA), Rio de Janeiro, Brazil

Abstract: Acquired by TAESA in 2010, the 500 kV Xingó – Angelim II Transmission Line (TL) is about 193 km

long, passing through Alagoas, Pernambuco and Sergipe states, on the Brazilian northeast. Initially projected to use glass insulators strings, the TL presented, since its commercial operation in 2002, many shutdowns related to natural pollution. Firstly, the totality of those cases occurred along 54 km of the asset, situated at the mountain range called São Pedro, in Pernambuco state, due to low rates of precipitation over the year and a high dew occurrence. The previous solution given in 2015 was to replace the existing strings to polymeric insulators solving the problem for 3 years. Then, last year, there was an increase of cases spread outside the previous location. The Engineering and the maintenance crew worked together to, at the same time, studying the reasons to the phenomena and classifying the polluted strings criticality by the analyses of the corona activity. As a result, the team could identify an important decrease in rainfall during 2016, comparatively to the previous year, as the cause of outages in that region. Finaly, with the new insulator employed, a different treat must be given to inspect them, where the engineering and maintenance crew is developing standardized ways of inspection.

Index Terms— Composite Insulators, Flashover, Pollution, Silicone Coating, Transmission Line.

I. INTRODUCTION

Failures in transmission line insulation related to natural pollution is not uncommon and occur in many areas in the world associated to issues involving environmental changes in specific zones, incorrect project parameters and others.

The outages related to pollution normally occur when the water drop dissolves but do not remove the salt laid up at the insulator. So, the leakage current flows over the wet polluted surface of an insulator forming high-resistance dry bands. Discharges across these dry bands usually extinguish by itself but, exceptionally, this process may develop into a flashover [1] as shown in fig. 1.

Fig. 1. Steps to flashover on insulator surface due to pollution. [5]

That situation is more dramatic when areas where the insulators are installed pass throw long periods of low precipitation. It happens because the rain has no sufficient intensity to wash the majority part of the dirt coated on the insulator surface improved by an environment with reasonable levels of humidity.

The Brazilian Northeast area, where the TL 500 kV Xingó – Angelim II is located, historically brings together all those aspects, specially influenced by El Niño [2] and others important atmospheric systems [3].

Despite the studies on natural pollution in energy sector have been conducted for decades around the globe, in Brazil this problem has increased with the transmission lines grid growth over the past years, especially in Northeast. This situation has attracted the attention of specialists in order to analyze the pollution on high voltage [4] and purpose ways to overcome it.

On this context the TL Xingó – Angelim II has passed, since its commercial operation, through several outages due to natural pollution on their insulator strings.

This case study brings an overview on the procedures and researches made to fix and prevent those occurrences along the entire asset, avoiding premature stress of the line and substation components and, consequently, resulting in maintenance of the reliability.

II. PROBLEM BACKGROUND

The TL 500 kV Xingó – Angelim II, was design to connect the Xingó Hydroelectric plant, in Alagoas state, to Angelim II substation, in Pernambuco state, crossing its 193 km length inside the northeast part of the country.

Since its operational start, in 2004, this power line had suffered a series of shutdown events in a specific area called São Pedro Mountain Range, on highlands of Pernambuco state.

This region is characterized by few periods of precipitation, usually from April to August, and a frequent fog and mist occurrences.

About 57 km of this TL, starting at tower 284 until 397, passes through that area as shown in the Fig. 2.

Fig. 2. Part of the TL influenced by pollution in São Pedro Mountain Range highlighted in green, between towers 284 and 397. The first measure taken to solve the problem was to wash systematically the glass insulator strings with live

line techniques, as shown in Fig. 3. However, considering difficulties to access the towers and a brief time between cleanings, the cost of this operation became unfeasible.

Fig. 3. Use of washing techniques in live line.

As a result, in 2007, the company applied the use of silicone coat over the glass insulators surface. About 13,800 insulators were replaced by new ones, in with 8,656 of them was covered by silicone coating.

Fig. 4. Glass insulators with silicone coat

Nevertheless, after three years using silicone coated insulators, a new case of outage related to pollution

happened at the tower 284, exactly where the hills are situated. At that opportunity, O&M engineering team had the chance to observe the coat condition after some years in

service. Analyzing the insulators (Fig. 5) the engineers could find signals of hydrophobicity loss property and

consequently reduction of their effectiveness, evidenced by the silicone surface flaking.

Fig. 5. Polluted insulator flaking from tower 284, after 3 year of installation.

In 2015, after some other outages, the tower 284 was affected again, showing the necessity to take new measures to mitigate that problem.

Fig. 6. Failure at the tower 284 insulator’s string occurred in 2015 due to pollution.

III. DATA STUDY

The O&M engineering team conducted studies in order to valuate new solutions applicable taking all the data available.

Firstly, the cases of fault caused by pollution were listed in order to know if the previous problem was still happening at the São Pedro’s Mountain Range, confirming the loss of the coat effectiveness, or if new areas were suffering from the same problem, revealing a pollution dispersion.

The Table 1 exhibits the failures since 2010, pointing all the towers as well as their model and phases involved.

An important information taken from the database considered was that all those outages happened at dawn or during the night period, reinforcing the influence of humidity.

The biphasic outages to ground suggest that both phases had similar characteristics.

Date (year)

Distance to nearest

substation (km)

Tower operational

number

Tower type

Affected phases

2012 32 331 X1A A/C 2012 31 332 X2B A/C 2012 56 285 X1A C 2012 56 284 XS8 A/B 2013 61/62 272/273 X1A/X2A B/C 2014 42 312 X2A B/C 2014 42 312 X2A B 2015 19 359 XS8 A 2015 56 284 XS8 B/C

Table 1. Historic of outage caused by pollution. [5] Then, the silhouette of the towers in order to confirm if the cases were happening in similar arrangements.

(Fig.7 and Fig. 8).

Detail  of   flashover

a) Single I strain set b) Double I strain set

Fig. 7. Typical I set from TL Xingó – Angelim II. [6]

a) Single V strain set b) Double V strain set

Fig. 8. Typical V set from TL Xingó – Angelim II. [6] The TL 500 kV Xingó – Angelim II was projected considering 8,320 mm (15.12 mm/kV) as creepage distance

to all transmission line. This value represents a light level of pollution, according to IEC 60815 [7]. In order to confirm spatially if the cases were occurring only in the hills areas, a graph was plotted considering

the altitude of each tower. The Fig. 9 shows the topographic profile, indicating the São Pedro Mountain Range highlighted. By this

image, it is possible to understand by the altitude why this area is more susceptible to fog and mist conditions.

Fig. 9. Topographic profile of the TL corridor with the São Pedro Mountain Range highlighted. [5] To better understand how the rainfall behaves, the O&M engineering team collected, from the Pernambuco’s

Climatology Agency – APAC, several data from each year, taking as a reference the town of Bom Conselho, in the middle of the hills (Fig. 10).

0  

200  

400  

600  

800  

1000  

1   25  

49  

73  

97  

121  

145  

169  

193  

217  

241  

265  

289  

313  

337  

361  

385  

Alti

tude

(m)

Tower identification number

Topographic profile of the TL's corritor

Fig. 10. Accumulated rainfall average in Bom Conselho town. [8]

IV. FIRST MITIGATION

The results from the study showed the necessity to focus the efforts on the São Pedro mountain range, as that area was the only one with evidences of insulation failure due to natural pollution.

The mitigation of the problem passed through the analyses of many factors as:

• Amount of strings involved; • Cost of implementation; • Cost of solution; • Experience in assets from other utilities; • Penalties applicable by the national regulatory authority; • Safety of linemen and asset; • Type of solution.

Considering the proven performance of composite insulators adopted in neighbor transmission lines at the

same region and the own experience, the O&M area recommended the replacing of all the 618 current insulator strings installed on the São Pedro Mountain Range, between towers 284 and 397, to extra high pollution composite silicone insulators. This recommendation included towers with suspension and strain arrangements.

The composite insulators chosen have near 15,000 mm of creepage distance, about 80% higher than the glass string in addition to a higher level of hydrophobicity and other properties as shown at Table 2.

TECHNICAL INFORMATION GLASS STRING COMPOSITE

Model Toughened Glass Extra High Pollution Polymeric

Mechanical load (kN) 120 120 Diameter (mm) 254 138/110 Insulating length (mm) 146 (3796 total) 3791 (unit.) Creepage distance (mm) 320 (8320 total) 14630 (unit.) Connection 16A 16A Weight (kg) 104 26

INDUSTRIAL FREQUENCE Dry withstand voltage (kV) 70 (unit.) 995

Wet withstand voltage (kV) 40 (unit.) 780

Dry flashover voltage (kV) 80 (unit.) -

Wet flashover voltage (kV) 50 (unit.) 1025

LIGHTINING Withstand voltage (kV) 100 (unit.) 1785 Critical positive 125 (unit.) 1980

J F M A M J J A S O N D Months

Rai

nfal

l (m

m)

2012 2013 2014 2015

impulsive voltage (kV) Critical negative impulsive voltage (kV) 130 (unit.) 2030

Table 2. Technical specifications from the glass insulators and polymer insulators. [1]

The use of polymer insulators increased the creepage distance from 15.12 mm/kV to 27.3 mm/kV, enhancing

the line to face levels of pollution between heavy and very heavy classification. After acquired, a joint effort provided the replacement of all old strings which happened in three days in de-

energized line works (Fig. 11).

Fig. 11. Replacement of insulators string.

V. RECURRENCE OF OUTAGES AND SOLUTION

After one year running with new polymeric strings, the power line faced again a new increase of its outages rates in 2016 having the same characteristics of the natural pollution ones but, at this time, out of the São Pedro mountain range.

Then, the maintenance crew adopted inspections in sample areas along the line using corona camera (Fig. 12 and Fig. 13) in order to properly check what was happening.

Fig. 12. Critical corona activity I string – Tower 139.

Fig. 13. Critical corona activity V string – Tower 012.

The results show that the level of corona activity has dramatically increased on the insulators surface,

confirming the suspiciousness of high pollution level phenomena. At this point, the main question settled is the reason why those occurrences have not happened before if,

apparently, any of the geographical conditions had not changed. The only thing that could justify this sudden raise would be a change in the rainfall behavior. The O&M

engineering crew focused then their analyses on rainfall in 2015 and 2016 (Fig.13 and Fig.14) to see what has occurred. The difficulty to get rainfall data from Sergipe and Alagoas states [9] forced the studies to be concentrated in Pernambuco one - assuming that the rain behaves is quite similar on those areas based on previous Northeast pluviometric maps.

Fig.13. Precipitation in 2015

Fig.14. Precipitation in 2016

From the graphics was possible to observe a rainfall decline in 2016 compared with previous year, except for Terezinha station, as well as an extension of the dry season in most weather stations.

Those evidences could lead the company to take actions to avoid new occurrences by replacing the toughened glass insulators remaining to extra high pollution polymeric insulators.

VI. MONITORING The application of this new technology, compared to ceramic insulators, brings the necessity for different and

conjugated techniques to prevent occasional failures and new outages. The corona camera used by the team perfectly helps the maintenance crew to analyses the dirt evolution on the

polymeric surface along the years. However, to check problems involving fragile fracture an accurate visual inspection will be requested as well as a combination of other techniques such as the use of Positron and X-ray in polymeric insulator samples taken out from the transmission line.

The O&M engineering team has been building with the maintenance crew a regular inspection plan on those insulators using all the techniques listed and revising procedures to discipline their use.

VII. CONCLUSION The 500 kV Xingó – Angelim II pass through a heavy natural pollution zone in Brazil not considered by the

project. A combination of this situation with a humidity concentration leads the asset to reach high outage levels over the years.

The São Pedro mountain range was studied in the first place because of all the outages were concentrated in this part in addition to higher levels of humidity generally found in mountains. The solution applied firstly was successful once the number of outages decreased right before the replace of the toughened glass strings to composite polymeric ones.

The spreading of the outages cases to other parts of the line with same characteristics suggested that a probable combination of humidity and pollution was affecting the rest of the transmission line. The logical conclusion was that the faults had not happened before because the hills were the weakest part of the line under humidity conditions. Once this part was treated, other weak spots started to shows up.

The way the O&M engineering department found to support this theory was to analyze the rainfall data in order to check any changing in the rain season noticed in the year after the first replacement.

Data from the graphs confirms a decrease of precipitations levels along the year, comparing 2015 and 2016, in almost all the weather stations observed. This lack of rain brought a rise of dirt accumulation on the glass insulator surface, which could provide flashover cases under certain humidity conditions.

To solve the problem the same solution taken in the first case replied, that is, replace the existing glass insulators strings to polymeric insulators.

The configuration and interpretation of the Corona cam images was crucial to determine the criticality of the strings to be replaced. It also developed the maintenance crew to inspect the new strings properly.

Finally, the new concept of inspection must be implemented in order to face some issues that the use of polymeric insulators brings and TAESA is equipping and training its teams to this new challenge.

VIII. REFERENCES [1] HAMPTON, BFi, 1964. Flashover mechanism of polluted insulation. Proceedings of the Institution of

Electrical Engineers. IET Digital Library. p. 985-990. ISSN 2053-7891. [2] DE MELO, Josemir C, 1999. O fenômeno El Niño e as secas no Nordeste do Brasil. Raízes, ano XVIII, nº

20, p. 13-42. [3] DA SILVA, VICENTE DE P. R.S. et al, 2012. Estudo da variabilidade anual e intra anual da precipitação da

região Nordeste do Brasil. Revista Brasileira de Meteorologia. Vol. 27(nº 2). p. 163-172. [4] DE MELLO, Darcy R., et al. Avaliação do Grau de poluição em instalações de transmissão, subestações e

distribuição. I Citenel. Brasília, 2002. [5] TAESA, 2015. “Estudo sobre a falta provocada por flashover nos isoladores das fases B e C na torre 284 da

LT Xingó – Angelim II.” Rio de Janeiro:TAESA. NTE.RT.0052.00.

[6] QUINTAS & QUINTAS, 2002. “Sistema de Transmissão Xingó - Angelim 2 - Projeto Básico IN-LT500-PB-0100,” Rio de Janeiro:Quintas&Quintas. IN-LT500-PB-0100.

[7] IEC, 2008. IEC T. S. 60815-1:2008. Selection and dimensioning ofhigh-voltage insulators intended for use in pollutedconditions-Part, 1. IEC.

[8] AGÊNCIA PERNAMBUCANA DE ÁGUAS E CLIMA, 2015. “Boletim Pluviométrico Diário - 07/06/2015,” Recife: APAC. [Viewed 6 julho 2015]. Available from: http://www.apac.pe.gov.br/arquivos_portal/boletinspluviometricos/Boletim_Pluviometrico_07.06.pdf.

[9] BARROS, Alexandre Hugo Cezar, et al., 2012. Climatologia do estado de Alagoas. Recife: Embrapa Solos-Boletim de Pesquisa e Desenvolvimento (INFOTECA-E).