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

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The 500 kV Xingó-Angelim II Line that runs through northeastern Brazil, connecting the Xingó Hydroelectric Plant in Alagoas State to Angelim II Substation in Pernambuco, suffered a series of shutdown events. Most of these shutdown cases  happened mainly in the São Pedro Mountains, in the highlands of Pernambuco. This region is characterized by limited precipitation, typically only from April to August, as well as frequent fog and mist. This case study refers to a section of that line that runs from Tower #284 to #397 in that area.

map of the Xingó-Angelim II Line
Towers #284 to 397 portion line. in São Pedro Mountains, highlighted in green.

This transmission line was built in 2002 using glass insulators. These had to be frequently washed in order to overcome the natural pollution. However, the systematic live line washing was too costly and too frequent, and proved untenable given the difficulties in accessing the towers. As a result, in 2007 13,800 insulators were replaced by RTV coated glass insulators.

live line washing
Live line washing
glass insulators pre coated with with RTV silicone
Glass insulators pre coated with with RTV silicone

Outages will occur in polluted areas where rain is insufficient to wash away the salt and other conductive contaminants that have accumulated on the surfaces while humidity is high. Under such conditions uneven wet conductive filaments are formed creating leakage current and forming high-resistance dry bands. The discharges across these dry bands are usually self-extinguished but, in exceptional cases, they can develop into flashover

Three years after the application of silicone coat another pollution flashover outage occurred at Tower #284 – located at the foothills of the inspected line section. That incident gave the O&M Engineering Team the opportunity to remove and examine the condition of the coated insulators. There was  a  clear evidence of surface flaking and loss of hydrophobicity, and consequently a reduction in the coating effectiveness.

Flaking on polluted insulator
Flaking on polluted insulator
Fashover sign
Fashover sign

REVIEW OF OPERATING DATA

The O&M Engineering Team conducted studies to evaluate possible solutions. They were looking at all cases of faults caused by pollution trying to see:

  1. if these were concentrated mainly in the São Pedro Mountains or happened in other areas of the line as well – this would confirm the loss of coating effectiveness
  2. when did these outages occur  – if at dawn or during the night it would confirm the influence of humidity
  3. which phases were affected – biphasic would confirm that both phases had similar characteristics
  4. type of insulators – to confirm if faults were concentrated on the same type of   insulator configuration
  5. data from the climatology agency – APAC – on the local rainfall patterns over the years

STUDY RESULTS

The São Pedro Mountains area was the only area with evidences of insulation failure due to natural pollution  Possible mitigation alternatives were compared in light of several factors:

• Numbers of strings involved
• Cost of implementation
• Experience at other utilities
• Penalties applied by national regulatory authority
• Safety of linemen and asset
• Nature of solution

O&M ENGINEERS RECOMMENDATIONS

Considering the positive service experience with composite insulators applied on transmission lines in the same region as well as TAESA’s own experience with them, a recommendation was made to replace glass insulator strings with an extra high pollution design of silicone insulators. This recommendation included towers with suspension as well as strain insulator assemblies. The composite insulators selected had almost 15,000 mm creepage distance, i.e. about 80% higher than the coated glass strings they would replace. They also offered superior hydrophobicity and other desired electrical properties, as shown in the table. Application of these polymeric insulators increased specific creepage distance from 15.12 mm/kV to 27.3 mm/kV, enhancing line insulation to face pollution levels classified between heavy and very heavy.

OUTAGES KEPT ON

Corona on I string

Why these problems did not occur before?

The answer lies in the meteorological data supplied for the Sergipe and Alagoas  states. The data indicated that there was a notable decline in rainfall in 2016 and an extension of the dry season at most weather stations. This evidence would lead the utility to replace the glass insulators with the same high pollution poymeric type used near the São Pedro mountains.

A year later, in 2016, the line experienced again  an increase in outage rate, only  this time the focus was on towers located outside the São Pedro Mountains where the glass insulators were still in place. This time the O&M teams used a DayCor®corona camera to check what was happening. Results showed immediately and clearly that there was corona activity on the glass insulator surfaces. This confirmed the suspicion of  unexpected high pollution phenomenon.

MONITORING WITH CORONA CAMERAS

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.

As of the end of 2017, the O&M Engineering Team had begun to establish a regular inspection plan for these insulators, using all optional techniques, as well as revised inspection protocols to be undertaken by maintenance crews.

CONCLUSION

The 500 kV Xingó-Angelim II Line passes through an area of heavy natural pollution. The project was designed irrespective to the local ambient conditions selecting insulators with low hydrophobicity and low endurance to pollution.  A combination of pollution and high humidity resulted in high levels of outages over the years. The São Pedro Mountain Range was selected as a case study, inter-alia, since most outages were concentrated there. The solution to use RTV coating proved successful for a period of time but it did not solve the problem. It was eventually decided to replace the toughened glass strings with a composite polymeric type  suitable for the local conditions.

The configuration and interpretation of the corona camera images was crucial to determine the criticality of the strings to be replaced. It also lead the maintenance crews 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.

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