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Field experience and laboratory results on the application of RTV coating on HVDC lines

  • Authors: M. Marzinotto, TERNA Italy – J.M. George, Sediver France – G. Pirovano, RSE Italy
  • Title: Field experience and laboratory results on the application of RTV coating on HVDC lines.
  • Presented at: CIGRE 2020 PARIS – cigre e-session 48 – Aug 24 – Sep 3 2020


The utilization of RTV pre-coated insulators is rapidly expanding worldwide thanks to the very positive laboratory and field returns.

TERNA started the installation of pre-coated glass cap and pin insulators in 2003 on AC lines up to the 380 kV voltage level and today there are more than 1 million units installed in the Italian high voltage grid. No flashover was reported through all these years of service and insulation washing was no longer needed on those line equipped with RTV insulators.

Every year tests for the evaluation of both coating degradation and ageing are performed on different units removed after different years of service from the lines.

The outcomes of such tests are different since they are strictly related to the environment in which the insulators are exposed. In the worst condition sensitive changes related to degradation or ageing have been pointed out, even if both laboratory tests and field experience confirm their high level of endurance.

Since 2008 TERNA decided to gradually expand the installation of pre-coated glass cap and pin insulators on HVDC lines considering the very good returns of the AC applications.

Since degradation and ageing are different in DC, specific tests were performed in laboratory to make some inference on endurance.

This paper reports different type of tests on RTV pre-coated cap and pin insulators for DC applications with the aim to characterize the behaviour on new and field returned units among which results from the multi-stress ageing test (known as 2000 hour Terna’s test), which represents a particularly significative test for the overall evaluation of the performances of the insulators. The results of such tests are compared with the results of tests carried out on naturally aged insulators (after 7, 8 and 9 years of installation) in order to quantify the extent of aging in operation and estimate the useful life of the RTV coating.

New insulator strings were tested in laboratory in different string configurations (“I” string, “V” string and quite horizontal string) and applying in sequence twice the 2000 h ageing test. Test results from laboratory and field aged samples pointed out interesting information.

A discussion on the comparison of such results in terms of hydrophobicity, hydrophobicity recovery and reduction of flashover performances (evaluated through the “quick flashover method”) is reported, since both hydrophobicity recovery and flashover voltage reduction are considered as operational degradation factors for ageing inference. Both tests on laboratory aged sampled and on field returned samples highlight the high level of reliability of pre-coated glass cap and pin insulators also for DC applications.

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