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Internal and External Defects in Polymer
Insulators - Research by STRI
Dr.
Igor Gutman
The
main sponsors of the STRI research on IR/UV insulator diagnostics
were the following power utilities: Statnett, Svenska Kraftnät,
Vattenfall (Scandinavia), Entergy (USA) and Tohoku (Japan). Other
power utilities that contributed to the funding were: Fingrid and
Elforsk (Scandinavia), AEP (USA) and Powerlink (Australia).
Dr. Igor Gutman of STRI, an accredited and independent laboratory
for R&D, testing and consultancy within electrical power transmission
and distribution, presented in the UGM for the first time detection
of internal defects in polymer insulators. A special example of
an external erosion corona effects is also presented below.
fig 4, 5, 6: 400 kV NCI: corona detection
of internal defects in a clean test site by DayCor II
--up>
Detection
of internal defects in polymer
insulators
A
test was designed to check possible degradation of NCIs due to internal
defects. Three types of defects were intentionally induced in 400
kV Silicon Rubber line insulators as follows:
- Core
rod delaminated with acid ingress
- Core
rod-to-housing interfacial separation with water ingress (fig
4 and 5)
-
Interfacial separation with a conductive outer core rod surface,
i.e. tracking mark (fig 6)
Each
of these defect type, manufactured by
caused the corona activity. Thus it is possible to detect pure internal
defects as shown in figs 7-8. This insulator was damaged during
a lightning impulse test, exactly where the loss of contact was
expected.
The
histogram below shows results obtained during the second year of
the test from a batch of seven defective test insulators. By analyzing
the results, Dr. Gutman concluded that the probability to have corona
activity depends on the average ambient humidity, the insulators
were exposed to. Generally speaking, the probability of corona presence
depends on the extent of damage and exposure to humidity.
Detection
external defects in polymer insulators
Fig
9 is an example of detection of external erosion on an insulator
of 145 kV voltage class, exposed to severe coastal environment.
In this case the corona activity was produced not only close to
the fitting (where erosion was also located), but also, in the middle
of the insulator in the area with not too high E-field stress. --up>
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