Vlf
Vlf
Hi
The problems mentioned are mainly related to polymeric cables.
It was discovered in the early 80s that DC testing prematurely aged Polymeric cables. The primary problem seems to be the "space charges" that develope during DC testing. They are generally caused by small discontinuities in the plastic insulation, these micropockets become charged up by the DC current on the cable. When the cable is earthed and discharged at the end of the test, these pockets retain their charge and microdischarges take place when AC supply is reinstated.
These discharges cause tree's in the insulation to be developed which can rapidly grow into a fault.
The space charge problem was particularly bad in the early years of developement of the polymeric cables however with the improvement of quality control in the cable manufacturing process has reduced the severity of the problem however I think you will find that no cable manufacturer recomends dc testing on polymeric cables anymore.
Travelling waves are a little more difficult to explain (mainly because I don't have as much experience with them) as i understand it when DC causes a breakdown a travelling wave is developed that can cause further damage.
I hope this helps.
If you need any more help please drop me a line.
Paul Tuson
Inlec UK
Goto Hengfeng to know more.
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