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DC Hi Pot test Vs AC VLF test for cables - Travelling Waves ...

Author: Geoff

Aug. 06, 2024

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DC Hi Pot test Vs AC VLF test for cables - Travelling Waves ...

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.

 

What Is the Advantage of VLF Hipot Testing?

 VLF hipot 'very low frequency hipot ' is an AC high potential instrument used for testing the dielectric strength of insulating materials at 0.1 Hz or lower. Instead of having to perform DC testing because high potential test equipment that operates at power system frequency are large, heavy and therefore not very portable, VLF high potential testing  makes testing cables and electrical apparatus in the field more practical and economical. 



 
Where DC testing can leave harmful space charges in microscopic voids of solid insulation, testing with a VLF hipot exposes cable defects without harming good insulation. When you perform cable or generator testing with a VLF hipot at 0.1 Hz, it requires 600 times less power than at 60 Hz and 500 times less power than at 50 Hz. The added benefit of this type of test equipment translates into portability benefits for field testing applications. The VLF hipot test is a go/no go stress test where the load either holds the test voltage or breaks down. What could be simpler for exposing defects in cables and generators?  

VLF sinewave test equipment can also be used in monitored withstand tests where diagnostic information can be collected for Tan Delta and Partial Discharge.

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Editor's Note: This post was originally published October, and has been updated freshness, accuracy and comprehensiveness.

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