DC Hi Pot test Vs AC VLF test for cables - Travelling Waves and Space Charges

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Roshan0710

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While going through online Catalogues/Details of AC VLF Cable Test Kits, i came across an advantage of VLF testing Vs DC Hi Pot testing as

?Harmful Space Charges are not injected into the cable insulation.
? No travelling waves are generated during testing.
Click to expand...

I don't have much details about the generation of Space Charges and travelling Waves.
How are these generated?
How are these negated in an AC VLF test?
Are there any further literature available on these?

Any info would be welcome.
I am an Electrical Engineer, but being on site, i sometimes get out of sync with the engineering aspect. I am re-educating myself.
 
Post your links.
I think traveling waves means reflections from the distant cable end due to energizing the cable with a step function (unless the hipot test ramps up the voltage).

IIRC, Bergeron did work on reflections from discontinuities in system impedances.

Hope this helps. . .
"Space charges can also occur within dielectrics. For example, when gas near a high voltage electrode begins to undergo dielectric breakdown, electrical charges are injected into the region near the electrode, forming space charge regions in the surrounding gas. Space charges can also occur within solid or liquid dielectrics that are stressed by high electric fields. Trapped space charges within solid dielectrics are often a contributing factor leading to dielectric failure within high voltage power cables and capacitors."
 
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Links

Links

I am Sorry for not posting links.
Most of the sites that showed up on the google search were pdf brochures/specs.
Here is a link on a website itself

http://www.atecorp.com/equipment/highvoltage/vlf-28cmf.asp

I understand that Space charges are formed in the dielectrics, mainly the insulations.
But there is no information on how traveling waves are formed.
Is it due to the High transients that develop on application of High Voltages?
 
from
http://www.physicsclassroom.com/class/waves/u10l4a.cfm
apparently traveling waves interfere with each other so you don't know what each place on the cable is seeing as to voltage.

If the test set in the link varies the freq. this even makes more sense.

I guess an analogy is standing waves in a microwave oven. With a rotating food tray this effect is cancelled, and with the test set, the varying freq. is like the tray rotating.
 
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
 
more info on travelling waves

more info on travelling waves

From what i have been able to gather,

Due to a variety of reasons, such as a direct stroke of lightning on the line, or by indirect strokes, or by switching operations or by faults, high voltage surges are induced on the transmission line. The surge can be shown to travel along the overhead line at approximately the speed of light. These waves, as they reach the end of the line or a junction of transmission lines, are partly reflected and partly transmitted.

Similarly on the application of DC High Voltage during Hi Pot testing, upon the switching on, high voltage surges are induced in the cable's conductors. These surges travel along the length of the cable and are reflected / transmitted too. These surges induce electromagnetic fluxes in the conductors which are contributing to the conductor degrading.

AC VLF tests use a 0.1 Hz Pulse wave that changes due to sinusoidal polarity. due to sinusoidal transitions between the HV Pulses, travelling waves are not generated.



With regards

Roshan
 
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