aw_5_partial discharges in power cables

7/28/2019 AW_5_Partial Discharges in Power Cables

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15y old 20kV XLPE Cables-

Some data from EDF

1. Failure rate of 2 fails per year per 100km (the equivalent

paper cable is 3.5, albeit 35y-old systems)

2. 50% were third party damage

presented at Japan seminar 2009 2

3. 45% were at accessories- joints & terminations

4. Only 5% in the cable (other than third party)



Terminations and joints

Partial discharge is the failure mechanism and isdue to poor stress control

Caused by• Bad construction with continuity of the core screen


no ma n a ne n o accessory

• Thermal cycling or water entering so losing corescreen continuity

• Cutting tool penetrating insulation- eg saw cutswhen removing armour



Discharge breakdown XLPE 11kV cable

Saw cut stillvisible at side

of failure site


Careless sawing of the armour wire at gland position led to a cut

30% into the insulation- but it still lasted 5 years to failure!

Fai ure site



Example of use – Discharges in dried-out

tape wound termination

Traces afterdischargesin dissectedtermination.




132 kV XLPE Cable ends- before….




Failed 132 kV termination

The core screen

termination-somewhat misplaced


Should be down here

And where is theporcelain housingand oil?



Modern stress relief cone


CD

BE

Diagram Key:

1 – XLPE insulation

2 – Copper conductor3 – Stress cone silicone rubber conductive insert, electrically floating

4 – Electric field equipotential lines (diagrammatic)

5 – Region of high electrical stress and surface damage

Voltage equipotential lines – stress cone electrically floating.



Failure path in Cable

End of core screen




Discharge damage in sister (not failed) cable

Discharge damageto the XLPEinsulation under the


graphite coating . Itwas taking over ayear to get to thisstage



cable fault in the length- 11 kV LDPE

Third partydamage cancause instant


with long termPD for someyears before

failure if damageonly to surfaceareas.



Water trees in the cable length

Water treeing failures have been common throughout the

world. Lead sheathed oil- paper cables were phased out and it was thought that ol mers didn’t need the


This is revealed by staining. Through most of the development

stage water trees are not carbonised. They are simply waterpenetrating the material at weaker areas – eg at crystalline/amorphous boundaries. But they create electrical PD at a laterstage

same protection from ground water.



Water trees in the cable length

Water trees are not carbonised& do not create PD until finalstages and then too late.

Picture- 11 kV cable after 6 years in


But there is no consensus on best method to detect water

trees- but probably a dielectric loss measurement.Better to avoid Water treeing with an effective sheath barrier, and not leaving ends exposed after pulling in.

Cause? End exposed too long afterpull in



400 kV cable stop joints

A major problem with 275 and 400kV self contained oil filled cable wasfailure at stop joints. Each circuit has a few- at points of ground elevationchange. The narrow oil channel collects copper particles- leading to PD.

CEGB research led to a PDmonitoring system, andfeeding all circuits with an


fluid, DDB

“Partial discharges in power cable joints” RJ Jackson,

A Wilson & DB Geisner, Proc IEE 127C, No 6, 1980



275/ 400 kV joints

1- Bubbleproductionand break-up

2- PD inBubbles


PD occurs if copper debris gets inthe oil gap

Phase resolved PD allowed identification of different typesof pre- breakdown PD in stages 1-2-3 – even in 1970s

3- gasfills gap



On line PD monitoring existing 275/ 400 kV joints


“Partial discharges in power cable joints” RJ Jackson, A Wilson & DBGeisner, Proc IEE 127C, No 6, 1980



Discharge marks on 400kV stop joint


Ref- IEE Proc 127C pp 410-429, 1980

aw_5_partial discharges in power cables

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