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)
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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
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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
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Discharge breakdown XLPE 11kV cable
Saw cut stillvisible at side
of failure site
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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
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Example of use – Discharges in dried-out
tape wound termination
Traces afterdischargesin dissectedtermination.
presented at Japan seminar 2009 5
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132 kV XLPE Cable ends- before….
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Failed 132 kV termination
The core screen
termination-somewhat misplaced
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Should be down here
And where is theporcelain housingand oil?
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Modern stress relief cone
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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.
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Failure path in Cable
End of core screen
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Discharge damage in sister (not failed) cable
Discharge damageto the XLPEinsulation under the
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graphite coating . Itwas taking over ayear to get to thisstage
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cable fault in the length- 11 kV LDPE
Third partydamage cancause instant
presented at Japan seminar 2009 11
with long termPD for someyears before
failure if damageonly to surfaceareas.
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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
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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.
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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
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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
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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
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fluid, DDB
“Partial discharges in power cable joints” RJ Jackson,
A Wilson & DB Geisner, Proc IEE 127C, No 6, 1980
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275/ 400 kV joints
1- Bubbleproductionand break-up
2- PD inBubbles
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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
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On line PD monitoring existing 275/ 400 kV joints
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“Partial discharges in power cable joints” RJ Jackson, A Wilson & DBGeisner, Proc IEE 127C, No 6, 1980
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Discharge marks on 400kV stop joint
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Ref- IEE Proc 127C pp 410-429, 1980