Multicore / Power Cables

[Linspire]

Dear all,

I would like to seek reason why the following test must be conducted with acceptance criteria,

i) partial discharge,
ii) high voltage at 66.5kV (AC) for 5 min,
iii) Sheath HV (DC) test, 25kV (1min),
iV) Hot set test,


From i understand,

i) to ensure minimum current losses,
ii) able to withstand such high fault voltage for certain period,
iii) No ideas.
iv) To ensure when short circuit fault current flow through, still maintain the original length


Hope there is engineer correct me and enlighten me.

 

[Linspire]

I also need you guys input each layer of screen/insulation on a cable.

 

[LiteYear]

I'll make a start for others to improve upon:

i) current losses are insignificant. More to do with cable deterioration - partial discharge leads to insulation failure.
ii) test cable insulation. Duration would allow for partial discharge effects.
iii) this is about "treeing" (

http://en.wikipedia.org/wiki/Electrical_treeing)

- DC can promote this effect much faster than AC
iv) no idea

 

[7anoter4]

According IEC 60502-2 Table 19 – Test requirements for particular characteristics
of various thermosetting insulating compounds Hot set test Treatment:
– air temperature (tolerance ±3 °C) for EPR,XLPE 250 dgr.C
this is actually the maximum short-circuit temperature rated for this materials then this test instead of short-circuit test.
I could not find a similar test on UL 1072 standard.

 

[Linspire]

Any engineers can share more on regards or explain more.

Thanks !

 

[rcw retired EE]

Here's my semi-technical, non-rigorous explanation:

i) partial discharge, - Good insulation has no partial discharge. This is the best test for verifying integrity of the insulation and cable assembly. For example, it might pick up some voids in the seam between the insulation and the extruded semi-conducting conductor screen due to improper extrusion technique during manufacture. Under high voltage stress, the voids concentrate the voltage gradient in a small area leading to partial discharge (small arcing) that acts like an etching tool that can eventually bore a hole in the insulation.
ii) high voltage at 66.5kV (AC) for 5 min,- AC withstand test, may be part of PD test. The AC over potential can trigger some PD activity.
iii) Sheath HV (DC) test, 25kV (1min),- verifies that the jacket over the sheath is good. If the jacket is cracked or scraped, the sheath may be exposed to deteriorating agents. A crack or pinhole may be picked up by the sheath test.
iV) Hot set test, verifies the quality of insulation or jacketing materials.

Second question on cable layers:

1. Conductor, multiple strands, operates at system voltage.
2. Semi-conducting strand shield. Usually an extruded layer, could be tape, that fills in the gaps between the individual strands of the conductor, minimizing irregularities that could concentrate voltage stress. It smoothes out the voltage profile.
2. insulation - Usually extruded, could be layers of tape with insulating oil. It is the major insulating component.
3.Insulation Shield - Thin semi-conducting extruded or tape layer between insulation and ground shield to smooth out voltage stress.
4. Shield - copper tape, or wrapped wires that form a concentric ground plane around the conductor and insulation. It's main purpose is to keep the voltage stress (Volts/mm) uniform in the insulation. Ideally the equipotential lines in the insulation will be perfectly concentric.
5. Jacket- mechnical protection over the shield. Protects shield against corrosion and mechanical damage.

There may be other mechnical protection layers.

 

[Linspire]

So nobody going explain further ?
I presume rcwilson has good explanation.