Cable Armour Damaged in No-Load Condition [Analysis] [Pictures Attached]

[Rejeckted]

Hello,

Cable Laid, Energized but No-Load Condition. How could the Armour even get damaged in such condition? Looks like some sparking took place? But how does spark take place in cable brackets in No-Load Condition (Just Energized). Induced Voltages will be only due to No-Load Current which shouldn't be that high.

Share your opinions

Thanks
Chris

 

[Rejeckted]

The Construction is 2XTaY(P) so Conductor >Insulation > Cu Wire Screen > Innersheath > Double Al Tape Armour > Outersheath

 

[waross]

Capacitance of the cable raising the voltage?
How far from the feed end was the damage?
Single phase?
Test voltage?
Was there prior damage to the jacket that could have allowed water to intrude and facilitate the development of water trees?

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[RRaghunath]

Is the cable armour earthed at both ends?
How long is the cable?
Are the two earth mats, sending and receiving end stations, interconnected through copper flats??
Was the cable outersheath integrity tested after laying the cable and before energisation??

 

[Rejeckted]

Cable was being Laid so its a newly laid. Yes single phase 25 KV Railway AC Electrification. They didnt mentioned the Vr, I understand that there will be ferranti effect.

Could it be that charging current through the copper screen (grounded) was high? and it got burnt? But then it is made to withstand S/C Current during fault so i dont think so.

 

[Rejeckted]

@Raghunath No the armour is Open Circuit, only the screen is earthed at one end. Please lemme know what if it was earthed or unearthed (like what is the point you are making because that will help)

Cable is nearly 700m but about 500m laid in Cable Brackets.

Are the two earth mats, sending and receiving end stations, interconnected through copper flats?? (No information, but again please make your point, i will clarify)

Was the cable outersheath integrity tested after laying the cable and before energisation?? No Testing

 

[Power0020]

I can only think of the sheath failure to ground at a bracket due to jacket punture during laying. as the cable is single point bonded and there is no current, the cable charging current will still be there and raise the sheath voltage.

Please clarify if the sheath failed near to the earthed end or at the far end.

There may be an instant of an EPR at the substation where the sheath is connected to earth and the transferred voltage flashed away from the substation. A visit to the station event recorder should highlight any abnormal zero sequence currents.

 

[Rejeckted]

@Power0020 Good Points.

Induced Voltage: Is the induced voltage so high in no load condition? When only charging current is flowing? There was no SVL Connected. Also You are talking about Induced Voltage in both Armour and Copper Screen right?

Earth Potential Rise: But how does the rise in potential traveled/ transferred or flash away to that point or shall I say propagate? You mean to say such EPR will be recorded in terms of Zero Sequence current a SS right?

Thanks

 

[7anoter4]

In my opinion:
1)the cable jacket was damaged when cable run.
2) the cable terminating -or jointing- wrong made-the grounding wire was in contact with live part [it could be moisture penetration, in a way, too].

 

[waross]

I suspect prior jacket damage that allowed water to infiltrate and form water trees in the insulation.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Rejeckted]

@7anoter4 I mean if termination or splicing made Live and Ground Short its a L-G Fault. Why would the fault travel to that specific location. Wouldn't it be at that specific point? And Cable Screen Wires are generally rated to carry fault currents for few seconds. But then again wouldn't the entirety of Screen Burn? Or could it be that at that specific point two screen wires were joined with welding or solder and then the High Resistance Caused it to spark/ arc at that point?

 

[7anoter4]

Destruction of the armature and shield is due to the electric arc and not due to the short circuit.

 

[Rejeckted]

And where did the arc come from?

Lets say oversheath it got damaged while laying, armour comes in contact with the bracket. This armour has some induced voltage as per faradays law due to no load current.

Now what about the arc? Where does it come from. Is is when the Armour touched Grounded Bracket the arc took place?

 

[7anoter4]

It is obvious the shield was energized by contact with the live part. Usually, it could be at the termination-the semi conductive layer was not well cleaned on another phase-for instance. [And the damaged shield was not well grounded].

 

[Kiribanda]

Hello Rejeckted,
What is your method of grounding of the 25kV AC distribution?

 

[che12345]

Dear Mr Rejeckted
R. "...Cable Laid, Energized but No-Load Condition. How could the Armour even get damaged in such condition? Looks like some sparking took place? But..."
C1. I (suspect?) it is a [sabotage!]. Not cable manufacturing defect or installation bad workmanship or etc.
C2. Image 1 shows 2 cuts. Image 2 shows 1 cut. Image 3 shows 1 cut on the outer jacket. These cuts are not along the length of the cable, but at an angle. It is likely "man made" not a sign of damage pulling along during laying.
C3. Image 4, 5 and 6 show that there a small semi-circle on the lower part of the jacket. This is evident that the cable had been punctured by a small diameter "nail etc" from outside of the jacket through the insulation layers and armours to the conductor. This is evident that short-circuit is only on one side of the cable. The "nail" may had been taken away (in order to avoid any evidence} after the dirty work.
C4. Spark/heat is generated that cause the explosion where the conductor short to the armour, which is earthed.
C5. Precaution: Insulation resistance test, over-voltage test and insulation resistance test again; then dis-charge the cable immediately upon completion of the installation. Repeat the above tests and dis-charge the cable ; immediately [before] energisation.
Che Kuan Yau (Singapore)

 

[Rejeckted]

@7anoter4 So basically the copper screen was not grounded rather live due to bad workmanship. My only worry is why does it cause fault to take place somewhere in middle of cable? Also remember there was no load

@Kiribanda Single End Bonded (Only the Copper Screen) (Armour Open)

@che12345 Nice theory mate. Just so you know the cable is passing HV Test. So was conductor really short to armour or copper screen?

 

[davidbeach]

If the screen wasn’t grounded properly the failure was going to occur at the point of greatest stress, where it was in close proximity to a grounded surface.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[Rejeckted]

@Davidbeach Thanks, really appreciate inputs from everyone.

So many experienced people here. I get to learn a lot.

 

[Kiribanda]

Sorry, my question was what is the existing grounding method of your 25kV distribution?
Is it solid grounded or high resistance grounded (HRG)?