Trapped charge on HV cable

[leoliu]

We have a long undergrouded HV cable and are considering how to de-energize the cable. Does anyone know if the trapped charges on cable will decay slowly (how fast?) after de-energizing without any additional equipment to drain the charges? thanks.

 

[itsmoked]

From a safety point of view I would never cut that corner... I would always assume a charged state because the results of a bad guess would be....

Others may have different opinions.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

I agree with itsmoked
There will be charges and they will probably last longer than you wish to wait.
In the bad old days, we used a hotstick with a ground wire on the end to discharge relatively short above ground cables and/or transformers.
A long underground cable may be damaged by this type of discharge.
Cables where substantial charges were anticipated were discharged with a resistor is series with the ground cable on the end of the hotstick.
The old wisdom was that an insulated cable with a substantial capacitive charge could be damaged by a discharge through a low impedance path.
There are now additional arc-flash issues.

My advise is that there are lethal hazards and the possibility of cable damage.
Do not proceed until you aquire up to date knowledge as to both personal safety and arc flash safety, and modern approved methods.
respectfully

 

[davidbeach]

MV cables and equipment exist in one of only two states - visibly grounded and energized. Don't assume anything about "unenergized", it doesn't exist.

 

[ausphil]

There are many variables as to how quickly the stored charge will decay. Our standard operating practice for 132kV paper/oil/gas cables was to isolate them from supply at both ends, wait 15 minutes, then apply the portable earth (or ground depending which country you read this in). This worked well because the insulation resistance in the paper cables did allow a good amount of discharge of the stored energy, down to a level where it was more than safe for the operator to apply the earth (ground). Plus at the time we mainly used magnetic voltage transformers for protection. This 15 minutes was calculated many years ago, and served well.

We came unstuck when we started to use XLPE cables, and also capacitive voltage transformers. Here you are automatically increasing the IR of the cable, plus you have eliminated the resistive discharge path through the primary winding of a magnetic VT and replaced it with the high IR of a capacitive VT. This showed itself when the operator isolated the cable, waited the standard 15 minutes, then proceeded to apply the portable earth, which drew an arc longer than he cared for whilst standing below it.

We did some rough calculations at the time, and found for the same stored charge, to discharge it to the same level that we believed would be safe to apply an earth to, the time rather than being 15 minutes (as for the paper cable) now became 14 days!!! We now tend to use a magnetic VT at one end of the cable now to allow the discharge to occur in a reasonable period, so we have eliminated the issue of arc-flash PPE requirements.

The calcs are pretty easy, you assume the maximum stored energy on the cable, then use the IR measurements from the cable (or at least the manufacturers IR levels, which will be higher then what you have in the ground anyhow) and the IR values of any VTs and CTs at either end. This gives a time constant from which you can tell how quickly the cable will lose the stored energy.

And as the above has said, don't assume it to be discharged until you can see that it is - earth or ground connected

 

[davidbeach]

With other circuits in the vicinity a disconnected, but not grounded, cable may never discharge. I believe that grounded parking stands are available for elbow connected systems so that you can move the cable onto the grounded parking stand and have the discharge happen within the elbow connector.

 

[IRstuff]

If it's not energized, it should be grounded or discharged through a reasonably low impedance. I forget what the mil standard is, but my recollection is that a high voltage node should be discharged to a safe voltage within about 15 seconds.

Having survived a zap from an ungrounded 25 kV chassis, I know EXACTLY what it feels like, or rather, I know what it feels like to land flat on my back afterwards.

TTFN

 

[fsmyth]

May be a dumb question, but why not use a suitably
sized resistor in line with the grounding electrode?
Could be selected to discharge the cable(s) in
whatever time period was desired. And if large
enough, could be left in permanently (like the
VT's mentioned).
<als>

 

[waross]

fsmyth
See my post of 1 Jun 06 14:34
You may be able to use the primary winding of a spare magnetic VT as a suitable discharge resistor.
respectfully

 

[benlanz]

I agree with other's comments. Safety first. Follow all standard IEEE safety practices.

If the system is left ungrounded it is likely that there will be a charge on the system for weeks if not longer. Relatively lossy dielectrics (Paper type) will discharge faster. However, the charge may never completely disappear as charge can be induced inductively and capacitively for other circuits.

However, after turning off the AC power to the cable discharging the cable is really not a big deal for line crews who are familiar with deenergizing circuits. (Provided you are only discharging the capacitive charge left on the cable) There will be an arc when the grounds are connected, but the charge displacement is relatively low. Always leave the grounds connected as relaxation charge and/or inductive/capacitive induced charge can build on the system.


Benjamin Lanz
Vice Chair of IEEE 400
Sr. Application Engineer
IMCORP- Power Cable Reliability Consultants