Trapped charge on HV cable
Trapped charge on HV cable
(OP)
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.






RE: Trapped charge on HV cable
Others may have different opinions.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Trapped charge on HV cable
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
RE: Trapped charge on HV cable
RE: Trapped charge on HV cable
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
RE: Trapped charge on HV cable
RE: Trapped charge on HV cable
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
RE: Trapped charge on HV cable
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>
RE: Trapped charge on HV cable
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
RE: Trapped charge on HV cable
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