11kV termination earthing through core balance CT
beniroli, Thu Apr 06 2017, 08:29AMHi All, first post and fairly new to the HV department in my company.....on a recent 11kV installation, an experienced HV jointer terminated the 3 single AWA cables. I am concerned that he insisted on running separate earth conductors up through the core balance CT and back down through it, even though the armour is made off on an insulated plate, and the copper screen is also made off below the CT.
I will try to attach a simple diagram pdf of how each core is connected, missing out the final heat shrink layer for clarity.
my concern is that a) it is conducive to partial discharge, earth conductors being irregularities around unscreened cable and no copper screen on cable going through CT, and b) the earth conductors didn't need to go through the CT at all. my understanding is that returning the earth back through the CT is only necessary if the cable screen goes through, i.e. to generate the earth fault current imbalance into the CT.
any thoughts, opinions, advice welcome.
Thank you.
]hvearth.pdf[/file]
Re: 11kV termination earthing through core balance CT
Dr. Slack, Thu Apr 06 2017, 11:52AM
Those loops of earth conductor only happen to be in the CT physically. Electrically, as they go up and then down again, they are not in any way electromagnetically coupled to the CT.
The insulation on those conductors is deemed to be sufficient to insulate them. The sharp edges on the CT core would probably look like sharper irregularities then the nice rounded earth conductors, so if the conductors can cope with the core, I'm sure they can cope with the earth conductors.
I think those loops of earth conductor are pushed through the CT for somewhere convenient to put them, as the guy has left them so long they'd get untidy if draped around somewhere else. Which suggests that the installation hygiene is not fully thought through.
Dr. Slack, Thu Apr 06 2017, 11:52AM
Those loops of earth conductor only happen to be in the CT physically. Electrically, as they go up and then down again, they are not in any way electromagnetically coupled to the CT.
The insulation on those conductors is deemed to be sufficient to insulate them. The sharp edges on the CT core would probably look like sharper irregularities then the nice rounded earth conductors, so if the conductors can cope with the core, I'm sure they can cope with the earth conductors.
I think those loops of earth conductor are pushed through the CT for somewhere convenient to put them, as the guy has left them so long they'd get untidy if draped around somewhere else. Which suggests that the installation hygiene is not fully thought through.
Re: 11kV termination earthing through core balance CT
beniroli, Thu Apr 06 2017, 12:20PM
Posted: Thu Apr 06 2017, 12:52PM
Firstly thanks for the response, i'll try to answer each bit separately.
"Those loops of earth conductor only happen to be in the CT physically. Electrically, as they go up and then down again, they are not in any way electromagnetically coupled to the CT"
- why do you say they aren't elecomagnetically coupled? because they cancel themselves out?
"The insulation on those conductors is deemed to be sufficient to insulate them. The sharp edges on the CT core would probably look like sharper irregularities then the nice rounded earth conductors, so if the conductors can cope with the core, I'm sure they can cope with the earth conductors."
- I agree about the sharp edges of the CT core, but the screen which is on the cable is deliberate and uniform for this reason I believe. so crossing the unscreened 11kV conductor with earth potential (either CT core OR earth cable) seems unwise.
"I think those loops of earth conductor are pushed through the CT for somewhere convenient to put them, as the guy has left them so long they'd get untidy if draped around somewhere else. Which suggests that the installation hygiene is not fully thought through"
- I know for a fact that these earth conductors were installed in this way on purpose with the thinking that the earth HAD to run up and back through the CBCT for it to work. THIS is what I think is a misunderstanding of how the earth fault detection works.
beniroli, Thu Apr 06 2017, 12:20PM
Posted: Thu Apr 06 2017, 12:52PM
Firstly thanks for the response, i'll try to answer each bit separately.
"Those loops of earth conductor only happen to be in the CT physically. Electrically, as they go up and then down again, they are not in any way electromagnetically coupled to the CT"
- why do you say they aren't elecomagnetically coupled? because they cancel themselves out?
"The insulation on those conductors is deemed to be sufficient to insulate them. The sharp edges on the CT core would probably look like sharper irregularities then the nice rounded earth conductors, so if the conductors can cope with the core, I'm sure they can cope with the earth conductors."
- I agree about the sharp edges of the CT core, but the screen which is on the cable is deliberate and uniform for this reason I believe. so crossing the unscreened 11kV conductor with earth potential (either CT core OR earth cable) seems unwise.
"I think those loops of earth conductor are pushed through the CT for somewhere convenient to put them, as the guy has left them so long they'd get untidy if draped around somewhere else. Which suggests that the installation hygiene is not fully thought through"
- I know for a fact that these earth conductors were installed in this way on purpose with the thinking that the earth HAD to run up and back through the CBCT for it to work. THIS is what I think is a misunderstanding of how the earth fault detection works.
Re: 11kV termination earthing through core balance CT
Dr. Slack, Thu Apr 06 2017, 03:41PM
The red/blue/yellow conductors are looped once each through the CT core, they couple one turn each.
The green/yellow striped earth conductors *appear* (correct me if I'm wrong) to go up through the hole (+1 turn) then back down through the same hole again (-1 turn) for a grand total of zero turns each through the CT. Therefore there is no magnetic, that is transformer, coupling. It looks like if you pulled down on a loop of earth conductor, the loop could be pulled out of the CT. That is, the conductor does not loop the core. As far as the CT is concerned, those earth conductors do not exist.
Dr. Slack, Thu Apr 06 2017, 03:41PM
The red/blue/yellow conductors are looped once each through the CT core, they couple one turn each.
The green/yellow striped earth conductors *appear* (correct me if I'm wrong) to go up through the hole (+1 turn) then back down through the same hole again (-1 turn) for a grand total of zero turns each through the CT. Therefore there is no magnetic, that is transformer, coupling. It looks like if you pulled down on a loop of earth conductor, the loop could be pulled out of the CT. That is, the conductor does not loop the core. As far as the CT is concerned, those earth conductors do not exist.
Re: 11kV termination earthing through core balance CT
beniroli, Thu Apr 06 2017, 03:49PM
Exactly right. so the earth conductors (green/yellow) can, and should have, been routed straight to the earth bar, NOT through the CT.
as I understand, the only reason to return the earth conductor back through the CT is if the screen is still on the cable as it runs through the CT. i.e. it has to return through to "generate" earth fault current in the event of one.
beniroli, Thu Apr 06 2017, 03:49PM
Exactly right. so the earth conductors (green/yellow) can, and should have, been routed straight to the earth bar, NOT through the CT.
as I understand, the only reason to return the earth conductor back through the CT is if the screen is still on the cable as it runs through the CT. i.e. it has to return through to "generate" earth fault current in the event of one.
Re: 11kV termination earthing through core balance CT
Dr. Slack, Thu Apr 06 2017, 06:36PM
They were not 'routed' through the CT, the loops were pushed into the space pf the CT.
The CT detects imbalance between the 3 phases, measures the neutral current if you will. Fault current phase to ground may, or may not, generate a neutral current, depending whether the neutral is floating or connected to ground. Current in those earth tails will be irrelevant to any fault currents that flow from the pashes to ground.
Dr. Slack, Thu Apr 06 2017, 06:36PM
They were not 'routed' through the CT, the loops were pushed into the space pf the CT.
The CT detects imbalance between the 3 phases, measures the neutral current if you will. Fault current phase to ground may, or may not, generate a neutral current, depending whether the neutral is floating or connected to ground. Current in those earth tails will be irrelevant to any fault currents that flow from the pashes to ground.
Re: 11kV termination earthing through core balance CT
Uspring, Fri Apr 07 2017, 11:00AM
Dr. Slack wrote:
Now think of a loop not enclosing the toroid. Wouldn't that also see the electric field, i.e. be coupled? The field path integrals might cancel, but why?
Put in other words: An AC driven toroidal winding would create an electric field outside the winding but no magnetic one. That could be caught by a loop outside the winding. Maybe you are right, but you got me puzzling.
Uspring, Fri Apr 07 2017, 11:00AM
Dr. Slack wrote:
Those loops of earth conductor only happen to be in the CT physically. Electrically, as they go up and then down again, they are not in any way electromagnetically coupled to the CT.I'm wondering about this. First think of loop of wire which encloses the toroid. Obviously this loop is coupled to the CT, since toroidal transformers are made this way. Say you put an AC current through the CT than this implies, that there would be (rotational) electric fields where the loop wire is.
Now think of a loop not enclosing the toroid. Wouldn't that also see the electric field, i.e. be coupled? The field path integrals might cancel, but why?
Put in other words: An AC driven toroidal winding would create an electric field outside the winding but no magnetic one. That could be caught by a loop outside the winding. Maybe you are right, but you got me puzzling.
Re: 11kV termination earthing through core balance CT
Dr. Slack, Fri Apr 07 2017, 04:28PM
There's electrostatic coupling, but that's not what we call transformer coupling. Perhaps I should have specified 'magnetically' rather than 'electromagnetically' at first.
I'm not sure what you mean by a loop 'enclosing' the toroid. Transformer coupling only occur when a conductor 'links' the core, topologically. That's why I said get hold of the earth loop, and try to pull it down. It turns out that being magnetically linked with the core is the same as passing through the core once in such a fashion that it's not possible to pull the loop of conductor out of the core.
A convenient way to see whether a conductor is linked magnetically to the core is to see whether the conductor encloses (that word again) the flux that's in the core.
Dr. Slack, Fri Apr 07 2017, 04:28PM
There's electrostatic coupling, but that's not what we call transformer coupling. Perhaps I should have specified 'magnetically' rather than 'electromagnetically' at first.
I'm not sure what you mean by a loop 'enclosing' the toroid. Transformer coupling only occur when a conductor 'links' the core, topologically. That's why I said get hold of the earth loop, and try to pull it down. It turns out that being magnetically linked with the core is the same as passing through the core once in such a fashion that it's not possible to pull the loop of conductor out of the core.
A convenient way to see whether a conductor is linked magnetically to the core is to see whether the conductor encloses (that word again) the flux that's in the core.
Re: 11kV termination earthing through core balance CT
klugesmith, Fri Apr 07 2017, 06:20PM
>> A convenient way to see whether a conductor is linked magnetically to the core is to see
whether the conductor encloses the flux that's in the core.
Sometimes a conductor is locally rather straight, and/or the current return path is not obvious.
Then it may be conceptually simpler to see
whether the core (closed flux path) encloses the current that's in the conductor.
from
klugesmith, Fri Apr 07 2017, 06:20PM
>> A convenient way to see whether a conductor is linked magnetically to the core is to see
whether the conductor encloses the flux that's in the core.
Sometimes a conductor is locally rather straight, and/or the current return path is not obvious.
Then it may be conceptually simpler to see
whether the core (closed flux path) encloses the current that's in the conductor.
from
Re: 11kV termination earthing through core balance CT
radiotech, Sat Apr 08 2017, 07:35AM
From what I see the CT is for a ground fault device. The routing of the ground leads is to prevent a sharp bend
at the bottom.
Not seen, but of interest, is how the single cables enter the (presumed) steel enclosure .
Are the individual cables secured with a non ferrous connector? Cables of ampacity in the hundreds
will heat iron ring surrounds to an extent to , melt or compromise the cable insulation.
I am not an inspector, but have installed cables to boxes, low and high voltage. An inspector
might grumble the box is too small to cramp the CT like that
radiotech, Sat Apr 08 2017, 07:35AM
From what I see the CT is for a ground fault device. The routing of the ground leads is to prevent a sharp bend
at the bottom.
Not seen, but of interest, is how the single cables enter the (presumed) steel enclosure .
Are the individual cables secured with a non ferrous connector? Cables of ampacity in the hundreds
will heat iron ring surrounds to an extent to , melt or compromise the cable insulation.
I am not an inspector, but have installed cables to boxes, low and high voltage. An inspector
might grumble the box is too small to cramp the CT like that
Re: 11kV termination earthing through core balance CT
Uspring, Sat Apr 08 2017, 08:36AM
Dr. Slack wrote:
Uspring, Sat Apr 08 2017, 08:36AM
Dr. Slack wrote:
I'm not sure what you mean by a loop 'enclosing' the toroid.I'll use klugesmiths drawing to explain. Imagine the red torus to be a CT, i.e. a toroidial winding and the blue torus a wire loop. The wire loop then "encloses" the CT. Not enclosed would be the case if the blue torus is removable from the red one.
A convenient way to see whether a conductor is linked magnetically to the core is to see whether the conductor encloses (that word again) the flux that's in the core.There might be flux outside the core. That depends on the u of the core and geometry. If there is, then it can go through a loop, induce voltage there and therefore the loop will be coupled. For a high u core that might be negligible.
Re: 11kV termination earthing through core balance CT
Dr. Slack, Sat Apr 08 2017, 06:47PM
If Klugsmith's red ring is the flux-carrying core of the CT, and the blue ring a wire loop, then I'd say the blue ring *links* the red ring, or the blue ring *encloses* the flux in the red ring. The blue ring does not enclose the toroid, or at least, I find that description ambiguous.
Flux outside the red ring? Let's KISS with the ideal case of infinite permeability and say there's no flux outside the red ring. In the real case, the flux outside the red ring will be negligible. It certainly won't turn an earth wire loop that doesn't link the CT flux into one that does.
AFAICS from the picture, the earth tail loops do not link the flux in the CT core, because they go up and back again. The three phase conductors, red, blue and yellow, do link the CT flux, as they go through once.
Dr. Slack, Sat Apr 08 2017, 06:47PM
If Klugsmith's red ring is the flux-carrying core of the CT, and the blue ring a wire loop, then I'd say the blue ring *links* the red ring, or the blue ring *encloses* the flux in the red ring. The blue ring does not enclose the toroid, or at least, I find that description ambiguous.
Flux outside the red ring? Let's KISS with the ideal case of infinite permeability and say there's no flux outside the red ring. In the real case, the flux outside the red ring will be negligible. It certainly won't turn an earth wire loop that doesn't link the CT flux into one that does.
AFAICS from the picture, the earth tail loops do not link the flux in the CT core, because they go up and back again. The three phase conductors, red, blue and yellow, do link the CT flux, as they go through once.
Re: 11kV termination earthing through core balance CT
beniroli, Mon Apr 10 2017, 09:28AM
From what I see the CT is for a ground fault device. The routing of the ground leads is to prevent a sharp bend
at the bottom. - I'm afraid not. the routing is purely because the installer thought they conductors had to run through the CT
Not seen, but of interest, is how the single cables enter the (presumed) steel enclosure . - see pdf from 1st post. and attached pic. they come from the gland. also tied to the copper screen.
Are the individual cables secured with a non ferrous connector? Cables of ampacity in the hundreds
will heat iron ring surrounds to an extent to , melt or compromise the cable insulation. - copper lug I assume.
I am not an inspector, but have installed cables to boxes, low and high voltage. An inspector
might grumble the box is too small to cramp the CT like that as shown in pdf, the pic is only part of the compartment, below floor is where cable made off.
beniroli, Mon Apr 10 2017, 09:28AM
From what I see the CT is for a ground fault device. The routing of the ground leads is to prevent a sharp bend
at the bottom. - I'm afraid not. the routing is purely because the installer thought they conductors had to run through the CT
Not seen, but of interest, is how the single cables enter the (presumed) steel enclosure . - see pdf from 1st post. and attached pic. they come from the gland. also tied to the copper screen.
Are the individual cables secured with a non ferrous connector? Cables of ampacity in the hundreds
will heat iron ring surrounds to an extent to , melt or compromise the cable insulation. - copper lug I assume.
I am not an inspector, but have installed cables to boxes, low and high voltage. An inspector
might grumble the box is too small to cramp the CT like that as shown in pdf, the pic is only part of the compartment, below floor is where cable made off.
Re: 11kV termination earthing through core balance CT
beniroli, Mon Apr 10 2017, 09:33AM
beniroli, Mon Apr 10 2017, 09:33AM
Re: 11kV termination earthing through core balance CT
beniroli, Mon Apr 10 2017, 09:35AM
beniroli, Mon Apr 10 2017, 09:35AM
Re: 11kV termination earthing through core balance CT
Uspring, Mon Apr 10 2017, 10:41AM
Dr. Slack wrote:
Uspring, Mon Apr 10 2017, 10:41AM
Dr. Slack wrote:
In the real case, the flux outside the red ring will be negligible.I agree with mostly with this. Depends on the core u. For larger u the outside flux decreases.
It certainly won't turn an earth wire loop that doesn't link the CT flux into one that does.The magnitude of coupling depends on whether there is a linkage or not. Granted, that in the not linking case the coupling is likely to be tiny.
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