TRANSMISSION lines

[domnic]

Transmission lines on a tower say 200 kv have grounded lines above them do the grounded lines end up touching the ground rod at my home ?

 

[Sierrasparky]

HUH?

 

[kwired]

Transmission lines on a tower say 200 kv have grounded lines above them do the grounded lines end up touching the ground rod at my home ?

Though indirectly, likely yes.

If they are on separate structures from local distribution lines then the bond between them is going to be at a substation somewhere.

If they would happen to be on same structure as a local distribution line then both grounded conductors of both systems will likely be bonded together on that same structure.

The chance of them being on same structure probably isn't all that great though. The chance of lower level transmission lines like 34.5 or 69kV being on same structure as a local distribution is greater though and is done quite frequently.

 

[Ingenieur]

Those are shield or static wires
lightening protection and they are grounded (obviously)

but usually not a I carrying conductor, ie neutral

eg
delta transmission w/shields to a local distribution substation
delta-wye transformer with a derived neutral
statics tied to station bed
so is the neutral
sometimes the neut will have a small Z to drop any gnd bed potential rise due to a fault or lightening strike
but often no grounding Z since the bed is so effective that it holds 0 potential even under a fault/lightening strike

so yes tied together indirectly thru earth
but no, little risk of it flowing I into loads
Llightening will flow seeking equi-potential, so it does not 'seek' your system
a phase fault will tend to return to the power source which is not your home

bigger risk of a phase lightening strike bringing problems in to the load

 

[Bugman1400]

Those are shield or static wires
lightening protection and they are grounded (obviously)

but usually not a I carrying conductor, ie neutral

eg
delta transmission w/shields to a local distribution substation
delta-wye transformer with a derived neutral
statics tied to station bed
so is the neutral
sometimes the neut will have a small Z to drop any gnd bed potential rise due to a fault or lightening strike
but often no grounding Z since the bed is so effective that it holds 0 potential even under a fault/lightening strike

so yes tied together indirectly thru earth
but no, little risk of it flowing I into loads
Llightening will flow seeking equi-potential, so it does not 'seek' your system
a phase fault will tend to return to the power source which is not your home

bigger risk of a phase lightening strike bringing problems in to the load

Are you saying that the static wires don't carry ground current?

 

[Ingenieur]

Are you saying that the static wires don't carry ground current?

what is 'ground current'?

ground fault?
capacitive coupling?
delta or wye transmission?

 

[Bugman1400]

what is 'ground current'?

ground fault?
capacitive coupling?
delta or wye transmission?

Yes, Ground fault.

 

[Ingenieur]

Yes, Ground fault.

delta, single line to ground, no
wye, yes, in parallel with earth, although it may not be terminated at the X0 if a separate neutral is carried so would carry it for a ways then shunt to ground to the X0

 

[Bugman1400]

delta, single line to ground, no
wye, yes, in parallel with earth, although it may not be terminated at the X0 if a separate neutral is carried so would carry it for a ways then shunt to ground to the X0

I don't often see pure delta config (un-grounded) transmission systems. The ground fault current typically has to come from somewhere. If not, then all insulation has to be L-L rated as well as the surge arrestors. I also don't typically see the neutral carried directly to anywhere unless its a dedicated industrial customer. Typically, the X0 or H0/X0 (autobanks) terminals are grounded to the mat along with the static wires. Is that not the case where you're at?

So, for ground connected loads, most imbalance would cause ground current to flow over the static wires. The imbalance could be from imbalanced loads or a fault.

 

[Ingenieur]

I don't often see pure delta config (un-grounded) transmission systems. The ground fault current typically has to come from somewhere. If not, then all insulation has to be L-L rated as well as the surge arrestors. I also don't typically see the neutral carried directly to anywhere unless its a dedicated industrial customer. Typically, the X0 or H0/X0 (autobanks) terminals are grounded to the mat along with the static wires. Is that not the case where you're at?

So, for ground connected loads, most imbalance would cause ground current to flow over the static wires. The imbalance could be from imbalanced loads or a fault.

PA

most lines that serve mine substations are delta with static lines, no other loads are served off of these (usually)
25-69kv range
if they are wye (connected delta to the mine xfmr primary with a wye high resistance grounded secondary, 15-25 A) they have the static lines up high and will carry a 4th conductor below the phases, only the static line is landed on our mat

 

[Bugman1400]

PA

most lines that serve mine substations are delta with static lines, no other loads are served off of these (usually)
25-69kv range
if they are wye (connected delta to the mine xfmr primary with a wye high resistance grounded secondary, 15-25 A) they have the static lines up high and will carry a 4th conductor below the phases, only the static line is landed on our mat

Gotcha...that makes sense now. I'm used to dealing with HV & EHV autobanks and highside g-wye distribution banks. Don't ask me why the utility g-wyes the highside of a distribution bank, it is very hard to coordinate with a ground source in the middle of a T-line.

 

[Ingenieur]

Gotcha...that makes sense now. I'm used to dealing with HV & EHV autobanks and highside g-wye distribution banks. Don't ask me why the utility g-wyes the highside of a distribution bank, it is very hard to coordinate with a ground source in the middle of a T-line.

our primary concern is isolation
not taking a utility fault/strike into the mine for 2 reasons
1 shock-raising frame/touch potential
2 ignition source for methane

we use 2 beds and both must be <2 ohm
the sub is set on one, station or primary bed
the second is a min of 50' away inline with the mine, neut or safety bed
electrically isolated but there is unavoidable coupling thru earth
the ngr high side is connected to the X0 and the low side is connected to the neut bed and used as the mine ground
no single phase loads allowed w/o a xfmr

so most 138/345/etc kv transmission lines are wye-wye from step up to dist xfmr?
and the shield/static carries the imbalance?
interesting
I would have thougt delta-delta with the gen end step-up being a Z gnd primary and the distribution end secondary being wye-n/g so single phase loads could be derived

learn something new every day
thanks

 

[Bugman1400]

our primary concern is isolation
not taking a utility fault/strike into the mine for 2 reasons
1 shock-raising frame/touch potential
2 ignition source for methane

we use 2 beds and both must be <2 ohm
the sub is set on one, station or primary bed
the second is a min of 50' away inline with the mine, neut or safety bed
electrically isolated but there is unavoidable coupling thru earth
the ngr high side is connected to the X0 and the low side is connected to the neut bed and used as the mine ground
no single phase loads allowed w/o a xfmr

so most 138/345/etc kv transmission lines are wye-wye from step up to dist xfmr?
and the shield/static carries the imbalance?
interesting
I would have thougt delta-delta with the gen end step-up being a Z gnd primary and the distribution end secondary being wye-n/g so single phase loads could be derived

learn something new every day
thanks

Most 345/115 or 345/230 or 500/230 subs have auto banks that are g-wye on both high side and low side. Distribution subs are often power xfmrs that are g-wye on high side and low side. Generator buses are typically z gnd and connected delta to the step-up and then g-wye on the high side (transmission side).

 

[Ingenieur]

Most 345/115 or 345/230 or 500/230 subs have auto banks that are g-wye on both high side and low side. Distribution subs are often power xfmrs that are g-wye on high side and low side. Generator buses are typically z gnd and connected delta to the step-up and then g-wye on the high side (transmission side).

Thanks
in that case the shield is a current carrying conductor under nml conditions
carries imbalance

 

[paulengr]

Transmission lines on a tower say 200 kv have grounded lines above them do the grounded lines end up touching the ground rod at my home ?

That depends on what you mean by "grounded lines". Transmission lines are designed completely differently from distribution lines. A transmission line is typically 69 kV or higher in voltage. They often have static lines which are ungrounded conductors carried above the current carrying lines. The static lines are connected to ground via lightning arresters which are typically blocks of zinc oxide that have a very high resistance at normal operating voltages but go to very low resistance when the voltage gets above operating voltages...this allows them to shunt lightning strikes to ground and that's the purpose of static lines.

This is different from Wye-distributed power lines used in distribution systems. Utilities frequently use wye connected transformers for power distribution because they use three individual single phase transformers (even though wye-wye contains circulating current issues). In this case there will be a fourth neutral/ground that is grounded at usually every pole. This multiground arrangement means that faults on the system will be distributed amongst all the grounds and you can get what is known as "stray voltage" which is primarily a problem for cattle that are more sensitive to current than humans.

Finally as far as "connected"...be careful of what you mean. Essentially the phrase "electricity always follows the lowest resistance" is flat out wrong. Electricity follows ALL paths proportional to the inverse of their resistances. So in the case of two ground rods as the distance increases the resistance increases as we expect but there are vastly more paths to follow and thus it turns out that the resistance is proportional to the inverse of the distance. Doubling the distance halves the resistance. So effectively all ground rods are electrically connected together and the further they are apart, the lower the resistance becomes. Normally this isn't really an issue because there isn't a second conductive path to a remote ground rod so no circuit and nothing flows.

In fact the original Edison electrical distribution system was pretty infamous because it managed to shock more than one or two horses and pedestrians because it used only a single line. One of Edison's arguments against AC power was the requirement to use twice as much copper (two lines) where Edison just used a single line and used ground as the return path. Needless to say today we've added a third (ground) wire for safety and single wire distribution is a thing of the extremely distant past, except in radio communications where effectively we couple the wire to vacuum and use space as the conductive medium.

 

[mbrooke]

I don't often see pure delta config (un-grounded) transmission systems. The ground fault current typically has to come from somewhere. If not, then all insulation has to be L-L rated as well as the surge arrestors. I also don't typically see the neutral carried directly to anywhere unless its a dedicated industrial customer. Typically, the X0 or H0/X0 (autobanks) terminals are grounded to the mat along with the static wires. Is that not the case where you're at?

So, for ground connected loads, most imbalance would cause ground current to flow over the static wires. The imbalance could be from imbalanced loads or a fault.

But technically even with un-blanced loads the ground current is small as the 115kv to 13.8kv transformers are delta wye. Any neutral current on the secondary is transferred to the phases as zero sequence currents do not pass.

 

[mbrooke]

Most 345/115 or 345/230 or 500/230 subs have auto banks that are g-wye on both high side and low side. Distribution subs are often power xfmrs that are g-wye on high side and low side. Generator buses are typically z gnd and connected delta to the step-up and then g-wye on the high side (transmission side).

Not in my area though. What would be the benefit of wye-wye at 115kv to 13.8kv? :blink: Around here generation step up is delta wye-gr (22kv to 345kv); wye-gr auto with a delta territory (345kv to 115kv) distribution delta wye-gr (115kv to either 13.8 or 23kv) and utilization wye-gr wye-gr (13.8 to either 120/208 or 277/480).

 

[kwired]

Thanks
in that case the shield is a current carrying conductor under nml conditions
carries imbalance

But you still need to have a line to neutral load connected before there is any current on the neutral. Transmission voltages typically have no load connections to the neutral, but I suppose there could be neutral if wye connected primary side of any transformers supplied has a connection to neutral, but a floating primary neutral or delta primary transformers will not have any connection to neutral.

 

[Bugman1400]

Not in my area though. What would be the benefit of wye-wye at 115kv to 13.8kv? :blink: Around here generation step up is delta wye-gr (22kv to 345kv); wye-gr auto with a delta territory (345kv to 115kv) distribution delta wye-gr (115kv to either 13.8 or 23kv) and utilization wye-gr wye-gr (13.8 to either 120/208 or 277/480).

The only reason I see for having a g-wye/g-wye 115/13.8kV distribution bank is phasing or perhaps a super high ground impedance from the soil (ie rock or pure sand).

 

[Bugman1400]

But technically even with un-blanced loads the ground current is small as the 115kv to 13.8kv transformers are delta wye. Any neutral current on the secondary is transferred to the phases as zero sequence currents do not pass.

Not sure what you mean. I've seen large ground current on the neutral of a distribution bank for unbalanced loads on the 13.8kV side. I wouldn't expect to see any ground current on the 115kV side. The xfer to phase imbalance on the highside would be in the form of negative sequence current.....yes?