Phase to Ground Coordination Curves: CTI issues

[D!NNy]

Currently I am doing a coordination study for Phase-Ground Fault curves for my clients recloser and Utility Recloser.....trying to maintain the 0.2Sec of CTI Clearing Time Internval between TCC at any given point of time.

Maximum symmetrical SC data for the phase to ground fault provided by the utility is 4kA @ 12kV on a 3 wire system. Please correct me if am wrong with the following analysis.

I am not able to maintain 0.2sec CTI at the maximum short circuit current. As the utility and client reclosers are at the same location there is no difference between the SC current.
My argument is in a 3 wire system when will possibly have the maximum SC current 4kA for phase to ground fault with out having the 4th wire. Maximum available SC current listed by the simulating software is as if there is short circuit current between the source and single phase conductor along the ground (perfect ground of zero resistance ) or 4th wire.

If i am correct why the utility is providing the single phase to ground fault current 4kA, In reality this may be very low as there is no zero resistance ground return path to the source or 4 th wire.

Thanks in advance for the suggestions.

 

[hurk27]

Currently I am doing a coordination study for Phase-Ground Fault curves for my clients recloser and Utility Recloser.....trying to maintain the 0.2Sec of CTI Clearing Time Internval between TCC at any given point of time.

Maximum symmetrical SC data for the phase to ground fault provided by the utility is 4kA @ 12kV on a 3 wire system. Please correct me if am wrong with the following analysis.

I am not able to maintain 0.2sec CTI at the maximum short circuit current. As the utility and client reclosers are at the same location there is no difference between the SC current.
My argument is in a 3 wire system when will possibly have the maximum SC current 4kA for phase to ground fault with out having the 4th wire. Maximum available SC current listed by the simulating software is as if there is short circuit current between the source and single phase conductor along the ground (perfect ground of zero resistance ) or 4th wire.

If i am correct why the utility is providing the single phase to ground fault current 4kA, In reality this may be very low as there is no zero resistance ground return path to the source or 4 th wire.

Thanks in advance for the suggestions.

The voltage sounds strange for a local distribution voltage, most common local distribution voltages is the 7.2/12.4kv WYE which you would base you 4ka upon the 7.2kv to the MGN, remember the MGN is also paralleled by the Earth every 1/4 mile and may be more often, which can lead for it to having a much lower impedance then the ungrounded lines, are you sure there is not a 4th wire? in some cases I have seen it as the massinger conductor for the telco and or cable lines, if this is a 7.2/12.4kv WYE system then it would seem very dangerous to not have a MGN as any fault current would try to go through Earth and this would leave a very big difference of potential around each utility grounding electrode in the event of a fault or even if the phases were unbalanced and would not be very safe? the voltage drop at each electrode is high but as a whole the Earth is a very great conductor, its our small connection to it that is not that great.

If this is a 12kv delta then unless the utility is using zig zag transformers to derive a ground, the delta should not have any reference to Earth therefor no fault path but that would not be right either, maybe someone more familiar with your system can chime in as I'm not sure if I'm talking about the same system?

 

[kingpb]

You don't need the 4th wire, and in a distribution system you won't. It will always be a 3 wire system. Neutrals are derived at transformers for customer use.

However, you do still have grounded structures and so forth, i.e. fence, railings, poles, dead end structures, etc. A conductor going to ground on one of these, albeit unlikely to be considered bolted, but never-the-less a L-G fault.

 

[D!NNy]

The voltage sounds strange for a local distribution voltage, most common local distribution voltages is the 7.2/12.4kv WYE which you would base you 4ka upon the 7.2kv to the MGN, remember the MGN is also paralleled by the Earth every 1/4 mile and may be more often, which can lead for it to having a much lower impedance then the ungrounded lines, are you sure there is not a 4th wire? in some cases I have seen it as the massinger conductor for the telco and or cable lines, if this is a 7.2/12.4kv WYE system then it would seem very dangerous to not have a MGN as any fault current would try to go through Earth and this would leave a very big difference of potential around each utility grounding electrode in the event of a fault or even if the phases were unbalanced and would not be very safe? the voltage drop at each electrode is high but as a whole the Earth is a very great conductor, its our small connection to it that is not that great.

If this is a 12kv delta then unless the utility is using zig zag transformers to derive a ground, the delta should not have any reference to Earth therefor no fault path but that would not be right either, maybe someone more familiar with your system can chime in as I'm not sure if I'm talking about the same system?

You are correct with the voltage 7.2/12.4kv WYE.
In my case it is three wire system, there is no multiple grounded neutral. Even if we do have these it will definitely have lower impedance than the ungrounded system but not low enough to make the fault current be as high as bolted fault current.
Yes there is no 4 th wire
I agree with ur comment on earth as great conductor, but there are so many distribution systems grounded out there if there is ground fault on one conductor how can fault current return back to appropriate source??

My system is a wye grounded at the substation but no 4 th wire carried to the distribution circuits? i concern is this cant be considered as a grounded system. correct me if am wrong.

 

[D!NNy]

You don't need the 4th wire, and in a distribution system you won't. It will always be a 3 wire system. Neutrals are derived at transformers for customer use.

However, you do still have grounded structures and so forth, i.e. fence, railings, poles, dead end structures, etc. A conductor going to ground on one of these, albeit unlikely to be considered bolted, but never-the-less a L-G fault.

I understand these are considered as L-G fault, but when can you have maximum L-G bolted fault current available in your system with out having a effective ground path to the source of the transformer?? Please suggest if i am missing something here.

Thanks

 

[GoldDigger]

If you connect the - end of three batteries together and to ground, then short the + of one battery to ground, how does the current find its way back to the correct battery - ?
It is not a problem. Same with multiple independent sources all grounding one wire.

If the grounded neutral from s wye source is not brought to the service point, that does not make the "delta" an ungrounded system, it just prevents you from using line to neutral loads.
The line voltages to ground are stable and well defined, you just have a high resistance (maybe tens of ohms) in the circuit in case of a fault to ground. For high (medium) voltages this can trip a branch OCPD, but at 120 or 240 it is less likely to.

 

[hurk27]

Ok without the 4th wire (MGN) you do not have a low impedance fault path back to source, this is a very dangerous setup, the only way I can see to make it safe is to have some kind of ground fault detector that would open a fault to ground in less then 1 cycle if this is even allowed, I find it very odd to have a grounded system designed to depend upon earth electrodes as a fault path, the voltage drop across an electrode is the same at any voltage, at 120 volts a 25 ohm electrode would fault 4.8 amps and see at 90 volts 3' from the electrode, but at 7.2kv you will fault 288 amps and see 6480 volts three feet from the electrode, if your re-closer is set for 4ka it may never trip out, your going to need ground fault protection to lower the ground fault protection level to a point it will open fast enough to remove the step potential that will be on all the grounding on the load side of this service.

I have to run but will add more when I get back.

 

[mivey]

Could be a single point grounded system. I believe gar said he is on a 3 wire system.

Also, the l-g fault can be higher than the 3-phase fault when close to a delta-wye transformer since it blocks the source-side zero sequence impedance.