Unusual Ground Fault Trip

[PWR]

A customer has an unusual distribution system that may be ill-advised but has been operating for some time. The utility services the site with a 4.16kV solid grounded source. The customer takes delivery through a 2000A breaker and then uses a delta delta 7500KVA transformer to step up the voltage to a 12.47kV ungrounded open overhead line distribution system. Some recent equipment additions connected on drops off the overhead line allowed one or more ground faults to occur on the 12.47kV system. Very little physical damage was done before the 4.16kV breaker tripped with a 50N (1600A primary pickup) indication. Can anyone offer an explanation of how any fault on the 12.47kV side could appear as a ground fault on the "other" side of a delta delta connection? The 50N relay is connected in the residual of three 2000/5 CTs with an equivalent 4A secondary pickup.

 

[tinfoil]

Are you certain that the 12.47 system was truly ungrounded? How were the various loads connected to it?

The 'ungrounded' 12.47 system was possibly grounded through one or more loads with the wye-point grounded.

Then any resultant phase-to-ground fault currents would be seeking any return path available back to the utility source, including perhaps the residual of the 50N, which I presume is solidly grounded.

 

[DrSpark]

It would be good to establish an occurance of a "ground fault" on the 12.47kVside before entering into a discussion about 50N tripping. As was stated the 50N is acheived via residually connected CTs. This type of protection responds to the residual currents (also called neutral or zero-sequence current)

In = 3I_0 = Ia + Ib + Ic

This protection should consider that the 3 CTs do not have identical characteristics and will perform differently for heavy phase-to-phase faults or for initial asymmetrical starting currents.

This can produce false residual currents.

The setting should therefore be above the line maximum unbalance currents.

 

[PWR]

I can confirm that it is indeed ungrounded. All loads are connected via three phase transformers with delta windings on the 12.47kV side.

I am aware of the limitation of residually connected CTs during heavy phase faults. There is no evidence of any phase to phase (or double line to ground) shorting. In fact, when the breaker tripped, an electrician observed arcing at a point where a grounded surface was brought within one inch of a bare phase conductor (7200V to ground).

With the delta delta connection supplying the overhead system, there is magnetic coupling only between the 4.16kV and 12.47kV systems. A ground fault on the 12.47kV system cannot (at least in theory) appear as a ground fault on the 4.16kV side. Then too, with the 12.47kV ground fault current limited to what small value is allowed by the system capacitance, the magnitude should only create a small fraction of the 1600A relay primary pickup value.

 

[DrSpark]

What is the type of relay that is being used for 50N? Is it electromechanical or electronic ? Has an harmonics measurement being done for the system?

 

[timohearn]

I think that a line to line fault on the 12.47 kV overhead line could result in enough current unbalance between the three phase currents on the 4.16 kV side of the transformer to help to explain what is happening.

 

[DrSpark]

The sparking from the viccinity of a grounded surface at the Breaker would be due to the discharging of charged capacitance of the overhead transmission line.

I would not take that as a confirmation of ground fault occurance.

I do not know the exact configuration of the system but have you tried an IR test at the 2000A breaker terminal and then at the 12.47kV Trafo Terminals? That should be able to isoalte the section with the potential ground fault.
I would even recommend disconnecting the supply source (4.16kV) and instead feeding it with a temporary 110V supply to help locate and isolate the fault.

 

[rbulsara]

what is the time delay on 50N..may be its too low to ride through transients...

 

[PWR]

The relay is an ABB Micro 50/51 solid state relay.

We are not trying to find a ground fault. The ground has been removed by restoring proper phase to ground clearance at several new equipment locations.

The question remains as to why the breaker opened in the manner it appears to have done. It is fortunate that it did open as the phase to ground arcing took place very near exposed bare busses and a phase to phase fault could easily have been started via ionized gases. There is clear cut evidence that phase to ground arcing occured at some point in time(s) at a minimum of three new equipment locations due to poor installation practices. Those have been addressed and I am trying to understand why/how the faulting was isolated with very little physical damage.

If you refer to my original description, note that the system is such that the 50/51N relay should be able to "see" only from the breaker into the 4.16kV winding of the transformer and nothing else.

 

[RRaghunath]

4A setting for instantaneous earth fault protection (even on delta winding primary side of the transformer) seems rather low. Even changes in cable/OHL capacitance currents or transformer in rush currents during charging / when the downstream fault is cleared and the voltage magnitude is restored could cause the relay operation, I think.

Further, unearthed systems give rise to over voltages in healthy phases during an earth fault in one of the phases. This can cause transformer overfluxing and give rise to harmonics which could cause maloperation.

There is also possibility of ferro-resonance, though the same doesn't appear to have been the case in this partcular incident.

Please tell me whether there is residual over voltage based earth fault protection for the system (as the system is unearthed) and whether the same has operated any time!

It is also known that un-cleared single phase to ground fault in unearthed systems can develop in to two phase to ground fault. You may look at this angle as well.

 

[DrSpark]

As rraghunath pointed out a l-g fault will lead to an increased (by 1.732 times) current to pass through the balance two phases.

Also if I read it right the 12.47kV loads are connected via delta connected transformers.. Eeach of these transformers tend to have inrush currents during switching on or during large load startings.

also De-rating a transformer doesn’t assure that it will perform correctly when subjected to non-linear loads. The magnetic and resistive properties of a transformer are enhanced when it is designed specifically for non-linear loads.
Therefore I would recommend providing a V/f overfluxing protection especially reading the types of loads that are being fed.
Also a comprehensive diagnostic system study and power measurement at various locations could help point in advance at any future problems.

 

[timohearn]

On medium voltage distribution systems that are ungrounded it is common for there to be ground detection lamps installed on the switchgear (switchboard). Does this systen have such monitoring? What does it indicate?

If this monitoring doesn't exist you may want to consider it.

 

[stevenal]

Any arresters on the transformer?

 

[gordonl]

I would think tour "ground" arcing evidence was caused by a L-L fault through earth as rraghunath suggested. The 50/51N could them have tripped on false relaying with the relatively heavy fault current.

What also sometimes occurs is that the phasing of the CT's is wrong, a CT is shorted, or failed, but not enough load current is flowing to trip the 50/51N relay. On the downstream fault the 50/51N trips on Gnd when sufficient non-zero sequence current flows.

 

[PWR]

The very limited damage argues against a double line to ground and thus phase to phase fault with inaccurate reproduction in the CTs.

The system has no protection or monitoring on the 12.47kV side. I had warned this customer to expect heavy damage when a fault occurred because of the design. Lo and behold, he has a fault, there is very little damage, and I can't explain why.

I suspect some variant of rraghunath's "overfluxing" due to high voltage excursions during the arcing may be the best explanation. The resulting non-linear currents might have then not added to zero in the residual CT connection.

 

[gordonl]

Depending on the size and bonding of the system your L-G-L fault could have a signifigant impedance in the ground path to limit the fault current. Could the 4160V system have been affected by the path of fault current? If the 4160V is within the system (between the two faults) you could get some strange occurances. This installation sounds like it borders on the criminal with no ground fault detection! A phase to phase cross country fault can start fires, electrify fences, and other wonderful things.