Direction of Ground fault current in 3 Phase 3 wire solidly grounded system 2

[NickParker]

Hi All,

When a single phase to ground fault occurs with a Bus tie open, where does the fault current go?

 

[jghrist]

As always, it depends. Where are the two sources located? If they are both local transformers, the current will all flow to the Main A transformer neutral through the ground grid. If the sources are from the same remote substation, some of the return current will flow through the earth to the remote substation, some will flow through each of the shield wire/neutral to the remote substation. If the sources are different remote substations, some current will flow through the earth to remote substation A, some will flow through the Main A shield/neutral to remote substation A, some will flow through the Main B shield/neutral to remote substation B, then through the Substation B ground grid and earth to remote substation A, some will flow through tower grounds of Main B and then to the remote substation A ground grid.

 

[RedSnake]

You can say that the current will take the easies way back to earth, meaning the way with least resistant.
If there is more then one way it will divide whit more current where the resistant is the lowest.
So, the shorter and thicker the cable is and with the right connection area, without joints the lower the resistance will be, the better.

We always have separate earthing cables for each enclosure and machine frames to main earth, no connections in series, but this is low voltage industrial.
And we always makes resistant measurements too.

“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

 

[jghrist]

You can say that the current will take the easies way back to earth, meaning the way with least resistant.

The current doesn't just go to earth. It has to go to the fault source, either the neutral of a grounded-wye transformer or the neutral of a grounded generator.

 

[RedSnake]

If the fault source "short circuit" is between a phase and the enclosure and the enclosure is earthed by a ground spit (to mother earth) and not from/to the generator or the transformer then there is no neutral or earth going back, so not necessarily.

Not sure I ever heard of a 3 phase, 3 wire solidly grounded system, maybe that is the problem.
Is that the same as 3 phase +PEN?
But then there needs to be 4 wires.

“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

 

[jghrist]

If there is no metallic connection between the enclosure and the source, then ground fault protection may be compromised because the fault current will be low. You could use sensitive earth fault protection, but I would assume that the enclosure ground is tied to a ground grid that is also connected to the transformer or generator neutral either directly or indirectly through a transmission line shield wire of cable shield. I'm guessing that the 3-wire solidly grounded means that there is no neutral taken to the switchgear, but that there is a ground conductor.

 

[che12345]

Dear Mr. NickParker (Electrical)(OP)22 Nov 22 15:52
[COLOR=]"...When a single phase to ground fault occurs with a Bus tie open, where does the fault current go?..."[/color]
1. It is possible to have 3-phse 3-wire with Neutral solidly earthed (but NOT distributed) generator/transformer with Y winding. The gen/trafo A and B may be close by or distance apart; with same or different earthing pits.
1.1 Attention: (a) the Neutral point of A and B shall be separated.
(b) a single EF CT shall be located as close as possible to the Neutral point
(c) the whole length of the earthing conductor shall be insulated.
2. In case of an EF occurs on Section A, irrespective whether the earthing wires are connected in two earthing pits or solidly connected together, only the EF CT located at the gen/trafo A will detect it. The EF CT in gen/trafo B will NOT detect it.
2.1 Attention: The earth-fault current may flow all over through conductive structure/ earth etc. etc.; but ultimately flow back to the gen/trafo A only. The EF current will NOT flow through EF CT B to the gen/trafo Neutral B.
Che Kuan Yau (Singapore)

 

[unclebob]

Fault current will use all provided ground paths to return to its source, the A Trafo, assuming it's secondary is wye and the neutral is grounded.

 

[bacon4life]

Not sure I ever heard of a 3 phase, 3 wire solidly grounded system, maybe that is the problem.

How about 3 wire "effectively" grounded?
Many transmission systems are 3 wire effectively grounded. At transmission voltage levels a metallic neutral/ground make little difference the overall impedance of the line.

I also have an area where with very low earth resistive and the 13.8 kV side of the transformer is solidly grounded inside the station, but only 3 wires leave the station. For most of the LG fault recordings I have looked at on these feeders, the absence of a neutral wire makes a relatively small difference in fault current.

[Anecdote]
When I first started out doing distribution modeling, I realized I had not been accounting for the fact that the urban power grid really is a grid of neutral conductors. Even when there are Normally Open switches between various circuits and taps, the neutral is bonded throughout the grid. "Knowing" that current flows inversely proportional to resistance, it seemed like the software should be modeling the interaction between two neutral conductors running down adjacent streets.

Turns out the portion of current that flows on each parallel path is based on both the resistance and inductance. The inductance between two conductors is proportional to the log of distance between the conductors, so ground paths closer to phase conductors will have relatively lower inductance. The inductance of a neutral conductor one parallel street away from a phase conductor is so high that almost no current flows along that neutral.