AUXILIARY SUPPLY FAILURE DURING FEEDER FAULT

[appunni]

Sir,
We have a substation where 110kv is stepped down to 11kv via a 10MVA ynyn transformer . Three 11kv feeders are used for ditribution. The 11kv and 110kv side nuetrals are solidly grounded . The substation auxiliary supply is maintained through an 11/0.44kv Dyn transformer of which the neutral in the 0.44kv side has been solidly grounded.
We experienced that the 0.44kv side fuses are blowing off for a particular feeder ground fault. We found that this is not happening for faults of other feeders. We checked the ground resistance of transformer neutrals and found ok. The 0.44kv lines and 11kv cables are seperately routed and we think no chances of induction.
Finally , we noticed only one thing. The particular 11kv feeder was feeding to one direction and other feeders are feeding lines to the other direction. The auxiliary transformer is situated in the same direction of the particular feeder.
Let us analyse in this way. For an earthfault in the particular feeder the fault current will flow through ground from the fault point to the transformer 11kv side neutral. As the auxiliary transformer is situated between the fault point and 11kv side neutral of the 110/11kv transformer, the potential of 0.44kv side neutral increased which led to blowing of fuses. Please note that the 11kv side of the auxiliary transformer is delta connected.
Is my analysis correct? If not, what may be the possibilities? What checks I have to conduct further? I expect your valuable comments.
With regards,
appunni

 

[jghrist]

The 0.44 kV neutral voltage would not increase with respect to the 11 kV neutral unless there were a high resistance to the 11 kV return fault current within the substation ground grid. The voltage difference between the two ground points would be the product of the current flowing through the grid between the points and the resistance of this section of the grid. This is assuming that both the 11 kV neutral (at the 110-11 kV transformer) and the 0.44 kV neutral are bonded to the same ground grid. Is this the same substation that you discussed in thread238-65358 where the neutral grounds are separate? If so, then there may be a potential difference.

There may be a high voltage between the ground grid and anything grounded near the fault, but not at the substation. This could lead to a fault on the 0.44 kV system. Is the feeder neutral solidly grounded at the substation? Is there a telephone line that is run with the feeder?

Even if the 0.44 kV neutral voltage were elevated with respect to the 11 kV neutral, this would not in itself cause currents to flow and fuses to blow. The 0.44 kV phases would increase in voltage by the same amount as the neutral. Elevated neutral and phase voltages may result in insulation breakdown and a 0.44 kV fault where the 0.44 kV cables are run next to something grounded to the 11 kV neutral.

The solution may be to bond the 11 kV and 0.44 kV neutrals together.

 

[busbar]

It is possible there may have been primary-to-secondary fault in the Dyn0 unit with a significant potential difference between the respective ground systems, resulting in primary-to-seconday insulation breakdown. That may have operated fuses on the yn0 side.

In this case, impedance between the two grounding systems {electrodes} may have contributed to the fuse operation. The 11kV ground fault may have raised one primary-side terminal and associated windings of the station-auxiliary transformer from a normally stable L-N potential to a temporary L-L voltage. Careful evaluation of the station-auxiliary transformer may be in order.

 

[appunni]

Sir,
In the book 'IEEE GUIDE FOR SAFETY IN AC SUBSTATION GROUNDING' (IEEE Std 80-2000) , it is mentioned that hazards are possible if low voltage neutral wires are connected to substation ground due to potential rise during ground fault current flow.
Please see section 17.9.3 in page no.110.
I request your comments about whether this is applicable to my case .
The transformer secondary neutral is solidly earthed. There is no telephone line passing nearby. This is the same substation I had already mentioned in which jghrist answered. Thanks in advance.
With regards,
appunni

 

[jghrist]

IEEE-80 Section 17.9.3 concerns hazards where low-voltage feeders serve points outside the substation area. This is because the substation grid (and low-voltage feeder neutrals connected to it) can reach a high potential with respect to points outside the substation. This does not apply to station service feeders that feed only equipment within the substation. The voltage between the grid and all points within the substation should be below the allowable touch potential.

 

[Bung]

I have twice before seen what seems to be a similar problem - only in my cases it was an earth fault on one feeder causing an earth fault relay to operate on an adjacent feeder when there were no known zero sequence infeeds on any of the feeders.

The first one turned out to be blown lightning arrestors at a pothead about a kilometre out of the substation. We never did get to the bottom of the other one, but I suspect a similar problem. The first one we found after I drew a pretty picture with all the lines, more or less to scale geographically, put in all the current paths and looked at all the volatge gradients that resulted. From that I could direct a field crew to where I thought they'd find the problem, and what it was. (Ever seen that sceptical look on a linesman's face when an engineer desk jockey tells him where to go and what he'll find? It's even better when that engineer turns out to be right).

So I suggest you draw a bit of map, put in all the current paths, and have a stab at working out the voltages. ATP helps, but back of an envelope guesstimates are usually quite good to at least indicate if a problem may exist or not.

Bung
Life is non-linear...

 

[DanDel]

appunni, am I correct in my understanding that your problem is that the secondary main 440V fuses are opening for a ground fault in one feeder of the the 440V system but not in any other 440V feeders? Have you checked the GF short circuit values and the coordination between these devices?

 

[jbartos]

Suggestion to appunni (Electrical) Jul 28, 2003 marked ///\\ We have a substation where 110kv is stepped down to 11kv via a 10MVA ynyn transformer . Three 11kv feeders are used for ditribution. The 11kv and 110kv side nuetrals are solidly grounded . The substation auxiliary supply is maintained through an 11/0.44kv Dyn transformer of which the neutral in the 0.44kv side has been solidly grounded.
///The is normally done.\\\
We experienced that the 0.44kv side fuses are blowing off for a particular feeder ground fault.
///The is normal for solidly grounded neutral system grounding.\\ We found that this is not happening for faults of other feeders.
///The ground return path has a relatively high resistance. A suitable solution is needed.\\ We checked the ground resistance of transformer neutrals and found ok. The 0.44kv lines and 11kv cables are seperately routed and we think no chances of induction.
Finally , we noticed only one thing. The particular 11kv feeder was feeding to one direction and other feeders are feeding lines to the other direction. The auxiliary transformer is situated in the same direction of the particular feeder.
Let us analyse in this way. For an earthfault in the particular feeder the fault current will flow through ground from the fault point to the transformer 11kv side neutral. As the auxiliary transformer is situated between the fault point and 11kv side neutral of the 110/11kv transformer, the potential of 0.44kv side neutral increased which led to blowing of fuses. Please note that the 11kv side of the auxiliary transformer is delta connected.
Is my analysis correct? If not, what may be the possibilities? What checks I have to conduct further? I expect your valuable comments.

 

[appunni]

To dandel: The outgoing feeders are 11kv feeders. The subtation auxiliary supply is of 440volts. The fuse blowing on the 440v side was found for a particular 11kv feeder fault. There is no 440volts outgoing feeder. Thanks.
with regards,
appunni