Earth Fault Calculations 3

[Rodmcm]

Arrangement is a Zn earthing transformer on 11kV busbars, feeder breaker with residual EF feeding at a distance a Delta/Wye transformer with HV fuses

If we assume that for a single phase EF the EF current is 3Io then as I see it the feeder EF relay will see 3Io and each of the phase fuses on the transformer will only see Io if the fault is between the fuse and the transformer.

Thus if you were tripping at, say, a 30A fault at the feeder then the fuse is only seeing 10A and, on a HV distribution transformer, may never trip and hence you have no discrimination.

Are these phase currents correct? Do you agree with my reasoning?

 

[RalphChristie]

Rodmcm

Let me rephrase:

You have a Main Transformer with a delta secondary. (11kV) A zig-zag transformer is connected to this delta winding to obtain a neutral point. The zig-zag trsf is high-impedance earthed. I assume there is no breaker between the Main trsf and the zig-zag transformer.
This set-up is feeding (through a feeder with residual EF-relay) a downstream transformer protected by fuses. An earth-fault occurs between the downstream transformer and the fuses.

You have to draw it out. Lets say the fault is on phase A and the fault-current is 3Io.
3Io will flow down to earth and up into the neutral of the zig-zag transformer. Each of the three windings of the zig-zag transformer will see a current of Io. Now on phases B and C this fault-current will flow through the main transformer's windings back to the A-phase (Io+Io=2Io) and up to the point where the zig-zag transformer's A-phase is connected to the system. The current Io from the zig-zag A-phase meets the two currents at that point (Io+2Io=3Io) and this current flows on the faulty A-phase to the fault-location.

Thus the fuse on A-phase will see 3Io.
The EF-relay will also see 3Io. (Due to the residual connection)

What is the fuse rating?
What are the EF-setting and the CT-ratio’s on the feeder?
To which value do you limit the earth fault-current?

For the fuse to blow it depends on the size of your fuses and the magnitude of the earth fault-current. If the fuse-rating is bigger than the earth-fault current it will not blow for an earth-fault.
The same with the residual connected EF-relay. Due to the fact that the relay is connected in the residual connection you can’t set it to low (normally 10% of load value) and if the load value is high, the setting can be well above the earth fault-current.

On high-impedance earthed systems earth-faults are limited to very low values, many times much below the load values, and to detect earth-faults can be a problem. One solution is to use sensitive earth fault relays (with ring-CTs) and set it to very low settings.

Hope it helps

Regards
Ralph

 

[Rodmcm]

Yes, your scenario makes sense but the main and distance transformer are effectively in parallel, thus I can't see how B+C adds to A and then onto the fault at only the maim transformer, some current must go from B and C to the distant transformer and back to the fault on A missing the fuses, I'll have to think on it. What happens if the main transformer is a star connected generator with no neutral connection. Does the same summing take place in the generator?

 

[RalphChristie]

Rodmcm

Current tends to use the shortest route. (or rather the easiest route, thus route with less resistance) It is easier to flow directly to the fault-location than to flow through the downstream transformer to the fault-location. Futhermore, the current has to come from the source, and then has to go back to the source.

One of the characteristics of a zig-zag transformer is that the way how it is winded tends to force the ground-fault current to have equal division in the three lines.(from IEEE Green Book) Thus the current has to flow from the source, it takes the path with less resistance, down to earth at the fault location, up in the zig-zag transformer where it divides into the three lines, and back to the source.

You can also check IEEE C37.91-1985 (IEEE guide for protective relay applications to power transfomers)
Grounding transfomer inside the main transfomer differential zone.
In the figure they show how currents will flow during external earth-faults, because the scheme has to be stable during such a fault.

In a star connected source (with neutral ungrounded) the same summing will take place.

Regards
Ralph

 

[Rodmcm]

Thanks Ralph