Auto Transformer Protection 1

[alingstone]

I have to apply protection to an 11/22 kV step up 3 phase auto transformer. The protection relay is a MiCOM with basic OC & EF elements. The autotransformer is solidly grounded so there is in effect a zero sequence source in front of thew relay. The source side is solidly earthed i.e the 33/11 supply transformer is Dyn11. The questions are,

1) Will the relay on the 11 kv side detect earth faults on the 22kv side.

2) Will the relay on the 11 kV side detect system imbalance, i.e. will the auto transformer ground pass imbalance current back to the star point of the 33/11 transformer.

3) Are there any special considerations for this application of protection?

 

[davidbeach]

Your transformer can be an autotransformer or it can be a Dyn; but there is no way it can be both. Please clarify.

 

[alingstone]

At the zone substation there is a 33/11 Dyn11 supplying the 11 kV bus. About 2 miles away from the sub on one of the sub's 11 kV feeders we're putting in a 2 MVA 11/22 auto stepup.

 

[RRaghunath]

alingstone,
Yes, the relay at 11kV terminals of auto transformer can detect phase as well as earth faults on the 22kV side.
If there is earth fault in the 11kV system (other than the 11/22kV Auto trafo feeder), the relay is not expected to maloperate (unless the transformer is designed with a tertiary delta winding).
With regard to unbalance in the system, I am not clear, what you meant (Do you expect unbalanced loading at 11 or 22kV level!).
Trust the above is helpful.

 

[alingstone]

With the solidly grounded neutral of the auto tranny the 11 kV system in effect becomes a 3 phase 4 wire system due to the star points being bonded through ground. Can the ground carry any imbalance current back to the source?

 

[waross]

What do you expect for unbalanced currents?
That depends on the condition of the ground and your grounding electrodes or grids. You will be using the ground for a neutral conductor. Have you considered running a neutral conductor?
My thought is that grounding may be adequate for conducting short term fault currents.
If you are expecting unbalanced loading and the resulting neutral currents on an ongoing basis you may want to consider a neutral conductor.
If you are anticipating any harmonic current to flow on the ground connection you may want to evaluate possible accelerated corrosion issues with the grounding system.
Without a neutral conductor you will be running a four wire system on three wires.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[alingstone]

All of the distribution transformers (i.e. 22/0.4kV or 11/0.4kV) are either phase to phase or three phase delta connected. So correct me if i'm right or wrong but theoretically with this arrangement there should be no unbalanced condition, i.e. the current magnitudes in each phase may be different but the vector summation will always be zero, is this right?

 

[Marmite]

Correct. The only current of any significance flowing back to the source via the ground will be in the event of an earth fault.
Regards
Marmite