Differential Relay for Star Delta Transformer

In theory, it should but the reaction time of whatever controlling device is monitoring teh CT output may not be fast enough for "real" protection.

A much better approach is to install true "ground-fault" breakers on the main circuits from the transformer.

You did not mention the capacity of this transformer, but true ground fault protection is required for feeds over some minimum capacity, I have not take time to review the codes for the correct value. Another option is to install ground-fault breakers on all main feeds from the primary panel to the secondary panels or motor control centers.

A very intelligent question! I feel that the protection will be triggered, provided the ground fault current is more than the bias set in the relay and the fault is within the protected zone.

I have actually checked this and realiseed that the protection does not trigger for the fault condition shown in the attached figure.I have just put down only the delta side of Star Delta Transformer

Any possible reason why it is not triggering though it should be as it is a fault within the protection zone.

In the above case where you have only one CT with in the delta, how can the protection zone be defined.I think from my knowledge any equipment between the two set of CT's (comparing currents on per phase basis) would be treated as a protective zone. On the same line how can the protective zone defined for the above delta case.

Refer to the drawing, I assumed the right side is conneceted to generator in Wye connection, other side of transformer is in Wye connection. When a ground fault occured (as shown) the the ground fault current, flows to generator neutral, back to transformer thru phase C (assumed the lower wire is phase C), no fault current flow to transformer (except fault in tranformer winding). Generator protection will operate.

Any current changed in delta shide will transformer to Wye side proportional to the XFMR turn ratio. Diff. relay will not see the different current, so relay may not operate.

But if fault in delta winding, a CT may detect ground fault current (assumed only 0.5pu for fault location is 50% of XFMR winding).

It is not common implemented due to difficulties in connection, and also zone protection is limited.

The similar intallation is applied on generator differential protection for generator winding connected in delta connection. One CT is inside the delta, while other CT is located in line.

Do you have any such links where I can read more about this?

Hi Naina,

Since you have no ground reference on the output of your transformer it is clear that an earth fault will not generate a current and thus no trip will occur.

There must be a ref point on the transformer side of the CTs for this system to work. There are meters with settable trip outputs for measuring the insulation resistance of an isolated system. I've just tried googleing for these with no luck but I know they exist as I see them frequently on the yachts with 3ph delta systems.

Regards

Chas

Please read "Protective Relaying - Principles and applications, by J. Lewis Blackburn, Marcel Dekker", page 297. The normal differential connection cannot be used when the CTs are inside the Delta. Two sets CTs connected in parallel are required to provide protection for ground faults in these winding.

Hi

I have got your point, now if the ct are moved out and placed on the line side (outside delta, measuring line currents) of delta how would the same fault be detected.

Naina