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What is the significance of grounding the tertiary delta in a three widing transformer? And how can it be coordinated with the neutral grounding in a 50MVA Yn0Yn0 D5 transformer?

Krishna...

1) What is the rated voltage and capacity of each winding?

2) Are both wye windings grounded? Solidly, or via an NGR or NGT?

Regard, Phil Corso (cepsicon@aol.com)

Sorry sir for the delay in response,
The voltage ratngs are 110/13.8/13.8 KV.

Yes both are grounded.110 Kv solidly grounded and 13.8 KV with NGR(5.3 ohm)

Krishna, in answer to your questions:

1) Reason for Delta-winding.
Apparently construction is that of a 3-phase, shell-type transformer, and the manufacture provided the tertiary to:

a) Provide a path for 3rd-harmonic currents.

b) Prevent circulating-current flow in the tank, when asymmetrical faults occur in either the 110kV or 13.8kV circuits.

2) Reason for Grounding Delta-winding.
Even though isolated, that is, not connected to an external circuit, it is grounded to preclude electrostatic coupling between it and the other two windings.

3) Consequence if Delta-winding Ungrounded.
Without an intentional ground abnormally high voltages could appear with respect to ground!

4) Protective Device Coordination, HV & LV sides.
Coordination between the 110kV, solidly-grounded neutral, and the 13.8kV NGR-grounded neutral are unnessary, that is, neither affects the other!

5) Delta-winding Protection, HV-side as Source.
The 110kV-side protective devices must respond to delta-winding faults! This should not be difficult, except for failures close to the grounded terminal! You will probably have to rely on a Bucholz relay!

6) Delta-winding Protection, MV-side as Source.
As above, except the 13.8kV-side protective devices must respond.

7) 3-Winding Differential-Protection.
Is this an existing installation, or a proposed one? If the latter what is being done?

Regards,
Phil Corso (cepsicon@aol.com)

Thanks once again for ur invaluable suggestion.
It is proposed one with transformer overall differential protection along with REF both on the HV and MV side. The corner delta is grounded with sensitive earth fault relay.
Any clue?

Krishna, you are on track! If you would like me to critique your final design, make sure you send the transformer's nameplatee data, especially Zps, Zst, and Zpt!

Regards, Phil Corso (cepsicon@aol.com)