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Differential protection for a T/F operates on the difference of current entering & leaving the T/F.
But current entering or leaving a T/F is never equal. Please explain?

Responding to Sunny's 06-Mar-08 (00:23) query... an excellent question!

The use of differential-relaying to protect a transformer against Transformer-Faults, is far more complicated than that used for generators because of:

1) Current-inrush upon energization, which is, by far, the most critical of all of the factors.

2) Mismatch due to differences in high-side and low-side CT-ratios, transformer types, saturation curves types, and burdens.

3) The transformer's impedance, which introduces not only a magnitude change but a phase-shift as well.

4) The phase-shift imposed by D-Y or Y-D winding connections.

The effects of almost all of the above are controlled in the design and application of differential-relays for transformer-fault protection. For very severe mismatch cases, auxiliary CT are added!

Let me know if additional detail is required!

Regards, Phil Corso (cepsicon@aol.com)

Please give some additional detail.

Sunny, perhaps an example will help...

Given: Two-winding transformer; 30MVA; Dy11; HV-side, 69.0kV, 300/5 CT; LV-side, 11.5kV, 3000/5 CT.
Find: Percent unbalance?
Procedure:
HV-side, Amps = 30,000 / (Sqrt (3) x 69.0) = 251A, CT sec = 251/60 = 4.18A, Irelay = 4.18A.
LV-side, Amps = 30,000 / (Sqrt (3) x 11.5) = 1,506A, CT sec = 1506/600 = 2.41A, Irelay = Sqrt (3) x 2.51 = 4.35 A

The restraint-coil current from the HV-side 4.18A, while that from the LV-side is 4.35A. Their difference, 0.17A, would flow through the relay’s operating-coil. The higher percent mismatch, referenced to the lower current, is (0.17/4.18) x 100 or 4.1%. In this example, the mismatch is well within the limits of typical transformer differential relays.

Keep in mind, however, that each manufacture has specific tolerance values, as well as selection options to minimize the mismatch even further. Also, to keep it simple, I did not consider the impact of inrush current, nor transformer tap-settings!

Lastly, please search the Control List archive for like topics, as well as text-books and reference documentation that cover the subject in greater detail!