Transformer Tripping on Load

what are your CTs ratio? maximum load. i think you can start from there. then check the wiring of the CTs, secondary and primary injection tests. if all is well then it could be coming from somewhere else since it is in earth.

Check the CTs used for the SEF and REF relay circuits. My first thought would be that either the neutral CT or one of the phase CTs has been put in with reversed polarity, or the secondary wiring of one of the CTs has been installed with reversed polarity. This will not affect phase overcurrent relays, but will result in an imbalance that shows up at the pickup point of your SEF or REF relays (probably 300 amps!). See the following for a simple explanation of REF relay function:

REF protection

Let's say the protection was set correctly. Let's say the transformer was correctly checked and it is no short-circuit problem.

Then it could be a resonance phenomenon. If transformer capacity reactance and the inductance reactance will be equal for a certain harmonic [may be the 3^rd] then the total reactance-for this harmonic frequency-will be zero and the current will be elevated. The added load-above 300 A-could produce the harmonic resonance.

Sounds like the transformer to me. Have a good look at it, it could have developed a fault since it was last checked, and chances are it will only get worse.

I am assuming your site has enough redundancy in its supply that you can pull the suspect transformer out of service for a thorough check?

I joined this company 15 days before

With seven 2.5MVA transformers I've got to ask, what's your role on site and are you qualified? You may be shocked to find out that many who ask these types of questions on CR4 don't appear to be.

Just one question: Has any work been done on either the transformer or protection? if the transformer has recently started to trip, like Jack I would be inclined the an internal fault.

please suggest me action plan for this ... it is need to test transformer /ct or protection ckt first.
Suggest you hire a consultant.

I'm sure your new employer will be impressed by your resourceful use of a group of random guys on the internet.

Isolate the transformer and carry out an earth loop impedance test before you start fooling with the protection. Get back to basics.

Do you have 4-CT arrangement or 5-CT arrangement for REF protection (assuming high impedance principle for REF protection)!

I have seen REF protection maloperation on load due to incorrect positioning of the neutral CT (4-CT arrangement). In some cases, the reason was that the neutral CT remained shorted in the TB on transformer (which means the relay is seeing the residual / load unbalance current leading to maloperation after a particular load threshold.

For REF protection with 4-CT arrangement, the neutral CT shall be positioned so that it measures unbalanced load current in addition to current in the neutral-to-earth circuit.

Your post says SEF and REF protection, that is confusing. If you mean SEF protection, this could be neutral CT-connected standby earth fault protection. This maloperation suggests that the CT is not in the neutral-to-earth circuit and instead is measuring the load unbalance. This could be happening due to incorrect positioning of neutral CT or if the SEF protection is supplied from residual connection (of phase CTs).

Hope the above is helpful. Please share your findings.