Transformer Differential Pickup Test

Vector group YNd11d11

Read and be familiar with the basic principles involved in the design of magnetic induction as applied in transformers.. consider the turn ratios regardless of applications whether it is for voltage, current, or differential..

Then take note of how you apply these principles as you conduct your testing...

My question is why in secondary and tertiary side the trip value (335mA) it double of setting value. why not the trip value is the same of setting (168.4mA)....

You may need to take note and consider which winding is used as a source of the current needed to exite the transformer...

You have changed the original transformer designed turn-ratio... By using the secondary windings as your input you changed the Turns/Volt that in my view also changes the required excitation current of the transformer .....

What did the original equipment manufacturer say over the phone?

It might be necessary to inject current in all 3 input coils to excite the relay inductance elements, to match typical installation conditions. With no details on the design and/or manufacturer of the protective device. it’s a little difficult to analyze your situation.

I am confused because you have a three winding transfo. YNd11d11 suggests that each winding is 3 phase, a high voltage primary Y connected [connected to power source] and two delta [d] connected secondaries. The implication is that 400V is primary, but 6736 amps on other 15 kV windings would require incredible current at 400V. It seems most likely you have 15kV star primary to 15kV delta transfo with a 400V delta winding for cooling fans and other auxiliaries or site power.

You then describe windings as primary, secondary & tertiary. The current for primary matches the current & CT ratio for the 400V winding.

A relay for a 1 amp secondary CT would set up 0.2 p.u. as 0.2 x 1 amp = 0.2 amp operate level into relay winding. For the relay 1 p.u. is 1 amp [relays are usually for 1 amp or 5 amp CT secondary windings].

Also a relay for a 3 winding transfo would be a complex beast with 9 or 10 CT terminals.

Is this a digital relay into which wrong CT ratios or current settings for each winding may have been entered?

If you expect the same current setting from the relay for each winding, the problem seems to lie with the relay [ or its settings] rather than anything else, since your test set is injecting into the relay not into any CT primary bars.

I once received a relay marked 5 amp which was actually 1 amp & had to be returned to maker - they are not infallible!

A diagram of the transfo primary, CT and relay connections would be helpful!