Power Transformer name plate with weird %Z naming 2

[TS Liew]

Dear friends, I've recently come across a power transformer with a very interesting way of putting its %impedance, especially the tertiary winding. The attached nameplate does not show the tertiary winding power rating but the %Z=243.9% (tap 10) meaning the voltage has to be 2.439 P.U rated voltage to achieved the rated full load current at short circuit. This is a strange way to rate a transformer in my opinion. Obviously the tertiary winding is not 200MVA, so why rate it at 200 MVA?

Shuen Liew

TS Liew (Electrical testing, Commissioning, Power system studies)

 

[waross]

A star delta transformer will take power from the healthy phases, through the delta winding and back feed into a lost or shorted primary phase.
The high impedance will greatly reduce the back feed current in the event of a ground fault on a primary phase.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[davidbeach]

It’s much easier to deal with a single rating of the transformer and then have impedances for each winding pair on the overall base. If H-Y on that transformer is rated for around 20MVA you have a reasonable impedance. H-X is likely rated for the full 200MVA and X-Y probably has a rating similar to H-Y, but with a different impedance. Look at typical system modeling software, you’re likely to only have one MVA rating for the whole transformer.

I’ll see your silver lining and raise you two black clouds. - Protection Operations