Effects of replacing an existing Dist. Transformer with one having lower % impedance (6.22 with 6.0)

[Shark96]

What would be the effects and measures that would need to be taken if a transformer is replaced with one having lower impedance (6.22 % with 6 %)?

 

[HamburgerHelper]

More fault current and better regulation. 6% and 6.22 is low. I have seen industrial setups where someone had specified too low of an impedance and a choke needed to be installed downstream to avoid needing to upgrade the switchgear to meet the fault duty.

Shouldn't be much difference to go from 6.22 to 6. Probably, just need to check relay coordination and available fault current, which probably has changed since the relaying Was last set.

 

[ScottyUK]

'Low' is subjective though - we don't know how big this transformer is.

 

[Shark96]

@ScottyUK its a 2.5 MVA 11kV/0.4 kV dry type transformer.

 

[waross]

At 6.22% impedance your available fault current will be 58013 Amps.
At 6% impedance your available fault current will be 60140 Amps.
The change in regulation may not be proportional to the change in impedance.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

Depending on the change in X/R of the impedance regulation could be either better or worse; no way of telling from just the impedance magnitude.

 

[prc]

I think it is not an issue. In a batch of transformers, there can be that much difference among units.

 

[ScottyUK]

In Europe most breakers capable of handling the output of that transformer will have at least 65kA breaking capacity, so the effect on equipment should be negligible.