Dry Type Distribution Transformer Loading

[Elmir]

I'm involved in a transformer replacement project where we are replacing two 1000/1500kVA PCB oil filled transformers with dry type transformers in a large mall.

The customers service voltage is 25kV. The 25kV is stepped down to 12.5kV through a 3MVA autotransformer. Downstream of the 3MVA autotransformer are three transformers, all delta/wye configuration.

Two of these downstream transfomers are the PCB units which feed a main-tie-main secondary switchboard. The third transformer is 1.2MVA which feeds a seperate secondary switchboard.

The customer is interested in upsizing the replacment transformers to 1500/2250kVA for future additional load/expansion. Various problems exist with upsizing the two transformers which include secondary switchboard rating, protection modifications, cable ratings, utility permission and so forth.

With already 3200kVA of transformer capacity connected downstream of the autotransformer, is it feasible to even consider upsizing the 1000/1500kVA units due to the restriction of the 3000kVA autotransformer?

I understand if the connected present day load is minimal thus permitting upsizing the replacement transformers, but I am worried about future loading. Any thoughts or comments regarding this are appreciated.

 

[rbulsara]

As for other "various issues", let the owner know what they mean and could cost.

Having sum of the load side transformer capacities exceeding the source unit is pretty common, as long as the peak load does not exceed its rating.

I would consider getting rid of the auto transformers and use 25kV to LV (user level), delta-wye new transformers.


Rafiq Bulsara

http://www.srengineersct.com

 

[jghrist]

The transformers serving the main-tie-main secondary switchboard may be sized to allow one transformer to carry all of the load. If this is the case, you don't have to consider full loading of two when sizing the upstream autotransformer.