Paralleling 3-phase transformers

[Mike Koivisto]

Can I parallel two 112.5 KVA 3-phase transformers? Primary @ 480 volts 250 amps. Secondary @ 240/120volt Delta. will the available output be 225 KVA?

 

[mivey]

Yes, with some limitations.
see previous thread for coverage of issues:
http://forums.mikeholt.com/showthread.php?t=86228

Add:
In addition to freq & voltage, make sure the phase shift is the same.

I did not read the whole thread, but the approximate derating for mismatched impedance is:
Derating = (Z2/Z1 * K1 + K2) / (K1 + K2)
where
K1 = capacity of unit with larger impedance
K2 = capacity of the unit with smaller impedance
Z1 = impedance of unit 1
Z2 = impedance of unit 2

 

[mivey]

Example:
Two 112.5 kVA, one at 5.6% imp and one at 5.8% imp:
(5.6/5.8*112.5+112.5) = 221 kVA