Open delta transformer with delta connected load

[TS Liew]

Dear friends,
I am curious about the subject above. Many literature discusses the power requirement of open delta transformer power requirement in the form where the load is star connected. In this case, the load current naturally has a phase shift of 30 deg. Thus causing a power factor of 0.866. As such, the power capacity of the open delta transformer needs to be at least 15.55% higher than the intended load to cater for the 0.866 pf. Consequently, a 3 phase 9kVA delta transformer loosing one phase thus becoming open delta would have 2/3(9kVA)=6kVA apparent power and 2/3 x 9kVA x cos(30)= 5.196KW real power. Below is a picture obtained from E4U courtecy of Dr. Vipin Jain.

Say a star connected load is pure resistive 9KVA. As a result the required open delta transformer supply source shall be 9 x 1.1555 = 10.4kVA.

What if the load is pure resistive delta connected instead of star? Would it be logical to assume the load current to be in phase with the voltage and as such a unity pf? Not requiring the open delta transformer to be 15.55% higher rating?



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

 

[stevenal]

A full balanced delta load will act like the wye load pictured. If your load was open delta, connected properly, you could skip the derating.