Good day,
I have a complex problem involving a three phase bank of three single phase transformers. The phase voltage on the star primary side is 6.9kV (12kV line-line) and the phase voltage on the secondary center tapped delta is 115 / 230 V using a 4 wire system. The center tap is between the red and white phases.
The bank consists of 2 power transformers and 1 lighting transformer. The lighting transformer kVA rating is less than or equal to 2 times the rating of the power transformer. The possible combination of transformers are 25 kVa, 37.5 kVA, 50 kVA and 75 kVA. For example a bank can have one 50kVA transformer (lighting TF) as the max size with two 25kVA power transformers.
I can work out what each transformer coil is rated at (ie 50kVA / 230, 25kVA / 230). How do I calculate the % current overloading in terms of of each transformer's coil (phase) current given the line current output of the delta secondary and knowing that the phase voltage is 230V (115 / 115 V at the center tap)? This would of course involve calculating the phase currents in the delta secondary and comparing each phase current with that transformer's rated coil current. But what complicates the problem is the fact that the bank is used to supply single phase loads as well.
(1)The three phase load is shared equally among the 3 TFs i.e. 1/3:1/3:1/3, (2)the single phase load 230 V is shared in the ratio 1/3:2/3:1/3 and (3)the single phase load 115V is shared in the ratio 1/6:5/6:1/6.
Points 2 & 3 do not add up to total of 1. In the case of Pt. 2 it is meant to signify that the center TF lighting load is twice that of the other two. In the case of Pt. 3 the center TF single phase (lighting load) is 5 times the other two.
This is shown in the first diagram to help explain. The center TF is the lighting TF.
The current readings are taken from the secondary side of the transformer as shown in the second diagram.
The aim is to find delta phase currents IRB, IBW and IRW using measured line current values IR, IW and IB for all combinations of transformer sizes to determine if these phase currents exceed the rated coil current of the individual single phase transformers (based on rated kVA and secondary phase voltage 230V) to determine if each transformer is being overloaded.
I have a complex problem involving a three phase bank of three single phase transformers. The phase voltage on the star primary side is 6.9kV (12kV line-line) and the phase voltage on the secondary center tapped delta is 115 / 230 V using a 4 wire system. The center tap is between the red and white phases.
The bank consists of 2 power transformers and 1 lighting transformer. The lighting transformer kVA rating is less than or equal to 2 times the rating of the power transformer. The possible combination of transformers are 25 kVa, 37.5 kVA, 50 kVA and 75 kVA. For example a bank can have one 50kVA transformer (lighting TF) as the max size with two 25kVA power transformers.
I can work out what each transformer coil is rated at (ie 50kVA / 230, 25kVA / 230). How do I calculate the % current overloading in terms of of each transformer's coil (phase) current given the line current output of the delta secondary and knowing that the phase voltage is 230V (115 / 115 V at the center tap)? This would of course involve calculating the phase currents in the delta secondary and comparing each phase current with that transformer's rated coil current. But what complicates the problem is the fact that the bank is used to supply single phase loads as well.
(1)The three phase load is shared equally among the 3 TFs i.e. 1/3:1/3:1/3, (2)the single phase load 230 V is shared in the ratio 1/3:2/3:1/3 and (3)the single phase load 115V is shared in the ratio 1/6:5/6:1/6.
Points 2 & 3 do not add up to total of 1. In the case of Pt. 2 it is meant to signify that the center TF lighting load is twice that of the other two. In the case of Pt. 3 the center TF single phase (lighting load) is 5 times the other two.
This is shown in the first diagram to help explain. The center TF is the lighting TF.
The current readings are taken from the secondary side of the transformer as shown in the second diagram.
The aim is to find delta phase currents IRB, IBW and IRW using measured line current values IR, IW and IB for all combinations of transformer sizes to determine if these phase currents exceed the rated coil current of the individual single phase transformers (based on rated kVA and secondary phase voltage 230V) to determine if each transformer is being overloaded.
