Transformers connected in wye-delta

[a_hargy]

Homework Statement

Three single phase transformers rated 100kVA, 13.2kV/2.4KV are connected in wye-delta on a 3-phase, 18kV line.

I need to find the maximum load that can be connected to the transformer bank and the outgoing line voltage.

Homework Equations

The Attempt at a Solution

I am completely stuck, so far I've got:
Voltage across primary is 2.4kV therefore voltage across secondary is 13.2kV
E_s=13.2[tex]\sqrt{3}[/tex]=22.86kV?

Any help would be greatly appreciated.

Adam

 

[Phrak]

This can't be right can it? Do you have a 2.4KV transformer connected to an 18KV line?

 

[a_hargy]

I'm not 100% sure if that's what the question is implying as I'm completely stuck on it.
Thanks for the quick reply though.

 

[Phrak]

Well, I don't know how 3 phase transformers are voltage rated, as I haven't studied them. However, if the voltage of a Delta is measured from phase to phase and voltage of a Wye is measured neutral to any phase, then you don't use the factor of sqrt(3).

 

[paranormal]

I would start by first confirming the information provided in the homework statement. It is correct that the voltage across the primary winding is 2.4kV and the voltage across the secondary winding is 13.2kV. However, the calculation for the secondary line voltage is incorrect. The correct calculation is 13.2kV x √3 = 22.86kV, as you have correctly stated.

Next, I would use the formula for power (P=VI) to calculate the maximum load that can be connected to the transformer bank. The primary voltage is 2.4kV and the secondary voltage is 13.2kV, so the voltage ratio is 13.2/2.4 = 5.5. This means that the current on the secondary side will be 5.5 times the current on the primary side. Since the transformers are rated 100kVA, the maximum load that can be connected is 100kVA/5.5 = 18.18kVA.

Finally, to determine the outgoing line voltage, I would use the formula for voltage ratios (Vp/Vs = Np/Ns) where Vp is the primary voltage, Vs is the secondary voltage, Np is the number of turns on the primary winding, and Ns is the number of turns on the secondary winding. In this case, since the transformers are connected in wye-delta, the number of turns on the primary winding is equal to the number of turns on the secondary winding. Therefore, the voltage ratio (Vp/Vs) is equal to 1. This means that the outgoing line voltage will also be 13.2kV.

In summary, the maximum load that can be connected to the transformer bank is 18.18kVA and the outgoing line voltage is 13.2kV. It is important to double check all calculations and formulas to ensure accuracy in your solution.