What is the power rating for a 3-phase transformer based on the secondary voltage?

We want to install equipment on a 60 Hz, 400 V Delta system. The equipment is not suitable to work on a 400 V Delta system, therefore we can place a transformer to create a star configuration and are free to define its specs. I have some trouble with the results from the transformer power rating based on the secondary voltage. I assume the following definitions:

V_LL = line voltage = line-line voltage

V_LN = phase voltage = line-neutral voltage

• 480 micro-inverters spread over three phases, can be either connected line-line or line-neutral, max. specs is 230 V and 1.5 A. 160 units per phase for 240 A phase/line current.

• 3 Charger/Inverter combination, one on each phase and can only be connected line-neutral, power per unit is 15 kVA. 127 V (US) and 230 V (EUR) versions are available and assume current scales proportionally to meet power rating.

For the transformers, I see two options, 3PH 440 V Δ: 380 V Y/220 V or 3PH 440 V Δ: 220 V Y/127 V.

3PH 440 V Δ : 220 V Y/127 V
We connect the Micro-inverter line-line. Therefore the power is P = V_LL * I * SQRT(3) = 220 V * 240 A * 1.73 = 92 kVA
The appropriate charger/Inverter is line-neutral, therefore 15 kVA * 3 = 45 kVA.
Transformer power rating 92 kVA + 45 kVA = 137 kVA

3PH 440 V Δ: 380 V Y/220 V
We connect the Micro-inverter line-neutral. Therefore the power is P = V_LL * I * SQRT(3) = V_LN * SQRT(3) * I * SQRT(3) = V_LN * I * 3 = 220 V * 240 A * 3 = 159 kVA
The appropriate charger/Inverter is line-neutral, therefore 15 kVA * 3 = 45 kVA. Transformer power rating 159 kVA + 45 kVA = 204 kVA

Are my calculations/assumptions correct? I would have expected the same power. See generic schematics.

I am interested in the power calculation/rating of the transformer as a function of line-to-line voltages, line-to-neutral voltages, etc. I am not asking weather the equipment would work in this design, as it is just for illustration purposes.

• Are the secondary voltages you specify in you transformer options line or phase? Commented Nov 22, 2022 at 13:40
• In the 380Y/220V version, 380 is line voltage and 220 phase voltage. For the 220Y/127V version, 220V is line voltage and 127V is phase voltage. Commented Nov 22, 2022 at 13:48

1 Answer

There's no problem with the KVA rating of the micro inverters.

It's (3 * 220 * 240 / 1000) = 158.4 kVA, in both the configurations.

• I see, as the micro-inverters are connected in Delta, 240A is phase current and therefore line current is 240*sqrt(3), there was the missing sqrt(3)! Commented Nov 23, 2022 at 19:40
• You got it, d2key! Commented Nov 25, 2022 at 3:13