Transformer protection betwen inverters and utility connection

[jerryls]

I have a PV system where the service is 208V 3P, and the available PV inverters are 480V 3P. These are three 12KW SMA inverters. Normally if the inverters were the same voltage as the service, I would combine the 3 inverters using a Main Lug subpanel, then connect the output to the utility line side through a fused disconnect. My question is on the OCPD's using the step down transformer. Do I need to add a separate OCPD between the main lug panel that is combining the inverters and the transformer primary? Then I would use a sub panel that has a main breaker as well as the individual breakers for each inverter. Also, regarding 450.3B, the primary side is the inverter side, but the secondary sees the utility 208V as a primary even though there should be no load on the other side. Thanks for any discussion regarding this.​

 

[James@CHA]

Reply

Reply

I have a PV system where the service is 208V 3P, and the available PV inverters are 480V 3P. These are three 12KW SMA inverters. Normally if the inverters were the same voltage as the service, I would combine the 3 inverters using a Main Lug subpanel, then connect the output to the utility line side through a fused disconnect. My question is on the OCPD's using the step down transformer. Do I need to add a separate OCPD between the main lug panel that is combining the inverters and the transformer primary? Then I would use a sub panel that has a main breaker as well as the individual breakers for each inverter. Also, regarding 450.3B, the primary side is the inverter side, but the secondary sees the utility 208V as a primary even though there should be no load on the other side. Thanks for any discussion regarding this.​

I've had this situation come up on several project recently, where the 250kW Inverter output is 480V, 3P, and utility source 208V, 3P. I treat the transformer just like i would any other transformer, providing primary and secondary protection, and complying with NEC 240.21 (b)(3).

In this situation i would use an MCB panel rather than the MLO. Using the MCB as the transformer primary side protection.

 

[GoldDigger]

I've had this situation come up on several project recently, where the 250kW Inverter output is 480V, 3P, and utility source 208V, 3P. I treat the transformer just like i would any other transformer, providing primary and secondary protection, and complying with NEC 240.21 (b)(3).

In this situation i would use an MCB panel rather than the MLO. Using the MCB as the transformer primary side protection.

For the purpose of normal power flow.direction, the inverter is on the primary side.
But from the view of a bolted fault at the inverter the current will be with POCO as the primary.
Definitely protect for both.

 

[jerryls]

Thanks guys, I agree with both comments, regarding using an MCB panel instead of just a main lug type in case of bolted fault on the inverter side. My question is do I need 250% amperage protection per 350.3B on the primary(inverter side)?

 

[James@CHA]

Your inverter can only produce it's maximum continuous output rating, so sizing the primary overcurrent protection based on the inverter output should be acceptable. The 250% per NEC 450.3(B) is the maximum rating of your Primary FLA.