Transformer Compatibility

[Mian Usama]

The main service panel is connected to a low-voltage high-leg Delta transformer, with measured voltages as follows:

L1–L2: 240 V

L2–L3: 240 V

L3–L1: 240 V

L1–N: 120 V

L2–N: 208 V

L3–N: 120 V

We have 2 × 60 kVA inverters that operate at 480 V. My understanding is that we would need a 150 kVA step-up transformer with primary Delta 240 V and secondary Wye 480/277 V.

Question: Is it also feasible to install a 208 V Delta transformer at the primary

 

[jaggedben]

...

Question: Is it also feasible to install a 208 V Delta transformer at the primary

No, you don't have 208 on three phases. There is no delta at that voltage.

You have a high-leg delta service and I strongly recommend you file the utility interconnection application early. They may want you to backfeed only the split phase of this service, rather than all three phases. Or they may not want you to backfeed this service.

(Is this installation in the USA?)

 

[jim dungar]

Are you asking about installing two different transformers?

 

[Mian Usama]

Yes, In Texas

No, you don't have 208 on three phases. There is no delta at that voltage.

You have a high-leg delta service and I strongly recommend you file the utility interconnection application early. They may want you to backfeed only the split phase of this service, rather than all three phases. Or they may not want you to backfeed this service.

(Is this installation in the USA?)

 

[ggunn]

Yes, In Texas

Where in Texas? I have done several systems like this in Texas; what jaggedben says is spot on. I have encountered all three situations with Texas POCOs. Ask them or read their published interconnection guidelines (if they have any) before you go any further with the design and planning of your system; they will tell you whether your 120kW should connect to all three phases, just the A and C phases, or not at all. Obviously, if they restrict you to A and C you will have to change inverters.

But assuming the POCO allows you to connect three phase, your 480V to 240V 3P transformer configuration is correct. The 208V transformer question is a bit of a non sequitur; you cannot use it instead of a 480/277V:240V wye:delta.

 

[Mian Usama]

Thank you all for your feedback. The utility has approved the installation of the 100 kW system. My final question is: should we simply connect the phases to the utility lines (L1, L2, L3)? Also, do we need to install a unidirectional or a bidirectional transformer?

Where in Texas? I have done several systems like this in Texas; what jaggedben says is spot on. I have encountered all three situations with Texas POCOs. Ask them or read their published interconnection guidelines (if they have any) before you go any further with the design and planning of your system; they will tell you whether your 120kW should connect to all three phases, just the A and C phases, or not at all. Obviously, if they restrict you to A and C you will have to change inverters.

But assuming the POCO allows you to connect three phase, your 480V to 240V 3P transformer configuration is correct. The 208V transformer question is a bit of a non sequitur; you cannot use it instead of a 480/277V:240V wye:delta.

 

[jim dungar]

Also, do we need to install a unidirectional or a bidirectional transformer?

All transformer are bidirectional.
In the case where there is an actual primary listed it is usually connected to the primary source, which is usually the utility.

Edited to correct to intent.

 

[jaggedben]

All transformer are unidirectional.
In the case where there is an actual primary listed it is usually connected to the primary source, which is usually the utility.

I think you meant they are all bidirectional?

My understanding is that some are okay to be energized from either side and others are not.

 

[jim dungar]

I think you meant they are all bidirectional?

My understanding is that some are okay to be energized from either side and others are not.

Yep. Darn spell check. I will edit my post to say bidirectional.

 

[pv_n00b]

While all XFMRs are bidirectional for power flow the way it is connected still matters. XFMRs are designed to limit the inrush current only when the energizing source is connected to the designated primary side. They will still work if energized from the secondary side but the inrush current will be several times higher and may trip OCPD. I've seen this in action when it can take several attempts to get the CB to stay on when energizing a PV system.

 

[pv_n00b]

Thank you all for your feedback. The utility has approved the installation of the 100 kW system. My final question is: should we simply connect the phases to the utility lines (L1, L2, L3)? Also, do we need to install a unidirectional or a bidirectional transformer?

The system you describe is 120kW, so if they have approved a 100kW system they have not approved the system you intend to install. Most utilities are sticklers about the total AC rating of the interconnection.

 

[ggunn]

The system you describe is 120kW, so if they have approved a 100kW system they have not approved the system you intend to install. Most utilities are sticklers about the total AC rating of the interconnection.

This. I would never spec a transformer for interconnection of a PV system that has a lower kVA rating than the AC kW rating of the inverter(s).

 

[jim dungar]

While all XFMRs are bidirectional for power flow the way it is connected still matters. XFMRs are designed to limit the inrush current only when the energizing source is connected to the designated primary side. They will still work if energized from the secondary side but the inrush current will be several times higher and may trip OCPD. I've seen this in action when it can take several attempts to get the CB to stay on when energizing a PV system.

General purpose power distribution transformers are not designed to limit inrush current. They are often designed to provide ease of access to the high voltage windings. The inrush is typically not a design factor.

Yes the magnetizing current will be greater if the primary winding is located closer to the core than it would of it was further away, as occurs with most reverse connected transformers. This is physics.