Grounding Transformer WYE Grounded / delta

[Varshini Guduru]

Hi there , I'm working on one of the Solar Distribution generation projects which requires a grounding transformer as per the Utility system impact studies . Till date , I have had designed/ used grounding transformer with G-WYE on the high side and delta (3phase, 3wire) on the low side with 480V. I have received a design from our outsourcing partners with 240V delta on the low side (We are using a 75kVA, 12.47kV WYE Grounded /240V delta) - that begin said to my experience , 240V delta would require a Neutral with center tapped (As in case of High leg delta ) , but could a 240V 3phase 3 wire delta with no neutral be used on the low side of the grounding transformer. Please advise . Thanks in advance.

 

[David Castor]

If this is being used solely as grounding transformer, the delta side voltage doesn't really matter much. A 240 V three-wire delta will work, since nothing is connected to it. If you are trying using to serve 120 V loads, then it will be problematic. You want a straight delta secondary. The delta winding provides the zero sequence current needed during ground faults on the grounded wye side. The utility wants to make sure your facility is tripped off for a ground fault on their system.

 

[topgone]

Hi there , I'm working on one of the Solar Distribution generation projects which requires a grounding transformer as per the Utility system impact studies . Till date , I have had designed/ used grounding transformer with G-WYE on the high side and delta (3phase, 3wire) on the low side with 480V. I have received a design from our outsourcing partners with 240V delta on the low side (We are using a 75kVA, 12.47kV WYE Grounded /240V delta) - that begin said to my experience , 240V delta would require a Neutral with center tapped (As in case of High leg delta ) , but could a 240V 3phase 3 wire delta with no neutral be used on the low side of the grounding transformer. Please advise . Thanks in advance.

The problem with providing a ground reference point is already done when you grounded the wye point of the primary of your transformer.
The delta-connected secondary can give you ground fault protection when arranged in a broken-delta configuration (one corner opened. The voltage in the break of the delta will be zero if all phases are healthy. Any ground on any line of lines on the secondary will clip the voltage of that line to the ground. The voltage available at the break in the secondary of the broken delta transformer will be the rated line-neutral voltage when a single line-to-ground fault occurs in the system.

 

[Varshini Guduru]

Thank you David and topgone ! This helps me .

 

[kingpb]

Use a zigzag. Smaller and cheaper than grounding transformer.

 

[jim dungar]

Use a zigzag. Smaller and cheaper than grounding transformer.

I consider a zigzag to be a grounding transformer, just like a delta-wye is.

My decision is usually based on availability. For some reason my last installs were all priorities rather than being thought out ahead of time.

 

[Varshini Guduru]

@kingpb : I see your point here , however any changes made to the grounding means that we use on our design would require the Utility to reconduct the Impact studies at their end using the new equipment and this would eat up more time . That's the main reason of not changing the grounding transformer to Zigzag. Thank you for the response .

 

[electrofelon]

@kingpb : I see your point here , however any changes made to the grounding means that we use on our design would require the Utility to reconduct the Impact studies at their end using the new equipment and this would eat up more time . That's the main reason of not changing the grounding transformer to Zigzag. Thank you for the response .

I am confused, so you were using a 12.47kv primary to 240 Delta transformer. Is this the utility transformer or your transformer? Then on top of this you're adding a grounding transformer? Why not just use a single 12.47 KV to something wye transformer? Aren't your inverters 480?