Transformer for Microgrid Application - Confused

[mrcole]

I am currently working on a microgrid system that utilizes Ideal Power inverters (30C3):

http://www.idealpower.com/wp-content/uploads/2017/09/Stabiliti_30C3_DAT-00009_Rev-K_June17.pdf

These inverters are 3-wire 480V delta on the AC port. The inverters will connect to a 4-wire 208 wye generator bank. Per the recommendation of the inverter company, I was told a zigzag autotransformer could be used for this to derive a neutral for balance relative to ground. I've never used a zigzag (inter-connected star) transformer before and am unclear how this will work/is connected.

Another configuration that came up from speaking with transformer suppliers was a 480Y-to-208Y. This configuration was also not very clear to me given the 480V delta output of the inverters.

Has anyone encountered something like this before? Thanks.

 

[Ingenieur]

a zig-zag is common to derive a N/G, we do it all the time
search for 'post glover'
they have some good white papers
http://www.postglover.com/wp-content/uploads/downloads/2014/04/GT210-10_Zigzag_Trans.pdf

 

[pv_n00b]

Let's see, you have a 480V 3ph 3W output from the inverter and you want to connect that to a 208V 3ph 4W system? According to the install manual, the inverter requires a 50A circuit breaker in the output circuit. If a 480V delta transformer is used then there will be no grounded neutral to trip the breaker in the event of a ground fault. So this is why they are saying a 480V delta will need a grounding transformer to make up a neutral. Or you can use a Wye/Wye transformer. You don't connect the neutral to the inverter.

 

[GoldDigger]

Let's see, you have a 480V 3ph 3W output from the inverter and you want to connect that to a 208V 3ph 4W system? According to the install manual, the inverter requires a 50A circuit breaker in the output circuit. If a 480V delta transformer is used then there will be no grounded neutral to trip the breaker in the event of a ground fault. So this is why they are saying a 480V delta will need a grounding transformer to make up a neutral. Or you can use a Wye/Wye transformer. You don't connect the neutral to the inverter.

On the other hand in a truly ungrounded system a ground fault will not result in any fault current or hazardous voltage (except for the pump up from some arcing ground faults.)
I bet that the inverter output circuits would not be happy with the voltages of a corner grounded delta though, hence their concern about a ground fault.

 

[Sahib]

On the other hand in a truly ungrounded system a ground fault will not result in any fault current or hazardous voltage (except for the pump up from some arcing ground faults.)

That is the main problem with ungrounded systems.

 

[ggunn]

Use a 480/277V wye to 208V delta step down transformer. Ground the center of the wye on the 480V side and that's your neutral.

 

[Ingenieur]

what is the load on the 480 side?
another xfmr to tie into a utility? what config d:y, etc
or is it utilized at 480 on site?
multiple 208:480 sources bused together?