Fronius Symo 20.0-3 480 (Delta to Wye Transformer), Neutral Wire

[jay_the_gemini]

Hi everyone,

Try to be concise as possible here. The client wishes to make use of the Fronius Symo 20.0-3 480 inverter. According to the installation manual, it is applicable on the following grids, that being the 480V WYE grid.





According to the electrician who I am creating the 3 line diagram for, he is suggesting that we go from the Inverter to a 480 Delta > 208 Y Transformer, then do a line side tap via a 208V fused disconnect. All was dandy until I did some investigation.

It looks like this inverter requires the Neutral. Therefore, how on earth can this possible work. the 480V Delta side of the transformer has nowhere to land a neutral, and the neutral that is on the 208Y of the transformer is already spoken for in the sense that it is going back to the neutral bar on the 208Y panel board.

Therefore, after scratching my head for awhile, I basically came up with 2 ideas..

a. This design is flawed, and it may need a new inverter selection
b. Use a 480Y to 208Y transformer which apparently is quite rare, but does exist.

Aside from option (a), am I correct in thinking this is an impossible design? Keep in mind that the inverter DOES require a neutral for voltage sensing.

Does anyone have experience with this inverter or this scenario in general? I emailed the electrician and let him know my thoughts, and that perhaps a different design was in order. But, maybe I jumped the gun and am simply not seeing the solution?

Please advise. Thanks.

 

[winnie]

The typical transformer to use for this application is a 208V _delta_ to 480V _wye_ transformer. Not the most common (480V delta to 208V wye is far more common) but certainly a standard transformer.

What confuses people is that the _primary_ of the transformer in this application is the 208V side, even though energy is flowing from the 480V side to the 208V side. The reason is that with common grid tie inverters the side of the transformer energized first is the _grid_ side, and the inverter takes its voltage reference from the grid.

In theory you could use a more common 480V delta to 208V wye transformer 'in reverse' and then use some sort of grounding transformer (eg a 'zig-zag' transformer) to derive the neutral. But this would be an ugly setup and would require purchasing an even less common (though still standard) transformer to derive the neutral.

-Jon

 

[ggunn]

Hi everyone,

Try to be concise as possible here. The client wishes to make use of the Fronius Symo 20.0-3 480 inverter. According to the installation manual, it is applicable on the following grids, that being the 480V WYE grid.



View attachment 19637

According to the electrician who I am creating the 3 line diagram for, he is suggesting that we go from the Inverter to a 480 Delta > 208 Y Transformer, then do a line side tap via a 208V fused disconnect. All was dandy until I did some investigation.

It looks like this inverter requires the Neutral. Therefore, how on earth can this possible work. the 480V Delta side of the transformer has nowhere to land a neutral, and the neutral that is on the 208Y of the transformer is already spoken for in the sense that it is going back to the neutral bar on the 208Y panel board.

If I understand correctly, you want to connect a 480V inverter to a 208 service. If so, your electrician is has got it backwards. A 480V wye to 208V delta step down transformer creates a neutral on the 480V side. It's separately derived, so you'll need a neutral to ground bond on that side. You don't need a neutral from the service to the 208 side of the transformer.

OTOH, if what you are doing is connecting a 208V inverter to a 480V service, then your electrician is correct. The 208V side of the step up transformer will be separately derived with its own neutral that needs to be bonded to ground. You wouldn't need a neutral from the service to the tranny.

 

[electrofelon]

If I understand correctly, you want to connect a 480V inverter to a 208 service. If so, your electrician is has got it backwards. A 480V wye to 208V delta step down transformer creates a neutral on the 480V side. It's separately derived, so you'll need a neutral to ground bond on that side. You don't need a neutral from the service to the 208 side of the transformer.

OTOH, if what you are doing is connecting a 208V inverter to a 480V service, then your electrician is correct. The 208V side of the step up transformer will be separately derived with its own neutral that needs to be bonded to ground. You wouldn't need a neutral from the service to the tranny.

I agree with Mr gun. You need a 480Y to 208 delta. One other option is to use an autotransformer, which may be cheaper and save the installation hassle of running a GEC.

 

[ggunn]

I agree with Mr gun. You need a 480Y to 208 delta. One other option is to use an autotransformer, which may be cheaper and save the installation hassle of running a GEC.

That's what I thought but it rose up and bit me. For one thing, from my transformer sources there was little or no price difference between isolation transformers and autotransformers. For another, autotransformers rarely have taps where you can fine tune the voltage when (as happened to me) the line voltage from the utility was at the top of its range and what I thought was acceptable voltage drop (voltage rise) was enough to push the voltage at the inverters out of their operating window. Adding buck-boost transformers is a PITA and the added expense on narrow margin projects isn't good, either.

 

[electrofelon]

That's what I thought but it rose up and bit me. For one thing, from my transformer sources there was little or no price difference between isolation transformers and autotransformers. For another, autotransformers rarely have taps where you can fine tune the voltage when (as happened to me) the line voltage from the utility was at the top of its range and what I thought was acceptable voltage drop (voltage rise) was enough to push the voltage at the inverters out of their operating window. Adding buck-boost transformers is a PITA and the added expense on narrow margin projects isn't good, either.

Good to keep in mind. 3 times (or 1/3) is commonly given as the rough break even point for autos vs isolation and 208 to 480 is getting there so I guess it makes sense there isn't much of a price break ( I have never priced it out).