Rotary Phase Converters and 120% rule questions

Status
Not open for further replies.

Ctay005

Member
Location
Orem, Utah, USA
I have a customer who is looking to get a larger system (about 35kW) installed to offset a good portion of his annual usage (around 150,000kWh per year).

His house is fed by a 400A main panel that has 2 200A branches. One of the branches goes directly to his wood shop out back and the other goes to his house.

In his shop he has a bunch of wood working tools and a few tools that require 3-phase. He's using 2 Phase-a-matic rotary phase converters to get this power.

He wants us to be able to offset 80% of his power and I just dont see how this could be possible given his electrical setup.

My first question is: Am I limited to 120% of the 200A branch going to his shop (where the panels and inverters will be installed) or can I go higher since he has a 400A service. It seems that since his shop is ran off of a 200A breaker, that 200A is what I will base the 120% rule off of. If that is the case, it would be better off for me to do 2 systems, one for his shop and another for his house. That way I could get up to 80A instead of 40A.

My 2nd question is. Would it be better to go with the smaller single phase inverters (my preference) or a larger 3-phase inverter (customer's preference). I don't know how a 3-phase PV system will work with his setup since he only has 2 machines running on his 3-Phase panel and everything else in his shop as well as his house is all the single phase.

I haven't dealt with 3-phase on a residential system so this is a new realm for me in the residential PV industry!
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Since the three phase is provided by a (very inefficient) rotary converter, I do not see that it would be wise to do anything with a three phase GTI system. The GTI really needs a direct connection to a stiff (low source impedance) and stable power grid. I am also not sure whether a rotary converter (one which is not a motor generator) will even function properly with a reverse power flow.

You are limited to 120% of the bus rating of the panel in the shop if connect the inverters through backfed breakers in the shop panel. If the shop calculated load is low enough you can reduce the size of the main breaker in that panel (or the breaker feeding it if the garage panel is MLO) to get more room between the 120% of bus rating and the POCO input size.
It may be helpful to connect on the supply side of the garage panel also, particularly if you can reduce the size of the breaker in the main that feeds that panel.

Why do you see a problem with offsetting 80% of his usage? Do you know what his historic usage is? Or are you just guessing based on his service size?
You certainly will not need to generate anything close to either the 200A or 400A figure.
 

Ctay005

Member
Location
Orem, Utah, USA
Since the three phase is provided by a (very inefficient) rotary converter, I do not see that it would be wise to do anything with a three phase GTI system.

This is what I was thinking. I just have to find a way to convince the customer of this now:D It didnt seem feasible to backfeed 3 phase power into a converter that looks to be running off of an extension cord to an outlet!

You are limited to 120% of the bus rating of the panel in the shop if connect the inverters through backfed breakers in the shop panel. If the shop calculated load is low enough you can reduce the size of the main breaker in that panel (or the breaker feeding it if the garage panel is MLO) to get more room between the 120% of bus rating and the POCO input size.
It may be helpful to connect on the supply side of the garage panel also, particularly if you can reduce the size of the breaker in the main that feeds that panel.

The 200A panel in his shop is a MLO and we can't downsize the breaker feeding it.

Why do you see a problem with offsetting 80% of his usage? Do you know what his historic usage is? Or are you just guessing based on his service size?
You certainly will not need to generate anything close to either the 200A or 400A figure.

We have his historic usage and he is using around 150,000kWh per year. He wants to offset as much as his usage as possible with our system and it would take a very large system to offset this.

It seems like the best solution currently is to do a supply side connection at his main panel in front of one of the 2 200A breakers.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
We have his historic usage and he is using around 150,000kWh per year. He wants to offset as much as his usage as possible with our system and it would take a very large system to offset this.

It seems like the best solution currently is to do a supply side connection at his main panel in front of one of the 2 200A breakers.

First of all...
Wow.

To produce 150,000kWh a year will take something like a 100kW solar system (plus or minus around 30kw depending on conditions).

On a 400A, 240V service you could backfeed 400A/1.25*240V = 76.8kW. So in the ballpark of 80% (depending). But you are pretty much only going to be able to do this with a supply side connection at the main service, or possibly by upgrading both his existing panels to 400A. (Supply side is probably cheaper and easier.) That is all assuming there is somewhere to put the panels, of course.

Note that most utilities interconnection agreements are considerably more complicated when you go above 10kW or so.
 
Status
Not open for further replies.
Top