Getting Around the Factor 1.0 Rule

goldenfab

New Member
Joined
Oct 16, 2021
Messages
7
I'm new here and am in the preliminary planning phase for a system that plan on installing. I was leaning towards IQ7 micro inverters with a small battery run inverter for grid down scenarios but after reading up on here I learned about the Factor 1.0 Rule https://www.victronenergy.com/live/ac_coupling:start

My primary reason for being interested in solar is to be prepared for long term grid down situations and to hedge against rising energy costs. I'm in sunny AZ and right now power is very reasonable compared to other places, off peak is $0.0523 and on peak is $0.08683 per kWh. I have 400A main service split into 200A for my main house and 200A sub panel to my detached workshop and guest house. My highest usage so far has been 78kWh in a day and I average closer to 50kWh when its cold which is when I use the most power. I'm thinking of a 10kW to 15kW PV array. Due to location of where I want the PV array I would like to back feed PV power into the 200A sub panel in the shop and put the battery inverter up at the house into the main 400A panel with an interlock so I can safely only create a micro grid when I'm disconnected from the utility grid. I'm not sure if this is a viable or code compliant setup?

I have a very well insulated house with hydronic electric heat pumps for heating and cooling and I'm planning on adding large water tanks so they can store heat energy during sunlight hours so my electric loads during the night when PV isn't producing could be very minimal, hence I don't have a reason to invest in a very big micro grid creating battery inverter and batteries. I like the idea of micro inverters because its reduces the single point failure of one inverter and sounds like it would be easier to add more panels in the future.

Is there a way around the Factor 1.0 Rule? Or am I going to be better off with a DC coupled hybrid system?
 

RCinFLA

Photon Sorcerer
Joined
Jun 21, 2020
Messages
1,025
Your only real option is to put some of your microinverters on AC coupling and the rest directly to grid on input side of hybrid inverter, which you will lose their production during grid outage but at least you have some of the microinverters providing power during grid outages.

The AC coupled inverter must have the ability to take GT inverter AC coupling power and push it to batteries when PV production is high and house loads suddenly drop off leaving overproduction by PV to be dealt with. This not only means the inverter VA rating must be large enough but also the batteries AH size must be large enough to take the backfeed power from GT inverter until they can be shut down or reduced in power. This process can take up to a couple of seconds to get under control.

There is also the issue the maximum GT power AC coupled into hybrid inverter cannot exceed the hybrid inverter grid connect relay amperage rating.

I advise against using DC coupling for day to day grid feeding. It puts constant ripple current stress on batteries due to the 120 Hz ripple current profile of the hybrid inverter supplying 60 Hz AC power. The batteries effectively act as large filter capacitors. There is also lower PV to AC power conversion efficiency compared to GT inverter alone. For DC coupling you have the conversion loss of the DC SCC and the hybrid inverter. This does not include any heating loss in batteries due to high 120Hz ripple current in and out of batteries.

There is one other option although I think it is too expensive. Besides having high voltage input PV GT inverter, and hybrid inverter, you add a high voltage input DC charge controller. Then manually switch PV panels between GT inverters and HV DC charge controller. This does not apply to microinverters though.

I also advise against using a high frequency hybrid inverter for AC coupling. A low freq hybrid inverter is more rugged for absorbing backfeed power surges from GT inverters. If you do use a HF hybrid inverter with AC coupling it should have a separate load dump capability to take momentary backfeed power surges from GT inverters. SolArk HF hybrid inverter has this option as well as option of putting GT inverters into Gen AC input so it can open the Gen connect relay to immediately dump the GT inverters.
 
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goldenfab

New Member
Joined
Oct 16, 2021
Messages
7
@RCinFLA , thank you for the reply. I'm still wrapping my head around these options and refining what I think I want and may comment back when I have something intelligent to say.
 
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