AC COUPLING - In Arizona

[Yianie123.]

Hello Everyone.  This is my situation.  I have 5.6kw of solar panels all on Micro-inverters.  My utility company has a crazy on demand charge for your highest kw hour used, between 4:00pm-7:00, for 1 day during the month,  in which they charge over $18 per kilowatt.  I want to be able to store access power in a battery and use it when I need it during those hours.  Inverter like Sol-ark are way too expensive and other inverter can do what I need but they do not use Current Transformer on L1 and L2, but you have to remove L1 and L2 and connect to the inverter and then your load to the inverter.  My point is that if you loose your inverter you lost power.  With Current Transformers, you lose the inverter you still have power.  AC Coupling is what I need but I need a way of not spending $2K for the inverter.  Any help will be greatly appreciated.

[dnix71]

It might be easier to install a timer on whatever load causes trouble during those hours. $18/kw for peak demand use doesn't sound all that high for residential 2. The smallest customers here in south Florida have the demand charge built-in, but you can sign up for load shedding devices that will dump your hot water and a/c if you want to. Otherwise the demand charge is 1/2 your bill if you aren't careful.
One of the churches in Fort Lauderdale installed gensets to run a/c on Sunday morning because it was cheaper to operate than to pay the peak charge for using all that a/c one day a week. The a/c never saw the grid.
I had a co-worker who got whole house solar with batteries, but still had to install a nat gas genset to have a/c after a hurricane. That kind of demand is beyond a battery backup.

[Yianie123.]

Thank you for your reply.  I was hoping for a battery charger inverter type device that can be programmed to charge during the peak solar hours and discharge during the on demand time with my utility company of 4:00pm-7:00pm.  It would be a plug and play system just using an outlet on the wall and the size of a computer.  There is a European type like this called balcony solar, where they add solar panels on your balcony, that connect to battery unit and plugs into the wall.  It charges the batteries and discharges when power is required.  I like simple.  Has anyone heard of something similair?  I guess I can try to piece it together, but their is risk and I would rather purchase a unit.  All input is welcomed.

[joestue]

Cheapest way i think you can deal with this is to wire your solar 240v micro inverters for two sets of three phases, and connect them directly up to a sine wave filter and a cheap 5hp VFD. don't worry about blowing the vfd up, the solar inverters will only produce power after the vfd turns on and produces 57-63hz and 200-265vac.. (or whatever the limits are)

Upgrade the 5hp to a 10 later, after the system works and it doesn't blow up.

Run your utility AC directly into the vfd inputs as well. a brake resistor is needed, able to handle the full rated wattage of the solar panels production. so initially, only hook up half the panels. or a third, or a sixth. it doesn't much matter.

if/when the solar produces enough power to overvolt the vfd, the vfd will shut down, and the solar inverters will shut down. nothing will be harmed.

so anyhow, you now have multiple options for getting power out of this system.

you can load the 400 volt dc bus (which will vary between 340volts dc when the power demand exceeds the solar, and the utility starts providing power)

or it will climb as high as 450 volts, the upper limit of what the vfd will turn the brake resistor on. the voltage at the 3 phase output will stay the same at the programmed volts and hz.

some vfds are better than others, there will be some variation in that 240vac but not much. maybe 235-245 depending on the voltage of the dc bus.


so, anyhow you can load the vfd from the dc bus at 338-450volts dc, which isn't really what you want to do, or you can load the 3 phases from the solar panels.


as long as the total current flowing through the three phases is less than the vfd ratings, you can have a single phase load of 30 amps on two phases, while 10 amps flows into all three. the vfd only has to handle the reactive current and the difference between what the solar produces and your load consumes.


so anyhow, what you then do is setup a bank of transfer relays to move your loads from the VFD phases to the utility phases.

the VFD will not be synchronized to the ac mains, so you have to have a double set of relays that can't accidentally connect the two systems together.


but basically, this will only cost you 250$ for a VFD, probably 100$ worth of copper if you do an air coil inductor for the LC filters.. 20$ worth of capacitors to clean up the vfd output.. and you'll need a 5KW 400 volt rated resistor.. furnace heating elements are ok for that.


and then there is the cost of the transfer panels.

a 12 circuit non automatic generator transfer switch could be installed by an electrician..

you then add 12 solenoids to drive the switches from a current sense transformer and some other data inputs to determine which circuits to transfer over to the solar, or not.

[joestue]

since no one here wants to run 350 volts worth of  batteries, you then need a bi-directional fixed voltage ratio buck converter.

the ratio can be set such that the point at which the brake resistor on the vfd turns on, the batteries are nearly fully charged.

[dnix71]

https://robinsun.com/pages/balcony-kits There were similar units sold in the Netherlands years ago. Hang them up and plug them in the to outlets inside. No guarantee the
solar will peak when you want it to. For that you must have batteries and a time programmable inverter.

[joestue]

So another serious method.. and potentially cheaper and safer..

is to do what i suggest with a vfd and sine wave filter.

but instead of dealing with the 400 volt dc bus of the vfd, you just wind a 3 phase 240vac to 40vac transformer, and charge the batteries from the ac output voltage of the vfd.

the way to control this, is to use the current flowing into the brake resistor as the trigger to change the taps on the transformers to charge the batteries at different rates or voltages.

now the question of how to get the power out of the battery...

and that's where you just run a lot of your loads off  of relatively cheap 48 volt sine wave output inverters, on a separate AC network from the VFD and the solar micro inverters. 


the transformers just have to be wound to the right ratio so that you won't ever use the grid (338 volts on the dc bus) to charge the battery. a small auto transformer can be used to decrease the grid voltage to ensure this never happens. -remember, the grid is only needed to powerup the vfd at startup, to provide an initial stable sine wave to get the grid micro inverters happy. once they start producing, the dc link voltage on the vfd rises, the ac voltage rises slightly, power flows into the brake resistor, and the power flowing from the grid into the vfd drops to zero.


there is zero risk of backfeeding the grid because the front end of the vfd is a dumb 3 phase rectifier.