PV voltage and more universal controller options?

jaggedben

Senior Member
As far as I know, the gateway supplied with the PW (I don't see a brand on it but I think it's a Tesla manufacture) does the same thing regarding shifting the Hz of the PV array up to lower output if (a) the battery is charged and (b) there's more solar available than needed. My computer's UPS monitors powerline Hz and it's easy to see the shift upwards to 61-62 Hz around 1:30pm when when the battery is charged and the sun is out. PV output will lower and raise (if available) to meet just the house demand. Around sunset, when there's no PV juice available, the Hz settles back to 60 as the battery takes over.
I was going to say that the difference is that what you're describing with Powerwall is only an 'on-off' control of the solar. However then I remembered that Enphase IQ micro-inverters do have the capability to throttle power with frequency shifting. That is documented here: https://enphase.com/sites/default/files/downloads/support/Design-Considerations-AC-Coupling-Micros-Battery.pdf

However this solution won't work too well for our OP if he wants to run DC from the array to mitigate serious voltage drop considerations.

The (legacy?) SMA feature is also full range 0-100% throttling of PV inverter output based on the battery inverter adjusting the frequency between 60Hz and 50Hz. (Note that there isn't an standard here, and these features aren't enabled by default on interactive solar inverters; one had better make sure that AC coupled equipment is all programmed appropriately to create compatibility.)

(BTW with Powerwall it is the battery units themselves that shift the frequency (it has to happen at the source), although I would wager there is some necessary communication required with the gateway involved as well. The gateway you have is Tesla manufactured and works only with Powerwall.)
 

ggunn

PE (Electrical), NABCEP certified
As far as I know, the gateway supplied with the PW (I don't see a brand on it but I think it's a Tesla manufacture) does the same thing regarding shifting the Hz of the PV array up to lower output if (a) the battery is charged and (b) there's more solar available than needed. My computer's UPS monitors powerline Hz and it's easy to see the shift upwards to 61-62 Hz around 1:30pm when when the battery is charged and the sun is out. PV output will lower and raise (if available) to meet just the house demand. Around sunset, when there's no PV juice available, the Hz settles back to 60 as the battery takes over.
The Tesla gateway does (as far as I know) do that with the frequency, but most inverters will just run flat out until the frequency hits their operational limit and then shut down. Some SMA inverters when programmed to do it will throttle back their output as the frequency shifts but they are the only ones I know of that have that capability.

Edit: Apparently the Enphase IQ series can do it as well; see above.
 

PWDickerson

Senior Member
SMA pioneered frequency shifting as a means of throttling back the Sunny Boy inverters to prevent over charging of the battery bank. Several other inverters are employing this feature as well. Keep in mind that the Sunny Island is a 120V inverter, so you have to install them in pairs, and it is best to install them with an auto-transformer to use them at their full capacity. The larger one is rated at 6 kW, so you will need a minimum of 4 inverters to meet the 15 kW target. I have done a system like that, and there ends up being a lot of gear on the wall!

AC coupling simply means that the solar is connected to the AC side of the battery inverter through a simple grid-tie inverter. When you do that, the PV inverter will push an unregulated charge backward through the battery inverter to charge the battery bank. The SMA system uses frequency shifting (increasing the battery inverter output frequency slightly) to provide a signal to the GTI so that it will throttle its output and essentially act as a charge controller to keep the batteries happy. It's pretty slick.

The Sunny Island inverter only has a single AC input, which is a drawback in a grid-tied system, but should be ok in an off grid system.

The drawback of an off-grid AC coupled system is that if something goes wrong, and the the battery voltage drops too low, most battery inverters will simply shut down... and then the PV inverter goes offline, so there is no way to recharge the battery without an external charger.

You should check out the Sol-Ark inverter to see if it is a good fit for the project. Is supports 500V strings, has a built-in charge controller, and it supports frequency shifting for AC coupled applications. Installing two in parallel might be a good option. The downside is they don't have the long history of SMA. The SMA Sunny Island/Sunnyboy systems we have installed work really nicely, though most are grid-tied.

I would like to second Electrofelen's question about the cost of bringing in the grid. If it is in the $100k range, it would probably be worth it. One of our off-grid client's recently hired us to rip it all out and bring in the grid because she is tired of the "small life" she has to live with such a system.
 

GoldDigger

Moderator
Staff member
It's in the Sunny Island manual. When the PV output approaches the demand of the load and the batteries are full, the SI will start to increase or reduce (it alternates between occurrences) the AC frequency and in response Sunny Boy inverters throttle back their output as the change increases to the point where they will shut down completely when the delta f reaches a minimum or maximum (I don't remember what it is). You have to get into the parameters of the SB to turn on this feature.
It alternates to try to average out line driven electric clocks to near the correct time?
 

tortuga

Senior Member
Our battery bank voltage will have to be configured to play nice with the inverters, and that right now is being proposed at 48V.

I am curious if you guys are running at higher voltages?
I second the 'DC - AC - DC' coupled system with the sunny islands for everything you described.

The title of the thread and the fact that everyone on here seems to run away from off grid as fast as they can, leads me to weigh in.
For a 1-27 kw (225A @ 120V) off grid simple system where distances can be kept to a minimum I suggest considering 120V DC nominal system voltage.
120 VDC was a common utility voltage for the first 60 years of the code, and some things made for 120 AC can run just fine on 120 DC.
Many UL listings for 125V are still AC/DC for this reason, plugs, sockets, motors, cooking equipment, tools with universal motors.
Also 120 DC is a standard military voltage, so it is really easy to get 120 DC breakers and panel-boards.
There is the bizzare limitation of 100 volts between conductors in article 706 for systems installed 'for' dwelling units, wich would require a work around or a code change.
 
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