Running SMA sunnyboy SB4000TL-20 without feeding grid

technoware · August 2015

Hello,

I have this inverter, but my country does not allow to feed into the grid at the moment. This inverter gives the option to turn the inverter into off-grid mode. I want to design the system in a way that during the day time, most of the current is utilised from the grid tie inverter and any extra can be taken from the grid. For example: If the output during sunny day for my inverter is 3500 Watts 220V AC, whereas the appliances I want to run are 2500 Watts. Now in this case, I don't know where the remaining current will go. Other scenario is when the output of the inverter stays the same, i.e 3500 Watts 220V AC, but the consumption is 5000 Watts 220V AC. So in this case, I want the appliances to take remaining 1500Watts from the grid. Currently, I have a UPS which is connected to a big battery. The maximum output of the UPS with this battery is 1000 Watts AC for 2 hours approx.
I have no idea how to design the system, so that I can utilise the power of Sun and this lovely inverter. I want to avoid using battery arrays, as this is not an efficient way and also the batteries have very limited life and higher costs. During my google search, I noticed someone recommended a circuit to someone trying to use a grid tie inverter in a similar situation.
Any suggestions are highly appreciated.

Thanks and regards

inetdog · August 2015

I wonder if the option to operate in off-grid mode is basically the same as the Secure Power Supply (SPS) feature available on the TL-US models?
If so, it cannot be integrated with grid power for load sharing in the way you describe, and in addition it cannot deliver anywhere near the full power output rating of the inverter on that SPS circuit.
In the US, this optional extra output is limited to 120V and 1500W regardless of the amount of solar PV input available.

It is theoretically possible for a grid tie inverter to operate in full grid tied (load sharing) mode with hardware and circuitry enabled that measure the power transfer to and from the utility and throttle the inverter back from full output as the power drawn from the grid approaches zero, but I have not seen the details of any inverter that actually does that.

MarkC · August 2015

I'd guess that the SMA US models are the only ones with SPS - since it provides 120 volt single phase only.

I've been "machinating" about using my SB4000TLUS in a pure off grid mode by direct tie into the output of my off grid generator/inverter system - which is a Prius/APC SURT6000/SURT003 iso transformer (240 volt split phase). I believe some type of load shedding would be required to ensure the total load always exceeds the solar production (don't back-feed the UPS). Charging a relatively large battery bank as example with excess power. I would guess that the quality of the power output of the APC UPSs would have no problem "fooling" the SMA SB, but would the two systems go unstable and interact destructively with each other? If the total solar production was quite small compared to the total power usage - could be a relatively safe try and then increment up to larger fractions of solar. Would the full isolation transformer on the UPS output effectively protect it? Is the Sunny Boy protected by it's internal shutdowns against various grid disturbances?

Of course - just buy the Outback Radian!!

As much as I'd like to try - bottom line is too much risk to my equipment.

technoware · August 2015

Yes,

SMA inverter has grid protection. The tolerance can be adjusted via sunny explorer, i.e limit the voltage and frequency of the AC. But the UPS is already adjusted to 50hz and 230Volts AC. This off-grid mode is not the same as in Sunnyboy US model, since US models have separate dedicated AC output in case of grid outage for certain time. SMA have recommended the off-grid mode in conjunction with their Sunny island inverter, which is then connected to batteries. But that is an expensive deal and with batteries there are losses. My main concern is that although there is a cut-out at the battery end of my UPS, which disconnects the charging when the battery gets full charge, how will the inverter respond in that scenario. Will it turn off when the power output is not fully consumed or will it stay on and go in idle state? Or in worst case, it will destroy the UPS/Battery and appliances?
Thanks

inetdog · August 2015

technoware wrote: »

Yes,

SMA inverter has grid protection. The tolerance can be adjusted via sunny explorer, i.e limit the voltage and frequency of the AC. But the UPS is already adjusted to 50hz and 230Volts AC. This off-grid mode is not the same as in Sunnyboy US model, since US models have separate dedicated AC output in case of grid outage for certain time. SMA have recommended the off-grid mode in conjunction with their Sunny island inverter, which is then connected to batteries. But that is an expensive deal and with batteries there are losses. My main concern is that although there is a cut-out at the battery end of my UPS, which disconnects the charging when the battery gets full charge, how will the inverter respond in that scenario. Will it turn off when the power output is not fully consumed or will it stay on and go in idle state? Or in worst case, it will destroy the UPS/Battery and appliances?
Thanks

When the SB is combined with a Sunny Island (SI) into a hybrid system, the SI will signal the SB when the SI output approaches zero or is about to go negative. The SI increases the frequency of its output and the SB responds by throttling back its output.
If you do not have the SI or a compatible signalling AC reference source in your system the SB will continue to produce full power until whatever reference inverter you use shuts down completely or its voltage rises above the upper tolerance limit of the SB.
I would not consider any such system safe unless SMA or others had specifically tested and approved it.

MarkC · August 2015

I actually discussed the concept of using a networking type, double conversion UPS (very clean, spot-on outputs) in conjunction with the SMA "US" models to build a "hybrid" system (conceptually AC coupled?). The response was it likely would work, but not recommended - no surprise. The advantage of the "Prius" generator is that if automated so that the SMA does not "continue to produce full power" scenario, the Prius will turn off 90%+ (maybe 100%) of the time during adequate solar production if augmented with enough (Prius) traction battery power to handle the cloud passings, etc. Again all in theory, but seems for those that want substantial off-grid power only a few times a year, (and own a Prius!), could be a worthwhile system design goal.

Note that although the UPS is double conversion, the DC/AC is only section used.

Again, not really interested in "experimenting" without some level of actual testing by a knowledgeable electrical systems designer!

technoware · September 2015

Net Metering has now been allowed in my country, but there is a mandatory requirement of Third party injury insurance, also called Indemnity insurance. Does anyone know about this? i.e cost per month etc and is it variable depending on the size of solar system?

Thanks

BB. · September 2015

For my country/state--All we had to have was our normal home owner's insurance.

Since everything was installed to code and inspected--As I understand--The system is just part of the electrical system of the home and no extra liability costs were incurred by us. And the utility was satisfied by having a copy of our home owner's policy with the initial utility paperwork (application). They have not ever asked again for evidence of insurance (something like 12+ years now).

-Bill

Running SMA sunnyboy SB4000TL-20 without feeding grid

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