Hi all,

I now have some 3 Years of (positive) experience with my Off-grid system. It's straight forward. However I wonder and am eager to learn how a Grid-tied inverter handles load shedding - a) during sunshine and exporting, b) during the night. How is battery SOC maintained? What if battery runs low?

Grateful for any lesson.

Beat

Edited September 10, 20232 yr by Beat

 

Hi all,

I now have some 3 Years of (positive) experience with my Off-grid system. It's straight forward. However I wonder and am eager to learn how a Grid-tied inverter handles load shedding - a) during sunshine, b) during the night. How is battery SOC maintained? What if battery runs low?

Grateful for any lesson.

Beat

This depends on the definition of "grid tied". The definition I know is that a grid-tied system always is connected to the grid, but can use PV to supplement the grid. Such a system will not produce any power when the grid goes down, no matter how sunny the weather is.

You mention SOC, so you're talking about a system that has batteries, a HYBRID system.

So, when the sun is up, that sort of system will try to service the load from PV. If there is PV over and SOC < 100% then the excess is used to charge the batteries. If you manage things correctly, you have a near full battery when the sun goes down, and then you have to manage things so that the battery gets you through the night. If you have 5kWh of battery that's challenging, if you have 10 kWh then that's much easier.

Battery running low? Well most hybrid inverters have a way of managing this. On mine (Goodwe) there is a simple rule amongst all the setup options. You can specify the on-grid minimum SOC and the off-grid minimum SOC.  Mine are set to 40 and 90. That means that when there is grid, my battery cannot discharge past 40% remaining SOC* If there is no grid it will discharge to 90% (10% remaining) and then turn off. Since the BMS is going to turn off at that point anyway, it makes no practical difference,

Here's my system Friday this week

Green line is SOC. The graph starts at midnight. You can see SOC slowly dropping (yellow line  is load). Then about 6:30 load kicks up (heatpump) and SOC drops faster. But it doesn't get lower than 54% remaining. Load starts increasing as the PV (blue line) comes up (I think this is the solar side of the inverter turning on and taking some power). Just before 10:30 you will see some spikes - that's kettle and dishwasher, but already SOC is climbing. 10:30 the pool pump kicks in. Peaks after that are 2nd heat cycle of the dishwasher blah blah it dosn't matter what it is, System services the load first from solar, 2nd from battery, but overall the SOC keeps climbing. From about 13:00 SOC is 100 and the system now draws enough PV to serve the load. From 16:15ish the battery starts discharging again, and we start another lap around the mulberry bush.

Hybrid systems physically disconnect from the grid when grid is down, but will run off of PV and/or battery as long as one of the other is available

Orange line is grid - down is buying from the grid, up is sending to the grid (doesn't happen very much)
Purple line is battery - down is charging, up is discharging.

10kWh battery, and we do all the water heating, run all the appliances, do all the pool pumping during the day light hours. 

The trick with this sort of system is to do as much electrical work as possible during the day.

 

* when I say "when grid is up", that really means "inverter feels like talking to grid". My inverter disconnects from the grid if voltage drops much below 180V. which became a suprisingly common thing in my neck of the woods.

Edited September 10, 20232 yr by Bobster.
gramma and spelllin and sterretjie and formatting. Eish

Hi @Beatdesigning systems and installing them has made my curiosity of the subject increase for obvious reasons. I came from your world of Off grid inverters from Voltronics & have also had a good education from guys like @BritishRacingGreenwho have exposed the gutts of these machines & actually the OG7.2, OG10 & all the high voltage Solar charge Controller Inverters in the Voltronics range use Grid tie technology to be able to blend power sources even though they are not able to export power.

To answer your question one must understand what it is to Grid tie. It's a machine that Sychnonises with the Grid in terms of voltage, frequency and waveform. They therefore "Tie to the grid" or attach themselves to the grid. They do this In order to export power to the grid. They do not have traditional Ac input & AC output ports, it does all it's work with one Grid or AC port. That Grid port is By-Directional because it can draw power as well as export power from that port more especially in the case of a Hybrid like the Solis Hybrid range & the Sunsynks/ Deye inverters. 

A lot more technical than my short summary. In scenario a) exporting during sunshine -- the machine will recognize that the grid has disappeared & will for a very short period continue the export but then eventually stops dead in it's tracks. That period is short it must Island or stop export & this is a design requirement & this is especially tested in labs for grid certification. You cannot have a system pushing power into the Grid when the grid is down. If it does continue to push power back then it's failed it's certification & won't get a pass. 

This stopping of export is very similar to a full battery scenario & zero load on an Off grid machine with a lot of solar panels attached on a sunny day. The PV Array ramps down as there is nowhere for the energy to go. 

b) at night the same happens except there is no PV. Some systems like the Hybrids can export battery power & the same fundamentals are tested. 

In all these scenarios the Grid tied machine or Hybrid relies on the CT coil as well as sensors at the Grid port to tell it that the Grid is present. Why a CT coil is especially important is that it is just a Glorified AC clamp like a Multimeter telling the Grid tie machine or Hybrid how much energy is being exported. So with a Grid tie or Hybrid you can export or dump energy into the Grid without a CT but then you have no control over how much energy is being exported. The tap is either fully open or fully closed. 

That is why a CT Coil or Meter is fundamental if you want to restrict yourself to powering non-essentials on a Sunsynk with zero export. The CT is metering the traffic & flow of the amount of export of electricity. 

This is a very watered down explanation no doubt a lot more to it but in Layman's terms this is what the various aspects involve. 

Thank you both for the interesting lessons.

However there are more questions in my mind:

How are the batteries managed? Does battery charging have priority over export? And: If PV input is insufficient for the actual load, is there a mix of grid and inverter power? How do you manage to maximize the use of solar power stored in the batteries and minimize power from grid?

And still another question:

With the hypothetical assumption that in a given grid section there are as many grid tied hybrid inverter that they can support all the load in case of shedding. With more and more such installations going on line we are going into that. The voltage would not drop, the inverters would not even recognize the grid down. For the power providers this becomes a problem. If they need to cut a line for maintenance, repair or extension work, they could not even kill it.

Edited September 10, 20232 yr by Beat

One supplier maintains that grid-tied refers to inverters that can export power.  All other ones are labelled "off-grid".  Blending is a feature of certain inverters

The presence of the Grid is always monitored. So this is not possible to take place where the Hybrids don't island. One must also remember that the Grid has a requirement to push GW of energy. Unless every household had a Hybrid it would be hard to keep the Grid tie machine on. However, you touch on a very important aspect called AC coupling. Where you take a Grid tied inverter and connect it to the output AC of a suitable inverter. That Grid tie inverter is looking for an AC power source & a suitable machine can provide just that. Under such a. Circumstance you then have a Micro Grid. 

 

The voltage would not drop, the inverters would not even recognize the grid down. For the power providers this becomes a problem. If they need to cut a line for maintenance, repair or extension work, they could not even kill it.

That is why the systems must island. They don't know why the grid is down, they just know it is. So they don't allow any voltage to pass, but they do monitor the grid so they can resume interacting when the grid is restored. This is why COCT have a list of approved inverters and require details of the install. 

Must installers will do the job conscientiously, of course, and the functionality is built into the inverter 

 

One supplier maintains that grid-tied refers to inverters that can export power.  All other ones are labelled "off-grid".  Blending is a feature of certain inverters

This is what I was getting at earlier, there is no agreed definition of "grid-tied". To me it is a system that cannot work without the grid. Thinking about it, the inverter needs some voltage and phase that it must match. "Off-grid" as I understand it, means that there's no grid connection necessary, but the system has some other reference, perhaps a generator, or perhaps the inverted output from a battery.

Anyway, experts (which I'm not) can't agree.

 

That is why the systems must island. They don't know why the grid is down, they just know it is.

That's the point. How can it know it? Only by the presence of suitable voltage. If that is maintained by sufficient other inverters it wont drop, hence it will not know the grid is down. We would have a mini grid.

 

That's the point. How can it know it? Only by the presence of suitable voltage. If that is maintained by sufficient other inverters it wont drop, hence it will not know the grid is down. We would have a mini grid.

I think that's right, but at some point there will be a connection a grid or some other AC power source, otherwise what are you going to do at night? To get true independence you have to have some other source of power. Really you can't go off-grid, you can just choose another grid.

 

One supplier maintains that grid-tied refers to inverters that can export power.  All other ones are labelled "off-grid".  Blending is a feature of certain inverters

That is also my understanding. Basically Off-grid inverters are hardware wise UPS with solar battery charger added and a programmable operating mode.

 

I think that's right, but at some point there will be a connection a grid or some other AC power source, otherwise what are you going to do at night? To get true independence you have to have some other source of power. Really you can't go off-grid, you can just choose another grid.

I'm afraid there is some misunderstanding. I'm not saying that this situation is desirable. I'm only giving some thought of what might happen when the grid goes down while sufficient inverters are exporting to support the actual load.

I understood that there are limits to the amount of grid-tied solar that can be linked to one shared transformer, something in the order of 25% of the total demand. I'd want to see the numbers of what the real offtake is of grid-tied solar relative to the grid capacity, nationally and in wealthier suburbs, but I don't expect it's that much. I mean, it's one thing to say that everyone and his granny is buying a grid-tied capable hybrid inverter for backup, and each such inverter will take its signal from the grid. It's something different to say that all those users have installed bi-directional meters and are in fact selling power back to Eskom. My gut feeling is that most folks are sticking with their pre-paid meters and limiting their grid export. Again, just my 2c worth, and I'd be interested to know from anyone in the know what's really happening out there.

Edited September 11, 20232 yr by GreenFields

 

I understood that there are limits to the amount of grid-tied solar that can be linked to one shared transformer, something in the order of 25% of the total demand.

Interesting point. Fact is that with the actual and prospective Eskom situation there is a strong tendency of more and more people want to go solar and become independent. The whole town of Mosselbay Municipality is going that way. But here comes the hick: If a power provider must limit the grid-tied inverters, the City of Cape Town gets into trouble. If they ban off-grid inverters as announced last week, and do limit grid-tied inverters that would mean no additional solar system allowed in a given grid section. What an awkward politics!

Edited September 11, 20232 yr by Beat

 

I understood that there are limits to the amount of grid-tied solar that can be linked to one shared transformer, something in the order of 25% of the total demand. I'd want to see the numbers of what the real offtake is of grid-tied solar relative to the grid capacity, nationally and in wealthier suburbs, but I don't expect it's that much. I mean, it's one thing to say that everyone and his granny is buying a grid-tied capable hybrid inverter for backup, and each such inverter will take its signal from the grid. It's something different to say that all those users have installed bi-directional meters and are in fact selling power back to Eskom. My gut feeling is that most folks are sticking with their pre-paid meters and limiting their grid export. Again, just my 2c worth, and I'd be interested to know from anyone in the know what's really happening out there.

There's also the question of what would happen in the situation @Beatis considering, and IF the inverters didn't shut down. Mine wouldn't, if it had battery power left, but it's set to not export. OK... I can change that, but with no grid on the incoming side it still won't export. This is a requirement because, in most territitories, if the grid goes down you can assume that somebody is working on it or soon will be, so you don't want anything putting power on those wires.

But let's say that this situation happens. Now my home is putting out power. Let's assume I don't cut it off (why would I do that? To ensure that my home keeps on going as long as possible), how far is that power going to get? My next door neighbour, who will have demand in his house? The end of the street? In the nearest (circuit wise) sub station and out again? OK... what if the other houses in my street with solar are busy exporting? There's probably not enough juice to keep the whole street running, and one by one (or maybe all at the same time) those inverters will trip because they are being overloaded. And there's there's three phases in my street, OK... only two used by houses, one is kept for street lights. So is it the case that my spare power can only get to houses on the same phase as me? It's hard to visualise what would happen - OK, hard for me - and I suspect this all suggests that there's more to setting up a grid than we might think.

Solar is actually making a difference, there's enough of it now. But the difference it makes is that it reduces load on Eskom and so reduces load shedding by maybe one stage. A lot of systems (shopping centers for example) are grid-tied to save money. Most people I know with batteries and hybrid inverters that CAN export, have done the sums and can't see how it's worth their while to export. I've said it before and I'll say it again - I would give my spare power away and want no payment for it, but I also don't want it to cost me anything, and that's not possible right now.

A simple 220 volt double pole NO/NC relay, battery charger would work just as well as a simple inverter.  Add a Solar charge controller and you are set 

Obviously not as easy as that