2 x SPA3000 chargers? Anyone tried?

[mike_hug]

Hi

I got sent an SPA3000 from Growatt for a warranty swap on an old SP2000. Nice

Wired it to 2 x GBLI5001 batteries with third party BMSs. Works like a dream. Most of the day I am sitting at 0W on my smart meter

Perhaps stupidly I bought another SPA3000 as I have another 2 GBL5001 batteries here

Wired that all up and seemed all good, but...

I was watching them both today, and one is discharging the battery and the other is charging! This is when the panels are not able to satisfy the house demand, and the SPA should discharge. The house CT is reporting *roughly* the same on both SPAs but the display on both dances around +30, -5, +10, etc continuously

So it's as if they are fighting each other

Is what I've done a mistake, and I should just give up and have one SPA3000?

I was sitting thinking and thought perhaps I could relocate the house CT on the second SPA to the mains input on the first SPA, but with the clamp the wrong way round. My logic being that if the first SPA is using power (charging) then the house must be exporting (so the reverse of the normal usage of the CT - hence fitting it the other way round). If the SPA is discharging then the house must be importing. Therefore if the first SPA is charging then the second SPA could also charge, and if first SPA is discharging then the second one could too. Then they wouldn't fight. Or am I mad?

Is there another way to make them play nicely?

 

[SeaGal]

Not sure about Growatt, but IMHO with my Solis, the internal feedback loop will do whatever is possible to get the grid CT value down to zero. So, unless the firmware is designed for parallel operation they will always fight each other.

The fix, if the Growatts use an RS485-based power meter, rather than a raw CT, would be to implement a MITM solution to adjust the power meter data (between CT and inverters) and control the interaction between the two inverters yourself.

 

[mike_hug]

Not sure about Growatt, but IMHO with my Solis, the internal feedback loop will do whatever is possible to get the grid CT value down to zero. So, unless the firmware is designed for parallel operation they will always fight each other.

The fix, if the Growatts use an RS485-based power meter, rather than a raw CT, would be to implement a MITM solution to adjust the power meter data (between CT and inverters) and control the interaction between the two inverters yourself.

That could be possible

But what do you think about my idea of putting the 2nd CT clamp onto the mains feed of the 1st SPA3000? I can't see why that wouldn't work...

 

[SeaGal]

I can't see why that wouldn't work...

I can't see how that would work

If the SPA is discharging then the house must be importing. Therefore if the first SPA is charging then the second SPA could also charge, and if first SPA is discharging then the second one could too. Then they wouldn't fight.

So, let's assume the house usage is 2000W and your panels are producing 1000W.

The first SPA will see from the grid CT an import of 1000W. It will start discharging at 1000W to get the grid CT to zero Watts. As it starts discharging, the second SPA will see from the CT on the AC connection of the first SPA a discharge of 1000W, which it will interpret as a charge of 1000W due to the CT reversal. As you say, this will cause the second SPA to discharge as well, attempting to get it's CT reading to zero.

But as soon as the second SPA starts discharging at 1000W, there will be an export on the grid CT of 1000W, so the first SPA will think it needs to charge at 1000W, which it will start to do. The second SPA will then see a charge of 1000W going to the first SPA, which it will think is an export, so it will then start charging at 1000W. At this point both SPA's will be trying to charge at 1000W each, so the grid import will now be 3000W.

As far as I can see, that would result it an extreme un-dampened oscillation between importing and exporting at a frequency determined by how fast the inverters switch between charge and discharge - my guess is around once every 10 to 30s.

 

[mike_hug]

I can't see how that would work


So, let's assume the house usage is 2000W and your panels are producing 1000W.

The first SPA will see from the grid CT an import of 1000W. It will start discharging at 1000W to get the grid CT to zero Watts. As it starts discharging, the second SPA will see from the CT on the AC connection of the first SPA a discharge of 1000W, which it will interpret as a charge of 1000W due to the CT reversal. As you say, this will cause the second SPA to discharge as well, attempting to get it's CT reading to zero.

But as soon as the second SPA starts discharging at 1000W, there will be an export on the grid CT of 1000W, so the first SPA will think it needs to charge at 1000W, which it will start to do. The second SPA will then see a charge of 1000W going to the first SPA, which it will think is an export, so it will then start charging at 1000W. At this point both SPA's will be trying to charge at 1000W each, so the grid import will now be 3000W.

As far as I can see, that would result it an extreme un-dampened oscillation between importing and exporting at a frequency determined by how fast the inverters switch between charge and discharge - my guess is around once every 10 to 30s.

Thank you. After sleeping on it, I agree

Growatt have said they can't be paralled :-(

They play nicely during the day. I'm sat at 0w from about 8:30 until 21:00. But I'm losing battery power to each other as they swap between charging and discharging from each other in the evening

Perhaps the way forward is to put a Contacter Relay on the 2nd SPA, externally controlled, so it is switched out of circuit before sundown, and only switched back on later when the 1st one is exhausted?

 

[SeaGal]

I just had another thought... Is the CT sensitivity configurable at all? I know you can double the output of a CT by looping the wire through it twice, but am wondering if it is possible to halve the sensitivity?

If so, then maybe (and this is a quick guess, haven't had time to think it through in much detail)... you could have both CT clamps on the grid supply, but effectively reporting only half the current import and export to each SPA. Then each SPA would presumably get its own CT reading to zero by supplying half the power needed to prevent import or, conversely, charge at half the rate each when you have excess output.


Regarding...

Perhaps the way forward is to put a Contacter Relay on the 2nd SPA, externally controlled, so it is switched out of circuit before sundown, and only switched back on later when the 1st one is exhausted?

... that could work too, but you'd only have half your battery power in the evening which is, presumably, when you need it most. Personally I'd implement such a switch off of one inverter using its data comms (assuming it has such control) rather than a big switch - but then, if you're doing that you could implement a MITM solutions as I mentioned earlier.

 

[mike_hug]

I just had another thought... Is the CT sensitivity configurable at all? I know you can double the output of a CT by looping the wire through it twice, but am wondering if it is possible to halve the sensitivity?

If so, then maybe (and this is a quick guess, haven't had time to think it through in much detail)... you could have both CT clamps on the grid supply, but effectively reporting only half the current import and export to each SPA. Then each SPA would presumably get its own CT reading to zero by supplying half the power needed to prevent import or, conversely, charge at half the rate each when you have excess output.


Regarding...

... that could work too, but you'd only have half your battery power in the evening which is, presumably, when you need it most. Personally I'd implement such a switch off of one inverter using its data comms (assuming it has such control) rather than a big switch - but then, if you're doing that you could implement a MITM solutions as I mentioned earlier.

Thanks

Google gave me this:
You can unclamp the CT and by trial and error (thickness of material) insert a piece of paper between the jaws where the CT meets, this has the effect of reducing the CT coupling efficiency and reduces its output.

Try a small piece of 80gram paper first and see what the results are then perhaps some cigarette paper which is very thin, the technique will make a difference.

 

[SeaGal]

Hmmm... maybe assuming they don't get too hot

Wondering if a resistor in series with the CT could have similar effect?

 

[mike_hug]

I tried a piece of photocopy paper. That INCREASED the reading on the CT. It was around +20w, with a single piece of paper it jumped to +300. To ensure no madness had crept in I added a second piece of paper and it jumped to +600.

My head is now confused and I don't know whether I'm coming or going!

Interestingly I turned one of the units off, and then the other discharged it's battery. But then started to charge from the grid. This is not behaviour I was expecting. I have it set to Bat First 00:30 - 04:29, and yes the clock is spot-on

Can you enlarge upon the Man in the Middle solution you mention?