Pleasantly unexpected function of Growatt 12kW LF inverter in conjunction with a grid tie.

[Roger Harding]

I have had a 3.15kW Solaredge grid tie system since April 2019. I recently installed a Growatt 12kw DVM MPV low frequency inverter with a 20.5kWh LiFePO4 battery for emergency use. I had previously tested the system by turning off my main breaker and having the Growatt backfeed my main panel. I was charging my Tesla to burn off the power being generated by my grid tie inverter, as I had heard bad things about having the grid-tie producing more power than your consumption.

Fast forward to yesterday, I finished building an EVE 304ah 48v pack and added it to the system. I charged the entire system overnight using the inverter and a Signature Solar 25a(18a) battery charger. This morning after my girlfriend left for work I switched the house over to battery power and proceeded to cook some breakfast on the electric stove to see how the system handled it. Shortly after that I received a phone call, and I lost track of how much power my Solaredge grid-tie inverter was putting out. I opened the Solaredge app, and to my dismay I saw I was producing .57kW, which I knew was more than my house was using. I rushed to the basement to make sure the Growatt wasn't having a meltdown, and to my surprise, my Victron Smart shunt said there was positive power flowing through it. I opened the ShinePhone app, and where it normally said Discharging, it instead said Charging. That excess power was backfeeding through the Growatt and it was charging my battery bank. I had no idea that the system was capable of this, but I had suspected that having a low frequency transformer the system would handle this better than a high frequency inverter.

Did anyone else know that this was something that the Growatt LF inverters we're capable of? I am overjoyed at this discovery! ?

 

[Roger Harding]

Currently backfeeding over 1500W into the batteries. Yes, this is going in through the Growatt inverters output wires. I have the input breaker off both on the inverter and in my main panel.

 

[solar8484]

It's an inherent "feature" in LF inverters. However, it's not safe unless the inverter is specifically designed for backfeeding from AC coupled grid-tied inverters and has a way to curtail/stop the grid-tied inverter power input when the battery is nearly full to prevent overcharging and destruction.

 

[Roger Harding]

It's an inherent "feature" in LF inverters. However, it's not safe unless the inverter is specifically designed for backfeeding from AC coupled grid-tied inverters and has a way to curtail/stop the grid-tied inverter power input when the battery is nearly full to prevent overcharging and destruction.

I just emailed Growatt to ask if these support frequency shifting. In the mean time I will monitor everything closely, including the output frequency of the inverter to see for myself if it may have that function.

 

[RCinFLA]

Sounds like you have GT inverter output and AC input feed to Growatt from main panel and you switched off main grid breaker in panel.

When Growatt detects AC mains input it syncs to AC input, closes connect/pass-through relay and runs in parallel with grid. If your GT inverter is producing power and pushing it into your main panel and you switch off main breaker to grid connection the GT inverter will continue to run feeding its power into Growatt AC input. GT inverter and Growatt both think grid is still present but whatever the GT inverter is producing must be consumed by AC output loads and charging, no more, no less or the system will collapse.

The Growatt has no way to modulate its AC input loading or control pass-thru AC output loading to match GT inverter production. If it continued to run it was pure luck that loads approximately matched the GT inverter production at that moment in time. I don't think Growatt 12kW supports AC coupling capability.

One test you might want to do is see what Growatt does when you have grid AC input then switch off Growatt AC input breaker. You should hear a click of relay releasing from grid and not have power from inverter showing up on output side of breaker to Growatt. It may take a few seconds before Growatt realizes the grid is gone. If Growatt has poor anti-islanding function it may not recognize grid AC input has gone open. I would not be surprised if this was the case. Growatt will quickly release from grid if grid goes down as this looks like an overload to Growatt.

 

[Roger Harding]

Sounds like you have GT inverter output and AC input feed to Growatt from main panel and you switched off main grid breaker in panel.

When Growatt detects AC mains input it syncs to AC input, closes connect/pass-through relay and runs in parallel with grid. If your GT inverter is producing power and pushing it into your main panel and you switch off main breaker to grid connection the GT inverter will continue to run feeding its power into Growatt AC input. GT inverter and Growatt both think grid is still present but whatever the GT inverter is producing must be consumed by AC output loads and charging, no more, no less or the system will collapse.

The Growatt has no way to modulate its AC input loading or control pass-thru AC output loading to match GT inverter production. If it continued to run it was pure luck that loads approximately matched the GT inverter production at that moment in time. I don't think Growatt 12kW supports AC coupling capability.

One test you might want to do is see what Growatt does when you have grid AC input then switch off Growatt AC input breaker. You should hear a click of relay releasing from grid and not have power from inverter showing up on output side of breaker to Growatt. It may take a few seconds before Growatt realizes the grid is gone. If Growatt has poor anti-islanding function it may not recognize grid AC input has gone open. I would not be surprised if this was the case. Growatt will quickly release from grid if grid goes down as this looks like an overload to Growatt.

I do not have the Growatt input from the mains turned on at all. The input to the inverter, as well as the output from my inverter are both coming from the same panel, and cannot be on at the same time. All of the excess power from the grid tie is going into the Growatt through the Growatt output, which is then converting it back into DC and feeding it into the battery.

As an experiment, I turned the grid tied inverter off, waited half an hour and then turned it on. The grid tie took a couple of minutes to activate, but when it did the fans in my Growatt instantly turned on, and it began charging the battery bank with 2.9kW of power from the grid tie inverter. This seems to be working really well, but I am still monitoring it to see what happens as the battery approaches full (it is 96% right now). I have two EG4 LL server rack batteries in the bank that have BMS communication with the Growatt, so I hope that triggers the Growatt to frequency shift, turning off the grid tie inverter. This may not be a feature of the Growatt, so I am prepared to shut things down instantly if something doesn't seem right.

 

[SparkyJJO]

My concern would be when the batteries are full. Some inverters have no way of regulating charging when backfed, so they'll charge until the BMS trips, and then, well, who knows. Maybe my avatar situation occurs and the magic smoke is let out.

 

[Roger Harding]

My concern would be when the batteries are full. Some inverters have no way of regulating charging when backfed, so they'll charge until the BMS trips, and then, well, who knows. Maybe my avatar situation occurs and the magic smoke is let out.

Yeah, I am monitoring it very closely for that reason. If I see the packs much above 99% and nothing has changed I will start charging my car again.

 

[Solar Guy Richard]

There are definitely settings where you can choose how to charge batteries - AC/PV or just Solar, and in what order. You can also choose what order your inverter will power your house depending on your use. A lot of people in California etc charge batteries during 'cheap power' times and use batteries during 'expensive power' times. Definitely a built in function - if you have any questions let us know we are very versed with Growatt.

 

[Roger Harding]

There are definitely settings where you can choose how to charge batteries - AC/PV or just Solar, and in what order. You can also choose what order your inverter will power your house depending on your use. A lot of people in California etc charge batteries during 'cheap power' times and use batteries during 'expensive power' times. Definitely a built in function - if you have any questions let us know we are very versed with Growatt.

There are, but those settings don't apply in this case. The PV power is not going in through the PV input, and the AC power is not going in through the AC input. It is set to Utility first, but that probably only applies to the AC input terminals.

 

[Roger Harding]

Here is what the Growatt screen looks like, showing that it is not operating in PV or AC input.

 

[GXMnow]

You are flirting with a dangerous situation.

If the BMS does shut off, the Growatt inverter will lose it's load that it is using to control the output voltage. In an ideal situation, it "SHOULD" see the fault and shut down, and cause the grid tie solar to also shut down, but it would only take milliseconds for the DC buss voltage to spike and destroy many components in the Growatt inverter. You have over 2,000 watts charging batteries, and then that load disconnects... That's bad news.

You really need some other means of cutting the charge power well before the BMS has to do a protection shut off. Unless an inverter is truly designed and built for AC coupling, this is not safe.

 

[Solar Guy Richard]

Ah I misread your information then. Are you saying the AC Out is backfeeding the batteries?

 

[Roger Harding]

You are flirting with a dangerous situation.

If the BMS does shut of, the Growatt inverter will lose it's load that it is using to control the output voltage. In an ideal situation, it "SHOULD" see the fault and shut down, and cause the grid tie solar to also shut down, but it would only take milliseconds for the DC buss voltage to spike and destroy many components in the Growatt inverter. You have over 2,000 watts charging batteries, and then that load disconnects... That's bad news.

You really need some other means of cutting the charge power well before the BMS has to do a protection shut off. Unless an inverter is truly designed and built for AC coupling, this is not safe.

There are 5 battery banks, each with their own BMS, and only two of them are connected to the Growatt communication. What I may do is shut off the grid tie, power down the three banks that are not connected to the Growatt so that they are not quite as charged as the EG4 banks and let them get a few % out of balance, then start this test over. That way if the Growatt triggers the BMS in the EG4 packs because they hit full and it doesn't do some kind of phase shifting to shut down the grid tie there will still be somewhere for that power to go.

 

[Roger Harding]

Ah I misread your information then. Are you saying the AC Out is backfeeding the batteries?

Yes sir.

 

[Solar Guy Richard]

Yes sir.

Did you purchase the Growatt from us? I wouldn't use it in this capacity as it is definitely not designed to do this.

 

[Roger Harding]

Did you purchase the Growatt from us? I wouldn't use it in this capacity as it is definitely not designed to do this.

It may not be advertised as such, but it is not giving me any problems at all. I would have figured that it would have instantly given me a fault warning as soon as output started backfeeding. The ShinePhone app even shows when it switches from Discharging to Charging, so in some capacity it knows what is going on.

It is too sunny today, so I do have some concern about where that excess power is going. I may continue this experiment on a cloudy day, with the BMS communicating battery banks at a slightly higher state of charge than the rest. I could even set my JKBMS in the new 304ah pack to not charge, only discharge, so that I have a very quick way to send a lot of power somewhere.

 

[solar8484]

It may not be advertised as such, but it is not giving me any problems at all. I would have figured that it would have instantly given me a fault warning as soon as output started backfeeding. The ShinePhone app even shows when it switches from Discharging to Charging, so in some capacity it knows what is going on.

Regular inverters don't have backfeed protection. Monitoring app has nothing to do with inverter protection functions.

It is too sunny today, so I do have some concern about where that excess power is going. I may continue this experiment on a cloudy day, with the BMS communicating battery banks at a slightly higher state of charge than the rest. I could even set my JKBMS in the new 304ah pack to not charge, only discharge, so that I have a very quick way to send a lot of power somewhere.

I suggest you don't disable charging in your BMS. Charging is what's preventing your inverter from getting damaged. If you set the BMS to not allow charging then your inverter will just shutoff if you are lucky or let out the magic smoke from backfeeding.

 

[Roger Harding]

Regular inverters don't have backfeed protection. Monitoring app has nothing to do with inverter protection functions.



I suggest you don't disable charging in your BMS. Charging is what's preventing your inverter from getting damaged. If you set the BMS to not allow charging then your inverter will just shutoff if you are lucky or let out the magic smoke from backfeeding.

I am only going to disable it on 1 of the 5 battery banks, not all of them. That way I can instantly enable charging if the Growatt doesn't shut down the grid tie inverter, giving the power somewhere to go. It is a 200a BMS set to 150a charging on a 15.5kWh battery, so it will easily suck up every watt my grid tie can produce.

Either way, this system was never built for daily use, just emergency backup power, which is why the Growatt has no PV hooked to it. If this allows me to still use my grid tie when the grid is down, great. If not, I will hook a generator to the battery charger to extend my run time.

 

[solar8484]

If this allows me to still use my grid tie when the grid is down, great. If not, I will hook a generator to the battery charger to extend my run time.

If you are motivated you may still be able to make it work safely by adding a contactor to the output of your grid-tied inverter and DIY a controller to only turn on the contactor if you are 1) on grid or 2) off grid and your battery SOC (assuming your BMS provides such info over RS484 or similar interface) is less than some safe threshold (e.g. <80%). Similarly, when off grid the contactor should turn off whenever the battery SOC is nearly full (e.g. >90%).