Would it be possible to couple a grid tie string inverter on the AC output of my dad’s off-grid system? He’s got a Victron Multiplus 3kva with a 5kwh lifepo4 battery and 2kw of panels connected to a MPPT 150/70. He's looking to add another string of panels for bigger loads during the day when the sun is out (washing machine, dishwasher, pumps etc) and dedicate the MPPT to charging the battery. I thought it might be a good idea to get a little 2/3kw Solis string inverter instead of a second MPPT and increase the power available? I guess if you connect a generator you just have to make sure the Solis is disconnected so it doesn't push back power? Will the CCGX pick up the Solis?

 

5 hours ago, gooseberry said:

Would it be possible to couple a grid tie string inverter on the AC output of my dad’s off-grid system?

That is supported yes. Remember there is a 1:1 rule. If the Multi is a 3kVA unit, you cannot put more than 3kW worth of AC-coupled PV on the output.

5 hours ago, gooseberry said:

I thought it might be a good idea to get a little 2/3kw Solis string inverter instead of a second MPPT and increase the power available?

Something I need to ask. On grid or off-grid? On the grid, sure, no problem. Off grid, that is different. You cannot assume that the extra power of the PV-inverter on the output can be added to that of the Multi. PV-inverters are picky about the quality they will tie with, so while it works well enough and you can charge batteries without problems, a large load switching on can cause enough of a disturbance that the PV-inverter will disconnect and restart (like it does when there is a grid outage), and then your Multi must be able to carry the full load  until the PV-inverter comes back. In other words, adding PV on the output does not increase your peak power capability.

Also, the Solis inverters are not officially supported. They work, they support frequency shifting for power reduction, but I am not aware that they can be configured specifically for microgrid use (like you can with a Fronius for example).

5 hours ago, gooseberry said:

instead of a second MPPT and increase the power available?

Aaaah, working from this, it seems you may be using the term power incorrectly. Adding more PV (on either side) will make more energy available, or in other words, it will lower the bill. But in both cases your peak power would remain 3kW.

Using a PV-inverter is more efficient for loads that run during the day time, that would be one reason to go for that over another MPPT.

5 hours ago, gooseberry said:

Will the CCGX pick up the Solis?

No. They use a completely different protocol. The protocol is modbus-rtu based as far as I know. The CCGX can only speak to Fronius inverters, or any inverter that supports sunspec via modbus-TCP.

Also, power limiting (to prevent feeding into the grid) does not work. It works only with Fronius and ABB.

Thanks! This is a completely off-grid system, closest Eskom lines are a few km away. So this is why it would need to be coupled on the output side of the Multi. The only AC that ever feeds into the system is from a generator which only gets fired up if there is more than two days without adequate sun.

Not sure if the price of a Fronius or ABB is worth it for this application? So will the CCGX just see a smaller load if the Solis picks it up? Wouldn't it be able to power a 4kw load if the Solis supplies 3kW and the Multi 1kW, assuming the Solis doesn't restart when the load comes on? And even if it restarts, the Multi can supply peaks higher than 3kw for short amounts of time, right? Long enough until the Solis comes back?

How would you charge the battery from the Solis? Would you be able to loop the output back into the AC-in of the Multi?

 

 

 

1 hour ago, gooseberry said:

Wouldn't it be able to power a 4kw load if the Solis supplies 3kW and the Multi 1kW, assuming the Solis doesn't restart when the load comes on?

In theory yes, just don't design the system around this. If possible, configure the Solis to allow wider voltage ranges that are more suitable to a minigrid.

Using a PV-inverter in this scenario would be done because it is more efficient (only one conversion, with an MPPT you have two conversions, one from PV to battery, and another from battery to AC), not to get a higher peak capacity.

1 hour ago, gooseberry said:

the Multi can supply peaks higher than 3kw for short amounts of time, right? Long enough until the Solis comes back?

Yes, it can supply a bit of overload for some minutes. The rules for AC-coupled PV is usually something like monitor for 60 seconds and if voltage and frequency remain in bounds then connect. So if the Multi can hold it for 60 seconds you will be fine. I have no idea what in of load it can handle that long, I do know that if it is overloaded the signal will not be as clean as it should be and the PV-inverter may not want to tie with it. I'd rather not take a chance with this.

1 hour ago, gooseberry said:

How would you charge the battery from the Solis?

You don't have to do a thing. The Multi sees power flowing backwards into its output and automatically uses this to charge the battery. If the battery is full, it increases the AC frequency to throttle the PV-inverter until it switches off (usually at 53Hz).

15 minutes ago, plonkster said:

You don't have to do a thing. The Multi sees power flowing backwards into its output and automatically uses this to charge the battery. If the battery is full, it increases the AC frequency to throttle the PV-inverter until it switches off (usually at 53Hz).

You must however load the PV-inverter assistant for this to work.

On 2020/06/22 at 4:37 PM, gooseberry said:

And even if it restarts, the Multi can supply peaks higher than 3kw for short amounts of time, right? 

 

On 2020/06/22 at 5:44 PM, plonkster said:

Yes, it can supply a bit of overload for some minutes. The rules for AC-coupled PV is usually something like monitor for 60 seconds and if voltage and frequency remain in bounds then connect. So if the Multi can hold it for 60 seconds you will be fine. I have no idea what in of load it can handle that long,

Previously tried to decipher the whole overload thing and came across this (link is to time stamp about Peak duration specifically but I advise watching the video from the start). If I understand it correctly Spec Sheet "Peak Power" (6kW for the 3KVA?) for 2 minutes IF the multi can maintain set output voltage.

I would think that it is probably a good way to shorten the multi's life if relying on peak output on a regular basis?

1 hour ago, introverter said:

6kW for the 3KVA?) for 2 minutes

It can do 200% for about a second, at least that is the info I have. Just long enough to start the average induction motor, not much more. It can run at 100% for maybe an hour, depending on ambient, when you run over 80% capacity it slowly heats up and depending on ambient it may overheat and shut down. The reasoning behind this is that you usually size the inverter for the peak load and not for the constant load, so this is aceptable practice.

Johannes (the guy in the video ) knows his stuff though, so he's probably mostly accurate... still I would not rely on 200% for a full minute, I'm almost certain it will shut down before the PV-inverter reconnects.

The following does not change anything about the multi going to bed before the solis shows up for the party. So readers that end up here curious about the victron+solis combo should skip the rest of this post...

tldnr: the multi will not maintain 200% for a minute.

12 hours ago, plonkster said:

Johannes knows his stuff though, so he's probably mostly accurate... still I would not rely on 200% for a full minute, 

When I was originally looking for the info I could not find anything on the peak/overload duration for the victrons (I was comparison shopping with axpert in the other corner) and only managed to track down the webinar... turns out this is one of those times when listening to what is said can possibly get you in trouble.  While Johannes talks about point 2 on the slide he refers to the Peak power value where I think he meant to refer to Continuous output value.

More recent posts on victron community ( 1   2 ) give this:

Over enthusiastic users, like me, want to know what the hardware can withstand while the Victron answer is actually, understandably, more about what does the inverter do since it is not just a simple passive component. (Johannes's webinar is not exactly meant as a statement about hardware spec)

I feel like a donkey trying to make sense of the Durban July but have now decided all of this means:

The firmware will generally do the following (in what follows "output voltage" refers to the inverter AC output voltage of 230V for example):

Any load greater than 200% of nominal power will lead to immediate shut down.

For any load up to 200% nominal power if the multi can't reach/maintain output voltage the firmware will shut down the multi if output voltage is not reached in 30 cycles/0.5 seconds  (AC cycle not physical start/restart/reset of inverter).

If a load between 150%-200% of nominal is started and the multi can maintain voltage for the 0.5 or so seconds the multi will not shutdown...yet. If the load remains at that level for longer than 0.5 seconds the multi will shut down... 

If the load reduces to (or starts at)  about 150% of nominal power and output voltage is reached the firmware will shut down the multi after 5 seconds even if the voltage can be maintained, unless the load reduces.

For a load (reduced to or starting at) between 130%-150% of nominal power even while maintaining voltage ouput the firmware will consider it an overload condition and shutdown the inverter after 2 minutes even if able to maintain output voltage, unless the load reduces.

A load between 100%-130% of nominal could be maintained for up to 30 minutes but the load/time is very much influenced by temperature and the firmware will likely shutdown the inverter after some minutes to an hour, unless the load reduces but it will probably not be classified as an overload shutdown it will be because of over temperature).

This is largely academic and unless I made any glaring/dangerous errors require no response.

PS. (If Johannes ever happens on this post and takes exception to my presuming to point out a possible error... please don't hack my VRM account and turn my multi into a pumpkin...)  😉

14 minutes ago, introverter said:

A load between 100%-130% of nominal could be maintained for up to 30 minutes but the load/time is very much influenced by temperature and the firmware will likely shutdown the inverter after some minutes to an hour, unless the load reduces but it will probably not be classified as an overload shutdown it will be because of over temperature).

I've run a tumble dryer on my 3kVA before, along with some other loads that pushed it to about 3.2kW. I can attest that the Multi handles this without a problem, but the red overload LED is blinking all the time. It can withstand this long enough to actually finish drying the washing (since it is not a constant load, the thermostat does kick it in and out, and the tumble dryer has two stages too, it only runs at the peak 2.2kW for a few minutes at a time).

Edited June 24, 20206 yr by plonkster

8 minutes ago, plonkster said:

 but the red overload LED is blinking all the time. It can withstand this long enough to actually finish the load 

My father's voice in my head saying "ag nee man...jy probeer mos nou om die ding te breek" would have me not repeat that ... then again a week of rainy cold weather with washing piling up makes one do strange things..

59 minutes ago, introverter said:

My father's voice in my head saying "ag nee man...jy probeer mos nou om die ding te breek" would have me not repeat that ... then again a week of rainy cold weather with washing piling up makes one do strange things..

It was a one-time experiment, not something I do as a rule.

I did however run the pool pump from the inverter once, and that is something that may happen more often in future. It was a bright Saturday morning and there was a power outage, the batteries were full and nothing else was happening... so I figured the 1.5kw pool pump can run so long. What makes this case interesting is the massive start torque of the pump. I didn't measure it, but the noise the inverter made clearly indicated it was a HARD start...

19 hours ago, plonkster said:

It was a one-time experiment, not something I do as a rule.

not to worry, my dad doesn't know what you did 🤣

19 hours ago, plonkster said:

I did however run the pool pump from the inverter once, and that is something that may happen more often in future. It was a bright Saturday morning and there was a power outage, the batteries were full and nothing else was happening... so I figured the 1.5kw pool pump can run so long. What makes this case interesting is the massive start torque of the pump. I didn't measure it, but the noise the inverter made clearly indicated it was a HARD start...

Not that you asked,  but I "clocked" my 1.1kW (1.5 HP) pool pump at about 29.9A (233VAC) on "inrush" (first 100ms)... which is possibly a bit quick to bug a multi but when switching to MAX/Min I have seen 25.2A so that peak can last a while (too lazy to switch the pump on/off for an exhaustive data set but I suspect  the first second or so should average about 15A)....

I would hazard that starting the pump not only says something about the multi but probably also the battery/PV?

I played with the idea of using a small Solis grid tied to bring the pool pump bill down (let eskom kick start the pump and then have the solis drive it down the hill) and then realised I can probably achieve the same result with my small multi, ESS and a cron job or two (still studying my way through that part).