It seems I’ve finally got to the bottom of why my Fronius Primo on the output of my MP2 disconnects when loadshedding ends: Apparently when the MP2 is preparing to reconnect to the grid it raises the output frequency to the point where all PV inverters will disconnect. Here’s the comment from Victron support:
That frequency shifting issue is how it has always worked. Yes it could be better and it will be looked at in the feature. As long as some AC input is present, the units keep “getting ready” to connect as soon as voltage is within limits. This means that to prevent overvoltage on the output, the PV inverters are shut down by periodically raising frequency. We hope to change this in de feature, but if that happens it won’t be soon.
So I took a look at the settings of my Primo:
(it starts derating at 51Hz at 60%/Hz, so output will drop to 0W at 52.66Hz)
(it disconnects at 52.8Hz)
And then compared them to the ESS assistant settings of my MP2:
So apparently my MP2 is raising the frequency to 53Hz just before it closes the relay to reconnect to the grid. This causes the Primo to enter state 105 “AC frequency too high” and disconnect. The startup process of the Primo is quite tedious. It goes through a whole self-test and eventually reconnects about 3-4 minutes later. In this time the MP2 is crapping its pants, because the wife saw that the sun is shining outside and has every kitchen appliance on simultaneously.
So I’m scheming that if I change the last frequency setting of the ESS assistant to also be 52.70Hz that the Primo will derate to 0W, but won’t actually disconnect. This will allow it to resume full output power faster and the MP2 won’t have to carry all the loads for as long as it would otherwise, and perhaps not trip.
Now the question is: What could possibly go wrong if I do this?
It is normal. The Fronius looks at VOLTS and Hz. When it’s below or above the parameters it will disconnect and reconnect only when everything is within the band again.
It will also disconnect should the voltage rise or fall to quickly. When you are inverting the output is very stable At 230 volt. When the inverter binds to grid, it will raise the voltage to whatever the Grid voltage is. In most cases the Difference is big enough to disconnect the Fronius. It will start going through the normal sequence before it will reconnect.
53 htz won’t normally make it disconnect, it will only throttle the production and not trigger a disconnect.
Raising the frequency from 50-53 hertz is called frequency shift and as the frequency goes up the Fronius starts cutting back. 52.7htz = 0 output.
In your case it might be Htz, but remember, too big a jump in the volts will also trigger a disconnect.
In one of my biggest installs, it happens with every single loadshedding and when the power returns. An industrial area with a average grid voltage of 252 volts.
I had to think about this a bit. This is why I think they do it:
The moment the grid returns, the Multi will synchronise it’s frequency with the grid, then wait 60 seconds (or however long the grid code says), and then when it reconnects to the grid the changeover is fairly seamless with no vector shifts and stuff.
That means that the moment the grid returns, there is 60 seconds where the Multi is forced to run at 50Hz. This means the PV inverter is no longer limited, it will run at full power now. Now imagine that the battery is also full. It’s going to overvolt the battery before it can connect to the grid.
So that’s the technical reason it does that. But I think it also depends on the kind of Fronius you’ve got there. Most that I have seen will reduce power to zero, but as soon as the frequency drops they will pick up again with minimal fuss.
However you look at it, for 60 seconds… things are in limbo. People need to learn that you cannot immediately turn everything on the minute the power returns
I’ve set Modbus to the highest priority under the Primo’s “Controlling priorities”, so won’t the Cerbo limit power output in the 60 seconds that it is running at 50Hz?
I’ve got dynamic power reduction as second priority and that is set to 0W, so I’m assuming the Cerbo must be talking to the Primo even during loadshedding (while its output is being controlled via frequency shifting) otherwise it would not produce any power.
No. When the Multi is not connected to AC, the modbus control is no longer done, in fact it sets the PV-inverter to max and then gets out of the way. Having two cooks in the kitchen is always a bad idea.
Also remember that we support non-sunspec inverters (with frequency scaling) as well, so this would not be a very generic solution either. On top of that, a Fronius takes up to 5 seconds to respond to a power reduction command from modbus. It’s not fast enough.
I’m quick to blame the wife, but as stage 6 became part of life many things were migrated from the non-essentials to the essentials circuit. I’ve currently got about half a dozen timers that I’ve set to run the big loads in sequence (geysers, pumps, etc.). Sometimes I lose track myself of what’s running when and I find myself looking at the Cerbo’s display wondering where all the kilowatts are going.
The rule in the kitchen during loadshedding is one device at a time, but accidents happen. If someone puts on the microwave at the same moment the dishwasher hits its heating cycle… darkness.
The downside of such a setup is that you need to manage loads even when there’s no loadshedding.
The MP2 5kVA has a 50A passthrough current limit, so when the grid is available and also when the sun is out and the Fronius is producing 5kW of AC we don’t need to limit loads.
Ironically, I often find myself worrying more about when loadshedding ends rather than when it starts. We run our heavy loads during the day when the sun is shining, and the Fronius normally doesn’t disconnect when loadshedding starts, but it disconnects every time when it ends.
Essentially if the current flowing on the controlling side exceeds the rating of the unit it will switch off the “controlled unit”
You can then slap all the loads that you won’t notice going off on the other side like your geyser.
You can also install a couple of them to build some intricate self loadshedding if you want.
With no LS the kitchen operates normally, the downside is we need to swap the devices between plug outlets.
Like my daughter last night, she visited during LS, we were out. We get a call: The oven does not work Mom. … exactly as intended.
EDIT: I can take the whole house onto Critical Loads … or I can add Critical Loads plugs here and there where things can get rough. That way I avert installing a 63a 220v inverter, the same size as the house main breaker see.