Offhand, I don't know of any add-on anti-islanding device although one could be developed. An inverter implements it by driving test signals out of phase with line voltage.
Sunny Island performs anti-islanding while on-grid, taking over the function GT inverters would otherwise have done. But it is more expensive than the inverters you're trying to adapt.
Someone on another forum proposed just a relay powered by line voltage:
Hi all, I'm playing with Victron's Hub-1 setup at the moment, but to really make it safe I need to add an anti-islanding device: Something that will disconnect the grid if the grid goes down. From what I can see, this isn't an easy problem to solve in a generic manner, mostly because of the dead ...
powerforum.co.za
That won't work, because it will remain connected during islanding. It would only detect a severe drop in voltage (a 240V relay will stay connected below 120V)
That was a Super Interesting Thread from a different forum posted by Hedges .....
https://powerforum.co.za/topic/112-anti-islanding-devices/
Here's a copy of what I will be studying again / from several of those forum members/ ...
Re: for Grid Lineman Protection when Feeding Solar Power to Grid, & protecting your legal liability butt:
::: from thread entitled: Anti-Islanding devices?
Copy of a few posts:
... to really make it safe I need to add an anti-islanding device: Something that will disconnect the grid if the grid goes down. ... From what I can see, this isn't an easy problem to solve in a generic manner, mostly because of the dead zone (when your incoming power is balanced very closely with your consumption). I haven't been able to find any commercial devices (except for a company in Germany) to do this sort of thing. ... One guy used two series-wired contactors for the job, powering the coil from the mains. The idea is that if the mains drops out, the contactor should disconnect. I'm worried that feeding back enough power might be sufficient to keep the contactors closed, that is: I don't see how the contactor cannot accidentally be powered by feedback power.
*** (Wondering IF this is True ??? ) ::: ... Getting back to your question regarding the contactor, Yes, it would work. For your invertor to keep the contator closed during a power failure, your invertor would need to supply enough power, to power the rest of the grid
That should be one beefy setup : ... although this solution might work in practice, legally this is not allowed, as the contactor might fail in the closed position, resulting in somebody's death.
*** (an Interesting Related Challenge ) ::: traditionally only the GTIs can be connected to the grid. But Victron has done something really amazing and all quatro and multiplus inverters are supported. You can run it in hub configuration where it can run connected to the grid, exporting power. ... What I really want to do is export just enough to power the rest of the house without any of it actually getting out on the grid. Confusing? Let me explain. Like many people, my inverter feeds only some circuits in the house, leaving the rest to run from the grid only. In other words, there is somewhere I can export electricity to where it can be used by myself (before the inverter, but after the grid connection), before it ends up going out on the grid (which I don't want, because I have a dumb prepaid meter). ... I ideally want a current sensor on the main incoming line, and I want the software in the inverter to balance things so that current on the incoming line hovers just above zero, thereby avoiding export. If I can convince someone at Victron to write an assistant for this use case... I'll be in business, but this sort of thing only happens if enough people want it. >>>
*** (This Looked more like a solution I have not yet fully understood: Need to study more / a 2014 post from South Africa ::::
It's relatively simple ... Put a current sense on your grid side, electronics must monitor amps and if it exceeds rating of your system the grid contactor must drop out. Mean while power continues thru the inverter powered contactor to the house load. ... the fact that the grid is down means that south africa is demanding power from you, excessive load. when you set the trip current it must supply the house and excess to the grid but any more than that it must drop the grid contactor out and protect the inverter and electricians working on the line. Extra electronics can be used to monitor a balance. IE Load is using 10A, inverter is supplying 20A and grid is taking 10A. 3 current sensors. now grid voltage goes down from eskom supply and grid becomes intensive load to your system. amp are up current load becomes excessive, contactor drops out. Now you monitor the grid voltage to see its down on the town side of the contactor. Make the contactor only pull back in when the town side voltage comes back up. It's a combination connect and disconnect for different reasons thru 2 or 3 contactors. ... Just remember when all contactors are out, voltages can be picked up on both sides of the contactors to signal your electronics or power your contactors on when you want them on and off. when all contactors are in it acts as one line, so breaking the line momentarily and sensing voltage as there or not will work. As soon as the voltage is back from Eskom that contactor or relay in back in. Flip flop contactors can also be used and there are more angles that can be wangled out but at more cost and parts. hope this helps.
And there is more to study on that thread if you are interested :+)