FrankcF

I think this is the only way to go for now. The VSD will provide a lot of relief...

@FrankcF thanks for the feedback. I think a lot of the end users who are not "technical" tend to then have issues because:
When manufacturers (battery or inverter) say, "all is well", they tend to be people who are just passing on the message; Sometimes if it's "over the phone", there's no documented evidence that the person ever said such a thing; Finally the sad part, which is the greater, is the amount of time spent, trying to get someone knowledgeable on the manufacturers side, to look into the issues that may be raised, which is frustrating to both the installer, reseller and end-user. Now talking of inductive loads, a 50kW system is quite large, relative to the average domestic household, but how then would this issue be addressed in the domestic scenario where a household has 5kW Inverter and likely a FW small battery but having an induction cooktop running of the system?

@FrankcFtypical explanation from Sunsynk....
FW have 2 battery profiles for their HV series of batteries. One is Atess HV and Sunsynk HV. Atess has 2 options for HPS and PCS. HPS uses High Voltage but lower VOlatge than the PCS. The PCS uses the HV+ battery which supports higher voltage range. 
My point is that FW only has a single Battery profile for Sunsynk. The voltage range is constant not as the Atess which has 2.
@Evan9707 You needed a Transformer based Low frequency inverter because you have High inductive loads. Typical of the Sunsynk is that is it High Frequency and this is not well matched for these loads. 
The magic is in the large transformer on the Atess Equipment...It makes the load not drop, the load stays Uninteruppted.

@Steve87 @Moffat @MikSA @root Guys, I had a lengthy debate with the director of technical at Sunsynk and it turned out well. They had one of their top techs log into the system and do a thorough check into the backends, which us plebs can't access. Here is their feedback

Findings on the practice.
The battery profile is not setup correctly , the nominal voltage for this battery is 411Vdc , yet the battery is only charging up to 331Vdc @99SOC , Please contact freedom won to log into the battery and check as well as update profile and adjust the battery profile. Now I have to contact FW AGAIN, and ensure they go out to site and let me know what they can do to resolve the issue. (Note, FW has logged into this battery multiple times and have always assured me they find no fault) 

Sunsynk uses high-frequency switching to create a AC sine wave. This design is prone to dipping since it takes time for the control-loop to catch up and tell those mosfets to giddy-up.
Your battery is a tad small for this size installation. You are barely over the 1C battery/inverter ratio. 1.5C is the minimum in my opinion.
Funny enough I have friend with the same problem on a 8kw Sunsynk. Also dips and is undersized (1.275C) on the battery.

Just bear in mind that the inverter will rejet AC input of a portable generator with hard N-E bond in place....

Good day @FrankcF let's see if i can explain in laymans terms the problem you are experiencing. You have what is called "floating neutral" due to the fact that you don’t have an earth reference from earth to the inverter(no incoming earth wire). You need to establish what we call a TN-C earthing system. The way to do this is firstly to install an earth spike in the ground shortest distance to your db board or inverter. Then you need to use 16mm² earth conductor( green/yellow wire) from the earth spike to the main earth bar in your db.Then from the inverter output terminals live to your Main breaker, neutral from inverter to neutral bar in db and earth from inverter to earth bar in the db. You now need to put a permanent wire across neutral and earth on the inverter output. If you now switch the inverter on it will be in islanding mode as you have no incoming power feed, measure between neutral and earth in your db and you need to get as close to 0v as possible but any voltage reading above 20v ac is an indication that you don’t have a good earth refrence from your earth spike.If you get any voltage above 50 volts ac between neutral and earth then switch the inverter off and install a second earth spike. There are ways to measure the resistance but you need specialized testers to do this( earth loop impedance or fall of potential method)
If a good earth refrence is not obtained your protection devices in the db board will not function correctly( long tripping times or not tripping when fault currents occurs) and is dangerous. Hope you have a better understanding and will be able to sort it out.

What you have is known as a floating earth, which can cause a lot of strange problems, susch as getting shocked by the water in your taps.

I also had that issue, and solved it by having an Earth Spike installed (10 metres from any bulding) and have a 16mm Earth copper cable run directly to the inverters external earting terminal (lower right side). 

Another thing to double check is that there is no possible problem being caused by the reversal of the live and neutral feed going to the DB board from the inverter.

Under normal circumstances, the Neutral and Ground should be almost the same voltage with referrence to the live feed. This is what result from a municipal Earth-Neutral common feed. You need to make your own Earth-Neutral bond.