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Nicholas Herbst avatar image
Nicholas Herbst asked

Overload at Mains Failure transition

Overload blues…

Hoping someone can chime in with some advice: we have a power backup installation at a retail store selling frozen meals - a bunch of high-end freezers, lights, Point of Sale.

2 x 5kVA Victron MPII on a Freedom Won 20/16. Massive overload spike each time load shedding starts, inverters trip, sometimes the system auto-restarts, sometimes not. If we do a grid fail simulation by tripping mains on-site no issue whatsoever. Overloads reported on VRM and verified with a clamp meter.
Tried installing older firmware (459), swapped MPII's for a 10kVA Quattro, no change. Looks like this isn’t an inverter issue.

Any thoughts as to what condition would be fundamentally different when the munic load sheds vs when we trip mains manually?

Backup loads are on a single phase of a 3-phase supply. Load is typically 2 - 3 kW under normal running conditions.


Load spikes to 50-60A (AC) at the transition of Mains Failure to Inverter.

MultiPlus Quattro Inverter Charger
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Alexandra avatar image
Alexandra answered ·
@Nicholas Herbst

A brown out before the blackout?

That is often the issue here, and lom detection as well. I assume though you have a grid code enabled?

See 2.3 of this page.

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Nicholas Herbst avatar image Nicholas Herbst commented ·

Thanks for the response @Alexandra

With the MultiPlus II's installed we have the South African grid code NRS097-2-1 configured, and when we tried the Victron Quattro 10kVA we had set the Grid code to "Other" with voltage threshold adjusted Inline to match the NRS097-2-1 parameters.

In both instances LOM Type B was enabled.

Do you suggest that we disable LOM detection and test?

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Alexandra avatar image Alexandra ♦ Nicholas Herbst commented ·

@Nicholas Herbst


There are two routes. Check if it is lom as described in the article linked, but also we have found here in Zim raising the disconnect voltage is also the way to go. Some systems lom is disabled as the grid connection has a high resistance and some we are using type B. What we do see with the grid side is the overload happens as it feedsback slightly to the grid as it is going down thats when you see the overload. So that will be the difference between you disconnecting and the grid disconnecting, the lines are open.

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shaneyake avatar image
shaneyake answered ·

What firmware version are you currently on? 489 is the latest for the Multiplus-ii
Can you update and see the problem goes away? There was an issue in the older versions.

How are you recording these spikes before tripping? Where are you measuring this?

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Nicholas Herbst avatar image Nicholas Herbst commented ·

We are running firmware 489.


The spikes are confirmed on VRM and via video using an AC clamp meter on the inverter output.

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shaneyake avatar image shaneyake Nicholas Herbst commented ·
This sounds like a load problem.

The main difference when you trip a breaker compared to a grid outage is the rate at which the voltage drops. You can try going non compliant for grid code but still keeping LOMs on but you can change the disconnect voltage to make it happen sooner. Raise it up to like 210V

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Nicholas Herbst avatar image
Nicholas Herbst answered ·

Our current solution to the problem involves a staggered delay on the freezer circuits.

The freezer circuits are transferred to inverter output (AC-OUT1) after 15seconds, when a Grid failure is detected. So sure, we cannot achieve UPS / seamless transfer on those circuits but the outcome is suitable and accepted by the client.

If anybody is experiencing a similar problem and would like to see the Circuit diagram to achieve this, please contact me.

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Alexandra avatar image Alexandra ♦ commented ·

Refrigeration compressors will do that. Locked rotor conditions and all. Nice solution.

What we normally do is size is to size about 5 times the rms rating to account for that. (Depending on the fridge unit type and age of course.)

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