question

Fredrik avatar image
Fredrik asked

Battery empty, cannot restart system because of capacitors blowing the fuses.

So i have three MP II 5kVA connected to a battery bank and AC connection. Just to run a test I disconnected the AC to simulate grid failure and let the loads empty my battery bank.

Here is the issue.

When turning on AC-IN again the charge of the capacitors (I believe) blows the 16A fuses in two of the phases (phase 2 and 3, never 1 for some reason).

I understand that the system is designed to have a DC connection to handle these surges in accordance with this thread : https://community.victronenergy.com/questions/122281/multiplus-ii-483000-ac-in-fuse-trips-when-no-batte.html

What I am looking for is a way to ensure that the system does not end up in this state where I cannot charge my battery bank as I cannot get the MP's up and running due to them blowing the fuses every time I turn them on.


Any suggestions for a way to allow this situation to occur (battery bank being blocked by BMS not allowing further discharge) and for the inverters to be powered down until reconnection to the grid and powered up again without blowing the fuse.
I believe a 20A fuse would be enough to avoid the problem, but that's not possible at the current installation.

Thanks in advance.

Multiplus-II
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6 Answers
Alexandra avatar image
Alexandra answered ·

@Rockabilly

You want to program the system to shut down before the batteries do. That way the inrush problem will be avoided.

Big consumers on ac2 out and they dont switch on until either ac in has been on after 5 or so minutes (rime delayed so there is time for batteries to recover and begin charge and the soc is above a certain percentage.)

I can't really be more specific without knowing how minimal the battery bank in your system is.

But general rule to allow for imbalances is a 10% SOC shut down and 15% start up for larger consumers if you have a min spec bank. This also allows a system self recovery if there is solar.

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Fredrik avatar image Fredrik commented ·

The battery bank at the moment is only 12 kWh, but will be 25 kWh soon.

But the idea is sound, do I do this in the cerbo GX or is is a setting available in the MPII?


Thanks for the answer

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Alexandra avatar image Alexandra ♦ Fredrik commented ·

@Fredrik

The shut down and restar is set on the inverters in the inverter tab using ve bus system configurator.

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Alexandra avatar image Alexandra ♦ Alexandra ♦ commented ·
@Fredrik

With 12kWh it was undersized. At 25kwh it is also undersized but better.

If those are Pylontec or pylontec similar then the bms ability is still undersized for the system

You need a battery bank that can provide at least 320A continuously.

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Fredrik avatar image Fredrik Alexandra ♦ commented ·
It is not Pylontech, it's, at the moment, a home made pack with a SEPLOS BMS at 100A. The planned setup will be 5 packs of 100A each so the 350A target will be met.


Out of curiosity and to learn. These 350A, how is that calculated? Is that for the MPII 48/5kVA specifically or is it a standard setting? I have a singe 24V 3kVA in the boat and those breakers are very touchy and it's never been blown in this manner. So I am guessing the Amp-requirement is dependent on what MP I am working with?

(Reason for asking is that I'm looking into using larger MP's than then 48/5kVA and it would be very good to know how I shall configure the battery bank to adhere to those requirements).


Thanks for the very helpful answers.

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Alexandra avatar image Alexandra ♦ Fredrik commented ·

Wattage of system ÷ nominal voltage of the system for the nominal continuous rating for batteries (dont use peak ratings for this on batteries)


In your case 3(5kva) ÷ 48v = 312A nominal current (peak rarings should allow for overload conditions which on mp2 should be 150% for a few seconds....)

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mvas avatar image
mvas answered ·

How did you calculate that a 16 Amp fuse was correct?

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Fredrik avatar image Fredrik commented ·

I did not calculate that it was correct, but it is what is installed where the system is set up, so it is what I have to adhere to.

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mvas avatar image
mvas answered · 
What I am looking for is a way to ensure that the system does not end up in this state where I cannot charge my battery bank as I cannot get the MP's up and running due to them blowing the fuses every time I turn them on.

You have two options ...

1) ALWAYS pre-charge the capacitors via the battery bank, which cannot ever go to 0% SOC.

and/or

2) install the correct AC-IN wire and the correct AC -IN fuse.

The existing design of the AC-IN is not acceptable and should be permanently fixed.

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Fredrik avatar image Fredrik commented ·
AC-IN wire is 6mm2 and is installed by an electrician.

AC-IN fuse, however, isn't that easy to change. I reside in Sweden and our residental buildings here are usually set up with a fuse between 16-25A (32 is super unusual).

Do I understand you correctly that where you reside your breakers, to the grid, are in the 32-50A range? 16A would give you 11kW draw from the grid, enough to power a house and 25A would give you 17kW. A 50A is 34kW of power and that seems like a lot to have for a single home.

Or is there something I am missing here?

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Rob Duthie avatar image
Rob Duthie answered ·

Hi

The correct AC in MCB protection as per Victron manual is 50A for a 5kVA models. i normal use 32A with no issues in my intstallation sites.

16A is to far light and will trip!!!

Install the correct cables required for the AC circuits, Use a registrated electrcian they will know what to install as per country regulations etc.

Regards

Rob D

NZ

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seb71 avatar image seb71 commented ·
It should be mentioned that in such case the wires also must be replaced.


A 16A MCB is normaly used in circuits wired with 2.5 mm2 wires. Can't just replace the 16A MCB with a 32A MCB if the wires remain the same.


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Fredrik avatar image Fredrik commented ·

It is installed with 6mm2 wires, as per regulation. In regards to the 50A for 5kVA, and this is to undersand, not to question.


When I read the manual it says "50 A (for 5 kVAmodel) and 100 A (for 8 kVA and 10 kVA model) or less". I interpreted the "or less" part as it would be quite OK for me to use a 16A fuse as long as I also set the the limitations in the MP to adhere to these specifications. Limiting the AC side to 16A per MP for example.


But when I ready what you tell me I understad it as you do not agree with my interpretation of that text. Is that correct?

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seb71 avatar image
seb71 answered ·

This setup does not have any PV panels?

If not, and if you can, add even a small MPPT with a few panels. That would charge the battery (once the Sun is up) regardless of the grid status.


Also are you sure that the MCB is tripping because of the inverter capacitors? They should not discharge so quick.

Maybe it's because the battery is empty and the inverters try to charge it from the grid. So is the charging current properly limited (for your 16A MCB) in the inverters settings?


Another thing to consider: on a tri-phase system, you should use tri-phase MCBs (I assume you have MCBs, not fuses). Or do you actually have 16A fuses?

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Fredrik avatar image Fredrik commented ·
The solar idea is sound for this installation specifically and I will try it out, but I'd also want to solve it in a way that does not require the panels.


I disconnected the battery bank to ensure that there was no charge surge happening and I've also set the AC limit to 10A in the MP's. I believe this isn't the issue.


Actually, where it is located now there is fuses, mounted 3 meters up requiring a lift to change every time I blow them out, so that is a bit of a hassle. =)
Thats not where it will be located in the end though so it is a passing problem. But yes, tri-phase MCB, is the way to go for actual installation on site.

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Daniël Boekel (Victron Energy Staff) avatar image
Daniël Boekel (Victron Energy Staff) answered ·

For big installations without DC PV I use a small DC power supply that can handle short circuits, this is useful for pre-charging the DC bus also.

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