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baxter avatar image
baxter asked

Minimum PV array to black start 3phase ESS

Hi,

i will have a 3phase ESS build up with Multiplus 2 48/5000 and a Fronius Inverter for the larger PV array oriented south. To remain the black start ability I would like to connect a smaller array (3 modules, roughly 1kWp in sum, oriented east) with a smartsolar 150/35 to the DC side of the Multiplus in addition. Would that PV power be enough to black start the Multiplus so they can establish a stable grid the Fronius Inverter can sync on and also start delivering power?

Thanks for your thoughts.

Br

Mike




ESS
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klim8skeptic avatar image klim8skeptic ♦ commented ·

Are you running managed batteries?

Any sort of BMS?

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baxter avatar image baxter klim8skeptic ♦ commented ·
Yes LFP with diyBMS
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shaneyake avatar image shaneyake baxter commented ·
1kw should be fine but have the BMS control the Multi. That way the BMS will never turn off completely the multi will just shutdown when SOC is low and start back up once you have enough charge.
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4 Answers
shaneyake avatar image
shaneyake answered ·
There is a Minimum SOC for ESS but also a Minimum SOC for shutdown.


If you turn off all the loads to prevent it from just shutting down again, even a small array like 1kw would be big enough to get the inverter to restart and then the AC-PV can do the rest.
I personally would probably do around 2kw with a 40kwh pack shouldn't take long to raise the pack voltage by 2v to restart the system especially if you are at the low end of the LFP voltage curve.

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basil katakuzinos avatar image
basil katakuzinos answered ·

1kw should be more than enough just keep the following in mind.

Ideally you want the inverter to cut out before the BMS cuts its supply. This way the battery is still on but the inverter is not using it, that way when the voltage rises above the threshold voltage the inverter should turn on again.

MPII will auto restart after a low voltage once the voltage rises enough. If the BMS cuts out though... depending on the model you may have to go manually activate it again.

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klim8skeptic avatar image
klim8skeptic answered ·

I would imagine that a 1kw array would be able to charge a battery that has disconnected due to low soc condition in a couple of hours to the point that it would be able to power the Multi and the AC coupled PV would restart.

This is assuming a lot though.

Does your BMS allow charging whilst the battery has been isolated?


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baxter avatar image baxter commented ·

Would it be possible that the Multi would first start just with the DC power deliverd by the PV array without using the battery, so the Fronius can kick in and charge the battery?

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klim8skeptic avatar image klim8skeptic ♦ baxter commented ·
You can not guarantee that a mppt will power an inverter via solar without a battery connected. The mppt are to slow to react to dynamic loads.


You have to have your system set up so the mppt can charge the battery, while the bms is still protecting the battery from low soc.

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marekp avatar image
marekp answered ·

@Baxter

It depends on many variables.

1. How big is your battery?

2. How fast you want to be able to get out of the blackout?

3. What is your minimum SOC at which your MP-II starts inverting.

4. How big is your initial load after the MP-II starts inverting again.

and more.

1kWp at the beginning of the day will produce maybe 30% of its max power. It could be max at the mid day but at full Sun. It will stay rather at 80-85% because of the heat degradation of PV power.

If we assume 10% min SOC, 20kWh battery and 40% power from MPPT PV, it will take around 5 hours to charge battery up to the level (10% SOC) at which MP-II would start inverting and producing microgrid for Fronius to start producing.

If I were you, I would give your 150/35 MPPT max it can take (2000Wp).


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baxter avatar image baxter commented ·
It is a 40kWh LFP. As fast as possible, latest at mid day so the bigger PV array can harvest enough energy.


Is the minimum SOC really of relevance in this scenario, as I have a grid outage. I thought the ESS is going below minimum SOC in that case and it starts inverting before reaching the minimum SOC (when DC input low restart is reached)?

Of course I will reduce the loads to a minimum in that case, but how long will it take for the small PV array to raise the voltage for restart?


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marekp avatar image marekp baxter commented ·

@Baxter

In ESS min SOC 20% is used when grid is present. When there is no grid SOC can go lower but it cannot go to 0. Inverter will turn off before 0% SOC is reached.

When there was no grid, and SOC min (eg, 5%) was reached, inverter turned off and you are siting in the dark.

When day starts and Sun is out and still there is no grid, your system waits for the battery to charge up to the point when inverter can restart.

At that point your load cannot be to high. With high load battery's voltage will sag and inverter will turn off again.

Your battery have to be charged up to the point when it can provide current for your load.

How low this start-up SOC must be depends on your load.

Let say it is 10%.

MPPT at the beginning of the day will not produce its full power.

If it is 30% of 1000Wp and your battery is 40kWh, it will take maybe 3 to 4 hours to raise SOC from 5 to 10%.

In Europe that system will be up around 8AM in the summer and around noon (or later) in the winter.

I still suggest 2000Wp for your 150/35 MPPT.


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