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ponzoa asked

Grid overvoltage causing issues with batteries ESS

I have a Multi II 48/10KVA 230V with a Fronius Primo (AC Coupled) and MPPTs connect to Pylontech US3000C batteries x 10.


The residencial area has a lot of solar and occasionally, the voltage on the grid gets to 253 which is the discconect limit on the Spain grid code so correct functionality. That is, at 253V, it disconnects from the grid.


The problem is this generally occurs when the batteries are fully charged and exporting excess power. Disconnecting causes the excess power to go to the batteries which are already fully charged and causing a battery overvoltage alarm.


It appears that the problem is caused because of the time it takes to ramp down the Fronius Primo and the 2 x MPPTs.


My feeling is that this is going to become an issue in many areas as solar grows and the grid is forced to up their voltages due to feed in.


I am thinking of changing grid code to one which accepts a higher overvoltage setting (UK for example) but wanted to know if anyone else has solved such issues and how it is solved. Suggestions welcome.


gridAC PV Couplingovervoltage
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Alexandra avatar image Alexandra ♦ commented ·

@ponzoa

Not sure that changing grid codes would be legal.

You could cap the battery voltage lower (0.4v) in DVCC. Or get a larger bank. Balancing on pylons starts at 90% so cell balance would not become an issue.

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ponzoa avatar image ponzoa Alexandra ♦ commented ·
Thanks @Alexandra, I'm aware it wouldn't be entirely legal, although it's likely to solve the issue. I would rather hear of other alternatives or find a way for the system to speed up it's ramp down vía frecuency shifting or comms with the MPPTs.


Apart from legal, what would be the implications of switching to say, the UK grid code?

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Alexandra avatar image Alexandra ♦ ponzoa commented ·

@ponzoa

For you, nothing. The equipment can handle the higher voltage. For the grid however it is an entirely different story. The reason for their limitations is for their protection, not yours.

The alternative for you is to allow your bank the ability to sink the current without damage, so either limiting charge or voltage.

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marekp avatar image marekp ponzoa commented ·
@ponzoa

In my country (Poland) the energy supplier would come and disconnect your house from the grid by taking down the electricity meter and you would be fined. Reconnecting would also cost you dearly.

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marekp answered ·

@ponzoa

Did you check if that high voltage is in the grid or just in your system?

Sometimes the wire from the grid to your house has large voltage drop because is not big enough.

That voltage drop should not be higher than 0.5V at full load.

Is the grid voltage over 253V even if you are nor feeding it?

If it is, complain to grid supplier because that is their fault not yours.

If your feeding the greed causes the high voltage, you can lower the feeding by redirecting your energy to hot water tank or pool if you have one.

Temporarily you can move your Fronius to AC-in side and it will take care of disconnecting when voltage is above 253V. MPPT chargers are limited fast enough.

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janieronen avatar image janieronen commented ·
That is important point. Always with over voltage / under voltage issues fit at least good quality RMS volt meter to grid metering point and track voltage average and peaks. If your grid-point is actually out of tolerance grid company usually comes and fits energy quality logger. Voltage levels are not only issue as the can be also massive harmonics with AC-connected PV etc which can also cause grid disconnect and will be shown in energy quality report.
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ponzoa avatar image ponzoa commented ·
Hi,


This only happens when exporting. Monitoring other installations in the area I have done, they also have high voltages but rarlely reaching the 253V. I feel that, the many solar installations exporting to the grid seems to be raising the grid voltage. Right now for example, exporting over 8kW, the voltage is fluctuating between 244V and 250V It's around 16:00 or 17:00 when the overvoltage cut off happens - sun sets this time of year at 21:40 here so plenty of sunlight left.

As the amount of energy exported is reduced (cloud passing) the input voltage lowers slightly.

I'd rather not move the Fronius to the AC-In side. Customer preference. It's been running well for some 2 years now and has only started having issues some weeks ago. All versions updated to latest.

Cable feed in is done in 16mm.

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marekp avatar image marekp ponzoa commented ·

On the supply line, when exporting, you will see increase of voltage at the house end compare to the voltage on the grid connection end. Grid could be 250V but your system at home will see 253V if the supply cable is to thin for the current flowing.

In some cases 16mm2 could be OK but it depends on how long is this cable and if it is Al or Cu.

I have 70 meters long supply cable and it is 35mm2 Alu and when exporting 10kW (3 phase) there is less than 1V difference between house end and grid connection end.


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ponzoa avatar image ponzoa marekp commented ·
16mm, Cu, 15m or less in length in this case. Grid feed in protected by an 85A fuse at the meter box.
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marekp avatar image marekp ponzoa commented ·
@ponzoa

It is OK than.

In Poland this high voltage problem is very "popular".

People are dealing with it by redirecting the excess energy to additional loads (water heater or AC unit) when grid voltage is close to 253V.

To speed up the frequency shift you can change the frequency at which inverter is turned off.


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ponzoa avatar image ponzoa marekp commented ·
I will see how it goes modifying by 0.4V in the DVCC and if I still have issues, modify the voltage limits in the grid code using OTHER LOMS B so that it is one or two volts above the 253V. Hopefully, the other solar inverters will disconnect at 253V if they are configured to the correct Grid code (not always happens, especially with Huawei invertoers) so it'll stop climbing and I won't disconnect in such a harse way for my batteries.


Fingers crossed and thanks for your feedback and support.

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