Controlling the maximum current output on the AC side of a Multiplus 2

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simon-t asked May 19, '22

Controlling the maximum current output on the AC side of a Multiplus 2

I hope to use a Multiplus 2 as a grid tied inverter. I live in rural Scotland. Although I have mains electricity, my DNO has put significant restrictions on the maximum size of a grid tied PV inverter I can have. Apparently the infrastructure around me is ‘fragile’ I am allowed a maximum of 7.36 KW with an ELS so that I never export more than 3.68KW.

I hope to get around this by having a large PV (circa 17kw) array to charge a suitably sized battery bank (in the style of attached picture but without the Genset).

My question is around controlling the output of the Multiplus. I plan to use the 8000VA model as specs say its continuous output power is 6400w, but its peak power is 15000w. I am worried the DNO might use the peak power as an excuse to say no. Can limits be set so that the output power of the Multiplus never goes above 7.36kw?

I know it seems a bit ridiculous, but the DNO have final say here, and ultimately a grid tied system will be so much easier to integrate into my house.

Thank you

Multiplus-II

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nickdb answered · May 19, 2022 at 11:28 AM

With the ESS assistant you can limit the inverter output power to anything you want.

This doesn't affect passthrough AC though.

The peak is transient anyway, it's not for sustained use.

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Modifications Space

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Answer 1 · 2022-05-19T12:43:38Z

Jason - UK answered · May 19, 2022 at 12:43 PM

@Simon_T If you have a limit on your export, the DNO will look at the maximum output of the grid tied PV array regardless to your battery storage system as if the batteries are full, their concern is you will then export your surplus energy to the grid with no locked / certificated means of limited that.

You would be better with a grid tied PV inverter or PV array (what ever is the smallest) no greater than the limit you have which will give you your certificated export connection to the grid, the have what ever size PV array connected to your batteries via a MPPT. Then you limited your feed in the remote console to 3.68W, that way your system as a whole only exports a maximum of 3.68kW, either by the grid tied PV system or any surplus energy form your PV array via the MPPT & Multiplus.

The grid tied PV system will generate what ever it can based on the solar generation and unless you have a interlock to stop it generating, it will need to keep discharging, whereas the DC-DC PV system will stop when the batteries are full and the Multiplus doesnt allow it to export.

Using the below layout, grid tied PV on AC out1 limited to the 3.68kW, Then have PV though the MPPT but off the top of my head, I am not sure if there is a limit other than limited by the MPPT for the DC-DC PV system.

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simon-t commented · May 19, 2022 at 01:48 PM

Thank you for your thoughts. It is certainly great food for thought.a couple of questions:

‘@Simon_T If you have a limit on your export, the DNO will look at the maximum output of the grid tied PV array regardless to your battery storage system as if the batteries are full, their concern is you will then export your surplus energy to the grid with no locked / certificated means of limited that.’

The 8000Va Multiplus 2 seems to have a maximum output of 6400w which is less than the 7360w I am allowed to generate. I can then use an ELS to limit my grid feed to 3680w. No matter what size of PV array/battery bank I have this cannot be altered. As a result, surely the DNO won’t care what the size of my PV array or battery bank is? Tell me if my logic is not sound?

For me this is all about the ability to self generate and self consume electricity using a grid tied system. I am not trying to generate electricity to export and sell

Second point:

You would be better with a grid tied PV inverter or PV array (what ever is the smallest) no greater than the limit you have which will give you your certificated export connection to the grid, the have what ever size PV array connected to your batteries via a MPPT. Then you limited your feed in the remote console to 3.68W, that way your system as a whole only exports a maximum of 3.68kW, either by the grid tied PV system or any surplus energy form your PV array via the MPPT & Multiplus.

The grid tied PV system will generate what ever it can based on the solar generation and unless you have a interlock to stop it generating, it will need to keep discharging, whereas the DC-DC PV system will stop when the batteries are full and the Multiplus doesnt allow it to export.

There is a potential hole in this logic. My maximum generation capacity becomes the Multiplus 2 and the PV inverter. Even with an Export Limitation system, I cannot theoretically generate more than 7.63kw. But maybe the DNO might accept the ability of the Multiplus 2 to have feed in turned off. It’s all great food for thought!

Thank you so much for taking the time to answer

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Jason - UK simon-t commented · May 19, 2022 at 02:33 PM

@Simon_T When you make your DNO application, they want to know the maximum potential generation, which is what they take as what you then might export to the grid. The installation needs to be signed off by a MCS or another certification scheme as part of the DNO application of which they have to state the maximum potential generation.

You have different ways of dealing with this proposed self-consumption situation.

1. Only tell the DNO about the grid tied PV system and make sure it is only capable of the maximum allowed export limit - 3.68kW, then once you have your PV system signed off and DNO approved, fit the Multiplus and what ever else you want making sure to commission the multi to limit the export should you add more PV. They will not be interested in anything else you do (unofficially) providing you never go above the 3.68kW export.
2. Same as option 1 but then add further PV to the AC side of things after you have a signed off installation but you will need a interlock to stop the PV inverter should your batteries have no capacity left to fill. If you have no interlock, the grid tied PV inverter has do discharge its energy therefore it will export to the grid which inturn wil take you over the export limit, then you have a problem with the DNO. If you have an PV inverter you can stop like a Fronius inverter which the multiplus can stop, then you are ok. I wouldn't rely on frequency shift to stop an inverter as that can cause problems with electronics anmd some of the cheap Chinese PV inverters don't stop on frequency shift anyway.

3. Forget the grid tied PV inverter and have a multiplus and all your PV DC-DC though a MPPT, then limit the Multiplus to a max export of 3.68kW but DNO's are a bit funny with ESS on DNO applications still because all they are concerned about is a maximum you can export from a certificated product, especially in areas of the UKPN that are constrained with too much microgeneration.

4. go for what I mentioned in my first post. :-)
Grid tied PV system no greater then 3.68kW and only stat that on your DNO application. Its a easy win because a PV system installed by a MCS contractor now greater than 3.68kW they can not reject and its automatically approved for connection to the network. Then add your Multiplus with battery storage and as much PV you want on the DC side via a MPPT and the DNO will never know, care or find out providing you install the grid tied PV system to AC out1, and limit the grid feed in on the multiplus to your allocated limit of 3.58kW.

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daniel-feist Jason - UK commented · Apr 19, 2023 at 09:32 PM

I have an existing 12.5kW 3-phase Solaredge installed by an MCS approved installed and all signed-off by the DNO. I have been exporting excess PV for the past 2yrs, but have never installed any batteries. I am now planning to install 3 Multiplus IIs along with batteries, with the existing AC-coupled PV inverter on AC-Out-1.

Based on your comments above, it seems your suggestion in this scenario is to install the 3 x Multiplus 5kVA, set the max feed-in on each inverter to 4.1kW and not submit a new DNO application, given export won't ever be above the existing approved kW. Am I understanding this correctly? Seems very reasonable, but I'm thinking that the DNO would likely think otherwise wouldn't they?

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