Grid tie solar - prioritise solar power?

[william hamilton]

Hi all, I am currently designing a solar system for my farm, aimed at reducing electricity bills.

I need the system to be connected to the grid, prioritising energy produced via solar, but using the grid otherwise. I do not want to export any energy to the grid.

I am trying to understand how I connect the inverter into the grid electricity supply, and most diagrams seem to show that is connected via a splice into the incoming grid connection line. As such I don’t understand two things.

1. How the solar generation is prioritised over the grid supply
2. How do I ensure 0 export, ensuring that all generated electricity is used on-site.


Can anyone help?

Thanks,
William

 

[grebaba]

I ran a 10 awg wire from my main panel though a 30 amp breaker to my ac in on my inverter.
Uses grid when solar or batterys are low, does not export back to grid.
Greg.

 

[hwy17]

This is standard SBU/SUB mode stuff for hybrid AIO inverters. 18kPV, Sol Ark, 6000XP, EG4 3k, etc. And older designs like Schneider equipment, Outback Radian, etc.

The question is are you ok with micro exports while the inverter is reacting to load changes, or do you want true zero export which is only possible with double conversion.

 

[william hamilton]

This is standard SBU/SUB mode stuff for hybrid AIO inverters. 18kPV, Sol Ark, 6000XP, EG4 3k, etc. And older designs like Schneider equipment, Outback Radian, etc.

The question is are you ok with micro exports while the inverter is reacting to load changes, or do you want true zero export which is only possible with double conversion.

I understand that an inverter can be set to different priority modes, however I don’t quite understand how it manages these if it is connected as below (I have removed isolators, meters and protection devices)

I still have a few questions that I don’t understand
1. How does the inverter override the grid? (Higher voltage?)
2. How is grid feedback prevented? - does one of the protection devices prevent it?
3. Is there alternative ways of connecting the inverter?

 

[hwy17]

How does the inverter override the grid? (Higher voltage?)

Yes. Minutely, but voltage is potential so it is the push pull force.

How is grid feedback prevented? - does one of the protection devices prevent it?

CT's are used to measure current flow at the inverter, and then software dials it in to zero or whatever's intended. Export is not perfectly prevented and very small blips will happen while the software is reacting to a load being suddenly turned off.

Is there alternative ways of connecting the inverter?

Double conversion is the extreme option, which prevents all export absolutely.

 

[zanydroid]

I do not want to export any energy to the grid.

Let’s take a step back. Why do you want to do this, especially when you don’t have a battery?

There are some legit reasons and some questionable ones.

 

[kommando]

1. How the solar generation is prioritised over the grid supply

On a Grid tied inverter that's how they come out of the box, all they produce is injected into the house/farm and any excess goes into the grid.

2. How do I ensure 0 export, ensuring that all generated electricity is used on-site.

Store the excess in a battery for later use, 100% a requirement for a dairy farm where use is highest during milking morning and evening and lowest during peak solar at midday.

You need to lookup G98/G99/G100 grid connections as you will need to meet these to connect to the grid.

G98 is not going to tackle a farm

Small large generation | Northern Ireland Electricity Networks

Fitting a wind turbine or solar panel? Find out more and apply for a connection to the grid.

www.nienetworks.co.uk

 

[1201]

is this a situation where you cannot export any at all because there is no interconnect agreement, or you can export tiny amounts but wont be paid for it. these are two very different scenarios with different ways of dealing with the problem

 

[william hamilton]

Let’s take a step back. Why do you want to do this, especially when you don’t have a battery?

There are some legit reasons and some questionable ones.

I am planning to use all electricity on site with a battery

 

[zanydroid]

I am planning to use all electricity on site with a battery

Ok let me re read the thread.

BTW your first SLD didn’t show the battery, and grid tie implies no battery. As such you’re a bit lucky to have gotten any engagement, and it’s kind of all over the place as it is.

Hybrid or off-grid are more appropriate (and the two have important distinction in terms of your interconnection arrangement with your power company, or lack thereof)

 

[kommando]

Northern Ireland is like the rest of the UK, you don't get paid much for exporting, you can export as long as you keep within the G98/G99 requirements you are given when you connect. But on principle I export nothing and any excess goes to batteries or heating water.

 

[william hamilton]

1. How the solar generation is prioritised over the grid supply

On a Grid tied inverter that's how they come out of the box, all they produce is injected into the house/farm and any excess goes into the grid.

2. How do I ensure 0 export, ensuring that all generated electricity is used on-site.

Store the excess in a battery for later use, 100% a requirement for a dairy farm where use is highest during milking morning and evening and lowest during peak solar at midday.

You need to lookup G98/G99/G100 grid connections as you will need to meet these to connect to the grid.

G98 is not going to tackle a farm

Small large generation | Northern Ireland Electricity Networks

Fitting a wind turbine or solar panel? Find out more and apply for a connection to the grid.

www.nienetworks.co.uk

This is a robotic dairy farm with a *relatively* consistent electricity usage profile, I have measured it with a ct sensor, max power in the day is 10kw (cooler, heaters and compressor all going at once). However almost all usage is below 6kw.

A g98 connection will allow me a 3.68kw connection, whereas a g99 connection will allow me 10kw. This is a single phase connection so I am likely to be limited to a g98, 3.68 kw connection.

Realistically I want a system with up to 10kw electrical output, but with more than 10kw of solar panels to charge the batteries with solar charge controllers.

The reason I am asking how the inverter interacts with the grid is because I want to know how much of an interest the DNO has in the connection if I am not exporting - as it is set out in the diagram above, even if export is set to 0 - there is significant potential for micro exports (and hence the DNO is likely to have concerns). However if my system is completely isolated from micro exports, then the DNO should not be concerned?

 

[zanydroid]

So as said before there is a control loop in a hybrid to adjust power to seek to achieve the export target (could be zero). The specifics depend on AC vs DC coupling. What level of detail do you need? There are slide decks out there showing it all the way down to the inverter level, including power factor matching… most people don’t need that.

SBU is usually an off grid concept. In that case the power supply source is controlled by a transfer relay when selecting between local solar/battery and utility. The split between solar and battery is done by a control loop.

 

[zanydroid]

The reason I am asking how the inverter interacts with the grid is because I want to know how much of an interest the DNO has in the connection if I am not exporting - as it is set out in the diagram above, even if export is set to 0 - there is significant potential for micro exports (and hence the DNO is likely to have concerns). However if my system is completely isolated from micro exports, then the DNO should not be concerned?

Not super relevant to you, but I was actually reviewing interconnection policy documents (the official ones, not blogs) in California when I couldn’t sleep this morning, and zero export with unintentional spurious export is a class of system you can get approval for. There are rules for system performance parameters and forms for it

Unintentional exports within the performance parameters for compliance are fine (since those guardrails exist and they are probably monitoring at the smart meter).

 

[kommando]

The reason I am asking how the inverter interacts with the grid is because I want to know how much of an interest the DNO has in the connection if I am not exporting - as it is set out in the diagram above, even if export is set to 0 - there is significant potential for micro exports (and hence the DNO is likely to have concerns). However if my system is completely isolated from micro exports, then the DNO should not be concerned?

Your 10KW inverter breaks G98, so cannot be connected to the grid without G99. They assume any inverter connected to the grid will produce 100% export if there is no house/farm load.

My solution for not wanting G99 was 2 systems.

On grid single G98 3.68 kw inverter, excess goes to hot water.

Off grid 6kw connected to two SMA Sunny Islands feeding batteries with 30kwh capacity.

I have a 3 way isolation switch so I can go fully off grid and the 2 systems combine with no grid connection. Fine when its sunny.

When its not Sunny I divide the 2 systems and charge the batteries and separately feed the house.

With full batteries I go off grid for as long as they last.

Winter is a killer, I am just to the east of you, solar in winter is 20% of summer if you are lucky. Turning your south facing field fences into solar panels would be good.

 

[william hamilton]

Your 10KW inverter breaks G98, so cannot be connected to the grid without G99.

My solution for not wanting G99 was 2 systems.

On grid single G98 3.68 kw inverter, excess goes to hot water.

Off grid 6kw connected to two SMA Sunny Islands feeding batteries with 30kwh capacity.

I have a 3 way isolation switch so I can go fully off grid and the 2 systems combine with no grid connection. Fine when its sunny.

When its not Sunny I divide the 2 systems and charge the batteries and separately feed the house.

G98 and g99 are both designed to allow a system to export electricity. If I want large system which is grid connected but is not set up for export, is there any way to avoid the power restrictions in these connections?

 

[william hamilton]

Your 10KW inverter breaks G98, so cannot be connected to the grid without G99.

My solution for not wanting G99 was 2 systems.

On grid single G98 3.68 kw inverter, excess goes to hot water.

Off grid 6kw connected to two SMA Sunny Islands feeding batteries with 30kwh capacity.

I have a 3 way isolation switch so I can go fully off grid and the 2 systems combine with no grid connection. Fine when its sunny.

When its not Sunny I divide the 2 systems and charge the batteries and separately feed the house.

With full batteries I go off grid for as long as they last.

Winter is a killer, I am just to the east of you, solar in winter is 20% of summer if you are lucky. Turning your south facing field fences into solar panels would be good.

Why did you not want g99 out of curiosity?

 

[william hamilton]

Not super relevant to you, but I was actually reviewing interconnection policy documents (the official ones, not blogs) in California when I couldn’t sleep this morning, and zero export with unintentional spurious export is a class of system you can get approval for. There are rules for system performance parameters and forms for it

Unintentional exports within the performance parameters for compliance are fine (since those guardrails exist and they are probably monitoring at the smart meter).

I have queried with NIE (my DNO) as to whether it is possible to have a parallel, grid connected generation system in place with no export and have asked what the performance criteria is (with respect to micro exports).

Will be interesting to see how this works, presumably sites that use generators in sync with the grid for peak shaving have an agreement like this?

 

[zanydroid]

Will be interesting to see how this works, presumably sites that use generators in sync with the grid for peak shaving have an agreement like this?

Exactly. I’m just a layperson but from skimming our governing documents I can see how they evolved from other on site generation, to solar and battery.

Another mode covered (and inherited from old school on site generator usage pattern) is momentary parallel, which is more like SBU operation. But IMO parallel with export limiting is better performing in many dimensions, if you can get approval. The momentary also has performance criteria

 

[kommando]

Why did you not want g99 out of curiosity?

G98 is inform only, G99 is ask first and only install after approval. All mine is DIY, 3 way switch was already in place for genny as grid was unreliable.