Hybrid Inverter Install Advice

[MikeJ]

I'm hoping to install a battery system and would like advice on a Hybrid Inverter.

I'm looking to use the inverter to supply up to 3.8kW (G98 limit I think) with the rest being made up from the grid. When grid is lost, I still want the inverter to work so need an grid isolations device (automatic island mode isolator), any recommendations?

Is there anything I need to be aware of when the grid is lost as there will be higher power devices in circuit (electric shower, cooker etc). Anything I can do to mitigate this?

Can anyone suggest an N-E bond relay for use in islanding mode?

 

[binky]

Have a read the posts in the solar section, it's a subject that has been covered several times.

 

[johnb2713]

Go for a higher power inverter, at least 6kW or more.

 

[MikeJ]

Go for a higher power inverter, at least 6kW or more.

Thanks, that is obviously an option. However, I've cash limited and therefore want to consider the smaller inverters and want to be sure it would work.

 

[johnb2713]

Thanks, that is obviously an option. However, I've cash limited and therefore want to consider the smaller inverters and want to be sure it would work.

It depends what your aims are, if you're wanting to cut down your use of peak rate energy, you need to have a bigger inverter. At 3.6kW a toaster and a kettle will see you importing from the grid, oven and hob, washing machine etc. What size battery are you considering? if for example you're looking at 7kWh remember the most you can save is around 6kWh per day. If youre slightly handy and not afraid of a bit of DIY you can save a fortune by building a battery yourself (see other threads from me).

 

[MikeJ]

It depends what your aims are, if you're wanting to cut down your use of peak rate energy, you need to have a bigger inverter. At 3.6kW a toaster and a kettle will see you importing from the grid, oven and hob, washing machine etc. What size battery are you considering? if for example you're looking at 7kWh remember the most you can save is around 6kWh per day. If youre slightly handy and not afraid of a bit of DIY you can save a fortune by building a battery yourself (see other threads from me).

Yes, looking around 7kWh battery. Aim is really a combinations of things, including peak shaving and power backup. We've had a few several-hour power cuts in recent months with the storms.

I see why the bigger inverter would be better. Why would the 3.8kW inverters be more popular then? Is there much more hassle going for G99 compared to G98?

Do you have any suggestions on how to island the inverter which may not have an island mode? I don't really want the expense of wiring in second consumer unit with essentials on it.

Thanks, Mike

 

[bladerunnerpv]

Hi MikeJ, for 48V battery Pylontech are popular, they interface with lots of different inverter makes.

There are often several power related things quoted on inverter datasheets. The maximum PV power input, it's output to the AC side and to the grid (3.6kW / 16A limit), and the draw capacity from battery. A nominal '5kW' hybrid inverter could be rated to 8kW PV input, supply 5kW (PV to AC) to the house but be limited to 3.6kW to the grid, 3ish to battery (AC to DC) and have as little as 2.5kW output from battery (DC to AC) to house! So keep an eye on the backup power capability the inverter can provide to the house.

There's a blog on deege solar site which talks about different options for backup power or EPS (IIRC)

 

[johnb2713]

Yes, looking around 7kWh battery. Aim is really a combinations of things, including peak shaving and power backup. We've had a few several-hour power cuts in recent months with the storms.

I see why the bigger inverter would be better. Why would the 3.8kW inverters be more popular then? Is there much more hassle going for G99 compared to G98?

Do you have any suggestions on how to island the inverter which may not have an island mode? I don't really want the expense of wiring in second consumer unit with essentials on it.

Thanks, Mike

I believe its because the DNO has to agree to a minimum export of 3.5 kW if thats what the customer requests, any more they can refuse. As far as I know the paperwork is the same just a different form designation and process. I try not to export, storing or using all what i generate.

 

[MikeJ]

Thanks @johnb2713 and @bladerunnerpv, all starting to make sense now.

So I should be able to find a 5kW inverter that limits export to 3.8kW to remain within G98. How do they know, via the current clamp in grid feed?

Presumable these would anti-island be definition? If so, I presume I need to setup my own isolation method if grid is lost to maintain house supply. Is there an easy method to do this?

 

[johnb2713]

Thanks @johnb2713 and @bladerunnerpv, all starting to make sense now.

So I should be able to find a 5kW inverter that limits export to 3.8kW to remain within G98. How do they know, via the current clamp in grid feed?

Most inverters have a way of limiting export. My Growatt you can set a % or zero for no export.

Presumable these would anti-island be definition? If so, I presume I need to setup my own isolation method if grid is lost to maintain house supply. Is there an easy method to do this?

Essentially a change over switch on the incoming suppy, one position selects the grid input, the other selects the inverters EPS output. Obviously loads may need to be reduced before selecting the EPS output.

 

[MikeJ]

I'm curious, if the hybrid inverter has a EPS output, does that mean that it cannot export to grid any excess power generated? If it does export on the non-EPS output, how does the changeover detect grid loss if there is export occurring?

If the inverter can set a limit export to within 3.6kW but is capable of more, do you need to apply for G99 but state that export will be limited or will G98 be allowed?

 

[johnb2713]

I'm curious, if the hybrid inverter has a EPS output, does that mean that it cannot export to grid any excess power generated?

No, not at all. The 'normal' connection provides the import / export capability. When the grid fails the EPS becomes energised and power can be taken from this output in complete isolation from the 'normal' connection.

If it does export on the non-EPS output, how does the changeover detect grid loss if there is export occurring?

Remember the inverter in grid tie mode synchronises to the grid supply, it does that by measuring the incoming supply voltage and monitoring import and export current. When the grid fails the export current would go sky high because your inverter would be supplying the whole of the grid load momentarily. Your inverter would recognise this colossal load and shut down it's output as per the regulations.

If the inverter can set a limit export to within 3.6kW but is capable of more, do you need to apply for G99 but state that export will be limited or will G98 be allowed?

I dont know the answer to this one, I suspect as long as the inverter is on the approved list, this may be the case that G98 may be OK. I'm sure one of the others will clarify this shortly.

 

[Phoenix]

Remember the inverter in grid tie mode synchronises to the grid supply, it does that by measuring the incoming supply voltage and monitoring import and export current. When the grid fails the export current would go sky high because your inverter would be supplying the whole of the grid load momentarily. Your inverter would recognise this colossal load and shut down it's output as per the regulations.

Theres actually more to it it than that, current wont go sky high as you are physically limited by the output of the panels, although if it was trying to power a large disconnected island, voltage would collapse down. But its also got to avoid creating an island out of a disconnected section of grid, where there could be a few similar systems, and potentially only limted load. As I understand it a few things are measured, which certainly includes voltage but also ROCOF (rate of change of frequency) as a set of islanded PV systems will have a lot less stability than the grid, unless the island is huge