question

Hi

Well I had the 150v MPPT from the previous smaller setup. It has been doing the job for a year now. The Current array is small. 15x275w panels. so 4.1Kw

The power consumption has just grown over time.

How much more power do you need? I want to add about 10Kw.

When is the additional power needed? During day time, or during night time, or both?

To give you a bit of background. The system is installed on a weekend farm. So the demand during the week is low. The idea has always been that the generator should not start during the week. And on weekends it is ok if it starts during the night but not during the day. So I want to stop the generator from starting up during the day, Currently I can't keep the battery @ 100% during the day.

What will new array size be? Additional batteries? Size? No extra batteries. The generator can start at night, I want to install roughly 10Kw

Are the phases fairly balanced, or are they unbalanced? Is additional power needed equally over all 3 phases? If not, how much difference? The three different phase supply three different houses. So worst case will be if the two inverter A/Cs are running in one house and nothing in the other houses. Then the in-balance will be around 2.6 Kw. The other big consumers is a 4Kw pump and a cold room but they are three phase.

I'm thinking that additional MPPTs are going to work the best but it will be very expensive in terms of wiring and additional components. MPPTs that are the same as the one I already have are R8300 each. Ill need thee more. And Ill need a R5000 combiner box for each. So thats around R40k. A Huawei 12Kw three phase inverter is only R20k.

Hi @Wilhelm1987,

Due to the 80m distance between the current system and the location of the add-on system, probably the most important aspect to consider, is voltage drop.

Considerations for options you have mentioned:

1) 150V DC back to system, with 2-3 MPPTS;

If you use the MPPT option, I would certainly recommend using the 250V, instead of the 150V MPPT. You want to get DC volts as high as possible, to limit % voltage drop over the distance.

For 10kWp array, with 325Wp panels, you will need 2 x 100/250 SmartSolar MPPT's.

Depending on the size panels you use, as well as configuration, the actual string voltage (corrected for lowest temperature) could be between 150 and 240V.

There will be 2 options for placing the combiner box: a) to have combiner box at panels, b) or at current system.

Option a will require less, but thicker cables from array to system, while option b will require more, but thinner cables, than option a.

You will have to calculate voltage drop as well as cost and practical implications, for both options, to see which is most feasible. You want to limit your total voltage drop to not more than 5 %.

2) Comms with 3 phase GTI?

Depending on which grid tied inverter you choose, you will have to check with the relevant supplier, with regards to maximum comms range. There are inverters that are quite comfortable with up to 100m range.

3) 3 x single phase PV inverters in 3 phase configuration.

Generally, 3 x single phase inverters, will be more expensive than 1 x 3 phase inverter. If loads are (constantly) very unbalanced, then single phase inverters of different sizes, could be more applicable. In this case, it would not be in 3-phase configuration, but each feeding into a single phase.

4) 3 phase PV inverter

With a grid-tied inverter, your DC input voltage will typically be in the region of 500-800V, depending on quantity of panels per sting as well as panel size & voltage. This is substantially more than the DC volts in option 1 (150-240V), and for this reason, in my opinion this would most probably be your best option. To eliminate (possible) comms challenges between the grid-tied inverter and the existing system, place the grid-tied inverter at the existing system and run high voltage DC from the panels to the grid-tied inverter.

Of course, if 3 x single phase inverters is relevant (option 3 above), then the same will apply.

A very important aspect to remember when choosing a AC coupled system in off-grid application, is to ensure the grid-tied inverter is capable of throttling, when the loads are reduced, as no grid feedback is possible.

The information I offered above is based on the information supplied, and is by no means complete and/or definitive. A lot of factors can influence, the various decisions to be made in designing the best-fit / ideal system for the given application / situation.

I have tried to answer as best I can, but if you need to discuss further, you are welcome to contact me via email: anita@allsolar.co.za.

Good luck!

Anita