Solar panel design and installation

[Isetech]

My understanding is that is reason for 2 MPPT's, is MPPT1 and MPPT2 work independent of each other.

It is better to keep the 2 strings on the MPPT 1 as close as possible, but string 2 and 2.1 on MPPT 2 can be completely different from MPPT 1.

This is what makes those home made DC control boxes so dangerous when connected to Sunsynk inverters with dual MPPT's.

There should be a separate DC control box for each MPPT and there should be a 4 pole DC isolator in each DC control box, isolating both strings, which should also be a metal box, considering they have been identified as a fire risk.

There should never be power on the bottom of an main switch/ isolator, ever. I leant the hard way.

The 8 kw Sunsynk 2+2 in the Sunsynk manual identifies as MPPT 1 (with 2 strings) + MPPT2 (with 2 strings)

The 12 kw 3 phase unit has 2+1 MPPT 1 2 strings and MPPT 2 (only 1 string)

I am sure this confuses a lot of people, like the 50 amp pass through current, very few people understand what the pass through current is all about.

Just I note, and I stand to be corrected here, I am not sure if you can use 2 different voltages on each MPPT input on the inverter.

[Andrew_van_Zyl]

Perfectly fine - just don't exceed the maximum.

[Isetech]

You cant exceed the max power, the Sunsynk inverter automatically caps the power as a default setting. we normally adjust cap to suit the plant.

I dont know about the 26 amp current rating, but I do know that you must never exceed the VOC, in fact I make sure we never go over the 450 absolute max.

Perfectly fine - just don't exceed the maximum.

[Isetech]

Reading the 8 kw installer manual I found this rather interesting ... page 76 "Inverter commissioning info" under solar ... ensure both MPPT's are balanced.

Just I note, and I stand to be corrected here, I am not sure if you can use 2 different voltages on each MPPT input on the inverter.

[Andrew_van_Zyl]

Not sure what they mean by "balanced" but in my experience inputs to MPPT's are rarely exactly the same as so many factors come into play like roof design, module layout, orientation, old vs new modules, shading, etc.

Only on my own roof do I have EXACTLY the same inputs into my 2 mppt's but that's only because I am blessed with a vast north facing roof.

In my humble opinion the mere fact that (Sunsynk) inverters have 2 mppt's is to deal with precisely the above inconsistent roof issues.

[Isetech]

This is where I got the information... one of the many Sunsynk training documents I have been reading since the beginning of 2022.

If you link 2 strings in parallel, they must be identical, but you can mix panels between MPPT's.

Copy and paste:

WARNING:

If the voltage is too high on the MPPT, you can burn out the front-end. DC to DC converter damaging also invalidates your warranty. Keep your Voc below 450V.

Regardless of the power of the solar panel, the inverter will only draw what it needs (basic ohms law).

Please refer to our training manual part one. When wiring to strings in parallel, please make sure the strings have identical power, identical size, and identical manufacturer. Do not mix panels if one string is only 8 panels and the other 9. This will create an imbalance and cause major efficiency losses. However, you can mix panels between the MPPTs since both inputs work independently.

[Isetech]

Important to take note of this:

6.1. WIRING PRINCIPLES In photovoltaic systems, the panels can be associated to generate more power, forming arrays. The modules can be connected in series or parallel depending on the application. When modules are associated in series, the total voltage increases and the current is the same. On the other hand, when modules are associated in parallel, the voltage is the same, but the total current is the sum of all currents.

What this means is that the 2+2 for the Sunsynk inverter the first 2 (string 1 and 1.1) will be connected in parallel and must be exactly the same. If you have 4 x 545 watt panels in string one, then string 1.1 must also have the identical panels. 4 x 545 watt panels.

the second 2 can also have 2 strings in parallel string 2 has 8 x 545 watt and string 2.1 must also have 8 x 545 watt

When the 8 panels are connected in series the voltage (49.75 VDC x 8) will add up but the current stays the same 13 amp, as soon as you put the 2 in parallel then the voltage stays (398 VDC) the same and current (13 x 2) adds up.

[GCE]

So you need to push as much power back into the grid to recover these cost. Having 6 panels on the roof with an 8 kw unit was a nice load shedding solutions, to top up your batteries without using the grid power, but will become a waste of money once they force you to register your system.

Just think if you install a 16 kw unit x R143.75 = R2300 monthly charge, with a feed back tarriff of only around R1.40 , you gonna need a lot of panels.

You may get a surprise using the above logic
Nersa restricts to 25% of CB size being allowed to feed back and at the end of the day municipalities are going to have to restrict feed back to enable transformers to stay loaded at at least 25% of capacity otherwise the network will be unstable.

If you look at Cape Town there is a limit to the amount of people that can feed back

[Derlyn]

It seems a bit one sided that the feedback is limited to 25% of the breaker size, the reason for which I fully understand, but the charges are calculated purely on the inverter size and do not take the breaker size into account.

[GCE]

It seems a bit one sided that the feedback is limited to 25% of the breaker size, the reason for which I fully understand, but the charges are calculated purely on the inverter size and do not take the breaker size into account.

So far I have only heard of KZN (from Isetech) charging per inverter Kw - In PE it is a str availability charge of R147 + vat and they will only zero on units used/fed back in the same month and there is a stipulation of same time period , in other words peak for peak or off peak for off peak

I would think that KZN will also only zero on KW used not on availability charge

Cape town they way I understand is also only on KWH used that feedback will credit - Availability stays fixed and not everyone will be able to get "cash back" for feeding excess , that is a separate issue - I am under correction