Solar panel design

[Isetech]

I am busy with a 12 kw 3 phase Sunsynk inverter

Looking at the solar panel calculations, there seems to be a lot of confusion when it comes to the max. amount of panels to be connected to the inverter. Lets start with the inverter specs.

Originally I was going to install JA 545 watt panels, however the roof space is only 14 m x 5 m , which doesn't allow for much wiggle room. So I have decided to go with 460 Watt panels, which will leave me with a little space to walk around, being a 2 story building you want some space to walk.

Spec sheet PV string data

Max. PV input power = 15600watt

15600/460= rounded off to 34 panels, because they are east west facing, we can go over the max power. the inverter will automatically cap the power.

Then we look at the PV input voltage range 550 Vdc ( 160 Vdc - 800 Vdc)

The MPPT input voltage range 200 Vdc - 650 Vdc

People say the VOC is the most important details when designing the solar. So what is the max VOC, can your panel voltage be 800 VDc or 650 Vdc?

I would normally just go with the 550 Vdc, it is in the middle.

The 460 Watt JA panel is 50.01 VOC

So we would be well within the specs if we put 11 or 12 panels per string.

Because the 12 kw inverter can have 2 strings on MPPT 1 and 1 string on MPPT 2, we would need to add up the voltage per panel per string.

MPPT - 12 x 50.1 = 600 Vdc which would still be within spec.

[Derlyn]

What about the current specs ?
What's the max MPPT current ?

You can connect 100 panels and still be within the voltage limits of the MPPT, but the max current rating of the MPPT is going to limit the number of panels.

Any thoughts ?

[Isetech]

I didnt finish what I started yesterday, had to go to a grand childs birthday party yesterday, will continue today.

What about the current specs ?
What's the max MPPT current ?

You can connect 100 panels and still be within the voltage limits of the MPPT, but the max current rating of the MPPT is going to limit the number of panels.

Any thoughts ?

[Isetech]

Before I continue, something that I dont quite understand the PV input voltage and the MPPT input voltage.

Why is the PV input voltage and the MMPT input voltage is not he same.

IF you have 160 Vdc - 800 Vdc of PV input voltage and the MPPT can only handle between 200 and 650 Vdc, then you going to damage the MPPT, right ?

If the MPPT only switches on at 200 Vdc, how will it work if the PV input is only 160Vdc ?

What am I missing?

[Isetech]

This voltage VOC can be a little confusing, I contacted the Sunsynk support and they indicated that I could have 16 panels per string (a total of 32 panels) on MPPT 1 (which can handle 2 strings).

This means you can connect a total of 48 460 watt panels to a 12 kw 3 phase inverter.

I shared my calculation, was then told I should contact technical support on Monday.

Then lets look at best case scenario:

Taking the 460 watt NOCT parameters in to account instead of the STC parameters and worse case inverter PV input voltage

460 Watt 47.38 VOC

PV input max voltage of 800 Vdc / 47.38 Vdc = 16.90 panels

This means that you can connect a total of 48 460 Watt panels to a 12 kw 3 phase inverter, anyone tried this?

So they telling me it is ok to use the NOCT parameters and the PV input voltage.

Good thing I record these discussions, imagine dropping off an inverter for a warranty claim, what do you record the chances are they will repair or replace the inverter?

What am I missing?

[Isetech]

To be on the safe side I am going to rather go with the panel STC parameters and the MPPT values.

I going to use the mid range 550 Vdc and

STC VOC parameter 50.01 Vdc

550/50.01 = 10.99

Because I have a buffer, we will go with 11 panels per string which would be well within the specs.


Total of 33 panels.

22 on MPPT 1 (which can handle 2 strings)
11 on MPPT 2 (which can handle 1 string)

[Isetech]

Now we know how many panels we are going to install we need to look at the max PV input power

As per the specs

MAX PV input power = 15600W

Panel = 460 W

15600/460= 33.9.

We fit a total of 34 panels, rather go back than up because we are already at max power.

Total panels to be installed = 33 x 460 watts = 15180W (within spec), we good to go.

We all know that you can put more because the inverter will automatically cap the power at the limit, I know this because we have an 8 kw inverter with 32 panels because of the location, shading, cloud cover angle seasons and a few other factors which were taken into account.

48 as suggested by Sunsynk customer service 460 x 22080W. I suppose it would be like having a V8 engine with an electronic management system installed, so when you stop at a robot and you are idling only 1 cylinder is firing compared to if you push the pedal flat on the floor, all 8 cylinders are firing at full performance. The power is only determined by the amount of current draw, if the panels are idle, the power is minimal, however if you switch on all the appliance in the building you have the full 15600 W capacity even in the middle of winter when the direction and angle and factors will reduce the panels capacity to produce power.

Now that we understand how that works, and why some of us add in a few panels to our installations.

[Isetech]

Now we move onto the current rating,

Inverter specs:

PV input current - 26Adc + 13Adc

Max PV Isc - 34Adc +17Adc

looking at the STC panel specs (worse case senerio)

Max power current imp - 10.92

Short circuit current Isc - 11.45

We are well within the specs.

2 x strings (MPPT 1) at 11.45Adc = 22.9Adc well within spec

1 x string (MPPT 2) = 11.45Adc

Could I add a panel or 2, I dont see why not, however I am more a 5 000 RPM kind of person, rather than a 7 000 RPM in the red line all the time kind of person.

We want these systems to last 10 years minimum and so far we are already 3 years in with some of our systems installed and no flashing red lights, tripping ELU's F56 faults and all the other crap I read about on groups. We must be doing something right.

[Isetech]

Lets look at an 8 kw inverter with 545 watt panels, this the most common setup we install.

Inverter PV data:

Max power - 10400W
PV input voltage - 370V (125-500)
MPPT range - 150-425Vdc
Startup voltage - 125V
PV input current - 26Adc + 26Adc
MAx Pv Isc - 44Adc + 44Adc
No, of MPPT trackers - 2
No. of strings per MPPT tracker - 2+2


Panel specs 545 watt using the STC values (to be safer)


Rated power - 545W
Max circuit voltage (VOC) - 49.75 Vdc
Max power voltage (Vmp) - 41.80Vdc
Short circuit current (Isc) - 13.93 Adc
Max power current (Imp) - 13.04 Adc

Before you start the calculation, you need to first check your current ratings, to verify they will be within spec

MPPT can handle 26Adc total for 2 string because you can connect 2 strings per MPPT, both 1 and 2 (2+2)

The panel can produce 13.04 Adc with a max short circuit of 13.93 Adc

Wil it work yes it can, not much of a buffer but it shouldn't be a problem.

Max power 10400/ panel wattage 545 = 19.08 panels total

The way I see this, if you had the perfect conditions, the panels facing North with a tracker to keep the panels at the right angle, no shade, middle of winter and no otehr factors that will affect the power put, then 19 would be a good number

As we know this is never the case, so we start looking at other specs to make sure we keep it within a reasonable level.

As I am sure that by now you are aware, everyone would have told you that VOC is the key factor when doing the panels design.

So lets look at the system design taking onto account the VOC

Once again lets look at 370Vdc the mid point.

370Vdc/49.75Vdc = 7.44 panels per string.

In this case we can go up because we are in the middle of the spec. so 8 would be ok.

Could you go 9, lets work it out

9 x 49.75 = 447.75 Vdc above the MPPT spec (425)

Will it work, yes it would, but you then run the risk of damage to the MPPT in mid summer on those days were the temp is 39 and the sun is dirctly on the panel at the right angle.

Would I do it, once again it depends on the site specific conditions, if for example there are lots of trees, direction is not north facing, the angle of the roof is not suitable etc etc.

As a customer who has spent a ton of money, taken a loan or used money form my pension to get a solar system installed, I would expect the installer to stay within the parameters of the equipment supplied. We all know that when it comes to warranty claims its like insurance they are going to expect to see pictures of the installation, get some form of design criteria and if you can provide a details regarding the specs which are over the recommended values, chances are your claim is out the window.

So what is the max amount of panels 545 watt panels I would connect to an 8 kw Sunsynk inverter

8 panels per string 2 x MPPT 1 = 16

8 panels per string 2 x MMPT 2 = 16

A total of 32 panels.

32 x 545 = 17440 watt which is around 7 kw more than the max DC power input.

Under normal conditions, by the middle of winter winter you could loose as much as 40 % efficiency (4 kw) plus shading, plus roof angle, direction etc etc I would say 32 panels would be a bit of an overkill.

As I mentioned you can cap the power in summer or the inverter will automatically cap it if it exceeds the max power.

My suggestion if you dont have ideal conditions, fit 5 or 6 panels per string and dont cut the rails, the inverter will collect all the data required right through summer and winter then make the decision to leave it as is or add a couple more panels per string, keeping them the quantity the same per string.

I am not an expert, so feel free to correct any information you feel might not be accurate. I can only share from my experience from what I have read in the manuals, been told by customer support, videos and the maize of information on groups and all the installations I have completed.

[Dave A]

You need to keep Voc of the strings below the PV Input Vmax of the MPPT.
This is because the way the inverter reduces the power being supplied by the panels is by increasing the voltage the MPPT operates at. Effectively the MPPT reduces power production by increasing the operating voltage in order to choke the current.
If the panels can push a voltage above the Vmax of the MPPT, things can (and one day probably will) go pop!

Next thing you need to take into account on this front is the edge-of-cloud effect. This can boost the Voc of the panels by 10%.
So unless you are trying to squeeze every last watt, set up your strings so that their Voc do not exceed 90% of the Vmax of the MPPT you are connecting the string to.
The rest of the equation relates to derating factors - angle, temperature, solar radiance per square meter. But if you leave some margin on the Voc and make sure Isc is less than Imax of the MPPT, relying on the standard label values of the panel should keep you out of trouble. If you are getting close to the limits, best you take a closer look at V-I curves and derating factors in the panel datasheets.