Higher PV DC Output voltage

[John Barsby]

Just been advised by a customer he had been told a higher DC output voltage (HJ Solar 250w) was better when shading was an issue.
Is this correct ?

 

[BruceB]

I cannot think of a reason that would be the case.
Probably rubbish unless anyone knows better?

 

[SolarCity]

Sounds interesting. But I don't see any reason why it should matter.

Voltage isn't affected so much by shading as current is though, so perhaps that has something to do with it?

 

[nowty]

Only thing I can think of is that those HJ panels might have an extra bypass diode. Would hardly makes a difference though.

 

[SibertSolar]

It could be a valid comment, depending on the inverter's MPP voltage range, the number of panels, their orientation, the sub-string structure of the panels, the number of bypass diodes and the nature of the shading - pretty specific circumstances relating to a variety of variables that are all installation-specific in terms of impact....not something anyone would want to generalise about though!

 

[Earthstore]

I think you would all agree here, that when there is shading, system design is paramount to negate it's affects, this can be done in so many ways, but just by using a higher voltage panel is not the answer.

 

[Gavin A]

depends on the system design, it can be relevant if it's a small system with string voltage close to the bottom end of the voltage range of the inverter, as activated bypass diodes might take it out of the voltage range, so it won't be able to cope with shading properly.

those panels also have 4 bypass diodes I think.

 

[John Barsby]

I have looked into this one now and the higher voltage is required to meet the minimum input voltage required by the SMA 3600tl inverter. using 4 strings (2 strings in input A and 2 in input B)
each string is 200v and 5A so 2 in parallel is 200v and 10A.

 

[jason121]

Try a single string inverter like Fronius or Steca output will increase

 

[John Barsby]

Try a single string inverter like Fronius or Steca output will increase

Hi Jason your right on an unshaded system but this customer has a lot of shading, so 4 strings will be used as I can't convince him to go for micro inverters.

 

[davidjacobs]

It does sound like the best option here is Micro Inverters. Is it the price that is putting the customer off? Perhaps a PV syst simulation to show the pay back increase would help?

 

[Worcester]

Someone's missing the point here, the 4 strings are only valid if attached to 4 different mppt's. In this case the two that are connected to each mmpt will affect each other, so effectively will be managed as a total of two strings not four. If shading ios that bad go for micro-inverters or power optimisers.

Note also that not all micro-inverters are equal, you also need to understand their individial mppt characteristics for them to be of any additional benefit over a well designed string system.- Bypass diokldes are there for a reason, and they work. - A single panel' Vout in partial shade could easily fall bellow the mppt value of a micro-inverter, meaning that even in very partial shade, it efffectively switches off completely, whearas it would still hvae been producing output in a string configuration.

 

[John Barsby]

Someone's missing the point here, the 4 strings are only valid if attached to 4 different mppt's. In this case the two that are connected to each mmpt will affect each other, so effectively will be managed as a total of two strings not four. If shading ios that bad go for micro-inverters or power optimisers.

Note also that not all micro-inverters are equal, you also need to understand their individial mppt characteristics for them to be of any additional benefit over a well designed string system.- Bypass diokldes are there for a reason, and they work. - A single panel' Vout in partial shade could easily fall bellow the mppt value of a micro-inverter, meaning that even in very partial shade, it efffectively switches off completely, whearas it would still hvae been producing output in a string configuration.

Hi Worcester, I think that the 4 strings would be valid even if there was single shade management as if one panel got shaded it could only possibly effect the other 3 in that string. If you had a larger number of panels in series and one got shaded it would effect more panels. With 4 strings of 4 you will get a better input into the inverters twin trackers.

 

[John Barsby]

It does sound like the best option here is Micro Inverters. Is it the price that is putting the customer off? Perhaps a PV syst simulation to show the pay back increase would help?

Hi David, Do you know where I could find some software to do this ?

 

[Gavin A]

Hi Worcester, I think that the 4 strings would be valid even if there was single shade management as if one panel got shaded it could only possibly effect the other 3 in that string. If you had a larger number of panels in series and one got shaded it would effect more panels. With 4 strings of 4 you will get a better input into the inverters twin trackers.

worcester's right.

dual string MPPT needs to work with just 2 strings in shaded conditions, doubling up like that is counter productive as the tracker will be trying to find the peak voltage on a shaded & unshaded string on the same tracker, where the peak voltage for each string will then be different - it will probably end up ignoring the shaded string anyway, and just sticking at the peak voltage for the fully unshaded system.

use 2 strings, and the shaded string tracker will optimise for the shaded conditions, so the bypass diodes will kick in and bypass the shade. At least it will if you use an SMA or Aurora with the full scan function installed to avoid the tracker ending up stuck on a false peak.

 

[davidjacobs]

Hi David, Do you know where I could find some software to do this ?

PV SYST and PV SOL are both pretty good.

 

[John Barsby]

Would be interested to know how much difference using 4 strings of 4 or 2 strings of 8 to a sma 3600tl would make.

 

[Gavin A]

all depends on the shading situation, and whether you switch on the optitrac global peak, but 4 strings of 4 is definitely worse than 2 strings of 8 - apart from anything else it puts the string voltage right at the bottom of the efficiency range, so you'll lose 1-2% throughout the year just from that.