Sunny Design: Polystring Configuration

This post was originally published in 2012. The tips and techniques explained may be outdated.

What happens if I have an east-facing PV array and a west-facing PV array connected to the same string inverter with one MPP tracker?

This is a fairly common question that SMA Technical Support receives from customers and installers. There is only one answer that is correct 100% of the time: Do the design!

Running a computer simulation like Sunny Design or doing manual calculations is the only way to precisely know what the optimum system size and expected yield will be.

One thing we can say for sure is that connecting east and west-facing PV arrays to a string inverter with one MPP tracker will not break the inverter or void the warranty (assuming the install and design were done per SMA requirements).  In most cases the inverter will operate and produce power.

How much power?

That is the where we have to take a closer look at the system behavior.

SMA Solar Technology AG has published a study involving “Polystring Operation”  which is defined as two or more strings at different orientations on one MPP tracker.

There are, of course, many possibilities for strings in different orientations. In this post I will focus on the scenario of a direct east-facing string and direct west-facing string connected to a single MPP tracking inverter.

The first thing to note is that the power produced by the PV array is directly proportional to the amount of sunlight it is exposed to. As the sun rises and moves across the sky, the power production on a south-facing array will produce a traditional bell curve graph. However, on a PV array facing due east, there is significantly more sunlight in the morning than in the afternoon.

Likewise, for a due west array, there will be much more sunlight in the afternoon. So, these strings are producing their peak power at different times. You essentially have two smaller bell curves, one for each string, instead of one larger combined curve. The east-facing string’s power curve peaks in the morning and the west peaks in the afternoon, with some overlap in the middle.

What does this mean?

According to the SMA study, “With non-uniformly aligned module a surface, less inverter power is required.”

Let’s say you have a 6 kW PV system, two strings of 3 kW in parallel, one facing east and one facing west. The peak power of the two strings is not going to happen at the same time, so the inverter is not going to see a 6 kW DC input. This means the system can be operated with a smaller inverter, which reduces the overall cost of the system.

How much smaller of an inverter?

That’s where the design is necessary; the exact orientation and inclination angles are the most important factors.

With arrays that are not facing due east or west and are facing some degree of southeast or southwest, the power curves for the two strings will overlap to a higher degree. The inclination angle of the array is also a factor. The steeper the angle, the sharper the rise and fall of the power curve, which affects the degree of overlap between the curves. These factors must be taken in to account when sizing the inverter.

Wait, won’t the two string voltages be different if they are facing opposite directions?  

No!  Luckily unbalanced string voltage is a completely separate issue.  As the SMA study says, “The currents from the PV modules may vary significantly in the substrings over the course of the day.  However, the voltage at MPP is practically identical.”  This is because it only takes a small amount of light for the modules to achieve their rated DC Voltage.

Overall, it is not an optimum system design to have strings at significantly different orientations. However, there are instances where different orientations are the only option or the desired option. The results of the SMA study shows that you can operate a string inverter with a single MPP tracker with strings at different orientations. One test that was done in the study compares a single MPP tracking inverter with a dual MPP tracking inverter, both with due east and due west facing strings and the same site conditions.  The results were only a 0.25% yield loss for the single MPP inverter.

Sunny Design Web Now Available

SMA has released an online version of Sunny Design with sizing systems using different string orientations, and Polystring Operation is a new feature! Sunny Design Web offers the same great features as the desktop software in an easy-to-use web format.

Visit SMA America’s YouTube playlist to view a short video introduction on Sunny Design Web.

 

29 Comments
  1. Colin Roberts
    Colin Roberts says:

    Hello,
    Thank you for posting this study. After reviewing this, i have several questions regarding the test parameters and limitations. Please let me know if you’re available to provide any more detail.

    Thank you,
    Colin

    Reply
  2. Md Shafiul Azam
    Md Shafiul Azam says:

    hello if i want to 2 input like different voltage and different current then what happened? or current same voltage same then what happened? Can i input different voltage or current? please tell me. i am very confused.

    Reply
  3. Beria
    Beria says:

    will 25000TL SMA inverter produce its maximum power if connected with 4 strings of 12 modules and 4 strings of 13 modules.?

    in other words is it okay to connect strings of different number of modules to a SMA inverter?

    Reply
  4. Jase
    Jase says:

    Hi,

    Speaking to a number of SMA installers, most of them don’t know what Polystrings are – which is a huge worry. Therefore it is difficult to get good advice on this topic.

    I’ve an SMA 5000TL21 with Q Cell G3Pro 12x255w modules on one string (North) & Q Cell G3Pro 6x255w modules on another string (West). I wanted to add a polystring (6×255 East) with the West string. I live in Southern Australia

    1-My existing panels are nearly 2 years old. If I can get identical panels, will the normal degradation of performance of the exisiting panels cause an issue vs the brand new panels?

    2-If I can’t get exactly the same panels (but can get similar specs with same brand but similar Voltage / Current, would this work as well (There seems to have been a model refresh despite the same model name, there is <0.5V and <0.5A difference in specs)

    3- I get some shading late in the day in the west. Will the new east Polystring cause issues?

    Thank you

    Reply
  5. Mark Jenkins
    Mark Jenkins says:

    Question for PolyStringing.

    I have a recently installed 4000 TL-US with two strings of panels attached to the A & B MPPT

    channels. Both arrays are 295W x 7 (30V/9.5A modules each) so 210V @ 9.5A each MPPT input. Both

    arrays are facing West (260 degrees) and tilt about 25-30 degrees. All is working pretty well,

    although I was expecting more AC power than around 3.1KW total from the array, but oh well.

    So my question is this. I want to add another 2Kw array on the East facing roof line, 80 D AZ, and

    tie it into the existing inverter. At first glance, one would assume to parallel combine the 2 West arrays

    on MPPT channel A, and place the East array on Channel B.

    However, the West array Max I could be 18-19 A, but the 4000TL has a max input of 15A per

    channel. I would be throwing away 4 amps, maybe 800W. I’m greedy and don’t want to lose this

    power.

    My thought would be to parallel combine one of the West arrays with the East array on Channel A, and have the 2nd West array on Channel B. This would give the A channel a max of around 14-15A, and not overdrive the inverter channel. I think this can be done, and I would make sure the array
    voltages’ on the East and West arrays are the same (same number of panels) Of course in the morning

    the East would light up long before the West array (my intention) but the East Voltage could be

    greater than the West, and how well would that balance. I suppose at some point the West array

    would catch up in V, but would I loose the power of the very early sun. I’m in Portland, OR at 45 d

    North Lat.

    Thanks for any advice you might provide

    Mark Jenkins

    Reply
  6. Luis Namba
    Luis Namba says:

    Hi guys. I have one question. A case where there are 24 modules pv, divided 2×12, west and east, how huch efficience it would lose/win if I connect 2×12 in parallel in MPPT A versus one in MPPT A and other in MPPT B

    Reply
  7. Ziyad Saeed
    Ziyad Saeed says:

    Fronius has the same answer. That the strings should be connected to a single inverter instead of two separate inverters. However, all new inverters come with atleast 2 mmpt trackers, so might as well use them. Or do you suggest to use only one of the mmpt tracker and leave the other one empty?

    Reply
  8. Guus Luppens
    Guus Luppens says:

    Hi,

    what if one string of the polystring is mounted on a flat roof with a small angle and the other string is mounted at the facade. When snow covers the top PV-area, the voltage will be zero. But the facade string will get sunlight. How does this affect the polystring output?

    Thanks

    Reply
  9. Handson
    Handson says:

    I would like to install a sunny tripower 25000 with 260watts panels. How many strings should I consider or should I consider the use of combiner boxes.

    Reply
  10. Tharushi
    Tharushi says:

    Is there any way we can change the maximum number of PV arrays in a subproject? According to the website we cannot add more than 3 arrays. Is there anyway this can be changed? I want to have 4 arrays.

    Reply
  11. Steve
    Steve says:

    My existing PV system consists of 15 Canadian Solar CS6-X 320P and a Sunny Boy 7700TL-US inverter. I have 9 panels connected to Input A and 6 panels connected to Input B at the DC disconnect. All panels face south at a 27 degree angle of inclination and are unobstructed. Since I installed this system in 2016, my power consumption has increased due to the purchase of an EV. I intentionally installed a larger inverter than I needed for the possibility of future expansion and also pulled an extra pair of #10 THHN wires from my Jbox on the roof to the DC disconnect. I would now like to add 4 new panels to increase production. Unfortunately, I am not able to purchase identical panels. I’m considering adding either Hanwha Q.PLUS LG4.2 345 or JINKO JKM320P-72. The voltage and current specifications of the Jinko panels are close but not an exact match to the existing Canadian Solar panels. The cost differential for the Q.PLUS 345W panels is insignificant. If I purchase and install the Q.PLUS panels and connect them to Input B at the DC disconnect, will the inverter be capable of taking advantage of the additional power of the higher wattage panels? I attempted to create this configuration in Sunny Design but was unable to set-up 3 strings with two different panel types. The new panels will be installed facing South adjacent to the existing panels. Thanks.

    Reply

Trackbacks & Pingbacks

  1. Homepage says:

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    […] There you will find 89354 more Infos: sma-sunny.com/us/sunny-design-polystring-configuration/ […]

  2. […] install was a single MPPT input or a dual MPPT input. I have read posts by tech advisors of SMA (Sunny Design: Polystring Configuration | SMA Inverted) and Fronius (http://www.energymatters.com.au/imag…olar-paper.pdf) that you dont have to use a […]

  3. […] raise a thorny subject: Interesting article here from SMA: Sunny Design: Polystring Configuration | SMA Inverted and their paper here: http://www.smainverted.com/files/201…-TEN122510.pdf This came out of […]

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