Sunny Design: Polystring Configuration

From on 12/04/2012 in Category Technology with 16 Comments
Sunny Design
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.

 

5.00 avg. rating (98% score) - 4 votes

The Author

avatar

My Title:
Sr. Technical Service Specialist
My Focus:
High level technical support of SMA communications and software products
My Hobbies: Electronics tinkerer, puzzle games like Sudoku, reading (mostly non-fiction, technical)

Tags

Related Posts

16 Comments

  • avatar

    Doug

    12/13/2012 at 5:53 am

    Interesting article.
    Now, when is SMA to start selling micro inverters? They were supposed to go on market last summer.

    Reply »
    • avatar

      Melissa Womack

      12/13/2012 at 8:27 am

      Hi Doug,
      We eagerly await the debut of SMA’s Sunny Boy 240 micro inverter, just as you do. It was previewed last summer and we expect them in 2013.
      Regards!
      Melissa

      Reply »
  • avatar

    Colin Roberts

    12/19/2012 at 2:23 pm

    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 »
    • avatar

      Melissa Womack

      12/19/2012 at 3:32 pm

      Hi Colin,

      Thank you for your inquiry. We would be happy to answer your questions regarding the parameter and limitations. Please e-mail all questions to SocialMedia@SMA-America.com

      Regards,
      Melissa

      Reply »
  • avatar

    […] 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 […]

    Reply »
  • avatar

    […] 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 […]

    Reply »
  • avatar

    Md Shafiul Azam

    03/15/2015 at 12:13 am

    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 »
    • avatar

      Justin Dyke

      03/17/2015 at 4:25 pm

      Hi MD,

      It’s best to consider the inverter’s input A and B as two separate inverters, they don’t have any impact on each other. It’s important to remember that voltage is additive in series (current stays constant) and current is additive in parallel (voltage stays constant). When your value is additive it does not need to be identical (because you’re adding them together), however, when your value needs to stay constant it needs to be identical or it will negatively affect the efficiency.

      Hope this clears things up for you,
      Justin

      Reply »
  • avatar

    Beria

    02/02/2016 at 7:16 pm

    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 »
    • avatar

      Justin Dyke

      02/04/2016 at 4:22 pm

      Hi Beria,

      Unfortunately the Sunny Tripower 25000TL is not a North American product and we don’t have the necessary knowledge to answer your question. Please let me know what country the inverter/system is planned for and I can connect you with a more local subsidiary that can help with your design question.

      Best,
      Justin

      Reply »
  • avatar

    Jase

    03/22/2016 at 6:27 am

    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 »
    • avatar

      Justin Dyke

      03/22/2016 at 10:10 am

      Hi Jase –

      Sorry to hear that so many of your local installers are not aware of the potentials with polystring system designs, there are lots of great potential system designs but it does take a careful eye into design specs to make sure you don’t incur losses.

      1. For the best performance in the polystring configuration, it is crucial for VMP of the strings to be as close to identical as possible. We can’t comment on module degredation rates, so it might be best to talk to the module manufacturer to inquire about the degredation rates they have seen in their testing.

      2. It’s not the current that matters, it’s the voltage that is crucial to have as close as possible to the same. However, you’re likely within an acceptable range.

      3. While it’s difficult to discuss shade impacts without knowing more, the short answer is yes. Shade will sag voltage on that string and thus impact the overall string voltage and potentially pull that inverter channel off the MPP, creating losses while the shade is present. Are there ways to reduce the shade impacts in system design or tree trimming?

      Reply »
      • avatar

        jase

        03/23/2016 at 3:59 pm

        Hi Justin,

        Unfortunately, the west shading is permanent. I live in a small valley, and there are neighbouring massive trees that can’t be removed.

        The shading happens on that string from about 3:30pm in winter, and 5:30pm in winter. That’s why I’m relucant to put on more panels on the west, and would rather put east panels to improve the spread of power generation in the day. Will the Polystring set up do this?

        Can I just accept, that with the shading & the new polystring set up, that those times that the shading occurs, this string won’t work? Or will it cause other problems?

        Is Polystring essentially a new series string in parallel to the existing series string?

      • avatar

        Justin Dyke

        03/23/2016 at 5:04 pm

        Hi Jase,

        We don’t want to say that the string won’t work, but it will most likely work below its optimal level and not make the amount of energy it would normally rated for. It would be prudent to research the costs for installing the added string and compare to a worst-case energy production to make sure that it makes financial sense.

        Yes, polystring is a way to say two strings of modules running in parallel to a common inverter input channel.

        Best,
        Justin

  • avatar

    Mark Jenkins

    04/15/2016 at 6:20 pm

    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 »
    • avatar

      Justin Dyke

      04/18/2016 at 11:11 am

      Hi Mark,

      The best way to check your system design options and review possible annual energy gains/losses would be to enter everything into Sunny Design and begin to make small changes to the planned system expansion to see what works best in the simulation. Sunny Design can accommodate three sub-arrays so you should be able to enter all of your information and begin to modify the system to see what is possible and where you may run into trouble.

      All the best,
      Justin

      Reply »

leave comment

 characters available