Understanding PV Wiring in Series, Parallel and Polystring

From Ronnie Raxter (guest post) on 03/05/2015 in Category Service with 8 Comments
Solar Module Stringing - SMA Inverted

Module stringing and the question of how many and in which orientations, voltage windows and, lest we forget, available roof space, play pivotal roles in system design. To properly design a system, it’s imperative to know how stringing impacts the total power delivered to an inverter.

With the vast possibilities in stringing, today’s inverter technology offers unforeseen abilities to capitalize on creative module stringing. Inverters like the Sunny Boy TL-US, with dual maximum power point tracking channels and built-in string combiners make it easy for customers without south-facing roofs to enjoy the same benefits from generating their own power.

See the power

A good way to visualize how electricity works is to relate it to something that we are already familiar with; let’s consider water.

Imagine a water tank that is full and with a half-inch opening at the bottom, the pressure from the full tank would force the water through the opening really fast. That same tank with a one-inch opening would disperse more water with less constriction.

Now, if the water tank was only half full, the pressure wouldn’t be as strong and it wouldn’t move nearly as much water. In this analogy, voltage is the water pressure, current is the size of the opening and wattage is the total amount of water that is displaced.

Sunny Boy TL-US Sample Stringing

An example of stringing the Sunny Boy TL-US with polystring configurations for both tracking channels.

Solar stringing 101

When wiring module strings together, which happens in series (e.g. positive to negative), voltage is increasing while current stays constant. When wiring multiple module strings together in parallel (e.g. positive to positive and negative to negative), current is increasing while voltage stays constant.

Looking at the adjacent image: Channel A and Channel B have two strings each that are wired in parallel on the DC combiner inputs at the inverter. The total number of modules on each channel is different, but the number of modules on each string within Channel A and B are the same (eight on Channel A, five on Channel B).

When wiring strings in parallel the current is additive, great for designing parallel strings with different orientations because the variable current will not constrict the other string. This is referred to as a polystring configuration.

Inverters like the Sunny Boy TL-US are ideally suited for systems using polystring configuration, a great design tool to have when southern roof space can’t fit the needed amount of modules or a home’s roof faces southwest and southeast. To stick with the water tank analogy, it’s like having two separate tanks so the pressure from one tank won’t adversely affect the other, yet the amount of water that comes out is the amount that’s needed.

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Ronnie Raxter (guest author)

Design Application Engineer

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8 Comments

  • avatar

    Tony Catlin

    07/12/2016 at 12:14 pm

    Hello Ronnie. I have just read your article “Basic Photovoltaic Stringing Terminology” and have a few questions. My customer is using a SunnyBoy 7.7. The design has 4 arrays each array consist of strings of 4, 14 (east facing), 13 and 8 (west facing). Do you reccomend combining the strings or can i run each string to the inverter. I’ve noticed in the DC disconnect that there are 2 inputs in the channel (A- has 2 inputs), can I run all the strings into the inputs of the inverter? Thank you for your time. I appriciate it.

    Reply »
    • avatar

      Justin Dyke

      07/14/2016 at 6:48 pm

      Hi Tony –

      I have shared your question and contact information with Ronnie and she should be in contact soon.

      Best,
      Justin

      Reply »
  • avatar

    Philip

    09/16/2018 at 5:25 am

    Hello, I have a question… I want 6 PV panels, two by two (east & west) in parallel and the three pairs in series. Is that posible?

    I hope to see in the morning The three east side panels preform well and in the afternoon the westside panels preform well. All three east west parallel PV-panel pairs will be connected in series to get higher voltage and go to my one input PV inverter. Is this a good, cheap and smart solution? Or will this not work?

    Thanks for your answer!
    Philip – The Netherlands.

    Reply »
    • avatar

      Alejandro

      09/17/2018 at 12:00 pm

      Hi Phillip! What you need to keep in mind when you string the array is never to violate the electrical limits of the inverter; Isc and Max Voltage. The inverter can handle very well different orientations through one input channel as long as the strings have the same length and the same type of modules. I recommend you take a look at our http://www.sunnydesignweb.com free software and at our YouTube Tech-Tips https://www.youtube.com/watch?v=z1sSMpqvneo&t=290s

      Reply »
  • avatar

    Solar Installation on Roof

    02/20/2019 at 11:52 pm

    Great explanation on how solar panel works. Thanks for the information.

    Reply »
    • avatar

      Alexandra

      03/04/2019 at 10:57 am

      Great to hear that you found it useful. Thanks for your comment and feedback! 🙂

      Reply »
  • avatar

    Alec

    11/06/2019 at 3:31 pm

    What bothers me with SMA is that the shutdown box output per channel is 20 amps but each terminal in the inverter is only 10amps this possibly losing half the production on that channel/string

    Reply »
    • avatar

      Mike

      11/14/2019 at 10:44 am

      SMA would recommend either using one string per box input or use parallel mode in the Sunny Boy if using two equal length strings facing the same orientation coming into the box input.

      Reply »

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