Inverter Layout Design - Best Practise

A suitable inverter layout design together with a proper stringing is crucial for a high and stable yield of your solar power system.

Below we give a best practise for a solar inverter layout design.


Step 1. Before installation. What you need to consider:

  • What type of Inverter: With transformer or transformerless?
    Most used today are transformerless inverters. Transformerless inverters have a higher efficiency and less weight compared to inverters with a transformer. However, for  a few types of modules, such as some thin-film modules, a galvanic isolation between the DC and AC circuits is required. In this case, transformer devices need to be used, since these devices have an appropriate separation between the AC and DC circuit.

  • Make a rough sizing and dimensioning of your strings and inverters:
    Calculate roughly how many modules you can use in each string and how many strings you will get. If you use polycrystalline modules you often obtain string lengths between 13 to 24 modules, depending on the allowed and optimal voltage range of the inverter. Monocrystalline panels have a slightly higher operating voltage than the polycrystalline modules, so the strings of mono-crystalline modules are little shorter. Finally much higher is the voltage of thin-film modules, and accordingly there are far fewer modules per string, but several strings in parallel.

     If shading on your roof occurs the modules should be grouped accordingly. Modules with similar shading should be grouped into the same string. Thus, shadow will design your string length and layout. Further, string lengths are determined by the roof geometry. Once the angle of your PV array change (eg installation of modules on two roofs with a different tilt angle), not combine the modules with different angles into a common string.

  • Dimensioning of the inverter:
    Very important is the appropriate size and dimensioning of the inverter matching with the photovoltaic array.

    Underdimensioned inverters cut the peak output of the PV array and yield et worth. An excessively large-sized inverter leads to higher cost and lower the return on investment.

    In Germany you also have to consider reactive power. Since 2012 all PV systems in Germany have to provide reactive power between 5% and 10%. The inverter must be designed accordingly larger since it has to provide the reactive power on top of the active power of the PV system.

  • Single phase or 3-phase inverter:
    In Germany, the maximum allowed unbalance between the phases must not greater than 4.6 KVA (kWp). Check your local regulation. 3phase inverters avoids any unbalance between the phase and should be preferred or even are mandatory for bigger systems.


Step 2: Selection of the inverter:

After making your initial considerations, you can now go into a detailed selection of the inverter. Besides a good price observe the following technical points:

  • Do you have shadow on your roof? MPP tracker.
    Shading on a part of the string or even on a single module in the string results in a power loss of the entire module strings. If you face a complex shading situation on your roof, you should rather use shorter strings and inverters with multiple independent MPP trackers (such as 3 MPP trackers per inverter). 

    Consider the shadow and move of the shadow over the day and year for your string design. Try to put modules with similar shadow situation into a common string. You can also use simulation programs to do a shadow survey, such as the freeware Google Sketchup.

    An MPP tracker ensures an optimal operating point of the string by changing its internal resistance. Shaded modules require a different operating point than unshaded modules.

  • Extra costs:
    Some inverters have all extras already on board. with others some extra costs are likely.

    Typical components which sometimes have to purchased on top are overvoltage protections (type 2), communication interfaces and monitoring. System monitoring is not mandatory, but useful and recommended to check the performance of the PV arrays and to detect any defects early. If your inverter doesn't have a monitoring tool on board, you can buy the monitoring box from the manifacture or also a neutral controller such as the solar log or even cheaper solutions like the "sun watch".

    Since 2012 you need in Germany also a power management for the PV system. It means if there is a grid overload, the power company can shut down your PV system with a remote control. The power management control unit need to be purchased separately.

  • Efficiency:
    High inverter efficiency is certainly one of the most important factor, and justifies a higher purchase price.

    In particular pay attention to the European efficiency and not the maximum efficiency. The European efficiency is the typical cycle of full load and partial load as it occurs in middle Europe. This information is much more realistic than the maximum efficiency.

  • Allowed voltage range of the inverter:
    Check the range of allowed MPP voltage as well as open circuit voltage of the inverter. The open circuit voltage is usually about 20% higher than the operating voltage. A large range gives more flexibility for your stringing.

    The string voltage has to be within the given range of your inverter. Also note that the string voltage depends on temperature. A low ambient or cell temperature leads to a high voltage. High temperatures at low voltages. Thus, the string voltage changes during the day and year.


Step 3: Design of the Inverter:
Your initial considerations are done. Now you can make to the final layout design.

  • Where to install the inverters?
    Keep the cable lengths short. For DC as well for AC. Short cables cut costs and lead to fewer losses. A good location is often at a centre point of the PV array. Keep also in mind that a clever stringing can also save cable miles.

    In addition, the inverters should be installed in a cool place, protected from sun and best also from rain with low air pollution.

    Think about fire safety. Never install the inverter on standard wood.  If you have a wood construction, use a fire-resistant wood surface between the inverter and your standard wood.

    Check carefully the minimum vertical and horizontal distances as given in the manual.

  • Voltage range:
    One of the most important points for a inverter layout design is to fit the optimal voltage range of the inverter with your strings.

    A distinction is made between the MPP voltage and open circuit voltage. The MPP voltage is the voltage under load (the sun). The open circuit voltage is the voltage with no load.

    The voltage is related to the string lengths (see below) and is also a function of temperature. With decreasing ambient temperature the cell voltage increases. With increasing ambient temperature the voltage decreases.

    The string lengths must be designed that they are within the given voltage range of the inverters.

    Take also a look of the efficiency / performance diagram of the inverter n the data sheet. The diagram shows the efficiency as a function of voltage and load. Depending on the topology of the inverter, the best efficiency can be at a low voltages point but for other topologies at high voltages. So it is quite individual and there is no general rule about the optimal voltage point. Your strings should ideally match the characteristic of the inverter.

Do not look only on the efficiency at full load. Most of the time in middle Europe PV systems run only under partial load. Therefore, the partial load range is more important than the efficiency at maximum load. The following table shows the norm load cycles in middle Europe. This load cycles are used for the determination of the European efficiency.

Typical Load Cycles in Europe: European Efficiency
Load P / Pnominal Percentage in Europe
5% 3%
10% 6%
20% 13%
30% 10%
50% 48%
100% 20%


Inverter Characteristics

Inverter characteristic diagram

 

Inverter characteristic diagram

Example of inverter characteristic diagram. The graphs showing the efficiency over voltage and load

Photovoltaic systems in middle Europe are mainly working under partly load between 50% and 70%. Therefore, you should design your string lengths that a good effiency is also covered for partly load.

In this example, the best range is about 650Volt. Note that the module MPP voltage in the manufacture data sheet is given under stadnard test conditions with 25°C. In reality higher ambient temperatures than STC (higher temperatures means a drop in voltage) and cable losses are likely. For this reason the string MPP voltage should be slightly high than the optimal voltage area of the inverter.

For this example: If you use polycrystalline modules with 30V MPP voltage the optimal string length is between 22-23 modules. This results in a string voltage at STC of 660 - 690V. In the real PV system, the string voltage for most time is slightly lower and you will match the best efficiency range of the inverter.

 

  • Current:
    When using only one string per MPP tracker the current correspondents exactly to the power of one module. You find the MPP current of the module in the module data sheet (expressed in amperes A). If you use parallel strings the current is add up according to the number of parallel strings. 3 strings thus means triple power.

    Also check the given current range of the inverter and ensure that the string current is within the range. The number of allowed parallel strings will be limited by the maximum allowed current of the inverter. Also note, it is recommended to use a string fuse to protect reverse current  if you use more than 2 parallel strings. The fuse rating should match with the maximum allowed reverse current of the modules as given in the data sheet. If you don't use a string fuse, the modules can be damaged in case of a failure.

  • Stringing:
    Strings can be connected to each MPP tracker individually as single string or as parallel strings. As described above, if you use parallel strings current will increase. Make sure to be within the allowed current range of the inverter.

    More important. In no case shall the minimum string voltage below the allowed MPP voltage and in particular the maximum string voltage not exceed the MPP voltage. To calculate the voltage, not just take the values ​​under STC (standard test condition)s, but check the extreme values which occurs at very low and high temperature (remember that the string voltage is a function of ambient temperature). So, check the open circuit voltage in the string at very low temperatures = maximum voltage occurring in the string; must be less than the maximum tolerances on MPP voltage. Also check the other extreme: The string voltage at high temperatures = minimum voltage occurring in the string; must be greater than the minimum allowable sine MPP voltage.

    Every inverter manufacturer offers in-house design software you can download for free from the manufacturer homepage. Use the software to design your system.

  • Sizing of the inverter:
    With your first rough design you already did a preliminary dimensioning of the inverter.

    The optimum inverter means low cost without any lost in output. This gives a dimensioning around the point where the inverter starts to cut the peak output. 

    Lets assume you have an expected maximum global irradiation at of 950W/m² at your site (you can get the daily, maximum irradiation for your roof with the free online tool PVGIS; find more below). In this case, an inverter dimensioning 95% of rated module power is suitable.

    Modules are rated at 1000W/m² and standard temperature of 25 ° C. A maximum global radiation of 950W / m² means that the module provides maximal 95% of its rated capacity.

    The maximum exposure usually occurs in early summer days (such as May or June) and the cell temperature is already relative high. A high temperature causes loss of the output . The losses due to temperature in middle Europe are often between 2% to 5%. At hot summer days even higher and at cold, sunny winter days lower  (for polycrystalline modules the loss with increasing temperature is typically 0.35% to 4% per ° C increase of temperature, look into your data sheet for details).

    Since you have the maximum irradiation at warm summer days, you can also assume that you have some losses in the modules. Therefore you can even dimension the inverter below the maximum expected irradiation. Like stated above, a value of about 2% to 5% is okay.

    To summarize, if you expect an maximum irradiation at your roof of 950 W/m² your inverter size can be about 90% of the module size. For a 11kWp system a 10kWp (KVA) inverter is suitable.

    How to determine the maximum daily radiation for your roof?
    Use the free online software PVGIS to evaluate the maximum for your site and module orientation / tilt angle.

    You find PVGIS here: http://re.jrc.ec.europa.eu/pvgis/

    Click the "Europe" symbol. Then go to "Daily Radiation". Select your location, select the new climate model "Climate-SAF", enter your location and PV layout parameters. Choose the early summer months like May or June and check the daily irradiation for your site.

    Find below an example of the daily radiation for a south-facing roof with 20 ° tilt angle in Berlin. In June, the maximum irridation for this site 995W / m² at  12.00 noon (means for a south roof in Berlin you can expect up to the nominal power of the modules of 1000W / m² and inverter size should be about 95% considering losses due to temperature).PVGIS Maximale Tageseintrahlung

 


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