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Solar inverter string Design Calculations

In this in-depth post, you will learn how to design, calculate, and size a 4.5 KW grid-tied solar PV system for your home and we will learn about solar inverter string design calculations using an excel sheet. To help readers understand the design process practically, examples and calculators are supplied for each formula used in this post to calculate various items, so that readers can read and practice at the same time.

To learn how to build your own solar PV system, you must first go through the procedures listed below.

Main objective

To Design a solar PV system with a 4.5 KW solar array installation that can produce and supply 6310 KWhrs of energy annually.

You might have varied needs for energy criteria. You can use the links provided above and below to calculate your individual energy needs and the size of your PV system.

Conditions of the site

In our situation, we wish to install a PV system on a home’s rooftop. The rooftop area is approximately 32 by 14 feet, which provides ample space for the array as well as the installation margins typically required by the building department.

Create a simple rooftop map showing the precise location of the PV installation area and its measurements. Include any roof edges near the array, the ridge, the eave, and any other edges, as well as any impediments or penetrations you need to watch out for during installation. Note the roof azimuth and tilt as well as the module arrangement.

In our case module orientation: portrait, Azimuth: 205 degrees, Tilt: 23 degrees.

Solar PV Module specifications

In our particular case, The Helios 6T-250 module was chosen for solar inverter string design calculations. Make a section for module specifications, noting the following values from the manufacturer’s spec sheet or nameplate found on the backside of each PV module.

Pmax: 250 Watts

V pmax: 30.3 Volts

I pmax: 8.22 Amps

V oc: 37.4 Volts

I sc: 8.72 Amps

Length: 66.1 inch

Width: 39 inch

Strategies for designing PV systems step by step are presented solely on a single page.

Required number of Solar PV modules

To calculate the number of PV modules in the array, divide the DC system size (4.5 kW) by the individual PV module wattage.

4,500 watts ÷ 250 watts per module = 18 modules

The DC system size is nothing but the array size we calculated using PVWatts. According to PVWatts, the 4.5 kW (DC) system in this survey can generate approximately 6,310 kWhrs of AC energy per year. Confirm that the DC system size you’re designing will meet your annual goal of AC energy production at your location, with your array orientation, azimuth, and tilt, when doing your own design.

It is now your turn to calculate the number of PV modules in the array based on your needs.

Document

Solar PV array layout

The modules in our demonstration are oriented in a portrait arrangement; yours may be in a landscape position. In order to convert a roof dimension from feet to inches, multiply it by 12:

32 feet × 12 = 384 inches

Divide the total length by the module width:

384 inches ÷ 39 inches = 9.85

Therefore, you can fit 9 modules in a row in your solar PV system

9 modules × 39 inches = 351 inches

Therefore, 384 inches of available space – 351 inches of required space = 33 inches of total space remain

33 ÷ 2 = 16.5 inches on each side

This is more than enough space to meet the building department’s 5 to 10-inch spacing requirement (for wind uplift at the roof edges).

Then, look at the other dimension to see how many rows will fit on the roof. Like before to convert a roof dimension from feet to inches, multiply it by 12:

14 feet × 12 = 168 inches

168 inches ÷ 66.1 inches (module length) = 2.54

Therefore, You can fit two rows of modules in your solar PV system

2 rows × 66.1 inches (module length) = 132.2 inches

Therefore, 168 inches of available space – 132.2 inches of required space = 35.8 inches of total space remain

35.8 ÷ 2 = 17.9 inches on each side

However, there are two strong reasons to relocate the array toward the ridge: (1) more roof space below the array helps break up and contain snow that slides off the array, and (2) more space along the eave provides a place to stand for installation and future maintenance. A good compromise, in this case, is to leave 12 inches of space at the ridge and 23.8 inches along the eave. Both of these exceed the previously mentioned 5-10-inch requirement.

Solar PV string layout

During solar inverter string design calculations, String size should be determined based on the total number of modules (divide into two, three, or more strings) and installation area (which arrangement fits best on the roof while meeting code requirements). For best performance, series strings must have an equal number of modules in each string.

18 modules total
9 modules per series-string
2 series strings wired in parallel

If the total number of modules does not divide neatly into equal-sized strings, either add or subtract a module or two or select a module model with a higher wattage rating so you may meet your power objective with fewer modules.

Solar PV string Wattage calculation

During solar inverter string design calculations, it is required to calculate the watts per string. To get that, multiply the number of modules in each series string by the module operating voltage (Vpmax), then by the module amps (Ipmax):

9 (modules in series) × 30.3 volts (Vpmax) × 8.22 amps (Ipmax) = 2,242 watts/series-string

To calculate the watts of parallel string, multiply the watts of each series string by the number of parallel strings.

2,242 watts/series-string x 2 (number of series strings wired in parallel) = 4,484 watts

Confirm that the total array wattage is near your system objective (4.5 kW)

Calculate the Maximum string voltage

Multiply the number of modules in the series-string by the module’s open-circuit voltage (Voc), then multiply by 1.25 (extreme temperature limit; this rate is mandated by the National Electrical Code, or NEC; applicable for all PV systems):

9 (modules in a series string) × 37.4 volts (Voc) × 1.25 = 421 volts

Calculate String ampacity

During solar inverter string design calculations, it is required to determine string ampacity. To get that, Multiply the short-circuit current (Isc) of the module by 1.25 (maximum irradiation value; apply to all PV systems) and then again by 1.25 (NEC continuous-use value; apply to all PV systems):

8.72 amps (Isc ) × 1.25 × 1.25 = 13.63 amps

Inverters:

While the typical electricity grid is AC and the majority of electrical equipment uses AC power, photovoltaic arrays produce DC. An inverter is required to convert the DC power generated by a PV array to AC power in order to guarantee that the power will enter the grid. This conversion is made possible by the circuit design of inverters, which generate alternating current by rapidly opening and closing the circuit. The voltage is then raised to the required level by the grid using transformers, which are covered later in the chapter.

Inverters come in two main categories: grid-interactive inverters, which are used in grid-connected PV systems, and battery or power inverters, which draw their power from batteries.

Grid-interactive inverters:

There are numerous models of this type of inverter, also known as a grid-tied inverter, which is used in grid-connected systems. They take the DC input from the array and adjust it so that it produces the AC output that the utility grid needs.

Only when the grid is present and operating within a specific voltage and frequency range will the inverter operate. The operating requirements of the device itself will determine whether an inverter charger can export power to the grid. For instance, a US manufacturer of inverter chargers has one model that can only be used with mains or DC power input and cannot export to the grid, and another model that has a software modification to enable the export of inverted AC power.

Inverter selection

We select a DC to AC inverter based on the overall estimated array wattage in our situation (4,484 watts or the DC system rating of 4,500 watts). Always choose an inverter with a nameplate rated greater than the array watts of your system. The sample inverter is a 5 kW Fronius IG Plus 5.0; 5,000 watts is greater than the array wattage of 4,484. Even if a manufacturer advertises that an inverter is compatible with systems greater than the inverter’s nameplate rating, follow this guideline.

Strategies for designing PV systems step by step are presented solely on a single page.

After choosing your inverter, create a section for inverter specs and complete it with the information as we have done for the specified inverter shown below:

MPPT voltage range: 230–500 volts
Maximum system voltage: 600 volts
Maximum DC input current: 23.4 amps
AC output voltage: 208/240/277 volts

Verify that the inverter specifications are compatible with your modules and string arrangement using the parameters you calculated in the previous stages.

Our selected inverter has an MPPT voltage range of 230-500 volts. The operating voltage of the string (9 modules in series x 30.3 volts = 272.7 volts/string) must lie within this range, known as the MPPT window.

Our selected inverter has a maximum system voltage of 600 volts. which must be greater than the string’s maximum voltage (421 volts).

The highest system voltage for string inverters in the US is typically 600 volts. The use of inverters with maximum voltages of 1,000 volts, which are popular in Europe, is not advised since many inspectors and utilities would not approve of them.
If your maximum string voltage is higher than the inverter’s 600-volt limit, you may simply shave off one or two modules from your string.

The maximum DC input current of the inverter we selected is 23.4 amps, which must be greater than our solar PV system’s total array current (amps). In our example, two series strings linked in parallel at 8.22 amps (Ipmax) each equaled 16.44 amps of current.

The AC output voltage of the inverter we selected is 208/240/277 Volts. While most commercial buildings utilize 208-volt or 277-volt electricity, the majority of houses receive 240-volt power from the utility. Your utilities can confirm this. Make sure the right voltage may be used with your DC-AC inverter. The desired voltage can be set via a separate input or switch on inverters with several voltage choices. Given that it is located on the AC side of the system, your electrician must set it.

All grid-interactive inverters perform these basic functions:
1. Converters the PV array’s DC power into AC power so that it can be used by local appliances or fed back into the grid via the meter. It is impossible to export electricity generated by a PV system into the grid without a grid-interactive inverter.
2. Make sure the voltage and frequency of the power being fed into the grid are correct. The inverter won’t release electricity to the grid if it can’t change the DC power to the proper frequency and voltage for the grid.
3. Use “maximum power point tracking” (MPPT) to make sure the inverter is locating the most amount of PV array power that can be converted to AC.
4. When the grid is not operating within acceptable voltage and frequency tolerances, the inverter automatically shuts down. This is made possible by built-in active and passive safety protections. In the section on inverter protection systems, this is covered.

Grid interactive inverters may be different in a number of ways depending on:

1. whether or not the inverter has a transformer.
2. the used transformer’s switching frequency.
3. how the PV array and inverter interface with each other.
4. inverter’s rated capacity.
5. whether a single string or multiple string PV power inputs are present in the inverter;
6. whether a single phase or multiple phases of power are intended for the inverter.
Inverter protection systems:

Only when the AC grid is operating properly and within predetermined operating parameters will grid-connected inverters function. The grid-connected inverter will disconnect and stop producing any power from the PV array if these requirements are not met. The inverter is configured to perform like the grid itself. These inverters’ MPPT software enables the PV output to be adjusted to best match the grid requirements at the time of power output.

Active and passive protection is typically included in grid-interactive inverters. The inverter switches off on over/under voltage or frequency in both forms. The inverter will disconnect if it cannot see the grid, such as when there is a blackout, because this protection is meant to serve as both self-protection for the inverter in the event of extreme conditions as well as protection for the grid itself.

Self-protection:

Incorrect connection: If an inverter is connected to a solar panel array incorrectly (for instance, with reverse polarity), it won’t work and will likely sustain damage. Even though some inverters offer protection against improper connections, most inverters’ warranties do not extend to such damage.

Temperature: Because inverters are sensitive to changes in temperature, manufacturers will designate a range of temperatures within which they must function. When temperatures exceed the operating range specified by the manufacturer, some inverters reduce their power output or shut themselves off. Even though the inverter may have over-temperature protection, it is crucial that it has enough ventilation and cooling because overheating can harm the inverter.

Too much DC voltage: Every grid-connected inverter will have a specific voltage range that it must operate within. When the maximum DC voltage they can withstand is exceeded, some inverters shut off to protect their electronics, but the inverter itself may still sustain damage. Other inverters lack this kind of protection.

Grid protection:

Grid-interactive inverters must be able to cut off from the grid if the grid’s supply is interrupted or if the grid is operating outside the predetermined parameters (for example, when the voltage or frequency is under or over-specified). In both of these situations, the inverter disconnects in order to stop supplying power to the grid when the grid is not functioning.

There has always been concern that if there were enough inverters connected to the grid in a particular area and the grid supply to that area failed (for example, a car hitting a pole and breaking cables), the inverters would interact with one another and the voltage and frequency would become a reference to one another, causing an inverter to continue exporting electricity to the grid because it would still believe the grid was “on.” In this case, it’s possible that the passive protection won’t work, allowing power to continue to be exported onto an inactive grid even though the grid is operating outside of the required voltage and frequency. The term “islanding” refers to this phenomenon. Consequently, both active and passive protection is needed. Electricity utilities must be certain that all grid-connected PV systems are disconnected from the grid in the event of a grid shutdown while technicians are working on it. This is known as “islanding,” which poses a serious safety risk. Of course, islanding increases the risk of electrocution for workers attempting to fix power lines and could harm transmission hardware.

The inverter’s passive and active protection addresses the “islanding” problem. Grid-interactive inverters are required by many standards to have both of these characteristics. The inverter’s ability to sense the grid’s voltage and frequency provides passive protection; for example, if the inverter notices that the grid is either over or under-voltaged or under-frequencyed, it will shut down. The inverter detects any frequency instability, frequency shift, or power variation that could change the voltage it detects in order to provide active protection. If the inverter detects any of these circumstances, it will turn off using this active protection.

The inverter will reconnect to the grid after a period of time, typically at least one minute after the condition that caused the protection device to activate is removed and the inverter synchronizes with the grid once more. Some nations mandate a minimum amount of time pass between the inverter detecting a stable grid and the inverter reconnecting for inverters sold within their borders. The inverter typically allows for programming of this time delay.

Mechanical fittings to support solar PV modules

For our particular solar PV system design, we require the following mechanical fittings:

The number of rails:
Number of support rails we want per Row: 2.00 Nos
Number of rows: 2.00 Nos
Number of rails required (2×2): 4.00 Nos

Rail Length:

9 modules per row x 39 inches wide
Total Length of modules in a row: 351.00 Inches

351 inches + {4x(number of modules in a row-1)} = 383 inches

This is the total length for each rail, which is made up of multiple rail pieces (typically 8 to 12 feet long) joined with splice fittings.

Number of Footers:

Typical footer spacing is 48 inches. This can be confirmed by a professional engineer and/or the local building department.
Start with end footers no more than 18 inches from each end of each rail, then space the footers as required by the racking manufacturer or the building department.

Space Left from each End of a Rail: 18.00 Inches
Space between each Footer: 48.00 Inches

383 inches total rail length – (18 inches space left * 2 sides) = 347 inches

343 inches / 48 inches footer spacing = 7.22 Nos footer + 1 End footer = 8.22 Nos Footer

Round up fractions to the nearest whole number. The next whole number of 8.22 is 9. That means a total of 9 Footers will be required.

Footer required for each Rail: 9.00 Nos
Total Footer required for the array (9 Nos Footer x 4 Nos of Rail): 36.00 Nos

End Clamp:

Two end clamps for the modules at both ends of each row.
Number of end clumps per Rail: 2.00 Nos
Total End clump required for the array: 8.00 Nos

Mid Clamps:

Two mid-clamps between adjacent modules in each row. Clamps are not used between rows.
Mid clump between adjacent modules per rail: 2.00 Nos
Total MID clump required for the array: 64 Nos

Installing Disconnect, Utility Net Meter, and others

For the system to be balanced, add all of the necessary meters and disconnects. Except for the disconnecting combiner box, these are typically mounted on the wall of the house at ground level and must be shown on the design in the same order in which they are wired. The PV array is shown here followed by the utility-grid tie-in in the following order:

• Disconnecting combiner box with rapid shutdown (typically on the roof; must be within 10 feet of the array)
• Emergency disconnect switch with rapid shutdown (such as Birdhouse). The rapid-shutdown switch is installed between the combiner box and the inverter, but it is not connected to the main DC power circuit from the array.
• Inverter DC-AC with DC disconnect
• AC disconnect
• PV generation meter (if required by your electric utility)
• Electric service panel main (household breaker box)
• Utility Net mete for your solar PV system (typically supplied and installed by the utility)
System grounding/earthing:

In order to ensure that a system’s exposed conductive parts (such as the array frame) are equipotential, or that there is no voltage difference between the components and the ground, grounding, also known as earthing, is used. PV panel metal mounting structures and metal frames can be grounded to prevent dangerous voltage levels in these areas. This prevents an electric shock from occurring to someone touching a conductive component. The importance of ensuring their safety cannot be overstated. People who will come into contact with the PV array, such as the system owner cleaning the modules, may have little knowledge of electrical systems. Countries have very different grounding/earthing regulations and standards, so the system and individual parts should be earthed in accordance with that nation’s requirements. The manufacturer of the module must specify that certain types of modules be earthed in order to guarantee the modules’ maximum performance. The installer must make sure that national codes are followed in addition to the inverter and module manufacturers’ instructions.

Inverter installation:

DC cable lengths should be kept to a minimum by installing the inverter as close as possible to the modules. (longer cables lead to larger power losses). Inverters should be placed in an area that is shaded, protected, and well-ventilated. They must not be exposed to temperatures higher or lower than those listed on their data sheet, which is typically between -25°C and 60°C. In addition, the wall on which the inverter is mounted needs to be strong enough to support its weight. Disconnection switches and over-current protection devices ought to be installed close to the inverter.Any time during the process of installing the system, including simultaneously with the mounting system, the inverter can be installed.

Interconnection with the utility grid:

Interactive distributed generation refers to the system in which small-scale power generators (like rooftop PV systems) are connected to the grid.

When necessary, the consumer draws electricity from the utility grid and the PV system. (as opposed to a stand-alone system where they can only use electricity produced by the PV array). This varies a little according to the metering setup: When net metering is in place, the PV system’s electricity output is used where it is connected, any excess is exported back to the grid, and extra electricity is purchased from the grid when the PV system isn’t producing enough. If gross metering is used, all electricity is exported to the grid, and the electricity needed by the load is imported from the grid, preventing any direct electricity flow from the PV system to the load.

An interconnection agreement contract is typically necessary in order to connect a PV system to the utility grid. Utility grid systems, as well as interconnection agreements and policies, vary depending on the country in which they are installed. The utility, which must consent to import the electricity produced by the PV system, is frequently another factor that must be taken into consideration in addition to local standards and laws. Certain local laws mandate that utilities purchase the electricity generated by PV systems.

However, in other places, utilities are left to make the decision. Different rules apply to safety concerns that arise when connecting the grid to various power generators around the world. Among these safety concerns is islanding and current overload.

when electricity is supplied to the grid while there is no power in the grid, which could pose a serious risk to electricians working near power lines. Inverters should turn themselves off when the grid is down because they are now constructed to prevent islanding. It is not unusual for the neighborhood utility to insist on performing its own inspection of the system before turning it on for the first time.

We used a 4.5 kW grid-tied system with a single string inverter to try and teach how to design, calculate, and size a solar PV system. There are 18 solar modules in the array, which are arranged in two physical rows of 9 modules each and two series strings of 9 modules each.