Calculations for string sizing are based on the specific voltage of your panels and inverter, as well as external elements such as temperature.
Each panel has a voltage output. The voltage that the panel sends to the inverter is this. We’ll have to examine a couple different graphs:
When the circuit is open, that is, when no current is flowing through it, the voltage supplied is known as the open circuit voltage (Voc). When the inverter isn’t turned on, it enters this state.
The voltage of the panel once it has been turned on and is running normally under load (Vmp) (current is flowing through the circuit).
Look for the rated MPP voltage range on the inverter spec sheet. This is the optimum operating sweet spot that I stated in the previous section.
Take note of the maximum DC input voltage as well. We’re particularly concerned about this since exceeding the maximum operational voltage can overload the inverter and cause the equipment to fry. (Unfortunately, we’ve seen it.) Your inverter’s warranty will be void if you exceed the maximum operating voltage.
In order to turn on the inverter, it must also meet a minimum DC voltage and a startup voltage requirement. This is usually not an issue because we want our strings to operate well beyond the minimum, in the MPP range, where they are more efficient.
Now that we have our numbers, it’s time to conduct some math. As an example, consider the 9 kW grid-tied setup I mentioned previously.
According to the spec sheet, Mission Solar 360W panels have a Vmp of 39.28 and a Voc of 48.08.
The MPP voltage range of the SMA Sunny Boy 7700W inverter is 270-480 volts. The operational voltage range is 100-600 volts (see the spec sheet for the lowest and maximum DC voltage):
Step 1: Find your minimum string size
To begin, we must determine the minimum number of panels that should be included in a string.
To do so, divide the low end of the MPP range (in this example 270V) by the panel’s Vmp (39.28).
Because you can’t put a fraction of a panel on a string, the result is 6.87, which must be rounded up to the next whole number. So, before temperature adjustment, your minimal string size is seven panels.
Step 2: Find maximum string size that doesn’t exceed operating voltage
To avoid overloading the inverter, we need compute maximum string size against the maximum DC input voltage.
Divide the maximum DC input (600V) by the panel’s Voc for this calculation (48.08).
Because we’re aiming to keep under a maximum threshold, we’ll have to round the result down this time. So we’re down to the final 12 panels.
Again, we haven’t compensated for temperature, so this number isn’t final.
Step 3: Check that max string size falls within MPP range
We estimated the maximum string size required to keep the inverter functioning in step 2. We’d like to double-check that this is also within our maximum efficiency range.
To do so, multiply the maximum string size obtained in step 2 (12 panels) by the panel’s Vmp (39.28).
We’re making sure this isn’t at the very top of the MPP scale (in this example, 480V). Everything checks out here because the voltage is 471V, which is lower than our aim of 480V.
If we got a number that was over the MPP range in this phase, we’d reduce the maximum string size by one and recalculate until we got a number that was within the MPP range.
We have a string size of 7-12 panels based on these estimations. However, this does not account for temperature, which can have a considerable impact on our results (colder temperatures lead to a rise in voltages and hotter temperatures will lower voltage).
Step 4: Account for temperature in your location
“Would excessive temperatures force us to fall outside of a safe working range?” we want to know now.
Assume we’re in Boise, Idaho. In the search bar, type in the following:
Set your units to Celsius using the dropdown menu. This will correspond to the units listed on the panel’s spec sheet.
Then go to the Almanac for this location’s records and average temperatures. I’m looking for the coldest day ever recorded (to account for the absolute worst-case scenario). The gray bar is as follows:
Returning to the solar panel spec sheet, we need to check for the temperature coefficient of Voc, which measures the change in voltage per degree Celsius away from the Normal Operating Cell Temperature (NOCT).
When the panel is tested in a climate-controlled setting, NOCT monitors the voltage at a specific temperature.
The NOCT for the Mission Solar 360W panels in our case is 44 degrees Celsius. The difference between the NOCT and the coldest day on record (-33.3 C) yields a value of 77.3 degrees Celsius below normal.
These panels have a temperature coefficient of 0.280 percent /C. This means that the panel will produce.28 percent higher voltage for every degree Celsius away from the NOCT.
To begin, multiply the panel’s Voc (48.08) by the temperature coefficient of Voc (.28 percent ). To account for this in the equation, change the decimal two places to the left because the temperature coefficient is a percentage:
This gives us the voltage change per degree Celsius, which we need to multiply by the temperature difference (77.3C) we discovered earlier:
Each panel will produce around 10.406 volts above its rated Voc of 48.08 on a record-cold day in Boise. On a record cold day, we need to combine those figures together to get the true panel voltage:
Multiply the true panel voltage by the string’s maximum number of panels (12), as computed in step 2:
On a record cold day, the array’s total voltage can reach 701.83V! This is significantly beyond the inverter’s maximum operating voltage of 600V, implying that frigid temperatures could drive your array into region that could destroy it.
That’s clearly not ideal (did we mention you could fry your system? ), so we’ll have to reduce the maximum string size to make it acceptable.
You should start removing panels from the string until you reach the operating range. Subtract the true panel voltage (58.486V) from the array voltage (701.83V):
It’s still over the 600V limit, which isn’t good enough. One more panel must be removed:
Perfect. After accounting for the temperature extremes the array may realistically be exposed to, we’re under the 600V maximum input threshold. We now have a maximum string size of 10 panels after removing 2 panels from our beginning position.
This is the all-important number. Your string size should be limited to 10 panels in these circumstances. In severe temperatures, anything larger has the potential to irreversibly harm your array.
“This accommodates for frigid temps…but what about temperatures above the NOCT that are abnormally hot?”
NOCT is typically measured between 44 and 46 degrees Celsius, or 111 and 115 degrees Fahrenheit. Most places don’t get much higher than this, although it does happen in few places, particularly around the Equator.
Warm temperatures, on the other hand, are less of an issue because they lower voltage. Our main concern is to keep below the maximum input voltage to avoid damaging the inverter. The decline in voltage caused by the warm weather only pushes us farther into the “danger zone.”
However, extreme heat will alter minimum string size, so make sure your minimum string size is still within the MPP range for maximum efficiency. To do so, perform the same calculations as before, but with the following values:
- Instead of Voc, use the panel Vmp.
- In place of the Temperature Coefficient of Voc, use the panel Temperature Coefficient of Pmax.
- Subtract the voltage compensation from the panel Vmp when computing the true voltage of the panel (instead of adding it to the panel Voc).
For example, Death Valley, CA has a record high of 56.7C, which is 12.7C over the NOCT. The Vmp voltage is 39.28 volts, while the Pmax temperature coefficient is -0.377 degrees Celsius. In this case, the math would be as follows:
Multiply by minimum string size for total input voltage
We mentioned 270V as the low end of the MPP range in Step 1, and you can see that 261.8V is just below that range. The ideal situation would be to increase the minimum string size to eight panels.
However, this explains the hottest day on record in Death Valley, which reached a scorching 134F! And the result isn’t inverter damage, but rather a system that functions at a slightly lower efficiency in harsh conditions.
As you can see, the cause for alarm is almost non-existent. Although 8 panels is officially the smallest string size, 7 panels will suffice.
How do you figure out how many solar panels you’ll need?
The minimum string size refers to the number of PV modules that must be connected in series to keep the inverter working during the hot summer months. Because the effects of high temperatures on module voltages are considered a performance concern rather than a safety one, the National Electrical Code (NEC) does not address them. Our customers, on the other hand, are concerned about whether the system we design is operational during the summer months, when their return on investment is strongest.
We must first determine the minimum output voltage, Module Vmp min, that each module will produce for the specified installation site in order to calculate the minimal string size. The minimal number of modules is then computed by dividing the inverter minimum voltage by the calculated module minimum voltage.
The module minimum voltage is determined using the installation site’s high temperature when the modules are projected to produce the lowest voltage. This is due to the fact that as a PV module heats up, it becomes less efficient and produces less voltage.
In solar PV, what is a string?
Individual PV modules are connected in series and parallel in a bigger PV array. A “string” is a group of solar cells or modules that are connected in series. In PV arrays, the combination of series and parallel connections can cause a number of issues. An open circuit in one of the series strings is one potential issue. The current from the parallel linked string (commonly referred to as a “block”) will thus be lower than the current from the other blocks in the module. This is electrically equivalent to connecting one shaded solar cell to multiple excellent cells, and the power from the entire solar cell block is lost. This impact is depicted in the diagram below.
Larger PV arrays may have mismatch effects. Despite the fact that all modules are identical and the array is not shaded, mismatch and hot spot effects can still occur.
If the by-pass diodes are not rated to withstand the current of the complete parallel linked array, parallel connections along with mismatch effects can cause issues. The by-pass diodes of series connected modules, for example, get connected in parallel in parallel strings with series connected modules, as shown in the diagram below. Due to a mismatch in the series linked modules, current will travel through a by-pass diode, heating it. The effective resistance is reduced when the by-pass diode is heated. The by-pass diodes that are somewhat hotter will now carry the majority of the current. These by-pass diodes then get even hotter, lowering their resistance and increasing current flow even more. Nearly all of the current may eventually pass through a single set of by-pass diodes. The diodes will burn out if they are not rated to handle the current from the parallel combination of modules, causing damage to the PV modules.
In paralleled modules, bypass diodes are used. Each 36 cell module normally has two bypass diodes.
In addition to using by-pass diodes to eliminate mismatch losses, a blocking diode can also be used to reduce mismatch losses. A blocking diode, as shown in the diagram below, prevents current flow from the battery through the PV array, preventing the module from charging the battery at night. Each string to be linked in parallel should have its own blocking diode when using parallel connected modules. This not only reduces the blocking diode’s necessary current carrying capability, but also stops current from flowing from one parallel string into a lower-current string, reducing mismatch losses in parallel linked arrays.
In an MPPT, how many strings are there?
Time and materials are saved by combining up to four strings of PV modules to a single inverter without the use of additional external combiner boxes. The NEC section 690.9 exception permits two PV strings to be connected to a single inverter input without a combiner fuse in each string. As long as the string wiring is suitably sized and no other current sources can back feed into the strings, this is possible.
If an inverter supports dual independent MPPT channels, it is possible to link up to two strings per MPPT channel without using combiner fuses in each string. As a result, an inverter with dual-MPPT channels can link up to four strings without the need for additional combining hardware.
The output power rating of most PV modules on the market has increased significantly in recent years, to the point where today’s small residential systems rarely require more than two strings. Larger residential applications, on the other hand, usually necessitate four strings. Commercial systems require a high number of strings, hence larger central inverters and external string combiners have been employed in the past. However, because there is a trend in the industry to use a large number of smaller inverters for these applications, a dual MPPT inverter would be beneficial in these designs as well.
Inverters with a single MPPT channel can only offer monitoring data for the full array. Data collection will be based on the entire array input, whether one, two, or four strings are used. The inverter may give monitoring information at the MPPT channel level with independent dual MPPT channels. As a result, monitoring data such as site status, energy production, and troubleshooting data has a greater granularity. This is significant because the loading of the two channels can differ depending on the system architecture. As a result, data collection is largely done at the string level for tiny systems (with one string per MPPT channel). Data collection is reported at the two-string level for larger residential systems (up to two strings per MPPT channel). As a result, in addition to delivering accurate energy harvest figures per channel, the user can see what is going on at each input at any given time. This can assist diagnose aberrant situations at a particular inverter input.
Understanding all of these features of MPPT will help you gain a better understanding of how energy harvesting works and, as a result, make solar systems more profitable.
In a solar farm, what is a string?
To have a working solar PV system, you must connect the panels to form an electrical circuit in which current may flow, as well as connect the panels to the inverter, which will convert the DC power generated by the panels to AC electricity that can be utilized in your home or supplied to the grid. In the solar energy sector. This is sometimes referred to as “stringing,” and each string is a group of panels joined together.
What method do you use to calculate strings?
The length() method of the Java string class can be used to calculate the length of a string.
Strings are objects produced in Java using the string class, which has a public member function called length(). As a result, the. (dot) operator can be used to access this method from any string variable.
In a string, how many modules are there?
6. Based on your array output, size an inverter and module strings.
You’ll be able to choose and size a central inverter once you’ve decided your goal array output size and chosen your solar modules. The various varieties of inverters, as well as their specifications and other features, are explained in the website’s primary tutorial. Inverters, like modules, are often identified by their wattage output. However, there are other specifications to consider, namely the array voltage range that the inverter can handle. We’ll go over a typical grid-tied system with one central inverter in this part.
Grid-tied inverters, such as these from SMA, are available in a wide range of shapes, sizes, and specifications.
The first step in choosing and sizing an inverter is to match your intended array watts to an inverter watt size as closely as possible. 2000 watts, 3000 watts, 3800 watts, 4000 watts, 5000 watts, and other standard inverter nameplates are available for purchase. If your home’s main service panel has a bus bar rated at 100 amps, the 3800 watt inverter is usually the largest inverter you may utilize without requiring a panel upgrade or other electrical adjustments.
In most cases, an inverter with a slightly larger output wattage than the array watts is chosen, however this isn’t always the case. After reading this section, you’ll understand that an inverter rated at up to 25% less watts than your array may be the best match for power efficiency. A 3,000-watt inverter, for example, might power a 4,000-watt array. If you plan to add modules to your array in the future, however, you should get the larger inverter. You should also purchase the larger inverter if you live somewhere that gets cold in the winter. (You’ll discover why shortly.)
Module PTC ratings and inverter efficiency are two specifications to keep in mind while you shop around and/or make sizing estimates.
Because manufacturers score modules using Standard Test Conditions, PV designers who want a more accurate assessment of array output watts utilize a different benchmark called as Practical Test Conditions.
The PTC wattage estimate is always 10-15% lower than the STC, or naming, wattage. A PTC value, on the other hand, is not always included on the product spec sheet. If the product has been evaluated and PTC certified, you may need to go elsewhere for the value. A database of PTC values for hundreds of modules is maintained by a California tax incentive agency, which you can obtain here. (Before beginning your module search, find out if PTC-rated equipment is required if you want to qualify for a tax credit, low-interest loan, or other incentive program.)
Inverter efficiency hovers around 95-97 percent, which indicates that 3-5 percent of the electricity generated by the array is wasted owing to heat, electricity used to drive the fan, and electricity used to operate the inverter display screen, among other things. While a few percentage points across models may not seem like much, it can add up to a significant difference in kilowatt hours generated over the course of ten or twenty years. You may wish to factor in inverter efficiency in your sizing calculations, just as you would with a PTC rating.
The most popular array configurations for home PV systems without battery backup are 2 to 4 strings of 7 to 12 modules apiece. After the strings have been wired, each string is connected in parallel to the next string. This indicates that the positive lead (or circuit wire) at the end of String 1 is connected to the positive of String 2, and so on, as shown in the diagram below. Similarly, the negative wire at each string’s end is connected to its corresponding wire in all other strings. The circuit voltage does not change when a parallel connection is made. However, the rated current or amperage of the module changes. If you have more than one string, multiply the current by the number of strings to get the amps that will go into the inverter. (In a series string, current remains constant, indicating that it is the exact quantity specified on the module spec sheet.) As a result, three strings of 6 amp modules will produce 18 amps to the inverter. The inverter will see 12 amps if there are two strings.
Multiple strings of modules are connected in parallel in this diagram. The dashed lines indicate that the array contains more modules and strings than shown.
Grid-connected PV arrays are often configured to generate a high DC voltage. As electricity travels downstream to the inverter, a high voltage reduces the inherent current losses. However, there is a limit to how high DC voltage can be in your home. Residential circuits are allowed to a maximum of 600 volts, according to the National Electric Code (NEC). It’s 1,000 volts in Europe.
Because sunlight across an array is rarely consistent, all grid-tied inverters are built to accommodate a wide range of voltages. On spec sheets, a common range is 150 to 450 volts, which offers you some leeway when sizing your module strings. The purpose of string sizing is to find the sweet spot within the inverter’s range, which is usually halfway to two-thirds of the way to the upper limit, or maximum voltage. Naturally, you don’t want to get too near to the edge, as this can put a strain on the inverter’s circuitry over time.
A module series voltage between 250 and 400 volts should make an inverter hum like a kitten for 10 years or more for a PV system of 3-5 kilowatts within the range already mentioned. However, selecting an inverter and string size is not as simple as many PV students and new contractors believe. Here’s why voltage is important to consider while choosing a string size:
- Solar radiation varies throughout the day based on the position of the sun. The array voltage rises slowly in the morning, peaks around noon, and then begins to fall after 2 p.m. As a result, the inverter will not turn on or start working until the array voltage reaches its lower threshold, or start-up voltage. You’ll waste precious energy every day if the threshold is too high or the array voltage is too low.
- On extremely cold, sunny days, the array voltage might easily exceed its regular limits. If the voltage rises above the inverter’s upper threshold or maximum voltage rating, the inverter may fail.
- The heat can cause the array’s voltage to drop much below normal values on extremely hot days. Even during high noon, if the voltage falls below the inverter’s lower threshold or startup voltage, the PV system will shut down.
- Although it is not always practicable, the voltage for all of your strings should match, which means that each string should have the same amount of modules. If you have a 20-module array, for example, divide it into 2, 4, or 5 strings instead of three. Different voltages will be produced by non-matching strings or modules positioned at different orientations and angles. This could result in a reduction in power efficiency.
- The voltage drop between the array and the inverter should not exceed 2% when sizing and running wiring. Because the inverter’s MPPT circuit measures voltage at the inverter input rather than the array output, it may function accurately. (In the wire sizing phase, you’ll learn how to measure voltage drop.)
- The array may be shaded or soiled.
- as the modules get older and less effective
- as a result of the voltage decrease
While this module size and configuration might just get it through the math marathon, a serious designer would probably reject the low voltage scenario. On a cold day, any of the following ways might help you get it higher without exceeding the inverter max:
- Change the array size (i.e. module count), but keep the module and inverter models the same.
- To make a single string with a lower maximum voltage, utilize a smaller number of higher-watt modules.
- To make two strings with a higher voltage, utilize a bigger number of lower-watt modules.
- Change to a lower-starting-voltage inverter that can still handle two module strings.
Due to shading, available space, or other factors, it’s not always possible to keep series strings even. You may need to set your strings at different azimuth orientations or altitude angles relative to the sun in some circumstances. Because this has an impact on the fluctuating voltage, it can make size calculations even more difficult. Regardless, such a design can be made to operate by keeping each array or string distinct from the others. Some inverters let you connect in four or more distinct circuits, allowing each string’s electrical properties to be isolated from the others.
When your series strings don’t match or you need to use more than one array, you have three options:
- Use a transformer-free inverter with a dedicated MPPT circuit for each of the input ports or channels.
- Each strand should have its own inverter.
- Each module should have a micro-inverter installed.
In terms of equipment and/or wiring, some of these solutions will be more expensive. Attaching a microinverter to each module, on the other hand, is the most prevalent option currently. This device converts DC to AC and performs the MPPT function right in the array, as shown in our inverter tutorial section. To get the job done, a microinvert costs $175-$200 each module, so expect to pay up to $2,800 for a 14-module array, as opposed to $1,750 for one central inverter. The transformerless inverter, which is commonly used in Europe and is more efficient in providing energy, is also gaining ground in the United States. It’s also a lot lighter because it doesn’t have the hefty copper windings. However, because a transformerless inverter requires ungrounded cables, extra code standards must be followed while wiring the PV system.
Let’s look at the spec sheet for numerous SMA Sunny Boy transformerless inverters to determine whether one of these items would work for our sample circuit to solve the voltage range problem:
It’s worth noting that the maximum DC voltage for all models is 600V. This should be sufficient to handle the maximum cold day calculation of 588 volts for a single string of 14 modules, but the razor-thin 12-volt buffer could be dangerous. In the meanwhile, the start-up voltage for 240V versions is 150 volts. The low-range voltage for the Fronius model is the same as this. If you planned to use two strings of seven modules with a regular working voltage of 205 volts, this voltage might be too low.
A slash mark (/) on the spec sheet, by the way, denotes two different specs in one column. The value on the left side is for 208V models, while the value on the right side is for 240V models. For 3-phase commercial and industrial users, a 208V inverter represents the normal AC operating voltage. Residential consumers receive single-phase 240V from their electric company. As a result, you can only purchase a 240V model.
In addition, these Sunny Boy TL inverters have a built-in plug-in outlet.
You can utilize this in the event of a power loss on the grid. You’ll have 1500 watts available to power important loads when the sun is shining, at least until the utility restores electricity. There is no need for a battery bank.
SMA also offers a data monitoring system that is compatible with all transformerless variants.
See The Solar Inverter and Optional Equipment for a more detailed look at inverter technology, including the various types available, their features, and specifications.