Everything You Need to Know About Solar Inverters
Updated: Jun 24, 2020
Engr. Jet Andal has 6 years of experience in the design and installation of residential, commercial and utility-scale solar PV systems. Together, and with the use of solar energy, let us help make the world a better place. You can click here to read all of our other blogs. For aspiring solar PV engineers, you can also check out his Solar PV Engineering Ebook on Amazon on this link.
Types of Solar Inverters: String, Central, and Micro
Although there are three types of solar solar inverters available in the market today, they are basically the same in terms of their functions. The main difference between the three is their size and their application.
String Inverters – these are commonly used in residential and commercial applications and their sizes range from 1 to less than a hundred kilowatts. These can be on-grid, off-grid and hybrid inverters.
Central Inverters – these solar inverters are used for utility-scale or solar farm applications. Their sizes range from 100kW to a few megawatts. In solar farms, a power station is used with central inverters to be able to connect directly to the grid. Central inverters are used on utility-scale applications because they are cheaper in per kilowatt cost and are easier to install (less total number of units installed compared to if string inverters were used). Although, solar PV systems using central inverters have a disadvantage of having one point of entire system failure.
Microinverters – these are, as their name suggests, much smaller than string and central inverters. They can only usually accommodate 2 PV modules and are rated at 500W. These are used for residential applications but their use in commercial systems is slowly gaining more popularity. Microinverters have an advantage compared to string inverters of having module-level MPPT. We will discuss more about this later in this chapter: Inverter Functions: MPPT. This adds an advantage of increased flexibility to accommodate different roof sizes, increased efficiency and less effects of shading. But it is more difficult to install and maintain because of more units needed.
Inverter Functions: Basics
The inverter, on the most basic level, converts the DC electricity produced by the PV modules to AC electricity. It can be considered as the brain of the solar PV system as it controls many aspects of its operations, from PV array production, data collection, grid management, and system protection. Solar inverters have the advantage of being “automated” in its operations and this is because of how far solar inverter technology has come.
It is very important to really understand how the inverter functions and its properties as a solar PV engineer because they greatly affect system design. Its input parameters, voltage, current, number of MPPTs, DC rated power, etc., all dictate PV array design, while its output parameters are crucial to compatibility with the building’s electrical system.
The inverter also has functions other than converting DC power input to AC. These include MPPT, grid management, and protection functions, all of which are very crucial in maintaining smooth and proper operation for the whole solar PV system.
Inverter Properties: Input Parameters
The input part of SMA’s STP 15000TL inverter datasheet is shown below:
Max. Generator Power – the maximum DC input power to the inverter. It is important to note that there is a difference between the DC rated power of the array and the DC power that actually reaches the inverter. The sizing of the PV array with respect to this parameter is discussed more on the chapter on PV Array Sizing Ratio.
DC Rated Power – the maximum DC input power on one of the inverter’s inputs. The value on the datasheet is much less than the solar inverter's rated output of 25kW because this is the DC input power per input and this inverter has 2 MPP inputs. The MPPT function of the inverter is discussed in the next section, 5.4 Inverter Functions: MPPT.
Max. Input Voltage – this determines the maximum DC voltage that your PV array can have. 1,000V DC PV array systems are most commonly used so most inverter manufacturers have solar inverters that have a maximum input voltage of 1,000V. As 1,500V DC solar PV systems become more and more popular, manufacturers will surely adopt this and release more solar inverters with a maximum input voltage of 1,500V.
MPP voltage range – this will be discussed in the next section.
Min. Input Voltage – this is the minimum input voltage that must be met before the inverter starts to get DC power from the PV array for conversion to AC. Having a smaller minimum input voltage means that the inverter will start converting power from the PV array earlier in the morning and later in the afternoon, where the irradiance and voltage are lower.
Max. Input Current Input A/Input B – the maximum allowable current on each of the MPPT inputs of the inverter. This affects how many strings you can connect to each input.
Number of independent MPP inputs/strings per MPP input – this will also be discussed in the next section.
Inverter Functions: MPPT
MPPT stands for Maximum Power Point Tracking. Remember that on the PV module’s IV curve, there is a point where the PV module produces the maximum amount of power and is most efficient. This point is called the MPP or maximum power point. The inverter’s MPPT function forces the PV array to always operate on this point.
The inverter does this by varying the resistance of the input circuit. Remember in Ohm’s Law that the voltage, current, and resistance in a circuit are related by the equation:
V = I *R
Where: V = Voltage
I = Current
R = Resistance
When the resistance of the input circuit is changed, the voltage and current values can also be changed. The inverter tracks the output of the PV array for each change in input circuit resistance and does a quick trial and error-like algorithm to find the resistance value that will produce the highest PV array power. In this way, the inverter is able to choose which operating point the PV modules operate on.
You may ask the question: If we already know the MPP of the PV module from the IV curve, why don’t we just set the input circuit resistance that will produce the VMPP and IMPP? This is because the IV curve is only for a specific irradiance and temperature, specifically for STC where the irradiance is 1,000W/m2 and a cell temperature of 25OC. Actual conditions vary and thus, the MPP varies as well.
Central inverters usually only have 1 MPPT input for the whole PV array. This is because solar farms, in which they are used, have a uniform PV array in which all the PV modules have the same tilt and orientation. This means that at any point throughout the day, the PV modules receive the same amount of irradiance and thus, have the same MPP. The