The inverter can be thought of as the “brain” of a solar PV system. This is because the inverter is the one that manages how it operates along with many other functions and protection features. In terms of a desktop computer, you may think of the inverter as the CPU or the central processing unit of the solar PV system. In this article, we will go through the basic functions of an inverter, and the different types of inverter used for solar PV applications. We will also go in detail about each of the inverter specifications and functions and compare how each type of inverter differs from each other based on these.
Basic Function of an Inverter
An inverter is a power electronic device that is not exclusively used for solar PV applications. Its most basic function is to convert DC (direct current) to AC (alternating current). The difference between the two and their specific applications are detailed below:
- Direct Current – this is described by a flow of electric current in one direction only. Sources of DC electricity can be distinguished through the presence of positive and negative polarities. The electric current from such sources flows from the positive terminal to the negative terminal. PV modules or solar panels and batteries produce this kind of electric current.
- Alternating Current – in this type, the direction of the electric current reverses its direction periodically in a sine wave manner. Conventional sources of electricity, like coal, natural gas and nuclear energy all produce AC through the use of generators. This is why this is the type of electricity that we use in our homes.
We use AC electricity in our homes because it has features that are useful in terms of electric transmission and distribution. One of these is the ability to increase or decrease its voltage to the required level. This is especially important in the transmission of electricity through huge distances because using a higher voltage level will result to lower energy losses. This fact, along with all conventional energy sources producing AC, is why our whole electric grid uses AC instead of DC.
However, with the advent of renewable energy technologies such as solar PV that produce DC, a device that can convert DC to AC is required to be able to integrate these energy sources into our electric grid. This is why these technologies require having inverters to be useful to us.
Types of Inverters
There are 3 types of inverters today that are used today: central, string and microinverters. All of these perform basically the same functions, the only difference being the scope of their applications.
Central inverters –
this type of inverter is the largest in terms of capacity and is the one that is most commonly used for utility-scale systems such as solar farms. Their sizes can range from 100kW to a few megawatts. These inverters are usually designed to connect directly to the electric grid, which is why they usually come in a package that includes a power station. They also have the advantage of being cheaper in terms of per kilowatt cost and easier installation and management.
String inverters –
this type of inverter is the one that is usually used for residential and commercial systems, with sizes that range from 1kW to 100kW. They also come in 3 different types: on-grid, off-grid and hybrid, the use of each depending on the project’s specific requirements.
this type of inverters is the youngest among the 3, but is now slowly gaining popularity and market share. As their name suggests, microinverters are the smallest out of all the 3 types, which limits their application to residential solar PV systems. However, as microinverter technology improves over time, it is also slowly being used for commercial and even utility-scale applications. Their sizes usually range from 250W to 1kW, which accommodates 1-4 PV modules.
Many people nowadays confuse microinverters with DC-DC converters and use the two terms interchangeably. However, DC-DC converters are not inverters and only converts the DC voltage and current that is produced by PV modules to levels that allow for maximum energy production. Because of this, DC-DC converters still require the use of inverters for proper functioning.
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Inverter Properties: Input Parameters
To be able to choose the best type, brand and model of inverters for your specific requirements, having a thorough understanding of the inverter’s specifications is a must. Here are the most important input parameters of inverters that differ for each type of inverter.
- Maximum DC Power or Maximum Generator Power – this refers to the maximum DC input power to the inverter. Its value depends on the power rating of the inverter, the number of its MPP inputs and how much more power than its rated capacity that it can handle.
- Central Inverters – for central inverters, the maximum DC power is usually a little more than the inverters’ total rated capacity. We would like inverters to produce power that is as close as possible to its rated capacity and because of losses in the inverter itself, this is not possible if the input DC power is exactly equal to the total rated capacity.
- String Inverters – for string inverters, the maximum DC power is also more than the total rated capacity, but the value specified on the datasheet is usually per MPP input. For example, if a string inverter has 2 MPP inputs, the maximum DC power that is shown in the datasheet is half of the inverter’s total maximum DC power.
- Microinverters – for microinverters, this is usually not specified on the datasheet. What manufacturers specify is the rating of PV modules that are compatible with their microinverters. Similar to central and string inverters, you will also notice that you can use a PV module with a power rating that is slightly more than the microinverter’s total rated capacity.
- Maximum Input Voltage – this refers to the maximum DC voltage that the inverter can withstand on its input side, which also dictates the maximum voltage that your PV array can have.
- Central Inverters – for central inverters, the maximum input voltage is usually 1,000V. However, some newer central inverters on the market already come with a maximum input voltage of 1,500V. These inverters allow the use of PV arrays with a maximum voltage of up to 1,500V, which requires fewer BOS components to be used.
- String Inverters – string inverters are similar to central inverters in terms of having a maximum input voltage of 1,000V. Before, string inverters usually had a maximum input voltage of 600V, but as the technology improved, 1,000V has become more common because of the significant reduction in the amount of BOS components that need to be used.
- Microinverters – for microinverters, the maximum input voltage is often not important. This is because as mentioned, manufacturers of microinverters specify the ratings of PV modules that their microinverters are compatible with. This also means that their microinverter’s input parameters are already designed to be compatible with the output voltages and currents of the specified PV modules.
Inverter Functions: MPPT
MPPT stands for Maximum Power Point Tracking. It is a function of inverters where they force the PV modules that are connected to them to operate on their Maximum Power Point, or with voltage and current values where they produce the maximum amount of power. The MPPT capabilities of each type of inverters differ significantly:
- Microinverters – microinverters have the best MPPT capability among the 3 types of inverters because each PV module is connected to its own MPP input. This means that each PV module is optimized to its own Maximum Power Point, which translates to the highest value of efficiency.
The basic rule is that you can not put PV modules that are on different tilts and orientation on a single MPPT because this will significantly reduce the inverter’s MPPT capability. Since microinverters have one MPP input for each PV module, microinverters can be used for residential applications with multiple roof segments.
- String Inverters – this type of inverter usually has 2-3 MPP inputs that accommodate 2-3 PV module strings. Because of this, PV modules are optimized at the string level instead of individually for microinverters. For string inverters then, PV module strings are limited to be mounted on the same roof segment to have the same tilt and orientation.
- Central Inverters – central inverters have the least amount of MPP inputs and are the most inefficient in terms of optimizing the power production of PV modules. However, this is usually not a problem since central inverters are usually used in solar farms, where the PV module’s tilt and orientation are uniform for all.
Inverter Properties: Output Parameters
- Rated Power – this refers to the maximum AC power that the inverter can produce and is usually included in an inverter’s model number. For example, SMA’s STP 15000TL inverter has a rated power of 15,000W or 15kW.
- Central Inverters – this type of inverter usually has a power rating that ranges from 100kW to a few megawatts.
- String Inverters – string inverters are the most versatile in terms of power rating, which can range from 1kW to 100kW with much smaller increments.
- Microinverters – the most common power ratings for microinverters are 250W and 500W which are designed for 1 and 2 PV modules, respectively. However, there are also some 1,000W or 1kW microinverters that are available for commercial applications.
- AC Nominal Voltage, Frequency and Number of Phases – this refers to the output AC voltage and its frequency.
- Central Inverters – since central inverters are used for utility-scale applications, they should produce the same voltage and frequency as that of the electric grid where it will be used. And because there are a lot of different electric grid standards all over the world, manufacturers can customize these parameters to match your specific requirements. In terms of the number of phases, they are always three-phase.
- String Inverters – string inverters have more leeway in terms of voltage and frequency because their voltage and frequency are specified in terms o range of possible values. For the number of phases, string inverters that are for residential applications are single-phase, while those that are for commercial and utility-scale applications are three-phase.
- Microinverters – most microinverters are designed to match the voltage and frequency of the residential electric grid since they are mostly used for residential applications. This is also why they are mostly single-phase. The only exception to these are the new microinverters that are designed for commercial applications.
- Inverter Properties: Efficiency
- Central Inverters – central inverters have the highest efficiency values among the 3 types of inverters. However, in practice, solar PV systems that use central inverters are the least efficient. This is because there are also other factors that must be considered, like the number of strings per MPP input, mismatch losses, shading, shading, and soiling, etc.
- String Inverters – string inverters basically have the same efficiency as central inverters but in practice, solar PV systems that use them are still more efficient because they have fewer strings per MPP input.
- Microinverters – because of their small package, microinverters have the lowest efficiency value but they are the most efficient when used in practice because each PV module in the system has its own MPPT. This totally eliminates the mismatch losses and minimizes the effects of shading and soiling.
Solar Energy: The Energy of the Future
We all know now how our energy consumption is hurting the environment because of our continued burning of fossil fuels. What’s worse is that we don’t just have to believe what the climate scientists are saying because we can see and feel its effects every day. The earth is getting warmer and warmer which causes us to experience more and more extreme weather calamities like typhoons/hurricanes, floods, heat waves, extreme colds and many more.
Fortunately for us, we already have the technology that we need to replace fossil fuels as our energy source. And with the continuously plummeting costs of PV modules and other solar PV components, this technology is now much cheaper than coal. Now, the takeover of solar PV on coal as our primary energy source is not anymore a matter of if, but when.
However, we are still a long way ahead and there are still a lot of things to do. And the only way for us to win this battle is if we work together.
Together, and with the use of solar energy, let us help make the world a better place. Visit our website, www.andalsolar.com, to join the solar revolution and help the world transition to this cleaner and better energy source.
For aspiring solar PV engineers, you can also check out our Solar PV Engineering Ebook on Amazon on this link.
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