This article aims to explain the differences between string inverters and microinverters, presenting their main differences, modes of operation, advantages and disadvantages for a system, lifespan, warranties, and other points relevant to this analysis.
In the field of power electronics, devices known for converting direct current to alternating current (DC-AC) are called "inverters". The input of an inverter consists of an external DC source (photovoltaic modules), and internally the inverter modulates the DC input voltage to a desired DC voltage through a boost DC-DC converter. The output of this boost converter serves as power supply for the power devices that switch and convert to alternating current.
In most inverter applications, the goal is to provide a sine wave as most motors and electrical components on the global market operate with a sine wave voltage. In applications, what would normally be seen from country to country is the nominal voltage of the system, 220 V single-phase or three-phase or 380 V three-phase, and a frequency of 50 or 60 Hz.
Based on inverter applications, inverters that work in parallel with the electric grid can be called grid-tie or on-grid. On the other hand, inverters that work disconnected from the grid or do not have the function of injecting power into the electric grid of the concessionaire are called off-grid.
To facilitate the understanding of some concepts for inverters and photovoltaic systems, below are some definitions commonly used in photovoltaic systems.
To understand what the maximum power point is, Figure 1 shows an example of an I-V curve of a photovoltaic module. By definition, the power of a circuit, component, or generator can be calculated in a simplified way as the product of voltage and current.
In an I-V curve, power can be obtained as the area of the graph below the point and above the origin on the axes. To exemplify, points A, B, and MPP (Maximum Power Point) are highlighted in Figure 1. Points A and B are generic operating points, and the MPP point refers to the maximum power point of the panel. Comparing the area of the graph, it is noted that the area of the MPP point is larger when compared to points A and B.
Based on what the MPP of a module is, the function of an inverter used in photovoltaic applications is to find this maximum power point, and for this, a function is implemented in the firmware to track this point. The implemented algorithm is called Maximum Power Point Tracker (MPPT).
Module Level Power Electronics (MLPE) has the same function as MPPT, which is to track the maximum power of the modules. The difference between the two is that MPPT processes the maximum power point containing several photovoltaic modules in series, while MLPE processes the power of the modules individually, and can be one or even two modules in series.
From the definitions mentioned earlier, a grid-tie inverter can be described as a DC-AC converter that is connected in parallel with the electric grid of the energy concessionaire and can inject both active power (kWh) and reactive power (kVARh). Its string nomenclature is related to the use of photovoltaic arrays that are connected to its input.
A photovoltaic array or string is defined as a series of panels that are connected to the input of an inverter. The number of modules in series normally varies between three (120-160V) up to thirty modules (1500V) and depends on the start-up voltage and the maximum voltage supported by the inverter components.
To process the electric power supplied by the photovoltaic array connected to the inverter input, the array is connected to the inverter input that contains the MPPT algorithm and will identify the best voltage and current operating point to obtain the best module performance.
To better understand the constructive and installation characteristics of microinverters and string inverters, the main advantages of each component will be presented, from the constructive characteristics to the installation characteristics.
For this study, the use of the Renac Power R1-3k3-SS model is being considered and is presented in Figure 2.
This string inverter has an MPPT with an input for MC4 connectors from the photovoltaic modules. This inverter has an input voltage of up to 550 V and works with modules up to 16 A and a manufacturer's warranty of up to 10 years.
To achieve high efficiency, string inverter manufacturers such as Renac Power usually have important information on their inverters' datasheets called "Rated Input Voltage" or nominal input voltage. That is, for the inverter to more efficiently convert the power from the photovoltaic modules, it is necessary for the maximum power point of the modules (sum of the Vmp parameter) to be as close as possible to the MPPT's nominal voltage of 360 V.
Regarding installation, a CC string box and an AC are typically used to protect against surges on the DC input and on the AC output. This protection is important to isolate the modules when maintenance is needed.
For this study, the TSUN MP3000 model is being considered, and the equipment is shown in Figure 3.
According to the datasheet, this microinverter has 6 MPPTs and works with MPLE technology. This equipment processes each of the 6 modules connected to its inputs independently. Therefore, unlike the MPPT application mentioned for the R1-3k3-SS string model, the nominal input voltage does not have a significant impact on the system's efficiency. It is sufficient for the connected module to be within the specified voltage and current parameters. In this case, the MP3000 microinverter works with up to 18.5 A and modules up to 60 V, and the manufacturer's warranty can last up to 20 years.
Unlike the string inverter, the microinverter does not require a CC string box, as the equipment can be disconnected from the CA breaker for maintenance purposes, and the voltage from each PV module is low. Therefore, the risk of an electrical arc is low compared to the string inverter.
In terms of electrical installations, the microinverter becomes simpler because it is installed under the PV modules. A conduit can be installed to carry the AC wiring directly to the customer's power panel, and a 20 A breaker can be installed in the power panel to isolate the microinverter when maintenance is required.
To compare the operation modes and efficiency of string inverters and microinverters and to exemplify the best equipment for each application, consider these three PV systems with 6 modules:
For the first system, considering that the customer has space for 8 modules but intends to install only 6 modules, the best option is to install an R1-3k3-SS string inverter, as only one MPPT is required. If the customer wishes to expand the system over the years, two more modules can be added because the inverter can support up to 8 modules. This way, the system can be expanded with a low investment.
For the second system, it is necessary to evaluate how long the PV module is shaded. If this period is close to the moments of highest irradiation during the day, the microinverter becomes the best option, as it will operate normally with the other 5 modules, and the shaded module will affect the system's production less. In the string inverter, the entire system's production will be reduced until the panels are fully exposed to the sun.
For the third system, as the customer has more than one orientation, if a string inverter is used, at least 8 modules should be considered, with four for each side to reach the minimum MPPT operating voltage, and an inverter with at least two MPPTs, one for each orientation. In the case of the microinverter, the system operates normally without the need to add more modules because each panel is processed individually by the equipment.
For a system with only one orientation, the string inverter can be used when there is a forecast for system expansion with low investment. In plants that have more than 6 modules, installation tends to have lower costs because with just one inverter, it is possible to convert the power from several modules. To increase a system with microinverters, one must acquire new equipment with new modules, and this way the system expansion has a higher cost.
For a system with multiple orientations, it becomes necessary to use inverters with more than one MPPT, and thus the microinverter becomes a viable option because it can process the power of the modules individually, there is no reduction due to the current limitation effect, and therefore, it increases the energy supplied by the system throughout the day. Additionally, in systems that have partial shading, energy production is increased by processing the modules individually.
Due to the fact that microinverters work under less stress over the years, the warranty of the products is greater when compared to string inverters. Normally, for the lifespan of modules of 25 years, one should consider replacing the inverter halfway through the time, while with microinverters, the trend is for their lifespan to be the same as that of photovoltaic modules.
Based on the advantages and disadvantages mentioned above, it is up to each designer to verify the financial viability of their project, always seeking the best solution for their client.
Author: Thiago Roberto Machado - 05/09/2023 - Technical Articles