Microinverter compatibility with modules above 500 W

The objective of this article is to answer a question that has recently arisen on the market: “With the arrival of increasingly powerful modules, what about microinverters?”

A similar subject was explored in the article “Compatibility between bifacial modules and solar energy optimizers“, which deals with compatibility between bifacial modules and power optimizers.

Bifacial modules are more powerful because of the gain in bifaciality. Making increasingly powerful modules compatible, whether due to the number and size of the cells or the fact that they are bifacial, is a challenge for which the inverter industry will have to prepare.

Microinverters began to become popular in Brazil around 2014, when Ecori introduced APSystems brand inverters in Brazil. Before that, the few microinverters available in the country were brought by curious people, imported unofficially, and used in a small number of projects.

More recently, microinverters from Chinese manufacturer Hoymiles appeared in the country, which quickly gained market share and also became popular with consumers. After all, what is so special about microinverters?

Its ease of installation is certainly one of the biggest differences compared to conventional wall-mounted inverters. We can also highlight other aspects such as the safety of the installations, due to the non-use of high voltages, with reduced risk of fire, and the ease of use in projects with modules in non-homogeneous operating conditions – this is more evident in places with a lot of shadows or on roofs with different pitches and different angles.

Microinverters are not that micro anymore. Previously intended for use with just one photovoltaic module, microinverters with two or four inputs are now common, for connecting up to four modules.

Despite having grown, they still preserve some characteristics in their “DNA”: they are flat, have reduced dimensions and are specially developed for installation close to the modules, normally accommodated below the modules and attached to the metal fixing structures.

Both microinverters shown in the figure above receive four photovoltaic modules at their inputs. There is, however, a simple difference between them. The Hoymiles MI-1X00 family models have four inputs and 2 MPPTs, while the APSystems QS1 model has four inputs and four MPPTs.

The advantage of a greater number of MPPTs is the fact that it allows independent power tracking, individualized for each photovoltaic module. With four MPPT inputs it is possible to individually maximize the power generation of each of the modules, reducing the so-called power mismatch, which is a power loss that occurs when modules are connected in series and do not have their maximum power point tracked. individually.

But let's get back to our topic: photovoltaic modules are growing. Modules with more than 500 W of nominal peak power (without bifaciality gain) are now a reality. Although they have not arrived in Brazil, modules with powers approaching 800 W have already been announced abroad.

This requires that the microinverter parameters be readjusted, especially the maximum allowable voltage and current. In this article we will consider the 500W, RS150-8-500M Risen modules that are already available in Brazil.

How to make modules and inverters compatible

The golden rule for compatibility between modules and inverters is to always respect the numbers shown in the data sheets. Normally, microinverters are specified in relation to their nominal power, their maximum allowable voltage and the maximum short-circuit current of the module to be connected.

Respecting these three numbers, there is no error. You can think about using a certain module whose peak power is slightly above the rated power of the inverter – and this is absolutely normal, as we know, because in photovoltaic projects the power of the inverter is generally 10% to 20% lower than the peak power of the modules.

Despite having higher power, the short-circuit current and open-circuit voltage of the module cannot violate the inverter specifications. Below we will analyze some cases of sizing high power modules with microinverters available on the Brazilian market.

Example 1: APSystems with four 500 W modules

In this example, we will make four 500 W photovoltaic modules compatible with a four-input microinverter and 4 MPPTs from APSystems. Project specifications:

• 1 QS1A microinverter (1.5 kW);
• 4 RSM150-8-500M modules (500 Wp).

The result of the PVSyst simulation, with the project located in the city of Campinas (SP), indicates a generation of 3269 kWh/year. The figure below shows, on the left side, the electrical input characteristics of the microinverter and the photovoltaic module. On the right side we see the result of the project dimensioning in PVSyst.

The green graph, at the bottom, reveals the occurrence of generation limitations for operational powers from approximately 1.6 kW. The generation limitation, caused by the microinverter power clipping, is caused by the factor of 1.33 found in the ratio between the module power (in STC) and the nominal power of the microinverter.

In other words, the module power is 33% above the microinverter power. Normally, this is not a problem in projects, as long as the components are correctly compatible and the energy generation is satisfactory, in accordance with the designer's expectations.

The data sheet from manufacturer APSystems specifies that the QS1A model is prepared to operate with photovoltaic modules from 250 Wp to 525 Wp. If clipping (or power limitation) is not a problem for the designer, that is, if energy generation is satisfactory with the chosen module-microinverter combination, the project can proceed normally.

The QS1A module and microinverter datasheets show that the two components are technically compatible. The maximum current supported by the QS1A is 13.3 A at each input, while the maximum (short-circuit) current of the photovoltaic module is 12.46 A.

Conclusion: it is possible to use the RS150-8-500M module with the QS1A microinverter without any concerns. Note: to carry out this study in PVSyst it was necessary to import the .OND file with the QS1A microinverter definitions, as this model was not yet listed in version 7.0.11 of PVSyst (revision 17793, updated on 15/09). This file was kindly provided by Ecori.

Example 2: Hoymiles with four 500 W modules

In this example we are trying to make four 500 W photovoltaic modules compatible with a four-input microinverter and two Hoymiles MPPTs. Project specifications:

• 1 MI-1500 microinverter (1.5 kW);
• 4 RSM150-8-500M modules (500 Wp).

The result of the PVSyst simulation, with the project located in the city of Campinas-SP, indicates a generation of 3255 kWh/year, lower than that obtained in the previous case – which is probably explained by the lower number of MPPTs of this equipment in relation to the previous one .

The figure below shows, on the left side, the electrical input characteristics of the microinverter and the photovoltaic module. On the right side we see the result of the project dimensioning in PVSyst.

Again, as in the previous case, there is generation limitation caused by the inverter's power clipping. Here we also find the factor of 1.33 in the ratio between the module power (in STC) and the nominal power of the microinverter.

In other words, the module power is 33% above the microinverter power. As previously stated, this is not a problem in projects as long as the components are correctly compatible and the energy generation is satisfactory, in accordance with the designer's expectations.

The simulation runs normally with this configuration, but one detail draws attention: the maximum current supported by the inverter is 11.5 A at its input, while the short-circuit current and the maximum power current of the module are, respectively, 12.46 A and 11.68 A. In other words, the module currents are higher than the maximum current specified for the microinverter.

Technically this module-microinverter combination can work, as the microinverter current is electronically limited during its operation, but the microinverter manufacturer must be consulted about the possibility of using this module power, since violating the specified current limit in the catalog may result in the loss of the product warranty.

O Solar Channel consulted Hoymiles and obtained the following position.

“The MI-1500 microinverter accepts modules of up to 506.25 W without losing the warranty, which can be extended up to 25 years for an additional cost. With powers above 506.25 W, the warranty cannot be extended. The open circuit voltage of the modules, in no case, may exceed the maximum allowable voltage of the microinverter, which is 60 V. In no case may the power of the module exceed 135% of the nominal power of the microinverter. As for current, the 11.5 A limit found in the datasheet is not a problem. The microinverter, even overloaded with a 500 W module, will not reach the maximum current of the module, so the equipment will work without problems with the limit of 11.5 A found in the data sheet”.

Conclusions

Photovoltaic modules are growing and microinverters need to keep up with this evolution. Two models of microinverters present on the Brazilian market were analyzed. The two devices are specified to operate with modules of a maximum of 525 W and 506.25 W.

Modules well above 500 W will require microinverters with expanded specifications. The specifications of microinverters and modules must always be analyzed to verify their compatibility. In the first case analyzed, it was found that the combination of 500 W module and microinverter works without any restrictions.

The QS1A datasheet points out that the maximum current of the Risen RS150-8-500M module is supported by it, indicating that operation can occur safely and without the risk of voiding the warranty. In summary, QS1A microinverter accepts 500W module.

In the second case studied, it was found that technically the MI-1500 microinverter is suitable to work with the RS150-8-500M module, but the manufacturer needed to be consulted. The equipment data sheet indicates that the current limit is lower than the operating current (in STC) of the chosen photovoltaic module.

The manufacturer, however, stated that its product supports modules up to 506.25 W without loss of warranty. In the two cases analyzed, the power of the modules is 33% above the power of the microinverter. PTo improve the performance of the photovoltaic project, it would be recommended to use higher power microinverters.

However, as already stated, if the module-inverter combination meets the generation expectations of the photovoltaic system designer, the power limitation imposed by inverter clipping is not technically a problem.