Centralized and decentralized topologies of photovoltaic inverters

The decision to select a photovoltaic inverter for commercial installations should not be based solely on the price of this equipment. This decision should be made based, in addition to the technical aspects, on a detailed analysis of all the costs that arise from the selection of this equipment.

First, the initial costs of the photovoltaic project (CAPEX – capital expenditures), which are partially influenced by the inverters, should be analyzed. Subsequently, the analysis of the operating costs (OPEX – operation expenditures), as well as the plant's generation returns, would be the logical steps to have a complete basis for comparison between the different types of project configuration.

In this study, we will compare one of the components that influence CAPEX, the connections/cabling component, in two different installation scenarios. The cabling costs will be assessed for a centralized topology, where all inverters are installed on a skid near the distributor's delivery point, and for a decentralized topology, where the inverters are positioned near the modules.

When analyzing the CAPEX portions of photovoltaic projects, we see that the costs linked to inverters are not the highest, as shown in Figure 1, however, the selection of this equipment directly influences the BoS (Balance of System) costs that incorporate all costs related to cabling, connections, structures, etc.

Associated with an appropriate topology for the installation, inverter selection can bring great savings to the photovoltaic project as a whole. For this study, data referring to the inverter was used Fronius Eco 27kW (Figure 2).

As a basis for the calculations that were carried out, an installation of 407 kWp was considered, consisting of 330W modules. These are arranged in 5 strings of 19 modules per inverter, with a total of 13 Fronius ECO 27kW inverters.

As everyone already knows, the sizing of the entire photovoltaic system must be carried out in accordance with the standard ABNT NBR 16690 Electrical installations of photovoltaic arrays – Project requirements, and it also references the ABNT NBR 5410 standard (Low voltage electrical installations) in order to complement the sizing of protection systems and alternating current cabling.

It is worth mentioning that the current capacity for direct current cables for photovoltaic applications are listed in the standard ABNT NBR 16612 Power cables for photovoltaic systems, non-halogenated, insulated, with coverage for voltages of up to 1,8 kV dc between conductors – Performance Requirements.

Assumptions were also adopted to carry out this dimensioning, in order to obtain a more simplified comparative basis.

In this case, connection components that are common to all topologies, such as cabling between photovoltaic modules, for example, were not considered.

Finally, below, Figure 3 and Figure 4 illustrate both topologies studied.

For these two topologies, it is important to initially note the different configuration, where the centralized topology presents the string box, and the decentralized one does not list it in its block diagram.

It is worth mentioning that this configuration is allowed when using the ECO 27 inverter, where it already incorporates the string box internally.

With the considerations presented previously, it was possible to determine the cable section for each of the topologies, considering the sizing criteria of the presented standards.

Thus, it was possible to obtain a section of 25 mm² and 16 mm² for the centralized and decentralized topologies, respectively. These cable sections refer to the sections indicated by the average distances shown in the block diagram.

Under these conditions, the decentralized topology will use, in this section described, alternating current (AC) cables. The centralized topology will use direct current (DC) cables suitable for photovoltaic installations in accordance with ABNT NBR 16612.

With the cable data in hand, it is possible to check the results for this original scenario with the average distance of 45 meters, and, additionally, compare them to a non-Fronius inverter that does not have integrated String Box technology, and installed in the decentralized topology. This result is shown below in Figure 5.

In Fig.5 the decentralized topology with Fronius ECO 27kW inverter was stipulated as a reference, thus presenting a value of 100% for cost.

For this original scenario, with an average distance of 45 meters, it was possible to notice that the centralized topology for the ECO inverter presents a difference of 8% more in costs when compared to the decentralized topology.

These differences tend to reverse with increasing average installation distances, as we will analyze below.

When the topologies are analyzed for a greater distance, now for an average distance of 80 meters, we see that the situation is reversed between the topologies when compared to the situation previously presented.

With this new scenario, the centralized topology with Fronius ECO 27kW inverters generated savings of 8,5% when compared to the decentralized topology with the same inverter.

This difference is largely linked to the number of cables used, since the alternating current side requires 3 phases, neutral and ground, while the direct current side uses a positive, negative and ground pole.

It is worth ing that these distances must always be evaluated as the voltage drop criterion for cable sizing must also be respected, making it necessary to increase the cabling sections for greater distances.

It is also noted that non-Fronius inverters presented higher cabling/string box costs than the other two alternatives. This is due to the need to have this additional component (string box) in decentralized installations.

For these reasons, it is possible to notice the break-even point between the topologies with Fronius ECO inverters, where both present equal values ​​for this fraction of the cable cost, which occurs for 52 meters of average distance, as illustrated in Figure 6.

The differences presented were based only on a portion of the BoS costs, and prices referenced to a cable and string box manufacturer, as well as analyzed for a specific situation for the study.

These differences do not reflect the total differences when analyzing the project encoming all costs (CAPEX). However, this study brings to light the importance of selecting photovoltaic inverters, as well as their installation topology.

The influences resulting from the selection of photovoltaic inverters are large and cause major impacts on the costs of the components for the balance of the photovoltaic generation system, which is why this analysis must be carried out in detail in order to seek the greatest profitability for your project.