Solar inverter with 2 MPPT operating with single input

This article shows that inverters with more than one MPPT input can be operated with a single input, even when their datasheet (datasheet) states that they physically have two or more MPPT inputs. MPPT is the acronym for maximum power point monitoring (maximum power point tracking), which is a feature built into all inverters for use in grid-connected photovoltaic systems. This feature is important to ensure the operation of photovoltaic modules always at the point of maximum power, enabling maximum energy generation by the photovoltaic system. Recently, inverters with two or more MPPT inputs have become common on the market. This means that modules connected to one input are decoupled from modules on other inputs. The inverters perform maximum power tracking separately for each input, allowing the modules of a project to be divided into independent groups. This is useful when we have projects with shadows or roofs with different slopes and orientations, for example. In some cases, however, we see inverters with multiple MPPT inputs being sold in photovoltaic generator kits with just one stringbox. How is this possible? In theory, if there are two MPPT inputs and if there is a need to use stringbox, two should be used stringboxes, one for each entry. This is true, but not always. It depends on how the inverter is being configured, as we will explain in this article. The important fact that we want to highlight in this article is that inverters with more than one MPPT input can have all of their inputs connected together, starting to function as single-input inverters. The advantage of this is that, in many cases, there is no need for independent MPPT inputs and connecting to a single input makes installation simpler and cheaper. In summary, even if the inverter offers two or more inputs, it still offers the possibility of working with a single input. When we analyze inverter data sheets we see in their descriptions the number of MPPT inputs that the equipment has. Typically, inverters sold in the micro and minigeneration market have between two and four inputs. Below we find, for example, the REFUone 3K-2T model from Refusol. This 3 kW inverter has 2 independent MPPT inputs and allows you to connect one string by MPPT.

Specifications table found in the Refusol inverters datasheet:

The most common way to parallelize the inverter's MPPT inputs is through a physical connection of the inputs within the inverter itself. Some manufacturers provide a special paralleling accessory to create a single input bus. In the figure below we see a system that has 2 strings which are connected in parallel in the stringbox, which is a fairly common case. THE stringbox In this example it has 2 inputs and 1 output. The inverter, originally having 2 MPPT inputs, operates with a single input, having its two MPPTs connected in parallel (that is, it becomes an inverter with just one input). Note that the diagram in figure 2 is single-line, that is, each input or output connection actually has two cables (positive and negative), which are not represented in the drawing.

In rarer cases, it is possible to find a system like the one illustrated in the following figure, in which the parallelism of the MPPT inputs is carried out outside the inverter. In this case, the stringbox must be designed appropriately. In the example below we have a system that has only 1 string and we want to connect this string to an inverter that has 2 MPPT inputs. This is technically possible as long as the manufacturer allows it and as long as the inverter is configured to operate in this way, as we will see in the case study shown in the next section of this article. In the diagram below, we see that the stringbox It has only one input and two outputs. Each output is connected to an inverter input connection and within the stringbox the two outputs are connected in parallel.

Parallelism of MPPT inputs carried out in the stringbox. 

Case study

This study is based on a generator sold by Aldo Solar. It is made up of 8 335 W photovoltaic modules and a 3 kW Refusol inverter. The supplier itself has some instructions on its website for installing this system by joining the two inputs, as seen below, highlighting that the two pairs of cables (2 positive and 2 negative) must have the same length and the same gauge in this project. in particular, since the output current of the stringbox must be shared between the two inverter input connections.

When we come across the information that a single array of panels must be connected using the inverter's two inputs, it becomes clear that in this case the independent MPPT inputs feature will not be used in this equipment. There is no problem with this, as the possibility of using the two inputs together is a function provided by the equipment manufacturer. This also allows the use of just one stringbox in the photovoltaic generator, which makes installation simpler. When we open the technical sheet of stringbox marketed or even the inverter instruction manual, we see that the two inverter inputs are being configured in parallel. THE stringbox sold in this generator kit is the SB 1E-2S from Proauto. This stringbox has entry for only one string and the two outputs that are in parallel. This way, the inverter's two MPPT inputs are connected in parallel and work as a single MPPT.

Table with data from the 1E-2S stringbox (1 input and 2 outputs) sold together with the REFUone 3K-2T inverter for use with 2 MPPT inputs connected in parallel:

The generator kit in question is being supplied with the inverter configured in parallel mode and, therefore, this connection possibility is possible, that is, the inverter physically has 2 MPPT inputs, but this function is not being used in this system. The figure below, taken from the inverter manual, shows that the two inputs strings are linked to a single “boost” block. In other words, the inverter operates with a single MPPT, with two strings connected in parallel at the equipment input.

Conclusion

Inverters for grid-connected photovoltaic systems may have one or more MPPT inputs, depending on the model. The number of available inputs is specified in the product data sheet. Under specific conditions, the use of multiple inputs is not necessary. Some equipment allows MPPT inputs to be grouped into a single input. If the equipment is marketed and intended to be used in this way, the design of the photovoltaic system must take this into account.