## 3 USPVPs Power Inverters
### USPVPs Power Inverters Topologies  
Nowadays, there is a wide variety of power electronics converter topologies oriented to solar plant applications. The state-of-the-art is extensive and there is not an homogeneous answer about which is the best solution [30; 31; 32; 33; 34; 35; 36; 37], providing very different solutions according to the needs of each specific application. There are many possible classifications of solar inverters according to different factors: galvanic isolation or not; single-stage or multi-stage; central, multi-string, string or module inverter; bi-level or multi-level converter; voltage source inverter (VSI), current source inverter (CSI) or impedance source; grid-connected or isolated; hard or soft switching, etc. [33]. In particular, this study focuses on the application of automated EL technique to large PV plants. Therefore, we will consider grid-connected high-power inverters. In these applications, VSI non-isolated inverters are commonly used. Besides, hard-switching is usually chosen because there are no space or weight limitations in the case of solar micro-inverters for domestic applications. According to the conversion stages inside the PV inverter, single-stage and multi-stage topologies can be found. Multi-stage topologies are suitable for direct current (DC) voltage adjustment, providing galvanic isolation through high-frequency (HF) intermediate stages if needed. However, specially in high-power plants, single-stage inverters are widely used due to the fact that strings are grouped reaching the highest possible voltage, up to 1500 V, and achieving a maximum power between 1 MW and 3.5 MW [38; 39; 40; 41; 42; 43]. The objective is to reduce the DC current at the rated power and, consequently, reduce the power losses in the DC installation. Finally, as the case of study are large PV installations, only three-phase topologies will be considered. Four-leg configurations or several single-phase devices in parallel are also possible, but they are not usually used in industrial environment, and thus they will not be studied in this paper. Within this context, the most representative topologies are described below (although the state-of-the-art is not limited to these topologies). Firstly, nowadays the bi-level three-phase inverter is the simplest and most used topology in the market. This topology, shown in Figure 2a, consists of only six full-controlled semiconductor devices (usually metal-oxide-semiconductor field-effect transistor (MOSFETs) or (insulate gate bipolar transistors (IGBTs)). In the following, several multilevel topologies for PV inverters are presented [44; 45]. These topologies can be extended in the case of 3, 5 or N levels, but, for the sake of simplicity, only the three-level cases are presented. The objective of increasing the number of levels is to add step levels in the voltage waveform, reducing the low-pass filter size and, therefore, improving the inverter response, as well as reducing its cost and size. One of the most used multilevel topology is the neutral point clamped (NPC) topology type I. Figure 2b shows an example of 3-level NPC topology; this configuration has a floating medium point in the DC-link that allows it to modulate 0 V in addition to \(+V_{DC}\) and \(-V_{DC}\). There are some variants on this configuration; Figure 2e shows one representative modification [46]. This configuration has some advantages over the previous one as lower harmonics introduction, smaller passive components, higher efficiency, and solves the leakage current problem inherent in PV panels. Besides, the new capacitor in parallel to the medium-point diodes facilitates the semi-buses balancing: the main problem in standard NPC topology. NPC topology type T is another of the most used topologies in PV applications. Figure 2c shows the T-type topology in a three-level example. This configuration reduces the number of semiconductor elements removing the antiparallel diodes required by the I-type topology.