## 4 Grid-Connected PV Inverter Topologies
### Common-Grounded Inverter  
In transformerless systems, the use of common-grounded inverters is one of the most used topologies to prevent the leakage current. In these converters, the negative terminal of the PV is directly connected to the neutral point of the grid; therefore, the general CMV is removed correctly [55]. Today, other common-grounded schemes have tried to reduce the number of DC voltage sources by using capacitors [56]. Switched-Capacitor based inverters use the virtual dc-link concept to obtain two times boost factor within a single-stage operation without using any auxiliary inductors. For example, in Reference [57], a novel topology for the single-phase transformerless grid-connected inverters family is presented. This converter is shown in Figure 7.  
This type of converter boosts the input voltage to double by using the one-stage SC, eliminating the disadvantages of two stages [58, 59]. The presented design employs a unipolar PWM method and only has two energy levels at its output. Using unipolar PWM, the THD and the size of the inductor is be reduced [60], but \(V_{CM}\) varies in the switching frequency of the inverter and its multiples [61]. The low impedance offered by the parasitic capacitance \((C_{PV})\) between ground and PV at these frequencies, causes a higher common current than bipolar PWM [62]. Particularly in this paper, the topology eliminates the leakage current up to a value close to 0 mA.  
The four modes of operation of the proposed inverter are shown in Figure 8. The topology always maintains a connection between the negative terminal of the panel and the neutral of the grid. The switches use a switching frequency of 25 kHz. The capacitors will be charged both in the positive and the negative half-cycle; this feature allows the use of metalized polyester film capacitors or MKT capacitors instead of electrolyte capacitors.  
The previous design has certain disadvantages; for example, the use of capacitors decreases the useful inverter life. Besides, it only has two levels of energy at the output (positive and negative), which increases the THD compared to inverters with more than two levels. On the other hand, the switching frequency can be considered high, causing the elements to wear and sometimes noise to the system. Compared to MLI, the topology must be used for low powers. SC-based multilevel inverters (SCMLI) are being addressed to increase the number of levels, decrease THD and increase the total power that this type of device can handle. New configurations of SCMLIs were suggested in the literature [63, 64, 65].  
Figure 7: Grid-connected inverter topology proposed in Reference [57].  
Figure 8: Operating modes of the proposed grid-tied inverter in Reference [57].  
Another type of common-ground based inverter is the one that uses floating capacitors. In Reference [66], a Type-I inverter with two switches in series during the positive cycle is proposed. The authors of Reference [67] analyze Type-II and Type-III inverters. In the Type-I topology, only one switch carries the load current during the positive cycle. For its part, Type-III topology no requirement of an extra diode as in the previous two topologies to form a topology similar to a conventional H-bridge. The three topologies use unipolar SPWM as a modulation technique to reduce system losses, THD and output filter size. In all three cases, only four active switches are used to realize the basic inverter circuit and reduce the leakage current until practically eliminate it. The reduction in common-mode frequency has also been achieved using transformerless topologies with multilevel neutral point clamp [68] or its variant, using split inductors [69].