## 4 Grid-Connected PV Inverter Topologies
### H5, H6 and HERIC Inverters  
Current source inverters are used in high-power applications due to its high reliability, inherent short-circuit protection, and regenerative capability [70]. The VSIs are dominant due to low cost, simple control, and mature technology. Usually, in residential renewable power generation systems such as PV systems, VSIs are of most employed [71]. The difference between VSI and CSI lies in the type of DC source from which they take their energy. The basic concepts and operations are different for each inverter. Some CSI topologies have been shown in recent research [72; 73; 74]. Nowadays, CSI topologies try to reduce the leakage current, such as H5 [75], H6 [76] and HERIC [77].  
In Reference [78], three similar topologies of transformerless current source inverters (full-bridge, H5 and H6 topologies) are compared to highlight their differences. The analysis makes a comparison considering the efficiency, the leakage current and the quality of the output current. In the paper the most important conclusion could be:...among the topologies examined, inverter H5 seems the best compromise in terms of efficiency, reduction in the leakage current, number of components, and current quality...  
In Reference [79] a novel transformerless dual-buck full-bridge grid-connected inverter (GCI) with H5-type (TDFGI-H5) topology for PV systems is presented. The topology shows that TDFGI-H5 has the advantages of the three-level output, no shoot-through problem, high reliability, and can completely meet the condition of eliminating common-mode leakage current. Figure 9 shown the proposed inverter.  
Figure 9: Circuit topology of the proposed TDFGI-H5 in Reference [79].  
Figure 10: Operating modes and its equivalent circuits of TDFGI-H5 in Reference [79].  
The modes of operation presented in Figure 10 depend on the direction of the grid current (\(i_{g}\)). Acting on the switches \((S_{1}-S_{5})\), TDFGI-H5 can be divided into four operating modes. In this case, according to the authors in [80; 81], the leakage current is defined as in Equation (15). However, during the operating modes \(u_{ccm}=u_{GN}\) and \(u_{GN}\) remains constant. Therefore, the leakage current is eliminated.  
\[i_{tcm}=C_{G}\frac{du_{ccm}}{dt}=C_{G}\frac{\Delta u_{ccm}}{\Delta t}. \tag{15}\]  
In the paper, unipolar SPWM is used, which, in combination with the topology and control strategy, keep the common-mode voltage constant. It is important to point out several features of this design. This topology has a more significant conduction loss than its BDFGI counterpart. The inductors used have a longer useful life than the capacitors. Current research is working to reduce the size of the storage elements since it makes the system more expensive, bulky and bright than VSI.  
Despite the advantages mentioned in the previous topology, the H6 topology is the only one that has bidirectional capability [82]. In Reference [76], an unusual topology is shown whose challenge is to find an efficient solution to the bidirectionality of this type. As a result, the research uses a hybrid modulation strategy, both for the rectifier and the inverter. Among the most relevant results is the possibility of extrapolating the modulation strategy proposed for similar topologies.  
The highly efficient and reliable inverter concept (HERIC) inverter is a cost-effective topology, which has low leakage currents and relatively high efficiency. Among various transformerless topologies, the HERIC is a promising candidate due to the simple structure and high efficiency. In Reference [77], a unipolar modulation strategy for a HERIC inverter is presented. The proposed method takes the advantages of the conventional UP-PWM, the UP-PWM with dead time, and the modulation strategy of reactive power capability. To further improve power quality, the effects of the dead time and minimum pulse width limitation were compensated through the hybrid UP-PWM scheme.