###### Abstract  
Grid-connected inverters (GCI) are commonly used in PV system applications to generate a regulated AC current to feed into the grid. Transformerless inverters are the most advanced inverters that are used in industry, which provide efficiency with smaller size and lower cost. This paper proposes a grid-connected single-phase transformerless inverter with the technology of common ground and the virtual DC bus concept. In this topology, the grid neutral is connected directly to the PV ground, which generates a constant common mode voltage (CMV), thus leading to the elimination of the leakage current caused by the PV array's parasitic capacitance. The proposed inverter has a buck-boost circuit with a flying capacitor to generate the DC bus for a negative power cycle, four switches, and two diodes. A unipolar sinusoidal pulse width modulation (SPWM) technique is used which reduces the output filter requirements. In addition, only one switch carries the load current during the active states of both the negative and positive power cycle, thus minimizing the conduction losses. One more advantage presented in the proposed inverter is its ability to charge the flying capacitor during all operation states due to the existence of the buck-boost circuit. Design and theoretical calculations were conducted in this paper to optimize the losses. Moreover, the PSIM simulation was used to validate the proposed topology inverter, verify the performance by showing leakage current elimination, and achieve unipolar voltage in the output bus. The simulation results show a peak efficiency of 98.57% for a 2 kW inverter, which agrees with the theoretical calculations. solar photovoltaic (PV); transformerless inverter; flying capacitor; common ground; buck-boost converter; leakage current; single-phase; grid-connected; common mode voltage (CMV) +
Footnote †: journal: Applied Sciences  
or a low-frequency AC side transformer. The galvanic isolation serves to improve the overall system safety to achieve the corresponding safety standards required for grid-tied PV systems; at the same time, these transformers lead to bulky inverters with higher cost and lower efficiency [5; 6]. Recently, for the residential market, transformerless inverters have been utilized due to their lower cost, higher efficiency, and smaller size [7]. The basic structure for the transformerless inverter was first presented in 1956 by [PERSON], where he developed the Full Bridge FB or H-Bridge inverter family for the first time. The structure of an FB inverter can be seen in Figure 1 [8]. The H-Bridge structure has many drawbacks regarding safety; one limitation is the leakage current produced by the parasitic capacitance formed between PV panels and the ground. The leakage current is generated by the variation in common mode voltage (CMV), which leads to high leakage current and high electromagnetic interference (EMI) [9; 10; 11]. According to the VDE 0.126-1-1 standard, a leakage current over 30 mA must trigger a break within 0.3 s, while the leakage current range should not be more than 100 mA and the fault breaking time should not be more than 40 ms, as shown in Table 1 [12; 13]. Additionally, the leakage current can be decreased by the type of modulation [14]. The leakage current path is shown in Figure 2, where \(I_{Leakage}\) is the leakage current, \(C_{pv}\) the parasitic capacitance and \(C_{in}\) the DC bust coupling capacitor. \begin{table}
\begin{tabular}{c c} \hline
**Leakage Current (mA)** & **Fault Discontinuity Time (ms)** \\ \hline
30 & 300 \\ \hline
60 & 150 \\ \hline
100 & 40 \\ \hline PV Array & Inverter & Filter \\ \hline \end{tabular}
\end{table} 
Table 1: Leakage current with discontinuity time according to VDE 0126-1-1. Figure 2: Common mode circuit. Many transformerless inverter topologies have been proposed in the literature to increase the efficiency by reducing conduction losses, decreasing the CMV to satisfy grid requirements, and decreasing the number of components to reduce the costs and size [15].