# Common-Ground Photovoltaic Inverters for Leakage Current Mitigation: Comparative Review
## 2 Classification of Common-Ground Converters
### Voltage-Source CG Converters (VSCG)
#### 2.2.1 Bucking VSCG
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Converter** & **Features** & \multicolumn{2}{c}{**Limitations**} \\ \hline  & & \(\bullet\) & The input current is discontinuous. \\  & & \(\bullet\) & In addition to the high input capacitance, one \\  & & \(\bullet\) & extra capacitor with a high capacitance value is \\  & & & needed to act as a flying capacitor to interchange \\  & & & the connection from the positive and negative \\  & \(\bullet\) & Low number of semiconductors is used. & half-cycles; this increases the failure rate and \\  & \(\bullet\) & No need for magnetic components; thus, \\  & & reduced costs can be acquired. & \(\bullet\) & Type 3 of these proposed topologies needs special \\  & & & reverse blocking IGBT (RB-IGBT) switches; this limits the switching frequency (\(<\)20 kHz). \\  & & & Possibility of DC current generation in type 2 as an unequal number of switches is conducted \\  & & & during positive and negative half-cycles. \\ \hline  & & \(\bullet\) & The input current is discontinuous. \\  & & \(\bullet\) & It can only offer two voltage levels; a large filter \\  & & \(\bullet\) & Asymmetrical inverter output voltage may be \\  & & & produced. The output inverter voltage is equal to \\  & & & the input voltage in the positive half-cycle and \\  & & & equal to the difference between the DC-link and \\  & & & the input voltage during the negative half-cycle. \\  & \(\bullet\) & It uses a low number of switches. & \(\bullet\) & The converter operation can result in the \\  & \(\bullet\) & Low capacitance value is needed for power & \\  & & decoupling purposes. & \(\odot\) & Very high DC-link voltage should be \\  & & & adopted; thus, more safety concerns \\  & & & should be considered. \\  & & & \(\odot\) & Large number of PV modules should be \\  & & & connected in series as it should be higher \\  & & & than the peak grid voltage; thus, \\  & & & inefficient MPPT operation is highly \\  & & & expected, especially during shading \\  & & & conditions. \\ \hline  & & & \(\bullet\) & The input current is discontinuous. \\  & & & \(\bullet\) & High number of semiconductors and passive \\  & & & components are needed \\  & & & \(\bullet\) & Inrush current may reach a high value during the \\  & & & charging of capacitors. \\ \hline  & & & \(\bullet\) & Two capacitors with high capacitance values are \\  & & & needed \\  & & & \(\bullet\) & The input current is discontinuous. \\  & & & \(\bullet\) & Asymmetrical operation during positive and \\  & & & negative half-cycles as an unequal number of \\  & & & switches is conducted during positive and \\  & & & negative half-cycles. This can result in DC \\  & & & current injection. \\  & & & \(\bullet\) & The switches should be over-rated to withstand \\  & & & the current during the parallel connection of \\  & & & capacitors. \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Features and limitations of voltage-source CG converters (bucking type). Except for the topology introduced in [50], all other converters in this category do not offer continuous input current. Therefore, improper MPPT operation is expected;
* To overcome the above issues, a front-end DC-DC converter is usually used with bucking VSCG converters. However, this results in increased cost and size;
* Among these topologies, the converters introduced in [51; 52; 53] can offer five voltage levels at the output. Thus, a reduced filter size is needed. On the other hand, the inverters introduced in [45; 50] can only offer two voltage levels. Therefore, large filter size is needed. Such high filtering requirements can significantly diminish other features related to the use of a low number of components. All other inverters belonging to this category can offer three voltage levels;
* Among the inverters offering five voltage levels, the inverter in [52] uses a fewer number of switches than the other inverters introduced in [51; 53].