techniques since the common mode voltages cannot generate a leakage current due to isolation. The bypass switching state (zero voltage level) prevent a reactive current flow between the filter inductor and The dc-link capacitor. Nevertheless the bulky transformer has several disadvantages (low power density and lower efficiency), making this topology less popular with time.
The transformer Hybridge, also known as an H4 inverter (shown in a two-stage), gets rid of the low-frequency transformer by splitting the grid inductor into the phase and neutral wires of the systems and using a bipolar PWM (two-level) to solve the issues of the switched common-mode voltage and leakage currents and by using a boost stage for a wider input voltage range. The downside is that the two-level modulation reduces the power quality at the grid connection and lowers the efficlency since there is a reactive current flow between the passive elements of the circuit at zero voltage through the freewheeling diodes as the dc-link cpacitor is not isolated from the grid at any time.
The Hybridge with the HF isolated DC-DC stage is composed of a MOSFET full-bridge inverter, rectifier. This approach greatly reduces the size of the converter, improving the power density compared to low-frequency transformer-based topologies. However, the additional converter stages introduce higher losses.
The three-level NPC inverter (3L-NPC) also has several modified and enhanced versions for PV string inverters [22]. The adventage of the 3L-NPC over the Hybridge is that it provides a three-level output without a switched common-mode voltage since the neutral of the grid is grounded to the same potential as the midpoint of the