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.
To overcome the problem of the reactive current transfer between the grid filter and the dc-link capacitor in transformer Hybridge string, Inverters during freewheeling, several proprietary solutions have been introduced by different manufacturers [18]-[20]. the H5 string inverter by SMA adds an additional switch between the dc-link and the Hybridge inverter to open the current path between both passive components, Increasing the elficiency and reducing the leakage current. The highly efficient and reliable inverter concept (HERIC) introduced by Sunways uses instead a bidirectional switch that bypasses the whole Hybridge inverter, separating the grid filter from the converter during freewheeling. The HG topology introduced by Ingeteam [21] adds an additional switch in the negative dc bar to the H5 topology. Two versions were introduced one with a diode connected In parallel to the dc side of the Hybridge of the H6 topology, called the HGD1, and the HGD2, which adds two auuxillary freewheeling diodes instead of one. Both allow freewheeling without interaction between passive components while enablling a unipolar output compared to the H5. The difference between the HGD1 and the HGD2 isthat in the former, the additional switches block the total dc voltage, while in the latter, they only block half
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 dc link. This enables transformer operation without the problem of the leakage current and modulation methods that do not use the potential of the converter. The main drawback compared to the Hybridge is that it requires total dc link of double the voltage to connect to the same grid. Hence, more modules need to be connected in series or an additional boost stage is required.
A full-bridge of two3L-NPC legs was introduced by ABB, resulting in the 5L-HNPC inverter also requires a symmetrical grid filter distributed between the grid phase and grid neutral wires. A special modulation technique can achieve a line-frequency common-mode voltage;hence, no leakage current are generated while enabling transformer operation.
The T-type or three-level transistor clamped string inverter was introduced by Conergy. The converter can clamp the phase of the grid directly to the neutral to generate the zero voltage level using a bidirectional power switch. For the same reason as the 3L-NPC, it can operate