Multistring Topologies
The main difference between the multistring and string configuration is that multistring is exclusively a two-stage system composed by more than one dc–dc stage [17]. Hence, all inverter topologies in the “String Inverter To-pologies” section could be used in a multistring configuration. Like with string inverters, the same combinations of isolated and transformerless configurations apply with or without symmetric grid filters. One of the first multistring inverters introduced in practice was the half- bridge inverter with boost converters in the dc–dc stage by SMA [17]. Other to pologies that have followed include the H-bridge, the H5, the three-phase twolevel voltage–source inverter (2L-VSI), the 3L-NPC, and the three-phase three-level T-type converter (3L-T). Figure 3 shows some examples of practical multistring configurations. The most common dc–dc stages used for multistring configurations are the boost converter and the HF isolated dc–dc switch mode converter based on an H-bridge, HF transformer, and diode rectifier.