String Inverter Topologies
The most common string inverter topology is the full- or H-bridge inverter. Several modified and enhanced versions have found their way into the market. The H-bridge with a grid-side low- frequency transformer features a simple power circuit, galvanic isolation, and voltage elevation provided by the trans- former, which enables a larger range of input voltages. This converter can be controlled with three-level carrier-based pulse-width modulation (PWM) techniques since the common mode voltages cannot generate a leakage current due to isolation. The bypass switching state (zero voltage level) prevents 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.