2 Solar String Inverters

Figure 2-1 shows the typical architecture of a solar string inverter.

Figure 2-1 Solar String Inverter Block Diagram

As Figure 2-1 illustrates, there are three major power blocks in the string inverter. The first stage is a uni-directional DC/DC converter stage that converts the variable string output to a stable high-voltage DC link suitable for the next stages, the second is a bidirectional DC/DC power stage the third a bidirectional DC/AC inverter stage. For single-phase systems the DC Bus voltage is typically 400V_DC. For three-phase systems the DC-Bus voltage is around 800V_DC or even higher up to 1500V_DC. This first DC/DC stage is also able to perform the Maximum Power Point Tracking (MPPT) for a complete string. It simply searches for the maximum power by changing voltage and current across a complete string. This DC Bus voltage is then converted to an AC voltage at the grid voltage level by the DC/AC inverter power stage. In today’s systems, the AC/DC is built as bidirectional PFC/Inverter to allow the operation of the DC/DC power stage that connects to a battery energy storage system, and allows to charge and discharge the ESS in both directions.

A more detailed block diagram of Solar String inverter is available on TI's String inverter applications page.