A grid connected system is connected to a large independent grid (typically the public
electricity grid) and feeds power into the grid & it’s a system without battery back-up.
A String Inverter is a special type of Inverter (electrical) that is used in Solar Photovoltaic power system to convert direct current into alternating current and fed it into the System. The technical name for a String inverter is "Grid connected String inverter". They may also be called synchronous inverters. Grid-connected inverters typically cannot be used in standalone applications where utility power is not available.
The String Inverter is designed to convert solar electric (photovoltaic or PV) power into utility- grade electricity that can be used by the local load or sold to the local power company. In order to operate, the inverter must have grid power available and connected. The inverter will automatically synchronize itself to grid. The inverter is provided with the isolation transformer internally with basic insulation.