Auto-calibrating dc link current sensing technique for transformerless, grid connected, H-bridge inverter systems

Power electronic inverters are commonly used for the interfacing of distributed generation systems to the electrical power network. These electronic inverters operate in a current controlled mode to inject unity power factor sinusoidal current into the network. To prevent possible dc current injection, a mains frequency isolation transformer is often employed at the inverter output. This isolation transformer is a costly component. An alternative approach is to use current sensing and control techniques to eliminate the dc current component. One method is to use a current controller to force the output dc current to zero. Current controllers are prone to errors associated with nonlinearity and offsets in the current transducers. This paper considers a novel auto-calibrating dc link current sensing technique that eliminates the errors associated with the current transducer, and helps avoid dc current injection into the grid when using a transformerless grid connect inverter system.