Modeling, analysis and control of bidirectional power flow in grid connected inverter systems

With the proliferation of alternate power sources such as fuel cells and photovoltaic systems in the distributed power system architecture it is important to design inverters to support seamless bi-directional power flow between the inverter and the grid. For this the inverter should work in voltage control mode in stand alone operation; current control mode in grid connected operation and when power is drawn from the grid the inverter should, typically, operate in power factor correction mode. This paper deals with the modeling and analysis of grid connected inverters to address control issues in the above modes together with seamless transfer between modes. It is shown that the filter network configuration changes between modes and greatly impacts overall system dynamics. Based on small signal modeling, new control strategies that ensure near unity power factor in bidirectional power flow and smooth transition between modes is presented. The control scheme is implemented on a DSP platform using state-space filtering techniques.