Control of grid connected cascaded H-bridge multilevel converter during grid voltage unbalance for photovoltaic application

With a large penetration of distributed power, transmission system operators (TSO) have more stringent demands regarding the interconnection of the distributed power generation system (DPGS) to the utility grid in order to maintain a stable power system. Among the new demands, power generation systems are requested to ride through grid disturbances and to provide ancillary services in order to behave as a conventional power plant. So, they should have the capability to sustain the utility network in the situation of grid voltage unbalance. Therefore, the control of DPGS under the grid voltage unbalance is important. In this paper, control of cascaded H-bridge (CHB) multilevel converter for grid integration of large scale photovoltaic power plants during grid voltage unbalance has been analysed. Operation for feeding constant power to the grid has been investigated and it has been found that it is not possible as loading of cells of CHB will be different which will cause the dc link voltages to diverge. A control technique based on negative sequence current control has been used to equalize the loading of cells.