A method is provided for controlling power flow within a wind park system for power transmission to a main power grid, the wind park system including two or more parallel-connected island wind park grids. The method includes the steps of: controlling a first voltage source converter as a swing bus in frequency control, the frequency control including controlling frequency to a steady state reference operating point for operating points within a power dead band and by frequency droop control when exceeding an end point of the power dead band, and controlling at least a second voltage source converter in power control, the power control including controlling power flow to a steady state reference operating point for operating points within a frequency dead band and by power droop control when exceeding an end point of the frequency dead band. A controller and computer program product are also provided.