Abstract: In current large-scale photovoltaic（PV）power plants,especially those with string PV inverters,because of the increase of large numbers of inverters compared to integrated PV inverters,there appears calculating and volume bottleneck and serious control command communication jam and delay in the automatic generation control（AGC）system.In order to solve these difficulties,this paper presents a new AGC hierarchical distributed control structure based on a new added middle-level AGC subsystem device in each PV array.An investigation is made on a scheme for the PV array AGC’s integrated implementation of product and technology,the array AGC power allocation algorithm and tactics in addition to the tactics for cooperation and coordination between array AGC and plant AGC systems.It is shown by field tests and simulation analysis that the technology proposed can solve the above AGC problems in a PV plant with string inverters.