Model Reduction Method for Distribution System Including Smart Inverters with Grid-Forming Droop Control

This paper proposes a bottom-up model reduction method for distribution systems that include smart inverters with grid-forming droop control. The proposed method consists of the following three steps: in step 1, the transfer functions of system components such as smart inverters and ZIP loads are derived; in step 2, the transfer functions developed in step 1 are combined with the transfer functions of upstream electric circuits comprising line impedances and transformers; and in step 3, the combined models found in step 2 are aggregated and the order of the aggregated model is reduced using the impulse energy approximation and the Routh approximation. The proposed model reduction method can generate an approximate aggregated dynamic model for a distribution system that includes smart inverters with grid-forming droop control when seen from the interconnection point for the target system. The validity of the proposed model reduction method is ascertained via computer simulations in the Matlab/Simulink environment.