Hybrid Admittance Model for Grid-Forming Inverters Considering Digital Sampling Process

For grid-forming inverters, digital control is commonly used with a sampling process that converts the current and voltage signals into the discrete domain. Inverter admittance models in the continuous domain without considering the sampling process have reduced accuracy and may lead to false stability predictions. To address this issue, this paper proposes a hybrid admittance modeling approach considering the digital sampling process, which can improve the accuracy of the admittance model and thus is suitable for stability analysis for inverter-grid systems. The hybrid admittance model for grid-forming inverters is developed by considering the physical characteristics of sampling, the discrete nature of controllers, and the continuous characteristics of the inverter plant. The sampling of the transfer function with zero-order hold is equivalent to its step-invariant Z-transform, which can avoid the infinite sum due to sampling. Compared to state-of-the-art models, the proposed hybrid admittance model maintains high accuracy near and above the Nyquist frequency. Finally, the proposed method is validated through simulation and hardware-in-the-loop experiments.