Abstract The modern power system has undergone a significant transformation with the high penetration of new energy sources and power electronic equipment. A distinctive feature of the power supply system dominated by new energy sources is the emergence of the heterogeneous inverter paralleled system (HIPS), characterized by the coexistence of grid-following (GFL) and grid-forming (GFM) inverters. Compared to a single-type multi-inverter paralleled system, HIPS needs to consider the device interactions and uncertainties introduced by differences between inverters. This paper focuses on the multi-dimensional resonance characteristics of HIPS and investigates the effects of interactions between different inverter types on system stability by establishing the HIPS interaction admittance matrix model. Since the GFL is synchronized by a phase-locked loop (PLL) and the GFM is self-synchronized by a power-synchronized loop (PSL), the effects of PLL, PSL, and delay are comprehensively considered when establishing the HIPS interaction admittance matrix. It enables accurate modeling of different inverter types. An improved Gershgorin-circle stability criterion (GCSC) is proposed. The introduction of a distance vector function F simplifies the analyzing process. A parameter sensitivity calculation method based on GCSC is proposed, and the stability effects of key action factors on HIPS are quantitatively analyzed. Finally, the effectiveness of the theoretical analysis and stability criterion is verified by time-domain simulation arithmetic and experiments. If the distance vector F is less than zero, the system is judged unstable. This method can determine system stability and visually analyze the oscillation point of the system. Compared to traditional methods such as the Nyquist criterion and modal analysis method, the GCSC reduces algorithm running time to 26% and CPU usage to 17%. The following conclusions can be drawn. (1) The high-frequency resonant instability of the grid-following inverter paralleled system (GFLs) in a weak grid is primarily caused by the instability of the current loop and PLL triggered by the excessive grid impedance. The current loop bandwidth BCL-GFL has a narrow adjustable range within the stabilization region, which is the most important parameter affecting the stability of GFLs. Moreover, the coupling effect with the LCL filter becomes strong as the grid impedance increases. (2) GFMs are susceptible to low-frequency oscillatory instability in a strong grid, mainly caused by small grid impedance triggering PSL and voltage loop instability. The voltage loop bandwidth BVL-GFM is identified as a crucial parameter. (3) HIPS, integrating GFL and GFM advantages, exhibits complementary stability characteristics. In weak grids, the GFM provides voltage and frequency support for the GFL to weaken the coupling influence of grid impedance. In strong grids, the GFL access increases the grid-side equivalent inductive reactance of the GFM, reducing power fluctuation influence. In addition, the time-domain simulation and experimental results show that the resonance points of the FFT analysis of the grid-connected currents are consistent with the stability analysis results of the GCSC. Focusing on the HIPS interaction admittance matrix model and its stability analysis, this paper quantitatively analyzes the key action factors. Further research is suggested to delve into discrete-domain modeling problems for complex high-order HIPS considering practical engineering aspects.
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2024, Vol. 39 
