Novel Grid-forming Control for PV Connected to Weak AC Grid

Traditional PV power generation usually utilises a phase-locked loop (PLL) to obtain the exact phase of the AC grid for maximum power point tracking (MPPT) control. However, the low short-circuit ratio (SCR) of the weak AC grid and the wide bandwidth control of PLL may destabilise the system. To address these challenges, this research introduces a pioneering power synchronous control technique that empowers PV power integration with a weak AC system while providing grid configuration capabilities. This approach involves the grid-side converter (GSC) replicating the inertia response of a synchronous generator by artificially establishing a coupling between the DC bus voltage and grid frequency, incorporating a sag characteristic. Additionally, the control mechanism regulates the amplitude of the converter voltage by manipulating the modulation ratio of the GSC through the voltage offset of the DC tank capacitor. This innovative approach achieves grid synchronism without the need for phase-locked loops, instead emulating the inherent characteristics of synchronous generators (SGs). Through a small disturbance analysis of the proposed method, it is determined that the energy stored in the DC capacitor can offer inertia support during system disturbances by autonomously adjusting the converter voltage amplitude, thereby providing damping for the system. Moreover, the PV power generation system provides inertia and damping effects to the system without compromising the MPPT characteristics, and the inertia and damping factors of the PV can be easily adjusted according to the system requirements. Nonlinear simulations were conducted to study the integration of PV with a large synchronous generator, considering sudden changes in active power reference and varying illumination conditions, to assess the efficacy of the proposed control.