Heterogeneous Synchronization Driven Frequency Stability Analysis and Enhancement in Low-Inertia Power Systems Under Grid Faults

Different from the synchronous generator (SG) whose frequency is intrinsically governed by the power imbalance, the frequency of the inverter-based generation (IBG) is controlled by the phase-locked loop (PLL). This fundamental distinction results in heterogeneous synchronization characteristics between IBGs and SGs. In low-inertia systems with IBGs and SGs co-existing, frequency stability mechanisms considering the synchronization heterogeneity remain inadequately understood. To address this research gap, a reduced-order heterogeneous synchronization driven frequency stability model is established for a simple two-machine system. Based on the model, three different types of frequency instability driven by heterogeneous synchronization are identified under grid faults. First, synchronization instability between the IBG and the SG directly triggers the system frequency instability. Second, synchronization stability induces active-reactive power coupling, resulting in a positive feedback control effect and propagating frequency instability. Third, the constraint of synchronization stability reduces the active power output capability of the IBG. This exacerbates frequency indicators, thereby leading to frequency instability. To avoid these types of instability, an active power reshaping phase locked loop (APR-PLL), which eliminates the synchronization stability constraint, is developed for frequency stability enhancement. Simulations performed on the modified IEEE 9-bus system validate the effectiveness of the proposed strategy.