Improving Transient Stability of PLL-Synchronized Grid-Following Inverters

This paper examines the issues related to synchro-nization of utility network through applications of three-phase grid-following inverters (GFIs). There are problems when trying to capture the positive sequence (PS) components of distorted and unbalanced grid voltage. From the grid signal, the PS components are extracted with the help of a dual second-order generalized integrator phase-locked loop (DSOGI PLL). When the grid voltage has lower order harmonics it fluctuates in both frequency and phase. To overcome these limitations, as suggested in the paper, notch filtering technique available in SOGI architecture, low-pass filter and modification of symmetrical components in type-3 PLL are recommended. This method preserves the performance of the DSOGI-GFI system. The paper also uses the GFI system with the Lyapunov-Function-based current controller described above. Current controllers use PLLs for phase and frequency estimation of three-phase systems in a dq-transformed frame. The proposed PLL is compared with other existing PLLs based on stability and transient characteristics. It also includes an experimental result that demonstrates the effectiveness of PLL.