FPGA Design of Isometric Sampling PLL for Variable Frequency Systems

Second-Order Generalized Integrator-based Phase-Locked Loops (SOGI-PLLs) are widely used for grid synchronization. In their standard configuration, a frequency feedback loop is used to adjust the resonance frequency of the SOGI, which enables frequency adaptability but increases implementation complexity. This paper presents an isometric sampling SOGI-based PLL that eliminates the need for trigonometric calculations and enables the SOGI parameters to adapt to frequency variations without modification. The isometric sampling method employs a variable sampling period technique ensuring a constant phase difference between consecutive samples by setting the sampling frequency as a fixed multiple of the input frequency. The proposed approach is implemented on a Field-Programmable Gate Array (FPGA) and experimental results demonstrate its effectiveness in managing significant frequency and phase variations in constant-frequency and variablefrequency grids. Structural simplicity, robustness, rapid dynamic response, and frequency adaptability are the attractive features offered by the proposed system.