An Isometric-Sampling PLL in Variable-Frequency Single-Phase Systems

Conventional SOGI-PLLs perform effectively when the power source maintains a stable frequency. However, the increasing prevalence of variable-speed systems requires real-time PLL adaptation to rapid frequency changes. Although frequency deviations can be tracked and used to update conventional SOGI parameters, this approach inevitably introduces significant transients due to the interaction between the SOGI and PLL. This paper proposes an isometric sampler that uniformly segments the AC voltage period, ensuring the product of sampling time and voltage frequency remains constant. Consequently, the SOGI parameters naturally adapt to frequency variations without parameter adjustments. Furthermore, since SOGI frequency responses along α- and β-components differ when using a single gain, optimal gains are separately derived by analyzing voltage disturbances. Experimental results verify that the proposed SOGI-PLL with isometric sampling achieves robust adaptation to wide-ranging frequency and phase variations in a single-phase voltage, while significantly improving transient performance.