An Improved Research on Frequency-Fixed Cascaded SOGI Single-Phase PLL

Abstract

The second-order generalized integrator (SOGI) with band-pass filtering characteristics is widely used in the design of advanced single-phase phase-locked loops (PLLs). But this type of PLL has insufficient ability to restrain dc biases, which limits their applications under certain conditions. Recently, a PLL adopting two cascade SOGI structures is called the cascaded generalized integrator PLL (CGI-PLL), which has perfect dc offsets blocking capability. However, the two output signals of the quadrature signal generator (QSG) formed by frequency-fixed CGI have different amplitudes, which makes the second harmonic oscillations appear in the estimated parameters of the PLL while the frequency deviates from the nominal frequency. Similarly, the in-phase output signal of CGI-based QSG is not synchronized with the fundamental frequency component of the grid voltage under frequency drifts, which is the reason for the phase offset error in the PLL outputs in the steady state. Some compensation strategies are added to correct the error that exists in the frequency-fixed CGI-PLL when the frequency deviates from the nominal value.