Improvement and Comparison of Single-Phase Frequency Locked Loops Based on Second Order Generalized Integrator

It is required for grid-connected converters to obtain synchronous information quickly and accurately when a short-circuit fault occurs in the grid. Compared with the phase locked loop (PLL), the second order generalized integrator (SOGI)-frequency locked loop (FLL) promises faster detection of the phase, amplitude and frequency of the grid voltage, but becomes susceptible to the effect of a direct current (DC) offset. Modified schemes based on the suppression method and the filtering method have drawback of slower response. A modified scheme based on the subtraction method is proposed with an addition of a SOGI block in the FLL. The basic scheme based on SOGI-FLL, the modified scheme based on the suppression method and the proposed scheme based on the subtraction method are analyzed and compared by transfer functions theoretically and simulations of PSCAD commercial software. Comparison results illustrate that the subtraction-based scheme promises faster detection of the phase, amplitude and frequency of the grid voltage, and better suppression of the DC offset and low frequency components. Taking account of respective transient and steady-state response characteristics, the subtraction-based scheme has the best comprehensive properties.