Single-Phase Type-1 Frequency-Fixed FLL for Distorted Voltage Condition

Phase-locked loops (PLLs) are widely used for the synchronization of power electronic based equipment due to their ease of digital implementation and satisfactory response. This article focuses on a single-phase PLL to enhance its performance under distorted supply voltage condition. In most single-phase PLLs, the loop filter used within the control loop cannot completely reject low-order harmonics. Thus, the distortion in supply voltage significantly affects the frequency and phase estimations, both in steady-state and dynamic conditions. To overcome these problems, this article proposes a type-1 frequency-fixed frequency-locked loop (FLL). Generally, when the frequency is fixed in a PLL or FLL, the estimated variables (frequency and phase) suffer from double-frequency oscillation and phase offset whenever the system experiences any frequency variation. In the proposed work, a modified low-pass filter with notch characteristics is used to reject the double-frequency oscillation while the phase offset error is compensated by adding the linearized phase error to the estimated phase angle. Furthermore, a selective harmonic filtering technique is incorporated to improve the disturbance rejection capability under distorted conditions. These advantages make the proposed FLL robust for grid synchronization of single-phase converters. Experimental results are provided to verify the performance of the proposed FLL.