An Enhanced Frequency-Fixed All-Pass Filter PLL for Single-Phase Application

All-pass filter (APF) offers a simpler implementation for orthogonal signal generation (OSG) and has widely featured in single-phase phase locked loop (PLL) applications. However, they have two prominent issues, the absence of filtering capability and reliance on grid frequency information for yielding 90° phase shifted signal. This paper proposes an enhanced APF-PLL based on frequency fixed configuration for abnormal grid voltage scenarios addressing both the aforementioned concerns. The suggested structure utilizes a modified transformation for attaining frequency-independent APF-PLL, while a frequency-fixed bandpass filter (BPF) is appended as a prefilter to serve the filtering objective. Thus obtained PLL structure, is then evaluated against some recently proposed OSG-PLLs. The simulation and experimental results demonstrate improved performance (dynamic and steady-state) in different operating scenarios.