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 degrees 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 frequencyfixed 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.