Single-phase frequency-fixed all-pass filter QT1-PLL with inherent dc offset suppression

The all-pass filter (APF) is an orthogonal signal generator (OSG) structure that is widely adopted in single-phase systems because of its simpler implementation. One of its applications is in the form of synchronous reference frame (SRF)-based phase-locked loop (PLL). The APF basically provides unity gain in the whole frequency spectrum while altering its phase according to the frequency. The APF is, however, devoid of a filtering capability. The dc offset poses a particularly serious challenge and this issue is usually addressed with the integration of additional filtering structures. This article shows the development of a two-stage modified APF (TSMAPF)-based OSG structure that is capable of suppressing dc offset while maintaining a frequency-fixed structure. The proposed OSG relies only on optimized parameter tuning without the need for auxiliary filtering units. The TSMAPF is then combined with a quasi type-1 (QT1) PLL to provide an innovative single-phase PLL with a fast dynamic response and the capacity to reject disturbances. Both simulation and experimental results verify the effectiveness of the proposed PLL.