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Improved Phase Detection for Single-Phase, Phase-Locked Loops

Fig. 1. PLL Freq. response without modification and with LPFs.
A crucial component of grid-connected converter systems is the Phase-Locked Loop (PLL), which tracks the grid voltage's frequency and phase angle. Therefore, accurate, fast responding PLLs for control and protection purposes are required to provide these measurements. Ripple noise in the estimated values, especially 2nd harmonic ripple which is common in 1φ PLLs, can lead to undesired effects: nuisance tripping of protection schemes, power factor misalignment, improper selection of the converter's mode of operation.

Common approaches to mitigate the 2ω ripple is to add Low-Pass-Filters (LPFs) to attenuate the noise ripple; these methods work, but cause unnecessary delays in parameter estimation, as seen in Fig. 1.

An improvement over this method would be to eliminate the need for LPFs altogether. Fig. 2 demonstrates how simple feedback of the estimated angle can be summed with the original Phase Detector output.

In this method, the 2nd harmonic term is completely eliminated under ideal conditions and results in as much as 88.2% ripple cancellation, as seen in Fig. 3.

This method also does not require the use of complex control schemes to improve performance; with a simple PI regulator the modified PLL system can achieve the same level of transient performance as the unmodified system, but without the noise ripple present, as seen in Fig. 4.

Fig. 2. PLL structure with summation modification in the Phase Detector.
Fig. 3. 88.2% reduction from modified implementation.
Fig. 4. Same transient performance between PLL systems with and without modification.
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