*3.1. Zero-Crossing Detection Methods*

An elementary method used to extract information about phase and frequency of the grid voltage is based on the zero-crossing measurement [62,65–67]. The ZCD structure is shown in Figure 3. When the grid voltage waveform crosses the zero, a counter provides in output information about the period and, consequently, the estimated frequency ωˆ of the grid voltage is obtained. The phase θˆ of the grid voltage is achieved integrating ωˆ. Despite the simplicity, this technique does not allow high dynamic performances. Indeed, the phase tracking can be fulfilled just for each half cycle of the grid voltage waveform.

**Figure 3.** Zero-crossing detection (ZCD) structure.

Since the grid voltage *e* is generally affected by power quality disturbances, a low-pass filter (LPF) is used to extract the fundamental frequency ˆ *f* of the original signal. No controller is employed in the detection and the method is not proper to track and to monitor the grid voltage in case of abrupt changes.
