*3.3. EPLLs*

In the last years, an alternative synchronization technique known as enhanced phase-locked loop (EPLL) has succeeded [23,74] due to high filtering performance. It consists of an adaptive nonlinear detection algorithm which provides two orthogonal signals synchronized with the grid voltage. The EPLL structure is represented in Figure 7. It allows to estimate the frequency, the phase and also the amplitude *EEPLL* of the input signal fundamental component. The EPLL operates as an adaptive filter (either a notch or a band–pass filter) whose frequency tracks the fundamental frequency of the grid voltage. *eEPLL* denotes the filtered signal tracking the grid voltage supplied in input.

**Figure 7.** Enhanced phase-locked loop (EPLL) structure.

The EPLL represents one of the most promising synchronization systems for single-phase applications since it provides: filtering capability in respect to the undesired harmonics, adaptive detection of the grid voltage fundamental frequency, proper estimation of frequency, angle and amplitude of the grid voltage supplied in input [24,75]. Some modifications of the EPLL have been also proposed in the most recent literature, in particular in [76] the structure of the EPLL has been modified in order to achieve a linear model.

The main differences of the EPLL compared to the PLL based on the Park transform occur testing the two systems in presence of grid voltage perturbations, in particular frequency changes and harmonics. It has been demonstrated that the EPLL exhibits higher filtering capability and shorter transients [23,77]. For all these reasons the EPLL represents the ideal candidate to operate in coordination with the islanding detection techniques discussed in the following Section.

#### **4. Islanding Detection**

In case of grid disconnection, the PVS operation depends on the power level provided by the PVS before islanding occurrence. In Figure 8 it is represented the PVS power stage for the islanding detection test. The grid utility breaker is denoted as Sg, two different breakers *S*<sup>1</sup> and *S*<sup>2</sup> are used to connect the PVS to the point of common coupling (PCC) and to connect the load. A variable RLC load is considered in order to assess the islanding phenomenon in case of different load powers and quality factors.

**Figure 8.** Photovoltaic system (PVS) power stage for islanding detection test.

For each islanding detection method, the non-detection-zone (NDZ) defines the area where the anti-islanding methods fail to detect islanding. As a consequence, the NDZ can be used as a performance index to assess the islanding detection methods [78–80]. However, in comparison with the previous version of the standard [81], the new standard [25] requires voltage and frequency ride-through capability of the PVSs which increases the NDZ of the islanding detection algorithms. Hence it has to be pointed out that ride-through requirements hazard the islanding detection techniques.

Traditionally the islanding detection methods were classified in remote techniques (based on communication signals) and local techniques. Considering the recent advancements of communication equipment and the requirements updates related to the PVS standards, in this study a different classification is adopted. The islanding detection methods are classified in four main categories: Communication-based methods [82], passive methods [49], active methods [25,50] and signal processing-based methods [55]. In addition, in order to improve the performance of the islanding detection methods and to satisfy the standard requirements, hybrid techniques are developing in the last years which are based on combination of the previous categories. In Figure 9 the main islanding detection methods are summarized.

**Figure 9.** PVSs islanding detection methods classification.
