**1. Introduction**

At the end of 2018, the world had 152 GW of installed photovoltaic (PV) electricity capacity. The best PV markets in 2018 were China with 44.3 GW, India with 10.8 GW, USA with 10.7 GW, Japan with 6.7 GW, Australia with 3.8 GW. The European Union (EU) has registered rise for the first time in years with 8.4 GW, but this growth is far from the 23.2 GW registered in 2011. However, the growth can be considered slow, some of the EU countries have already achieved high PV penetration due to past installations such as Germany with a PV overall capacity of 45.5 GW by the end of 2018, Italy that exceeds 20 GW, United Kingdom with 13 GW, Spain with 5.6 GW, Belgium with 4.3 GW and Switzerland with 2.2 GW. It is estimated that overall, the PV systems (PVSs) had contributed to the 2.9% of the global electricity demand in 2018 and that the climate change impact is of 590 millions of tons of CO2 saving every year [1–3].

Due to the photovoltaic prize reduction and availability of loan products, a significant portion of PVSs have been recently installed also in absence of governments initiatives especially for residential applications. A performance evaluation of residential PVSs in some European countries is presented in [4]. Considering the period 2014–2016, the highest specific yield in kWh/kWp has been registered in Italy in 2015.

The PVSs inverters price has diminished around 0.10 \$/Wp in the last decade [5]. In addition, the design optimization of the PVS converters has facilitated the reduction of the total cost of ownership [6]. Nevertheless, the increase of PVS grid-connected installations implies several management challenges depending also on the point of interconnection between the PVS and the grid [7–10]. In this scenario advanced control features of the PVS inverters can contribute to overcome some of the grid management challenges due to high penetration [11–13].

Looking at the residential applications, the PVSs can be single-stage or double-stage. In case of single-stage PVSs, the PV array is directly connected to the inverter avoiding a boost DC/DC converter. Single-stage transformerless PVSs represent the most promising technology due to lower weight, higher efficiency, smaller size and limited cost than double-stage PVSs or single-stage architectures

coupled to low-frequency transformers [14–18]. Focusing on a single-stage PVS, a review about some of the main control issues is presented in [19], however the analysis is limited to current and voltage control methods and maximum power point tracking (MPPT) techniques.

About the most important issues to be considered in grid-connected PVS there are the synchronization with the grid and the detection of the islanding condition. Synchronization deals with PVS high power factor operation, since the synchronization algorithms objective is to provide grid voltage information about amplitude, phase and frequency in order to generate a current/voltage reference which is in phase with the grid voltage [13,20–24].

Synchronization deals also with the grid voltage monitoring. According to the grid-connection requirements [25], the PVSs connected to the low-voltage distribution grid must operate without causing step change in the RMS voltage at the point of common coupling (PCC) exceeding 5% of rated value. In addition, the synchronization parameters limits for grid-connected PVS are: 0.3 Hz for the frequency difference, 10% for the voltage difference. Abnormal conditions can arise on the utility grid which require a prompt response from the grid-connected PVS, hence the information provided by the synchronization system are fundamental for this purpose [26–28].

Unintentional islanding phenomenon is verified in case of grid power outages when the PVS continues to supply the local loads. Unintentional islanding can cause damages to the local electrical loads, to the grid-connected PVS inverter, to the technicians during the maintenance operations. Numerous improved islanding detection algorithms have been proposed in literature in the last years aiming to detect islanding phenomenon in all possible cases [29–35]. However, many these algorithms are not designed peculiarly for PVS.

In case of low power residential PVSs and in particular in case of single-stage systems, the PVS inverter is commonly in charge of the islanding detection, hence the anti-islanding functionality represents one of the main challenge in the PVS inverters design [18]. The anti-islanding protections must be implemented on the basis of the international standards requirements for distributed power generation systems (DPGSs) [25,36–38]. In particular, it is required that unintentional islanding be detected in less than two seconds as already established in the previous guidelines for PVSs [39–41].

After a disconnection due to the islanding detection an improper reconnection event is not improbable if the PVS breaker connects the system to the grid when the PVS voltage is out of phase. In this hypothesis a second disconnection can occur due to the PVS protections action. Hence the reclosing procedure has to be managed in strict coordination with the PVS synchronization system. For this reason, synchronization and islanding detection issues must be analyzed together.

About synchronization systems some books have been published such as [42]. Few review papers can be found in literature [23,43–46]. In [43] all the main families of synchronization techniques (also including the artificial intelligence techniques) are classified showing advantages and disadvantages. Basic concepts about phase-locked loop (PLL) techniques are explained in [44]. Reference [45] is devoted to three-phase applications, reference [23] is devoted to single-phase application, while [46] is oriented to design issues.

About islanding detection methods many review papers have been published in literature considering different DPGSs [47–57]. Reference [47] provides a review of the islanding detection methods for high power DPGSs. In [48] an extensive review of the islanding detection methods is provided focusing on some performance indices evaluation and in particular on the detection time. Reference [49] is focused just on passive methods, reference [50] is focused just on active methods. In [51] the focus is on active and passive methods and a new active methods is proposed for a three-phase PVS. The same islanding detection methods are discussed also in [52] also including the hybrid detection methods. However, hybrid methods are categorized just as combination of active and passive methods. In [53] some active islanding detection techniques are compared on the basis of a new index assessing the non-detection-zone (NDZ) size. In [54] the active techniques are classified in two categories: techniques introducing positive feedback in the control of the inverter and techniques based on harmonics injection. In [55] the islanding detection methods based on different signal

processing techniques are discussed in detail. Reference [56] provides a comprehensive review of the islanding detection techniques particularly oriented to recent intelligence-based methods. A similar approach is adopted in [57]. Intelligence-based islanding detection is out of topic in relation to the present study.

All the analyzed papers share as starting point a first classification among remote islanding detection techniques (based on communication) and local islanding detection techniques. Currently this classification can be overtaken considering the availability of communication protocols and local communication interface equipment by most PVS inverters.

No studies combining the analysis of the synchronization systems with the islanding detection techniques can be found in literature for PVSs or for other DPGSs applications. Nevertheless, many islanding detection schemes are based on modification or employment of additional synchronization systems to assess the unintentional islanding condition. In addition, independently of the adopted islanding detection technique, two synchronization systems are required in order to manage the reconnection procedure to the main grid by a PVS after an islanding event. Starting from this issues, the aim of this study is to provide a combined analysis about synchronization and islanding detection techniques which need coordinated operation and proper integration in the PVSs control structure.

Focusing on single-stage single-phase PVSs, the present study aims at giving an update of the most-recent trends about synchronization techniques and islanding detection methods, in particular: in Section 2 there are summarized the main goals of the single-stage PVSs control systems. The most adopted and the highest performance grid synchronization methods are analyzed in Section 3; while Section 4 is an overview of the islanding detection methods. Section 5 is about coordination between synchronization and islanding detection systems. Finally, the conclusions are presented in Section 6.

## **2. Control System Functionalities of a Single-Stage photovoltaic Power System**

The overall control structure of a single-stage PVS is shown in Figure 1 where it is assumed that the PVS can be connected to a local load, to the utility grid or it can be part of a smartgrid. The control functionalities can be classified in basic control functions and ancillary control functions. The basic control functions are the maximum power extraction, the grid synchronization, the current and the voltage control, the unintentional islanding detection. High power factor operation and harmonic rejection are achieved by proper design of current and voltage controllers and of the synchronization system. The current/voltage control reference signal is provided by the PV source power control which consists of a maximum power point tracking (MPPT) algorithm and a DC voltage controller. The MPPT algorithm is in charge of the maximum power extraction.

**Figure 1.** Single-stage photovoltaic systems (PVSs) control functions.

The ancillary control functions are the ride-through capability, the voltage and the frequency support to the local loads or the main grid. The ancillary control functions are out of the scope of the present study.
