**1. Introduction**

As well known in electrical engineering, user appliances work in the best conditions (i.e., performance, efficiency, and lifetime) when fed at a rated voltage or within a small voltage deviation from that value [1]. Not only the loads' section but similar considerations can also be drawn for production, transmission, and distribution systems (e.g., generators, transformers, lines, reactors, and shunt capacitors). Indeed, also in these cases, the voltage on components is to be maintained within a limited range for avoiding various negative effects on the system operation [2,3].

During a standard operative scenario in transmission grids, undesired voltage fluctuations at the grid nodes are mainly caused by variations in absorbed power, which is variously requested from the different loads connected to the network [4]. On the other hand, temporary out of service of any network component (lines, transformers, etc.) are responsible for significant variations in supplied voltage and even the loads' disconnection [4]. For the reasons expressed so far, the transmission system operator (TSO) must guarantee an adequate voltage control service, i.e., a complex of measures for achieving a suitable voltage control in the different network nodes [4]. Clearly, each TSO operating on a specific grid can implement a peculiar voltage control service, which is definitely not stiffly established. Being the high voltage (HV) nodes mainly influenced by the flows of reactive power [4], the voltage control is typically performed by regulating the reactive power flows by means of

different reactive power resources. For giving some examples, it possible to mention FACT devices, synchronous generators, synchronous condenser, transformer tap changers, static VAR compensators, capacitor banks, and capacitance of lines and cables [5].

During the last decades, several countries have implemented hierarchical voltage control systems, practically based on the reactive power provided by conventional large power plants [6]. In this regard, a review of the adopted systems is reported in [5]. As a matter of fact, the standard voltage control systems have been designed and developed for traditional electric power grids, which are characterized by a unidirectional energy flow (from large production centers up to the loads). Nowadays, this assumption is not valid anymore, where the electric power systems are experimenting a great structural change by moving from the centralized production paradigm to the distributed generation model [7,8]. The latter is characterized by small–medium size generation centers, typically based on renewable energy sources (RES), which are conveniently (sometime randomly) distributed throughout the territory. The large penetration of RES leads relevant consequences on several aspects (power quality, power losses, and voltage profiles), as discussed in [9–11].

The RES scenario in EU-28 is constantly evolving [12]. Just to give some data [12], at the end of 2014, the installed RES power has been equal to 369,511 GW, more than a third of total installed power (37.8%). In detail, RES power plants are capable of contributing to the production of 930 TWh, another time almost one-third of the total production (29.2%). This data can be summarized not only in the significant growth rate of RES in the last years but also in the fact that RES growth is the only one having a positive index among the different resources. There are several technologies that contribute to renewable generation: among others, hydroelectric, wind, biomass, and photovoltaic [12–14]. Particularly, the last references provide some important reports for the context frame.

Nowadays, the secondary voltage regulation (SVR) for HV networks is based on traditional fossil power plants and large hydroelectric power plants, while most of the new renewable energy plants are not taking part in this task [5]. By considering this aspect, the existing voltage control systems are experiencing a decrease in the control capability, especially when the power of conventional thermal plants is replaced by the one produced by distributed generators (DGs) [5]. For what concerns the connection, medium voltage (MV) and low voltage (LV) distribution networks are the standard end-point for RES power plants, whilst the HV transmission system represents the future connecting point for high-power renewable sources. In this regard, the Italian case represents an interesting example, where 6.4% of PV plants are directly connected to the HV grid by the end of 2014 [15]. The Italian case is noteworthy also for the high presence of nonprogrammable RES power plants, whereas the Italian TSO implements a hierarchical voltage control by adopting the production plants as actuators [5].

Independently from the voltage level of interconnection, the introduction of RES technology can determine important consequences on the voltage control [16]. The RES operation outside the constant unitary power factor is commonly accepted as a standard requirement, independently from the interconnection at transmission or distribution level [17–20]. Particularly, last references are aimed at defining regulations and guidelines for network code.

In this regard, the authors have largely studied the contribution of photovoltaic (PV) plants in supporting the network voltage: in [21], the voltage control functionality is guaranteed by modulating the reactive power and bounding the injected ramp of active power, while PV power plants are regulated for behaving as STATCOM devices in [22]. As proposed in [23], PV units can provide reactive power compensation as ancillary service, whilst the dynamic reactive power compensation can be obtained by suitably controlling power electronics devices [24]. By regulating active and reactive power of PV systems, the adaptive droop-based control algorithms are useful to minimize losses and increase the capacity installation, while avoiding overvoltage [25]. For what concerns the integration of PV systems into a residential area, the papers [26] and [27] offer some interesting proposals, while the mix of RES and electrical storage is conversely described in [21,28–30]. Extending the concept of RES away from the PV power plants, several voltage controls have been proposed in literature [31–35] for the wind farms. Particularly, [36] provides important considerations on how to regulate the wind generators for emulating a hierarchical voltage control. A similar perspective is the one provided by the so-called virtual power plants (VPP) [37]. In such a case, multiple distributed generation plants are connected and managed for behaving as a virtual smart network (VSN). For the electrical grid point of view, the VSN constitutes a single provider of main services (energy and capacity) and auxiliary ones (regulations, reserves, etc.) [38–41].

The present paper wants to analyze an innovative control strategy to conveniently integrate RES power plants (i.e., large PV) into the voltage control system of the transmission network. Such a strategy is based on a hierarchical control architecture, successfully implemented in several countries [6,42–44]. A crucial device for this control system is the reactive power regulator (RPR), which is implemented in the so-called SART (in Italian language "Sistema Automatico per la Regolazione della Tensione", translated as "Automatic Voltage and Reactive Power Regulator") [45] in the Italian Grid Code. In this paper, the algorithm adopted by the control system is extended to coordinate the reactive powers of all generators that participate in the voltage control, despite their size and position in the network (and therefore applicable also to distribution networks). The final effect is the control of the voltage of the bus connecting the power plant to the rest of the system, through the reactive power of the generators. By comparing simulations and data from measurement campaigns on conventional operating power plants, the paper is initially aimed at validating the proposed methodology. Once the methodology is proved on standard cases, the control strategy can be proficiently transferred to the PV test case.

The paper is organized as follows. Section 2 describes the proposed architecture for the voltage control, while Section 3 is focused on modeling aspects, particularly on the differences in representing RES (where large PV power plant is a particular case) and traditional power plants. Then Section 4 discusses the main topic. Once demonstrated, the validity of the proposed methodology by comparing simulation results and measurements data from traditional power plants, such a methodology will be proficiently extended to the PV case. Finally, Section 5 provides a discussion about the obtained results, while Section 6 outlines the conclusions.
