*4.3. Case C) PV Power Plant of 46.8 MWp*

The considered RES power plant [56] is a large PV plant with a peak power of 46.8 MWp. Particularly, the plant is composed of 82 photovoltaic subfields, each one rated about 0.57 MWp. Every subfield has its own inverter with a rated power of 0.5 MVA, connected to the 20 kV-internal distribution network by a transformer. The topology of the considered network is depicted in Figure 18, whereas all components are interfaced by using two different types of cables (in Figure 18 the two different types are drawn with different style according to Table 1). The lengths of all cables are in the order of hundreds of meters. All lengths have been taken into account in the mathematical model.

**Figure 18.** The topology of photovoltaic (PV) power plant, plant C.



A single 132/20 kV transformer is used to step-up the voltage to the final HV bus, thus the single POC to the HV network. From the transmission grid point of view, the POC is seen as a unique generator similar to a traditional power plant. This PV power plant is traditionally operated at unity power factor, which is no longer considered satisfactory by the TSO. The presence of several generators based on inverters suggests the possibility of using their reactive capability for supporting the HV network voltage control through the proposed control. Additionally, in this PV power plant, a step in the reactive power level is applied as in the previous cases, while only simulation results are shown. In this regard, Figure 19 reports the p.u. reactive power (i.e., rated power as basis) of 6 representative PV inverters, where in total they amount to 82 unities. Each generator injects an amount of reactive power according to its rated power and its residual capability; therefore, the differences are almost

negligible. By considering only one size for the inverters, the results are coherent. The voltage outputs of the considered inverters are shown in Figure 20, while the voltage profile at the POC is highlighted in Figure 21. The time responses show stable behaviors, whilst the dynamics result in accordance to the expected time constants. Finally, it possible to highlight the absence of cross dynamics disturbances between the different PV subfields.

**Figure 19.** Reactive power profile (in p.u.) of six representative PV inverters (simulated data).

**Figure 20.** Voltage profile (in p.u.) at the six representative PV inverters output (simulated data).

**Figure 21.** Voltage profile (in p.u.) at the POC (simulated data).

#### **5. Results and Discussion**

By focusing on Case A and Case B, the good correspondence between simulated results and experimental data validates the mathematical models, then the way for representing power distribution grid and control architecture in DOME platform. This verified methodology can be then applied to the RES power plant (i.e., Case C), where the results are indeed consequent and thus capable of well-representing the dynamics of this kind of controlled power plant. It is interesting to highlight that both simulations and experimental data show a first-order behavior in presence of a reactive power reference step. On the other hand, the controlled voltage can reach the no-error steady-state condition without oscillations. In such a way, the group of generators (whether in parallel configuration or in star configuration) behaves as a single generator in a primary voltage regulation or as a group when coordinated by means of a secondary voltage regulation. Furthermore, Case C actually shows a behavior similar of what is observable in the presence of a standard power plant operating for a secondary voltage regulation. The latter is an important aspect in which the authors are interested in drawing attention. Finally, the proposed control methodology is not only applicable to the example of this paper but also it can be used when small–medium generators are interconnected to whichever distribution grid's topology.

#### **6. Conclusions**

The proposed work has presented a coordinated voltage and reactive power control architecture. The control algorithm has been originated from well-known techniques, already widely used in the transmission network of many national systems for conventional power plants. The novelty is the capability of the algorithm in being adaptable to different technology, size, and topology of generating plants connected to the HV network. A mathematical model of the control system has been discussed and then implemented in DOME (a python-based simulation tool). Firstly, this implementation has been used to simulate the dynamical behavior of traditional power plants, and experimental data have been compared to validate the control system model. Thus, the validated control system model has been applied to a large PV power plant, where numerical simulations have verified the behavior. The performed simulations have demonstrated how a large controlled PV field exhibits an asymptotically stable behavior, in terms of voltage and reactive power. Moreover, the PV dynamics (i.e., time constants) are fully comparable to the ones of large traditional power plants, which are involved in coordinated voltage and reactive power controls in HV networks. Therefore, the reactive capability of PV power plants and large power stations can be synergistically exploited, while keeping a uniform dynamic performance.

The proposed control scheme is finally characterized by several advantages: it is capable of ensuring a fast response, while performing a perfect tracking of both the HV bus voltage and RES reactive powers. In addition, there is no steady-state error and the system dynamics does not highlight oscillations. Each generator participates in the control by sharing its reactive power (in absorption or injection), where the provided quota is proportional to its capability at the point of operation. An important note to be highlighted regards the communication data. The proposed control strategy is based on the communication of a single control signal (i.e., the reactive power level) to all the generators involved, thus making its implementation rather simple. The transmitted *qliv* value is coherent to the reactive power level's control signal that is already in use in the coordinated voltage and reactive power control of transmission networks. This makes the proposed control architecture fully compatible with existing ones. As a further development, authors are investigating how this control architecture can be scaled and integrated into distribution networks. In a glance, the proposed control is characterized by the two important pros: a) only one signal is requested for regulating multiple-generators and b) it is strictly alike to the secondary control already used in transmission system, thus consequently it is compatible. The proposed solution is feasible when promoting the integration of large PV systems in the HV networks' control strategies (i.e., voltage and reactive power).

**Supplementary Materials:** The following are available online at http://www.mdpi.com/1996-1073/13/10/2441/s1, File S1: Case A.dm, File S2: Case B.dm, File S3: Case C.dm. Three additional files (Case A, Case B and Case C) are made available during the submission process. Full datasets and full experimental details are provided in these three files. The three files are the inputs for performing simulations then reproducing the results in DOME, the Python based simulation tool used in this paper.

**Author Contributions:** Conceptualization, M.C., D.B. and G.S.; Data curation, M.C. and R.C.; Formal analysis, R.C.; Methodology, D.B. and G.S.; Software, M.C.; Supervision, G.S.; Validation, D.B.; Visualization, R.C.; Writing—original draft, R.C.; Writing—review & editing, M.C., D.B. and G.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.
