**Massimiliano Chiandone, Riccardo Campaner, Daniele Bosich \* and Giorgio Sulligoi**

Department of Engineering and Architecture, University of Trieste, 34127 Trieste, Italy; mchiandone@units.it (M.C.); riccardo.campaner@gmail.com (R.C.); gsulligoi@units.it (G.S.)

**\*** Correspondence: dbosich@units.it

Received: 11 April 2020; Accepted: 9 May 2020; Published: 13 May 2020

**Abstract:** The increasing presence of nonprogrammable renewable energy sources (RES) forces towards the development of new methods for voltage control. In the case of centralized generation, the hierarchical regulation or secondary voltage regulation (SVR) is guaranteed by coordinated voltage and reactive power controls in transmission systems. This type of regulation loses effectiveness when the generation becomes distributed and based on small and medium sized generators. To overcome this problem, it is important that also distributed generators, typically based on RES, participate in the voltage regulation. By starting from the methodologies already applied, this work wants to present a new method for involving distributed generators in SVR. The novelty is given by the application of an existing methodology to the new configuration of electrical grids characterized by a relevant distributed generation. The aim is to control the distributed generators (DGs) as coordinated sources of reactive power for conveniently supporting the voltage regulation. In this paper, a real large photovoltaic (PV) plant is considered. The power plant is composed of several PV generators connected through a distribution network. With the algorithm proposed, the set of generators can be treated as a single traditional power plant that can participate in the hierarchical voltage regulation. The reactive power of each single generator is coordinated in a way similar to the SVR used in several national systems.

**Keywords:** distribution network; hierarchical voltage control; reactive power control; RES; SVR; PV
