Abstract
MicroGrid (MG) is becoming an important system for a massive and reliable integration of renewable energy sources. Hence, due to its benefits, photovoltaic (PV) systems are the most suitable and widely used in MGs. However, for a reliable integration of PV panels, they must be interfaced by smart power electronics devices that allow the implementation of advanced control solutions. In addition to operating as active power generator with maximum power point tracking (MPPT), the new PV systems (PVS) should behave as ancillary services providers by participating to the grid regulation, such as frequency stabilization, voltage profile control, harmonics compensation and so forth. This paper aims at reviewing the control and operation of PVS in AC MG. Control structures, active power control, MPPT and inner loops are presented and discussed. Design, analysis and control of single-stage three phase PVS in AC MG that is able to operate at MPP or as a dispatchable source are made. The control structure is validated through dynamic simulations. © 2016 IEEE.

Abstract
Photovoltaic systems (PVS) based MicroGrid (MG) should be able to operate at the maximum power point (MPP) in order to extract the maximum available power during atmospheric conditions changes. In addition to this, the PVSs should be able also to operate below the MPP in order to participate to the MG regulation such as grid frequency stabilization, voltage profile control and grid management support. This paper discusses the state of the art of operation and control of PVS in MG. Design and control of a single stage single phase PVS connected to a low voltage AC MG are presented and analyzed. A comparison between two current control methods, Proportional integral with grid voltage feed forward (PI+FF) and proportional resonant with selective harmonics compensation (PR+HC) is also made. The control structures are validated through dynamic simulation. © 2016 IEEE.

Abstract
The large scale integration of inverter-based renewable generation in isolated power systems is posing stability concerns as a result of the displacement of the conventional synchronous machines (SM). In this sense, the integration of battery energy storage systems (BESS) connected to the grid through power converters operating as grid-forming units is mandatory in order to ensure system stability. Therefore, this paper aims to perform a dynamic stability analysis of an isolated power system regarding the installation of a BESS, where it is intended to determine the minimum required grid-forming power capacity of the associated power converter that guarantees system stability under several operational scenarios. Moreover, the expected interactions between the grid-forming inverter and the conventional SM are also addressed. © 2019 IEEE.

Abstract
The hierarchical control scheme for three phase parallel connected voltage source inverters (VSI), forming a low voltage AC MicroGrid (MG)is presented and analyzed in this paper. The proposed control scheme consists of two controllers, a local controller and a centralized controller. The local controller consists of inner control loops that are composed by a voltage and current proportional resonant controllers with selective harmonics compensation, and the power sharing controller that includes the droop control and the virtual output impedance loops in order to share properly the active and reactive power between the connected VSIs. In this system, the centralized controller is designed to restore the magnitude and the frequency deviations of the AC bus voltage produced by the power sharing controller. Simulation results for three parallel connected VSIs forming a low voltage AC MG that operates in islanded mode are provided to show the effectiveness of the proposed control scheme. Active and reactive power are properly shared as well as good harmonics compensation is achieved when both linear and non-linear loads are connected to the load bus.

Abstract
The total losses volume represents a substantial amount of energy and, consequently, a large cost that is often included in the tariffs structure. Uneven connection of single-phase loads is a major cause for three-phase unbalance and a fundamental cause for active power losses, particularly in Low Voltage (LV) networks. This paper analyzes the impact of load unbalance on LV network losses. In the first phase, several load scenarios per phase are considered to characterize how losses depend on load unbalance. The second phase examines the data collected per phase on a set of real networks, aiming at illustrating real-world cases. The third phase analyzes the effect that public lighting and microgeneration may have in the load unbalance and on the subsequent energy losses. The results of this work clearly demonstrate that it is possible to reduce three-phase unbalance (and losses) through a judicious distribution of loads and microgeneration.

Abstract
During the last years, several plans for offshore grid development in Europe have been under discussion as a result of the need for the integration of large amounts of offshore wind power, providing additional transmission capacity and building adequate conditions for the single European electricity market. From the system operation viewpoint, technical constraints justify the need for adopting High Voltage Direct Current (HVDC) technology with Voltage Source Converters (VSC) for offshore grid development rather than conventional AC connections. Additionally, reliability and flexibility of operation is pushing for the development of the Multi-Terminal DC (MTDC) grid concept, by which a set of offshore wind farms is connected to a set of onshore AC nodes via a (possibly meshed) DC grid structure. This concept is in line with the envisioned plans for the development of the pan-European Transmission System. A broad range of aspects related to MTDC grids planning, operation, and technology was dealt within the TWENTIES European research project (2009-2013). In particular, this paper presents some results regarding the impact of MTDC systems on the AC systems they are connected to: specific focus is devoted to provision of advanced ancillary services (primary frequency controls, Fault Ride Through capability to AC faults, reactive power support), to the possible contribution of the MTDC grid in enhancing the operational security of AC systems and in restarting them after partial or complete blackouts. To emphasize the advanced functionalities that can be provided for overall system operation, this MTDC grid is referred to in the paper as "DC Grid" (DCG). Simulation results illustrate the potential benefits for the AC systems deriving from the described advanced control functionalities of DCGs. The work has been carried out within TWENTIES Working Package 5.