In future power grids, a large integration of renewable energy sources is foreseen, which will impose serious technical challenges to system operators. To mitigate some of the problems that renewable energy sources may bring, new voltage and frequency control strategies must be developed. Given the expected evolution of technologies and information systems, these new strategies will benefit from increasing system observability and resources controllability, enabling a more efficient grid

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... on. The ELECTRA IRP project addressed the new challenges that future power systems will face and developed new grid management and control functionalities to overcome the identified problems. This work, implemented in the framework of ELECTRA, presents an innovative functionality for the control room of the cell operator and its application in assistance with the voltage control designed for the Web-of-Cells. The voltage control method developed uses a proactive mode to calculate the set-points to be sent to the flexible resources, each minute, for a following 15-min period. This way, the voltage control method developed is able to mitigate voltage problems that may occur, while, at the same time, contributes to reduce the energy losses. To enable a straightforward utilization of this functionality, a user interface was created for system operators so they can observe the network state and control resources in a forthright manner accordingly. Energies 2018, 11, 1659 2 of 23 as the representative grid architecture of the future power systems. The WoC consists of a group of interconnected substructures, the "cells". A cell is defined as a group of loads, generators and Distributed Energy Resources (DERs), within a geographical area, which can integrate different voltage levels [8] . The main characteristic of a cell is that it seeks to use local resources to solve local problems, thus requiring a certain amount of flexibility to counteract unexpected generation or load deviations [9] . The proper operation of the cells together with new mechanisms of close collaboration between them will help to make the future power system more stable and secure. A new set of functionalities were developed in ELECTRA to adapt some of the frequency and voltage control mechanisms that exist nowadays to future power systems, in particular to the WoC concept, and also to develop new ones [10] . The work presented in this paper is solely focused on the visualization of the voltage control mechanisms developed in the project, as well as on the user interface that was created for the cell operators in order to use the voltage control mechanisms in a straightforward manner. Today's control rooms are composed by multiple monitors, varying with the complexity of the system being monitored, which display different types of information, such as distribution management systems data, networks alarms, e-mails, etc. Operators also have to get in contact with crews and other operators through radio, phone or e-mail and sometimes the procedures end up not being very efficient [11] . With the evolution of distribution networks and its growing complexity, the control room must be redefined or else the risk of operational errors may occur more frequently [12] . In [13] , it was identified some sources of operator errors due to the lack of situational awareness and identified some improvements and measures to prevent design errors that provoked problems in past situations. Given the large amount of grid measurements that are expected to be collected in the future, grid observability potential will increase significantly. Some works have dealt with the increase of renewable energy sources, which increase the complexity of the system and its uncertainty, and made recommendations concerning the way this problem should be treated [14, 15] . Thus, advanced grid control rooms will be required to enable system operators to integrate in an efficient manner all the information received from grid equipment, filtering or extracting and visualizing a clear portrayal of the grid state, creating a good situational awareness of the system [16] . The information that appears to the operators must be well detailed, prioritized and presented in a simplified way, so that it facilitates their work in understanding the problems of the network and how to manage them in the best way without disregarding system safety [11] . In the ELECTRA project, this subject was also addressed, and in the view of the WoC concept, the system operator should have the responsibility to supervise a highly automated system, while having some degree of control over the system and intervene when necessary [17] . Considering this, a user interface was specifically developed in ELECTRA for the voltage control. The aim of the user interface is to provide an enhanced grid observability to system operators, so they can have some degree of control over the resources, instead of relying only in automatic Optimal Power Flow-based solutions or local voltage control approaches (e.g., droop control [4, 18] ). The structure of the voltage control developed in ELECTRA as well as the detailed operation of the Post-Primary Voltage Control (PPVC) algorithm are explained in Section 2. The user interface for the control room is presented in Section 3. The case studies and results obtained are presented in Section 4. Finally, the conclusions are presented in Section 5. ELECTRA Voltage Control Scheme Two voltage control functionalities were developed in ELECTRA for the WoC: the Primary Voltage Control (PVC) and the PPVC. The PVC aims at mitigating voltage deviations in the connection point of the device while PPVC restores the voltage to their optimal values minimizing active power losses. The PVC developed for the WoC is based on the utilization of a grid impedance estimation function to calculate the necessary active or reactive power to be injected/absorbed to reduce the difference