The recent technological advance in the fifth generation of telecommunication networks (5G) has led to a evolutions in many domains, including connected cars, manufacturing and electricity. A technological domain that had large benefits from this advance is the Electrical Power and Energy System (EPES). Despite the simplicity and efficiency that 5G brings there are also underlying risks that are slowing down its adoption. These risks are caused by the presence of convergence connectivity interfaces in legacy infrastructures that were built with no security in mind. Specifically, EPES systems are often targeted by cyber criminals to cause massive blackouts in entire cities or countries that in turn lead to societal impact, such as consumer discomfort. In this work we propose a cyber-security measures for 1) early-stage detection of cyber-security incidents and 2) protecting against them through applicable security measures. The proposed measures are applied to a Hydroelectric Power Plant (HPP) of the Public Power Corporation (PPC). The cyber-attacks are performed in a 5G-enabled smart meter that measures power production and transmits measurements to PPC’s control center through the use of 5G Network Function Virtualization (NFV) technologies, such as network slicing. To protect against the attacks, cyber-security measures are applied and incorporated in a cyber-security platform, that was developed within the PHOENIX H2020 project. The measures are used to detect the attacks and perform necessary mitigation actions for restoring the HPP operation.