Future power systems face several challenges: (i) the high penetration level of renewable energy from highly variable generators connected over power converters; (ii) several technologies for energy storage with very different time constants, some of them using power converters as an interface to the grid; and (iii) a pan- European transmission network facilitating the integration of large-scale renewable energy sources and the balancing and transportation of electricity based on underwater MTDC transmission. All of them have an element in common, high power converters that decouple the new energy sources from the pre-existent AC power systems. During a system frequency disturbance, the generation/demand power balance is lost, the system frequency will change at a rate initially deter- mined by the total system inertia. However, future power systems will increase the installed power capacity (MVA) but the effective system inertial response will stay the same nowadays, this is because the new generation units based on power converters creates a decoupling effect of the real inertia and the AC grid. The result is deeper frequency excursions of system disturbances. A considerable reduction in the ability to overcome system frequency’s disturbances is expected, the inertia response may be decreased. The inertial response of the system might be negatively affected with devastating consequences for system security and reliability.