Power system operation is based on ensuring balance between the energy supplied and consumed. Traditionally, the balance of electrical energy has been achieved by continuously adjusting the produced amount of energy in power plants according to the load. However, due to the introduction of intermittent renewable energy sources, liberalisation of electricity markets and electrification of the areas of transport, heating etc. requires energy storage. That allows to decouple the production and consumption of electrical energy, improves efficiency, sustainability and reliability of electricity supply and helps to reduce costs.