The correct functioning of the electrical system is based on a fundamental cardinal principle to guarantee its stability and safety, namely that at any moment in time the power generated by the production plants must be exactly equal to the power absorbed by the consumption units (plus losses). This instantaneous balance has, as is known, a very sensitive “indicator”, the network frequency. In an electrical system, if a generator runs at a slightly different speed than the others, it tends to progressively move further and further away from the remaining part of the system, until at a certain point, the drift becomes so large that it completely loses its pace. It is said that it “runs away”, as it can no longer be held back by the elastic bond with the others. Therefore, all the machines must be interconnected via the network, which performs an elastic function between the various generators, and they must all move synchronously with each other in order to keep the entire system at the exact same speed.
Small fluctuations in the average value of the frequency are generally physiological, they occur continuously due to stochastic variations of the loads and do not produce significant effects. Other times the frequency disturbances can be unexpected and rather significant, as could happen due to the sudden outage of a large generator or the loss of an interconnection line. Technical network constraints, in fact, affect the possibility of transferring power on the network. Violation of the current limits of a component (in particular of the lines) can lead to the disconnection of the overloaded component, with consequent spilling of the power onto other nearby connections, which in turn can overload, causing the intervention of the respective protection systems. Then there are frequency limit values, in particular below 47.5 Hz and above 51.5 Hz, for which the alternators disconnect from the network in order to avoid failures. At that point, the lack of energy fed into the grid would further increase the power imbalance between generation and load, and if this were not promptly compensated by the plants that remained in operation, it would lead to other generators disconnecting from the grid. The natural consequence of all this would be the triggering of a rapid chain shutdown process, the effect of which culminates in a blackout. The Italian blackout of September 28, 2003 (the most serious event of this kind in the history of Italy) certainly remains in the collective memory, which occurred due to a short circuit caused by a tree falling on an import line with Switzerland.
Over the past ten years, there has been a substantial change in the electricity system. The objectives of decarbonisation and containment of greenhouse gas emissions, set by the European Commission with medium and long-term reference scenarios, have so far irreversibly changed the national and continental electricity context, which is still in continuous evolution. These policies currently set even more challenging objectives, foreseeing the achievement of a renewable share of 32% of the energy produced by each country by 2030, and an almost complete decarbonisation (95%) of the entire European energy system by 2050. One of the main characteristics of some types of renewable energy plants is the non-programmability of production profiles. The producibility of plants such as wind, photovoltaic or flowing water hydroelectric is by their nature strongly dependent on the availability of primary resources (wind, sun, water) linked to climatic conditions, seasonal variations and the alternation of day and night. Because of this characteristic, it is not possible to control their production when required, except to reduce the power supplied, thus giving up the “free” energy that they could provide. In addition to the already significant uncertainties due to fluctuations in demand and failures of system components, there is also the difficulty in predicting the production of such systems. Safe management of the electrical system implies the need to maintain the stability of the electrical network, that is, to ensure that the system reacts from the first moments to the occurrence of sudden disturbances, avoiding encountering operating states that can cause it to shut down. In the event of a disruptive event represented for example by the loss of a generation plant or a line, the electrical system is the site of a transitory phenomenon in which the electrical parameters (in particular the network frequency) undergo oscillations with respect to the nominal values. The greater the extent of the disturbance, the greater the associated transitory phenomenon. The system’s ability to “resist” an imbalance between generation and load in the very first moments downstream of the disturbance without excessive variations in the network frequency is measured by the network inertia parameter. Traditionally, network inertia is provided by conventional thermal groups that represent the vast majority of “rotating” generation. In general, it can be stated that the progressive decommissioning or exclusion of conventional production units from the electricity market, in favor of FRNP generation plants interfaced to the network via power electronics, reduces the number of rotating generators on the network, resulting in a double negative impact:
– Compromising the reserve available for regulation;
– Reduction of the overall amount of inertia in the network.
When generation from FRNP covers a significant portion of the load, in addition to the lack of regulation capacity (power reserve margin), examined so far, an inertia deficit may also occur in the system.
The progressive decarbonization of the electrical system, therefore, determines the need to request new services that were not necessary before, or that were not necessary to request because they were provided free of charge by the electrical system, such as inertia, guaranteed by large rotating machines connected to the electrical system for the generation of electrical energy and which is missing due to the progressive reduction of dispatched rotating power. In order to guarantee a flexible electrical system, it is necessary to procure network services from all the resources available to provide them, opening the services market and incentivizing participation in new resources, such as distributed generation, Storage Systems (SdA) and demand. Among the new frequency regulation services we find synthetic inertia and fast reserve. These services are important to guarantee a better dynamic response of the electrical system in the first moments following frequency transients, currently provided by traditional generators connected to the system.
The introduction of storage systems would also favor the integration of non-programmable sources, constituting at the same time an important resource for the stabilization of networks, both interconnected and isolated. There are many technologies that differ on the basis of the conversion method used for the accumulation of electrical energy (mechanical, potential energy, electrochemical, electrostatic, etc.). Another classification method is based on the performance characteristics offered by these systems. Therefore, SdA can be distinguished with: energy performance, characterized by having to exchange power continuously for a few hours, therefore having good autonomy and a low value of the power/energy ratio; power performance, which work by supplying a very high power for short times, therefore with reduced autonomy up to a few dozen minutes, and characterized by a high value of the power/energy ratio. Similarly to the distinction between centralized and distributed generation, it is then possible to distinguish between centralized storage systems, i.e. very large-scale systems starting from tens of MW, installed in HV and to support the transmission system, and distributed systems, of smaller size, from kW up to a few MW and installed close to the user.
Among the possible technological solutions available or under development for the storage of electrical energy, which are best suited to carry out this type of application, electrochemical storage systems, in particular lithium-ion batteries, are of particular interest. These systems are known for their great versatility of use and modularity, but also and above all for their ability to provide high power responses within very short times. This property makes them particularly suitable for containing frequency deviations. This can be exploited not only to generate the power necessary to perform the primary regulation functions, but mainly as an inertial contribution in the first moments after the disturbance, allowing a more stable and flexible operation of the network itself. Among the main advantages, electrochemical storage systems are characterized by the speed and precision in the delivery of power, especially when compared with the action of speed regulators commonly used in traditional systems. Having response times of less than a second, they therefore show superior performance for this type of services compared to those of thermoelectric systems. Being able to quickly transition from the charge to the discharge condition, they can cover a regulation band with a width equal to the sum of the maximum charge and discharge powers.
By introducing appropriate control logics, storage systems can: provide the regulation margin by supplying or absorbing energy from the grid, if an upward or downward regulation is required respectively; emulate the inertial response of rotating electrical machines, providing the so-called “synthetic inertia”.
