The apparatus PRESTO (Primary REgulation of STOrage) has been developed within a research activity collaboration among the Energy Department of
Politecnico di Milano (SmartDGlab research group), MCM Energy Lab, ELVI S.p.A. with the technical support of the FIAMM Energy Storage Solutions,
which is expert producer in batteries and storage systems.
The project focuses on new regulation schemes/functions devoted to manage Dispersed Generation (DG) integrated with energy storage systems in
order to provide ancillary services to the main grid.
By means of DG it is possible to exploit renewable energy resources spread on the territory and reducing the use of fossil fuels. Nowadays,
DG is integrating into the electrical system according to a “fit and forget” approach; since the current distribution network is designed as a
passive systems (i.e. not able to receive high amount of generated power) high DG power injections may affect the quality of supply and the
system stability. As DG penetration increases it will become a technical and economic imperative that DG participates in the provision of
ancillary services needed for a secure and reliable operation of the power system. This is important for the simple reason that if DG only
displaces the energy produced by central generation but not the associated flexibility and capacity, the overall cost of operate the entire
system will rise.
The new scenario implies new rules for the active users connected to the distribution network. Until now, DG power plants don’t offer any
ancillary service for the network operation. Nowadays, demand is growing from large utilities (DSO and TSO) to exploit DG as a service for the
system. In this way generators connected along the distribution system are a resource useful to improve the stability, safety and power quality
of the electric grid. They represent a regulation resources dispersed along the feeders and they can potentially offer a network control in
remotely points of the distribution network normally not controlled. Furthermore, the ancillary services extension to DG connected to the
distribution level is essential also for a better integration of the DG itself and for an increasing of the hosting capacity of existing networks.
These services involve both the transmission and the distribution system management.
A wide primary regulation reserve can be guaranteed only if DG is coupled with a suitable Energy Storage System (ESS). In this case, the
integration of DG and ESS represents a PRESTO unit, i.e. a generation and control dispersed resource for the primary regulation of the system.
The most important system services offered by PRESTO units can be divided in two categories: reactive power modulation service and real power
modulation service.
These control services are different in term of impact on the electrical system.
DG injections at distribution level alter the voltage profile along the feeders: in particular it is no longer monotonous and over-voltages at the
DG’s Point of Common Coupling (PCC) may occur (i.e. violation of EN 50160 prescriptions). The reactive power modulation services mainly allow to
control the voltage of the system.
By means of the PRESTO unit extension, the reactive power can be modulated according to local measurements at the PCC. This voltage control
strategy has been considered also in the Italian rules for the connection of active users to the distribution system.
The second ancillary service offered by the PRESTO unit is the real power modulation service which is mainly required by the TSO to control the
system frequency and guarantee a network stability. An ever higher penetration of intermittent DG units, replacing the traditional power plants,
causes a weakening of the system, especially a reduction of the total rotating inertia and a decreasing of the margin for the primary frequency
reserve, i.e. with lower capability to support the system in case of frequency oscillations. Because of this, the margin of power available for
the frequency control provided by traditional plants could become insufficient to face a sudden power imbalance (e.g. due to loss of generation or
load increasing).
In order to achieve a suitable network reliability, the PRESTO unit solution, which involves ESS, has to be considered. The goal is to provide
suitable margin for the frequency regulation service as much as a traditional power plant. Therefore, with the approach proposed, the participation
to the primary frequency regulation could be extended also to renewable power plants.
The apparatus has been installed and tested within the ALPSTORE research project in the Technocity area; in particular the experimental campaign
last for three months (may- june – july 2014), collecting data about voltage level and frequency transient in the LV distribution grid.