## 3 The VPP Architecture  
The proposed VPP architecture has been conceived for the management and the dynamic integration of active prosumers participating in advanced DR programs. With respect to the classical VPP approach, which mainly considers the aggregation of several DERs owned by different stakeholders (i.e., end-users, utilities, and independent electricity producers) into a single virtualized entity [1; 8], the domain of the herein presented VPP architecture is limited to the physical domain of single users. According to this approach, each VPP aims to virtualize all the DERs that are connected to single nodes of the electrical grid, i.e., to single Points of Delivery (PODs). As depicted in Figure 4, VPPs thus represent an aggregation of all the energy assets of users at each POD of the electrical grid. For each node of the electrical grid, different possible aggregations of DERs are considered, including RES-based generators, Electrical Energy Storage (EES) units, flexible and controllable loads (e.g., heating and cooling systems, lighting systems, and controllable appliances), Plug-in Electric Vehicle (PEV) charging stations, and Medium Voltage (MV) to Low Voltage (LV) transformer units. It is assumed that different combinations of DERs can be potentially connected to each node of the grid depending on the type of prosumer. As shown in Figure 4, the diversity and number of DERs,  
Figure 4: Schematic representation of the physical aggregation levels of DERs and prosumers in the electrical grid domain. EES: electrical energy storage; DG: distributed generation; MV: medium voltage; LV: low voltage. as well as the diversity and number of end-users, define the type of prosumer, here classified as: small residential user, large residential user, industrial or large commercial user, and complex private system. Small residential users consist of a single group of homogeneous end-users (e.g., a family), with a small DG unit (e.g., a PV power plant), in terms of few kW, domestic appliances, and possibly a domestic wall box. Large residential users consist of multiple groups of homogeneous end-users (e.g., more families), with a small- to medium-size DG unit, up to 100 kW, several domestic appliances, and multiple PEV charging stations. Industrial or large commercial users consist of multiple groups of heterogeneous end-users (e.g., employers, customers of commercial activities, and others), and are equipped with a medium-size DG unit, typically more than 100 kW, at least one MV to LV transformer unit, several types of loads, and multiple PEV charging stations. Complex private systems consist of multiple groups of heterogeneous end-users, and are equipped with multiple small to medium-size DG units, multiple MV to LV transformers, several types of loads, and multiple PEV charging stations. The virtualization of each prosumer, at each node of the distribution grid, is provided by the VPP architecture schematically described in Figure 5, which defines all the agents involved in the VPP architecture. Three different domain levels are defined in the system architecture, namely: the Smart Grid domain, the prosumer domain (i.e., the VPP domain), and the third-party domain. The Smart Grid domain includes all the agents and entities involved in the management and control of the  
Figure 5: Service-oriented multi-agent scheme of the proposed Virtual Power Plant (VPP) architecture. electrical grid, such as the Distribution System Operator (DSO), the Transmission System Operator (TSO), the electricity market, independent operators, and market aggregators. The prosumer domain contains all the energy assets owned by the prosumer, which represent an economic value in terms of consumption, production, reliability, maintenance, and quality of service. This domain also includes the system administrator, who is responsible for the strategic management of the system, by defining operation constraints, strategic schedules, and optimization objectives.