*2.2. Load Profile Smoothing*

Due to the growing share of RESs and their variable generation, the power system operation can be disturbed. It could happen especially for power systems with a significant share of renewables where difficulties in the balancing of demand and supply occur. Therefore, smoothing, which embraces reduction of rapid changes in the load profile, is another example of the AS that can be provided by the DRAs to assist the power system operation [24].

At the PCC, the load profile can be shaped by controlled operation of active resources incorporated into the DRA structure. The load profile smoothing could improve balancing, currently provided by the transmission system connected CDGUs because the smoothed profile is characterized by a smaller variability and milder up and down ramps. A visualization of load profile smoothing is presented in Figure 3.

**Figure 3.** Load profile smoothing.

## *2.3. Balancing and Reserves*

Electrical energy consumption must be equal to its generation at all times. The frequency of the grid is the best indicator of this balance. DERs managed by an aggregator are able to support the maintenance of the frequency within acceptable limits by quickly responding to its deviations. Such services are currently provided mainly by large CDGUs located in transmission networks, but properly aggregated structures may also be present on the balancing markets [25].

The balancing services may be also provided by compensation of balance deviations inside an aggregated structure in order to obtain a determined generation/load profile for the whole cluster. Some resources may be able to respond to other entities' deviations caused, for example, by variable weather conditions or rapidly changing demand.

The reserves are required to maintain the power system's balance in both the short and the long term. In this context, DRAs may additionally share a part of its capacity to cover TSOs' needs and be remunerated for the willingness to provide reserve services and for their activation.

#### **3. Structure of the Distributed Resource Aggregator**

#### *3.1. Roles and Structure of an Aggregator*

The main roles of the DRA coordinator are to select suitable assets located in distribution networks and to group them into a cluster in order to strengthen the significance of resources distributed on a small scale and thereby allow them to participate in wholesale markets, such as the balancing and AS markets. The operation of the aggregated assets is managed by the DRA coordinator in order to optimize the overall profit of the whole cluster. The structure of the DRA comprises assets located in the distribution level: passive loads, ALs, ESs, controllable and noncontrollable power generating resources. Figure 4 presents the proposed structure of the DRA and its cooperation with other entities located in the distribution level.

**Figure 4.** The proposed structure of the distributed resource aggregator (DRA) and its cooperation with other entities.

New technical and legal requirements should be introduced to allow the DRA to operate in the power system. These requirements should consist of basic technical parameters (power, response time, regulatory range, etc.) and legal conditions relating to inter alia, resource ownership, aggregation areas and financial settlements. After defining the aggregated structure, the DRA coordinator submits its technical capabilities to the DSO and makes agreements for the provision of the ASs due to the power system conditions and demands.

The DRA coordinator is responsible for supervision, coordination and control of the operation of resources. The coordinator has the ability to select suitable resources and create an optimal structure oriented towards the desired operation pattern which aims at a maximum profit while assisting the power system's operator.

In order to improve the power system flexibility, the DRA operation may prevent the negative impact of intermittent RES generation (left side of Figure 5) and stabilize operation of base load and combined heat and power (CHP) generation units (right side of Figure 5). The DRA can adjust output power from *PMIN* to *PMAX* to provide ASs and indirectly assist the TSO in the balancing of demand and supply.

**Figure 5.** The proposed position of the DRA operation in the power system.

#### *3.2. Modelling of Distributed Resources*

In the presented concept, four groups of distributed assets were modelled and can be incorporated into the aggregator's structure: intermittent RESs, dispatchable generating units, ESs, ALs. The proposed model is based on mixed integer linear programming (MILP). For further simulation purposes, 15 min intervals of a 24 h time span were assumed.

## 3.2.1. Renewable Energy Resources

RESs represent a group of noncontrollable generating units whose production is dependent only on weather conditions. For the purpose of further simulations, wind and solar generators were implemented in the simulation model. Figure 6 depicts adopted generation profiles for those RESs.

**Figure 6.** Generation profiles for wind and solar sources assumed for the simulation purposes (own development based on [26,27]).

The possibility of RES generation curtailment was also assumed, and is expressed by Equation (1).

$$\forall r \in R, \; \forall t \in T: P\_{\text{min}r} \le P\_{\text{cur }r}^t \le P\_r^t \tag{1}$$

## 3.2.2. Dispatchable Generating Units

The electricity production from units like gas, biogas, biomass, etc., is dependent on the amount of provided fuel; therefore, their output power can be controlled within their operational limits (Equation (2)).

$$\forall \mathcal{g} \in G, \ \forall t \in T: P\_{\text{ring}} \le P\_{\mathcal{g}}^t \le P\_{\text{max}\mathcal{g}} \tag{2}$$
