*2.1. Vehicle to Grid (V2G)*

V2G is the technology consisting of energy transfer in two directions, either from the vehicle to the grid if the energy stored in the battery is high, or from the grid to the vehicle when the energy stored in the battery is low [20]. The general block diagram of V2G structure is depicted in Figure 4. Since the energy flows to the vehicle from the grid, and from the grid to the vehicle, there occurs a bi-directional energy flow. During the energy flow, conversion operations are made with power electronics circuits to fit the type of power [21]. For the purpose of taking the voltage level of the current grid voltage to the appropriate level, firstly an AC/DC conversion system is carried out, and then the reduction is done with a descending converter. To transfer the DC energy in the battery to the grid, firstly, an additional operation is done with an increasing converter, and then the DC/AC conversion process is carried out. These operations are performed bi-directionally depending on the amount of energy in the battery. The purpose is to meet the energy demand by managing the energy in the battery or the grid [22].

The integration of plug-in hybrid vehicles (PHEV) with smart grids is also drawing significant attention of the scientific community because of its potential role in enhancing grid efficiency [23,24]. Hybrid electric vehicles usually charge their batteries through fuel, whereas electrical vehicles charge their batteries from the grid, thus having significant differences in charging circuitry and strategy [25]. However, both are of considerable interest in V2G technology. The EV is plugged into a charge station, which is connected to a power converter, followed by a transformer connected to the main grid. A control and monitoring unit is present to provide accurate input to the converter by comparing the reference signal and the outputs of the power converter (because the converter is bidirectional, both its sides can be considered as the output side). Given that electric vehicles are integrated into electric energy, they are considered as an alternative energy source for the grid. Moreover, they are also used as uninterrupted power supplies (UPS) [26]. The vehicles consisting of V2G technology are generally charged at times when electricity production is higher, or at times when the price of electricity is low

at peak load times, selling the energy to the grid at peak load times with high prices or when there is energy demand. In addition, this system also provides extra energy to the grid, increasing the reliability and efficiency of the energy system [27]. The carbon emission level of V2G technology is very low, and it also works in compliance with renewable energy sources. One of the problems experienced by electric vehicles in transferring energy to the grid is the coordination with the grid operators, and another problem is the bidirectional energy and communication infrastructure [28]. Several modes of communication, such as global positioning systems [29,30] and cellular networks [31–33], are merged with the vehicles to take advantage of the vehicles in every way possible. Different intra-vehicle and inter-vehicle communication and wired or wireless protocols are established to analyze the viability, although they are still at their primary stage, requiring more contribution for greater enrichment [34].

**Figure 4.** Block diagram of the V2G structure [22].
