## 6 Grid Impact
### Transformer Congestion and Line Deterioration in Distribution Networks  
The transformers (medium voltage and especially low voltage) and conductors belonging to distribution networks were not designed to charge EVs. There is a significant impact on overloading of these elements when EV penetration is high. Thus, it is important for future cities and neighborhoods to consider EV charging when sizing their electrical systems [134; 142].  
EV geographic aggregation strategies can impact distribution networks and their elements (transformers, feeders, and branches) unless charging coordination strategies are implemented [128; 132].  
Other authors have concluded that electrical systems do not have the appropriate dimensions for meeting future PHEV demand. Although there are no real data yet, many simulation analyses have been performed to obtain these conclusions [121].  
In Figure 20, it can be seen that there is a real risk of PEV loading in distribution networks causing deterioration of low-voltage transformers and lines [119; 130]. These effects include reduced efficiency, temperature increases, and premature failure of the insulation/windings or core structure. The power electronic systems of battery chargers produce current harmonics that deteriorate transformers, impacting the efficiency and safety of smart grids. The incorporation of local RES can help to solve these problems [120].  
The imbalance between phases occurring when an EV is charged in only one phase can produce overload in a line or transformer phase while the rest are unaffected, which directly impacts transformer failures [137; 104].  
These phenomena depend on the network zone, as overloads can be produced by a high concentration of EVs at a point while not affecting others. A detailed analysis is necessary to determine the underlying causes [131; 143].  
Other studies have indicated that even if EV penetration increases, with advanced smart charging strategies it would not be necessary to make new investments in generation and transmission, although it has been suggested that there would be an impact on transformers and lines during distribution [122; 126; 130].  
Simulation studies have shown that PEV penetration levels between 17% and 31% increase transformer currents from 37% to 74% [125]. These studies have concluded that this level of penetration generates transformers losses when chargers work at level 2 or 3. For future cities or equipment replacement, it is necessary to consider that transformers and conductors must be oversized. Studies have indicated that PEV penetration of 50% could reduce transformer life by 200% to 300%. On the other hand, if loads are controlled, the lifetime can be increased by 100% to 200% compared to the previous case. Moreover, PEV penetration of 30% would increase demand by 10% in a Belgian network, exceeding the capacity of transformers and conductors. In these cases, it is proposed to deal with voltage drops by means of load-tap charging transformers or by using a capacitor bank.  
The advanced use of V2G will make it possible to smooth out these effects [125], as EVs can behave as auxiliary services for network management [127].  
Recent research has evaluated the impact of artificial intelligence in analyzing optimal charging and discharging scheduling in V2G systems, concluding that the operating costs of the electrical network can be significantly reduced with the use of simple algorithms [144].