## 3 Key Contributions to Smart Grids
### Power Quality Support  
Power quality in smart grids is important for several factors. First of all, the equipment connected to the grid should be fed with uninterrupted energy and the supply/demand amount should be balanced. In addition, it is important that the systems operate at optimum energy levels. This is important for reducing electricity costs and contributing to the environment. Optimizing the electrical power means that the sensitive loads in the smart grid are less affected. Problems such as system malfunctions and overheating will be reduced. In addition, voltage problems on the network side and current problems caused by loads need to be compensated. Some power quality problems can be solved with the auxiliary functions of EV charging stations, such as harmonic reduction, voltage regulation, and reactive power mitigation [96]. In addition, EVs can be used as energy generation units for several concepts to mitigate power quality problems in smart grids. Figure 6 presents various concepts of EVs connected to a smart grid for the compensation of common power quality problems such as voltage sags and current harmonics. Smart grid-connected EVs can support the network during power quality problems. As shown in Figure 6a, EVs are connected to the point of common coupling (PCC) in order to compensate for the current harmonics caused by non-linear loads in the smart grid. The shunt connection of EVs through the converter interface is able to compensate for the current harmonics as well as for the power injection capability [97]. For instance, ref. [98] investigated EVs as a part of an active power filter to diminish THD values by less than 5% according to the IEEE 519-2014 power quality standards. The harmonic mitigating system behaves like a controlled current source, and the injected current is in phase with the system  
Figure 6: The potential compensation functions of EVs connected to a smart grid. voltage. Common methods such as synchronous reference frame and instantaneous active power theory are used to extract the harmonic signals for the control process [99]. In case of short-term voltage sags in smart grids, it is desired that the electrical devices remain connected to the network during grid faults. This condition, which is necessary for system stability, is defined as the fault ride-through (FRT) capability [100]. As shown in Figure 6b, the voltage generated from an EV-based system connected to the smart grid is fed to the PCC by injecting series voltage in order to keep the constant voltage on the consumer side. This concept is also called dynamic voltage restoration [101]. In this concept, the injected voltage is computed as the difference between consumer-side and grid-side voltages. The system detects voltage sags to inject controlled series voltages from an EV energy unit through a DC-AC conversion interface. Then, the generated voltage is filtered and fed to the PCC by the transformer in order to provide the FRT capability during voltage sags [102]. In addition, the dynamic voltage restorer is able to compensate for the voltage swells in addition to voltage sags to protect sensitive loads against hardware damage. Figure 6c presents a hybrid concept for the compensation of both current harmonics and voltage sags. In the hybrid concept, called the unified power quality conditioner, an EV feeds both a series inverter and a shunt inverter with a common DC link connection. The unified power quality conditioner is an advanced custom power device that can also compensate for power quality problems like voltage swells, voltage harmonics, unbalances, and reactive power in addition to current harmonics and voltage sags [103]. Although it is a multifunctional structure, it has disadvantages in terms of cost and control complexity compared to an active power filter and dynamic voltage restorer. EVs can play a significant role in the compensation of reactive power as well as in supporting active power in a smart system.