The transportation industry is transitioning from conventional Internal Combustion Engine Vehicles (ICVs) to Electric Vehicles (EVs) due to the depletion of fossil fuels and the rise in non-traditional energy sources. EVs are emerging as the new leaders in the industry. The supply systems in the world are invariably AC systems. This necessitates using an AC-DC converter in the battery-charging system. With grid connection, several important power quality parameters need to be considered. The power systems are usually designed keeping in mind the increase of load in the future.
1. Power Factor Correction
PFC can be done using two 
methods: (a) passive PFC and (b) active PFC. The passive method uses passive components such as inductors and capacitors to filter out the harmonics. The passive method is suitable for low-power applications (less than 100 W) since it leads to lower efficiency, high cost, and weight due to the size of line-frequency inductors and capacitors. On the other hand, active PFC methods use switching regulators to correct the wave shape to obtain sinusoidal grid current at unity power factor. This method is complex but is suited for high power levels. Active PFC can be done using converters that operate in continuous conduction mode (CCM), critical conduction mode (CrCM), and discontinuous conduction mode (DCM).
The general requirements of a PFC AC-DC converter topology include a simple power stage with a low number of components, less distortions in the input current, and the ability to achieve a near-unity power factor. Kolar et al. 
and Friedli et al. 
have provided an exhaustive review of the various topologies of PFC rectifiers. Conventional Boost PFC Rectifiers (CBRs) operating in CCM are among the most popular PFC topologies 
. Integrated PFC controllers such as NCP1650 by ON Semiconductors 
also work as a CBR. With Silicon Carbide (SiC) and Gallium Nitride (GaN) MOSFETs and diodes, the issue of output diode reverse recovery is mitigated 
. The design of the memory elements of the CBR is the same as that of a conventional boost converter. In addition to the CBR, dual boost bridge-less PFC rectifiers 
, totem-pole bridge-less PFC rectifiers 
, and interleaved boost PFC rectifiers 
are more commonly used 
in several products that need PFC. On the three-phase system side, Vienna rectifiers have become very popular 
since they have inherent PFC capability. Several control techniques can be used to achieve a unity power factor. Examples of control strategies include hysteresis control 
, average and peak current mode controls 
, model predictive control 
, sliding controller 
, and one-cycle control 
2. Harmonics and THD
EV battery charging systems are made of multiple power electronic converters and analog electronic systems. These are non-linear loads to the grid and hence cause harmonics in the grid current. The harmonics will be very significant when multiple vehicles are being charged at the same time 
. Not only will the harmonic pollution of the grid current be higher in that part of the power system, but it will also reflect on all its connected systems and cause distortion in those parts. The presence of harmonics affects the operation of all the connected equipment and can also cause failures with considerable financial implications. EV charging can cause unbalance in the power system. Due to the unbalance, negative sequence components can be produced, producing a second-order harmonic ripple in the DC link voltage. This causes distortions in the grid input currents. Equation (1) can be used to explain the effects of harmonics 
on the DC-link. The presence of even-order harmonics on the DC-grid side will cause the generation of odd-harmonics on the grid-side, leading to power quality issues. In Equation (1), α
represents the negative sequence components, and Idc
represents the DC current.
Other effects of harmonics on the various components of the power system are well known, and their solutions have been investigated for years. PFC converters are to be controlled such that the harmonics are mitigated well and the THD levels are within the permissible limits. In addition, strategic placement and optimal sizing of variable passive filters can also help in harmonic reduction. Alame et al. 
have comprehensively analyzed the effects of harmonics due to EV charging on the various components of a distribution system. The authors have explained transformer loss modeling, temperature rise modeling, and lifetime modeling. A sample case study on a 1500 kVA distribution transformer has been provided to show that the percentage of harmonic currents increased with an increase in the battery state of charge. Further, the impact of EV charging on the distribution system has been analyzed by considering the IEEE 33-bus system charging four EVs at different buses using PV-based distributed generation units. The analysis has been performed using the decoupled harmonic power flow algorithm to obtain the effect on voltage quality and current THD. A centralized control flow has been proposed as an optimization problem to mitigate the effects of harmonics.
3. Other Detrimental Effects
Due to the diverse charging rates of EVs owing to slow, fast, and ultra-fast chargers, several negative effects can be observed. These include stability issues, unbalance, and overloading 
. Dharmakeerthi et al. 
have concluded that using fast charging stations can cause issues in the grid since they can significantly reduce the steady state voltage stability of the grid. In addition, the harmonics and inter-harmonics produced due to charging rates will also affect the power system’s critical components, such as transformers, breakers, cables, and meters. Alshareef and Morsi 
have shown that fast charging stations can significantly affect and cause voltage flicker in distribution systems.
Despite these issues, EVs will be integral to our system since the advantages outweigh the limitations. Hence it is necessary to address these issues while designing a charging station. Nguyen et al. 
have provided the topology of a photovoltaic inverter used as an active filter to mitigate power quality issues during simultaneous fast charging of five EV batteries. The proposed system models a bidirectional DC-DC converter acting as a charger and a DC-AC converter connected to the AC grid. Control structures for the control of both converters to implement harmonic mitigation have been explained, and the corresponding simulation results have been provided to validate the proposed scheme.
Once the AC power has been converted to DC and the power quality is maintained, a DC-DC converter is required to regulate the voltage, match the battery voltage level, and implement charging control. These DC-DC converters form a significant research area, specific to the simultaneous charging of multiple batteries.