# Planning of High-Power Charging Stations for Electric Vehicles: A Review
## 2 Target Identification
### Power Converters and Architectures  
The battery charger can be installed inside the vehicle (onboard) or externally (offboard). Both charging systems take AC power from the grid and convert it to the desired voltage level. Onboard battery chargers are limited in size, weight, and volume and are therefore suitable with Level 1 and Level 2 chargers. Onboard battery chargers operate from a single or three phase AC outlet and are typically capable of only unidirectional power transfer; however, in some configuration cases, bidirectional power transfer can be achieved [99]. Level 3 chargers, due to their capacity ratings, are typically installed outside the vehicle (offboard battery chargers) and operate on 3-phase AC power supply with higher power rating than onboard battery chargers. The external charging system is typically made up of two stages: an AC/DC converter that faces the connection point to the supply network, followed by a DC/DC converter that provides the DC interface to the EV charging system. These two stages typically allow bidirectional power flow [100]. In Figure 3, a schematic representation of most common charging system topologies for both onboard battery chargers and offboard battery chargers is shown. The topology usually includes a first stage with alternating current-direct current (ac-dc) conversion that guarantees a sinusoidal grid current, and a second dc-dc conversion that ensures current and voltage on the EV battery, preserving the battery lifetime, in both onboard and offboard chargers. The rectifier stage can be performed by a half-bridge, full-bridge, or multilevel diode bridge, whereas the second stage is commonly performed by resonant power converters, because of the potential to achieve at the same time both higher switching frequency and lower switching losses [99]. An electromagnetic-interference (EMI) filter is responsible for maintaining the power quality on the power grid side [101]. The battery management system (BMS) ensures safety measures in the battery pack charging, both in terms of control and protection. Power control units implement the control algorithms, which can include PID controllers, fuzzy logic, and neural networks [102]. Finally, the protection is guaranteed through circuit interrupting devices (CID) and circuit breakers [100]. Based on the different battery charger typology adopted (level 1, level 2, or level 3), different architectures can be adopted in terms of common bus configuration. Two main categories can be identified, which are common AC-bus configuration and common DC-bus configuration. In AC-bus configuration (Figure 4a), an MV/LV transformer steps down the voltage from the main supply point, then a common AC bus feeds the EV charging stations and ancillary units, such as renewable energy source (RES)-fed power plants and storage units. The advantage of this configuration is a simple implementation as well as the possibility to feed generic appliances with low-voltage AC power. In a DC-bus configuration (Figure 4b), a single AC/DC rectification stage is connected directly downstream of the MV/LV transformer, and a common DC bus is distributed among the charging station appliances, such as charging stations and ancillary units. The main advantage of this configuration is the elimination of synchronization and reactive power issues [103]. A promising approach is represented by the utilization of solid state transformers (SST) (Figure 4c). This approach enables the direct connection to the MV line with the elimination of the LF transformer. It essentially covers the functionality of the LF transformer and AC/DC conversion stage with enhanced power density, with additional benefits in terms of bi-directional power flow, fault current limitation, and fault isolation [103].