• **Level 3 Charging**

Level 3 is for fast charging and it operates as a commercial refueling station (i.e., less than 1-h charging time) similar to the conventional gas station which can be installed at city main roads and highways. It is supplied from a three phase circuit with 480V or higher voltages. It is available only as an off-board charger because the charging power is high and may exceed 100 kW. It is clear that level 3 charging is not suitable for home charging. It has a high installation cost, and this represents a potential issue. It is expected that public chargers will use Level 2 or Level 3 for fast charging in shopping centers, parking lots, restaurants, hotels, theaters, etc. High charging power represents an advantage from charging time point of view, but it may generate a peak demand and overload the distribution network equipment in addition high installation cost.

#### 2.4.2. Wireless Charging (WC)

WC enables EVs charging without physical contact or cable between the power supply and battery. Advancement of WC will reduce the required onboard battery capacity which will decrease EVs price and mass, which will result in reduction of EVs energy consumption. WC may become a future alternative for traditional conductive charging. WC has a potential to be used for charging electric bus batteries [37]. It can operate at different voltages (level 1, 2, and 3). The highest efficiency recorded for WC is 90% [38]. There are three main technologies of WCS: inductive, resonant inductive, and capacitive WC [39,40].

Inductive wireless charging (IWC) will be explained to provide the basic idea of this technology. The IWC contains AC/DC converter that converts AC power supplied from electricity grid to DC. Then, it is converted again to AC power with high frequency fed to transmitting (i.e., primary) coil. All these components are in the street underground. The EV contains receiving (i.e., secondary) coil that receives power from transmitting coil by electromagnetic induction through the air gap. Then the AC power is converted to DC by AC/DC converter and charge the battery [41]. A simplified diagram of IWC that explains the main concept is shown in Figure 4. IWC can be classified into static inductive charging and dynamic inductive charging. For static inductive charging, EV must be stationary during charging. However, dynamic inductive charging allows WC while EV is moving [42].

**Figure 4.** A simplified diagram of inductive charging.

Current WC is designed for unidirectional power flow from grid to vehicle, but future development of this technology is to enable EVs to discharge power to the grid wirelessly to provide electrical services. The advantages of this technology are electrical safety, no cables needed, and user convenience. The challenges of this technology are the high infrastructure cost compared to conductive charging, and low power transfer efficiency between coils [40].

#### 2.4.3. Battery Swapping

Battery swapping station (BSS) is a charging station at which the empty EV battery will be replaced by a fully charged battery in few minutes [43]. Battery swapping may be used with electric buses that have a high capacity battery which will take a long time to be charged by traditional conductive charging. This technology requires a large stock of batteries owned by the BSS or a third party and rented to EV owner. BSS contains a distribution transformer, AC/DC converters to charge the batteries, batteries, and battery swapping equipment [44,45]. Some studies considered that BSS can use bidirectional chargers to provide electrical services by V2G mode [46,47]. The challenges of this technology are battery standardization, high infrastructure cost, and large space for BSS. In 2013, Tesla company revealed a battery swapping system that can swap the battery in 90 s [48].

#### **3. Charging and Discharging Methods**

Charging methods can be classified into two main categories, unidirectional and bidirectional charging. In unidirectional charging the energy flow is only from the grid to EV (i.e., uncontrolled charging, delayed charging, and controlled charging). In bidirectional charging, energy can flow from the grid to EV and from EV to the grid or loads as buildings, or homes (i.e., V2G, V2B, and V2H). Figure 5 presents the classification of different charging and discharging methods. Figure 6 shows how the total power demand profile of distribution system will change with different EV charging or discharging methods [10].

**Figure 6.** Total power demand profile with different charging and discharging methods. (**a**) Uncontrolled charging; (**b**) Delayed Charging; (**c**) Controlled Charging; and (**d**) V2G, V2B, and V2H.

#### *3.1. Uncontrolled Charging*

This is the simplest method to charge EVs and the current used way. The EV is plugged in for charging at the maximum power rating of EV charger until the EV battery is fully charged (i.e., state of charge (SoC) is 100%), similar to any electric device with a battery (i.e., laptop, smart phone, etc.). Several studies concluded that uncontrolled charging of EVs may result in severe negative impacts on distribution networks such as increase in peak load demand, overloading of transformers and cables and shorten their life, increase voltage drop, increase system unbalance due to single phase chargers, increase power losses, and increase harmonic distortion [49,50]. Moreover, this type of charging limits the EVs acceptable penetration level, because EV owners charge their vehicles when arriving home from work, which usually coincides with peak hours as shown in Figure 6a. The figure shows the total power demand of residential consumers during the day [51] before connecting EVs (in grey) and how the total power demand will change when EVs are connected to the distribution network for charging. Furthermore, it will accelerate the need for infrastructure upgrade. This charging method is also known as dumb charging, uncoordinated charging, and unregulated charging.

#### *3.2. Delayed Charging*

The severe impacts of uncontrolled charging can be alleviated with the use of delayed charging. In this case, the utility has different electricity prices during the day with low price at off peak time (i.e., time utility wants EV owners to charge at) and high price at peak time (i.e., time utility does not want EV owners to charge at). This method controls the charging time and not the charging power (i.e., EV charge at charger maximum power rating). With the optimal design of electricity tariff prices during the day, it can work as an incentive for EV owners to charge their vehicles on low price times which maximize both utility (i.e., distribution system operator (DSO) or distribution company) and EV owners benefit and result in valley filling as shown in Figure 6b. Although this method is a very simple technique to flatten the load profile and shift the load to off peak time, non-optimal design of electricity price, can incentivize a large number of EV owners to charge at off peak times, which may result in a second peak especially at the beginning of off peak time. Moreover, this method ignores EV owners' preferences and needs to charge their EVs at different times during the day. This charging method is also known as off peak charging, and indirect controlled charging with time of use pricing [52,53].

#### *3.3. Controlled Charging*

Although delayed charging has less impacts on distribution networks than uncontrolled charging, it has limitations. These limitations can be surpassed using controlled charging. This method controls the charging time and charging power of EV depending on some distribution network parameters like total power demand, transformer loading, voltage stability, power losses, etc., or to minimize the charging cost. In this technique, EV acts as a controllable load. Various studies proposed controlled charging algorithms for maximizing EV owner benefit by charging cost reduction and maximize utility benefit by distribution network stress and losses reduction, enhancement of power quality, and shifting the EV load to off peak hours which result in valley filling [54,55] as shown in Figure 6c. This charging method is also known as coordinated charging and smart charging.

Controlled charging can be classified as centralized, decentralized, and autonomous control architecture [56]. In centralized control, a central controller collects data such as electricity prices, system loading condition, EVs status, and owner preferences; based on this data the controller determines the set points of controlled EVs. Although centralized charging control usually results in optimal use of the system and enables EVs to participate in many electrical services, it has few disadvantages. There are privacy issues because all users data can be accessed by the central controller, the need for expensive two way communication infrastructure, large data amount must be processed, which is a high computational burden, any problem in the central controller or loss of communication can lead to severe consequences, and a large number of messages should be communicated in a short period which can result in communication issues. Moreover, most of the utility companies do not use real time pricing, which makes this method inapplicable currently [56].

In decentralized charging control, the control entity (i.e., DSO or aggregator) sends signals to EVs like electricity prices to incentivize them to take a specific action such as reducing their charging power or stop charging at peak hours. In this case no need to send EVs private information to the control entity. Although decentralized control methods need a cheaper processing and communication infrastructure compared with centralized control methods, they have few disadvantages. The optimal utilization of the system is not ensured, their ability to participate in ancillary service markets is limited and they are vulnerable to variations in customers behavior [56].

In autonomous charging control, the charging power is regulated based on local inputs such as voltage and battery SoC without any communication between EVs and control entity [57,58]. Usually, autonomous charging control is classified as decentralized control with no communication. Although this control architecture is the single option for distribution systems with no communication facilities and it can enable the integration of a larger number of EVs to the power grid in its current status, it has few disadvantages. Their ability to participate in ancillary service markets is limited, they are vulnerable to variations in customers behavior and the system cannot be operated optimally [56].

#### *3.4. Vehicle to Grid (V2G)*

This refers to the capability of EVs to supply power to the distribution network and operate as distributed energy storage devices. Bidirectional EV chargers are used to enable the electric power to flow in both directions from grid to vehicle (i.e., charging) and from vehicle to grid (i.e., discharging). When there is surplus electric energy (off-peak times) EVs will charge, which is called grid to vehicle (G2V) mode, and when there is a deficit in electric energy and consumption is higher than generation, EVs will supply power to the distribution network and this enhances the system reliability and efficiency [59]. So EVs can be seen by utilities as load and source. Because EV power and energy are limited and approximately have no effect on the power system, usually an aggregator is the responsible for aggregating EVs power or energy, which are located in geographical area to participate in electrical services.

The interaction in this case is between EV and power system management organization like DSO, which has to operate the power system in a reliable way. V2G can provide many grid services such as frequency regulation, spinning reserve, enabling the integration of more RESs, and peak load shaving and valley filling [60] as shown in Figure 6d. Other benefits of V2G is generating revenues for EV owners, and decrease emission and operating cost if grid services were provided by traditional nonrenewable generation [60]. Although this is a very promising technology regarding providing electrical services to the power system, it has many challenges and barriers. This technology is very complex and needs many infrastructure changes because it needs bidirectional chargers and continuous two-way communication between EVs and system operator or aggregator. Moreover, it is expected that continuous charge and discharge of EV battery may lead to battery degradation and shortening its life. In contrast, new studies concluded that EV can participate in V2G without battery degradation [61].

#### *3.5. Vehicle to Building (V2B)*

V2B is similar to V2G but in V2B there is no communication between the vehicle and the grid, and it only communicates with the building so the energy stored in the EV battery can be used to supply the building loads only. The use of V2B mode can be very effective during peak load times and outage conditions. EVs are used as an energy storage device and operates in two modes: G2V and V2B. It operates in a G2V mode to charge the battery at low cost when the grid is lightly loaded and there is surplus electric power generation. It operates in a V2B mode to supply the building loads when the electricity price is high at peak hours [62]. By doing this V2B is providing peak shaving and valley filling which is utility benefit and reduce building expenses by discharge at periods with high electricity prices and reduce building demand from the grid. This method is simpler than V2G as it operates behind the building meter, but it provides less electrical services to the power system. V2B involves one building (i.e., many homes) and many EVs.

#### *3.6. Vehicle to Home (V2H)*

This is very similar to V2B but in this case, only one home is involved and not a whole building as in V2B and only one EV. Normally EV absorbs power from home and can supply power to home when needed. It needs a simple architecture compared to V2G and V2B and can provide less electrical services to the power system compared to them. It can flatten the house daily load profile and reduce house consumption during peak hours with high electricity prices. Local energy production from photovoltaic (PV) or small wind generation can be effectively used by storing the excess generation in EV battery and use it when needed [63,64].

#### **4. Negative Impacts of Electric Vehicles on Electric Power Systems**

Uncontrolled charging of EVs with high penetration levels is expected to introduce unwanted negative impacts on the power system. The possible negative impacts are the impact due to increase of peak demand, voltage deviation from the acceptable limits, phase unbalance due to the single-phase chargers, harmonics injection, overloading of the power system equipment, and increase of power system power losses [65]. A large number of studies have investigated these impacts because the estimation of EVs charging impacts is based on several conditions [65] and have many uncertainties that must be considered and modeled properly in the study to accurately estimate the impact of EVs charging on the power systems, the main key variables are [66,67]:
