**Review of Positive and Negative Impacts of Electric Vehicles Charging on Electric Power Systems**

### **Morsy Nour 1,2,\*, José Pablo Chaves-Ávila 1, Gaber Magdy <sup>2</sup> and Álvaro Sánchez-Miralles <sup>1</sup>**


Received: 23 July 2020; Accepted: 4 September 2020; Published: 8 September 2020

**Abstract:** There is a continuous and fast increase in electric vehicles (EVs) adoption in many countries due to the reduction of EVs prices, governments' incentives and subsidies on EVs, the need for energy independence, and environmental issues. It is expected that EVs will dominate the private cars market in the coming years. These EVs charge their batteries from the power grid and may cause severe effects if not managed properly. On the other hand, they can provide many benefits to the power grid and get revenues for EV owners if managed properly. The main contribution of the article is to provide a review of potential negative impacts of EVs charging on electric power systems mainly due to uncontrolled charging and how through controlled charging and discharging those impacts can be reduced and become even positive impacts. The impacts of uncontrolled EVs charging on the increase of peak demand, voltage deviation from the acceptable limits, phase unbalance due to the single-phase chargers, harmonics distortion, overloading of the power system equipment, and increase of power losses are presented. Furthermore, a review of the positive impacts of controlled EVs charging and discharging, and the electrical services that it can provide like frequency regulation, voltage regulation and reactive power compensation, congestion management, and improving power quality are presented. Moreover, a few promising research topics that need more investigation in future research are briefly discussed. Furthermore, the concepts and general background of EVs, EVs market, EV charging technology, the charging methods are presented.

**Keywords:** electric vehicles; uncontrolled charging; delayed charging; controlled charging; V2G; V2B; V2H; peak shaving; valley filling; congestion management; renewable energy sources

#### **1. Introduction**

Greenhouse gases (GHGs) emissions, global warming, and climate change are getting significant attention worldwide [1]. Countries aim to diminish the use of fossil fuel which is the main reason behind these issues. Most of fossil fuel consumption is in electricity generation and transportation sectors [2]. In 2014, 35% of the total energy consumption was by the transportation sector [3]. Electricity generation can cut fossil fuel usage by moving to renewable energy sources (RESs) instead of traditional nonrenewable generation and transportation sector can cut fossil fuel usage by moving to electrified transportation. With this transformation in the transportation sector, carbon dioxide (CO2) emissions can be reduced [4,5]. The advancement in EV technology can lower the reliance on fossil fuels and leads to emission reduction [5]. Charging the EVs from a power grid with high share of RESs generation contributes more to decreasing pollutant emissions [6].

Electrified transportation like metros, electric trains, trolleybuses, trams, etc., are widely used in many countries public transportation and is considered a mature technology, but private electrified transportation like private EVs have not had large adoption in the past due to limitations in battery technology in terms of heavy weight, high price, short life, and long charging duration. Due to the breakthroughs and recent developments in battery technology, EVs are proposed as a competitor to traditional vehicles driven by gasoline due to the fast decline of EV prices [7]. Moreover, EVs produce no pollution (locally) and have a very high energy conversion efficiency compared to traditional vehicles because the internal combustion engine (ICE) is less efficient than the electric motor (EM). Furthermore, several countries already initiated governmental programs to increase EVs market share by financial subsidies and tax exemptions to motivate people to buy EVs. Additionally, many car manufacturers produce various types of EVs that have different specifications and prices. This gives the buyer a lot of options to choose from depending on his budget and car usage (i.e., long distance or short distance).

These EVs can be charged from large charging stations, street chargers, workplace chargers, and private home chargers [8], which will be supplied from the distribution network. It is expected that this new load which draws a large amount of electric energy from the power system in a short time will have harmful effects on the distribution network if not managed optimally and will foster the need for large infrastructure upgrades, which are an economic burden for utility companies. Therefore, comprehensive studies should be executed to understand and assess the characteristics of EVs load and probable effects they may have on the electric power system, especially the distribution networks [9], and consider this new load in the design, operation, and planning processes. Additionally, techniques to minimize and alleviate these negative effects and for optimal integration of EVs into the power grid should be developed [10].

A large number of research studies have investigated and assessed the impacts of uncontrolled EV charging on distribution networks due to uncertainties related to these studies. Many uncertainties should be considered like charging start time, the charger power rating, the charging location, EVs battery capacity, EVs battery state of charge (SoC) [11] when started charging, penetration level, and distribution network status. The studies assessed the impacts of uncontrolled EVs charging on distribution networks total power demand [12], transformer loading and life [13], cable loading [14], voltage profile [15], power losses [16], voltage and current unbalance [17], and harmonics distortion [18]. Moreover, various studies proposed solutions for optimal integration of EVs by delayed charging, smart charging, vehicle to grid (V2G) technology, vehicle to building (V2B) technology, and vehicle to home (V2H) technology, and how it can provide electrical services like frequency regulation, voltage regulation and reactive power compensation, peak shaving and valley filling, integration of RESs, spinning reserves, and improving power quality [19,20].

This paper is organized as follows: Section 2 presents a general background of EVs, EVs market, EVs types, the benefits and challenges of EVs, and EV charging technology. Section 3 presents the charging and discharging methods; for instance, uncontrolled charging, delayed charging, controlled charging, V2G technology, V2B technology, and V2H technology. Section 4 presents the negative impacts of uncontrolled EVs charging on electric power systems. Section 5 presents the positive impacts of controlled charging and discharging of EVs on electric power systems. Section 6 presents a few promising research topics that need more investigation in future research. Finally, conclusions are presented in Section 7.

#### **2. Electric Vehicles Technology**

An EV is a vehicle that is powered or driven, at least in part by electricity such as trams, metros, electric cars, electric trains, trolleybuses, etc. Most of the mentioned technologies are mature and widely used, except the private EVs (i.e., electric cars), but they are getting huge attention in the last years. EV technology is not new as it appeared in the 19th century powered by lead–acid batteries and it was not used a lot because the fuel-powered cars, i.e., internal combustion engine vehicles (ICEVs), showed better performance at that time because the energy density of fuel was better than lead–acid batteries. Usually, EV terminology is widely used to refer to private electric cars and not to other electrified transportation methods like trams, metros, etc.

#### *2.1. Electric Vehicles Market*

Based on the 2019 global EV outlook report of the International Energy Agency (IEA) [7], there is a very rapid growth of EVs market. The report shows that the EVs stock crossed 3 million in 2017 and exceeded 5 million in 2018. China represents the largest EVs market, then Europe and the United States after that. Norway achieved the highest deployment of EVs with a market share of 46% followed by Iceland 17% and Sweden 8%. It is expected that the number of EVs will increase significantly in the coming years. The forecasts for EVs number at 2030 between 130 and 250 million. There are many EVs in the market now from several manufacturers (BMW, Chevrolet, Ford, Hyundai, Kia, Mercedes-Benz, Mitsubishi, Nissan, Tesla, Renault, Volkswagen, etc.) with different specifications (battery technology, battery capacity, electric motor power, electric range, and onboard charger power rating,) which gives a wide range of options for people interested in buying an EV. Although the fast increase in the EV market, it still represents a small percentage of the global passenger light-duty vehicles market currently. This means that EVs have a long way until they become capable of making a significance in GHGs emissions and oil demands.

It should be mentioned that large scale adoption of EVs will have economic impacts and they should be viewed from two points of view, the EV owners' point of view and the power grid point of view [21]. From the power grid point of view, a high-power load represented in EVs will be introduced, which means additional expenses in fuel and generation capacity [22]. The grid power losses will rise, although they can be reduced by the use of controlled charging approaches [23]. Several studies showed that the electric power system will have a significant power loss due to EVs charging with different penetration levels. The lifespan of transformers and cables in the distribution grid may decrease due to excessive uncontrolled charging. A study in [24] showed that controlled charging of EVs results in reducing the peak demand and 60% saving in the system cost. Therefore, the improvement in EVs charging infrastructure and charging strategies need attention to enhance the power grid economic aspects. From EV owners' point of view, various benefits can be achieved like reducing the operating costs due to lower electricity prices compared with fuel and higher efficiency of EMs used in EVs compared with ICEs used in ICEVs [25]. On the other hand, the high initial cost of EVs due to the expensive batteries used compared with ICEVs represents a considerable challenge. Mass production of EVs, advancement in battery technology, new charging infrastructure and smart charging strategies, and providing rewards and incentives to EVs owners may control the high initial cost of EVs [26–28].

#### *2.2. Types of Electric Vehicles*

There are many types of EVs categorized according to the energy converter (i.e., ICE or EM) used to propel the vehicle, the power source (i.e., battery, fuel cell, or gasoline), and if it charges from an external source (i.e., charging station or home charger) [4]. A brief explanation of those types is presented in the following subsections. The basic structure of different EVs types is shown in Figure 1.

**Figure 1.** Basic structure of different electric vehicles (EVs) types. (**a**) Hybrid Electric Vehicle (HEV); (**b**) Plug-in Hybrid Electric Vehicle (PHEV); (**c**) Battery Electric Vehicle (BEV); and (**d**) Fuel Cell Electric Vehicle (FCEV).

#### 2.2.1. Hybrid Electric Vehicle (HEV)

HEV is similar to ICEV but with oversized EM and battery. The battery can be charged by regenerative braking and by the ICE at light loads. Usually, battery and EM drive the vehicle at lower speeds and the ICE drives the vehicle at higher speeds. Furthermore, EM can assist the ICE at high load and enhance vehicle performance and efficiency. HEV has lower GHGs emissions and fuel consumption than ICEVs. In this type, no charging from distribution network by EV charger is used [29,30]. Therefore, it does not result any negative impacts on the power system due to battery charging and cannot provide any electrical services. There are different structures of this type [29]: series, parallel, series/parallel, mild, and complex HEVs, in addition to series/parallel plug-in hybrid electric vehicle (PHEV) which is explained in the next subsection. The basic configuration for parallel HEV is shown in Figure 1a.

#### 2.2.2. Plug-in Hybrid Electric Vehicle (PHEV)

This is a HEV, but its battery can be charged by regenerative braking, by ICE, and EV charger supplied from distribution network as well. It is characterized by larger EM power, smaller ICE, and larger battery capacity compared with HEV in order to extend the electric range. It can operate in all-electric mode and use EM only which results in zero GHGs emissions. The capacity of the battery specifies the range of electric operation [29,30]. This type usually has a small battery capacity. Therefore, it is expected that it has a limited negative impact on electric power system. Moreover, its ability to provide electrical services is limited. PHEV can be in any hybrid configuration. The basic configuration for parallel PHEV is shown in Figure 1b.

#### 2.2.3. Battery Electric Vehicle (BEV)

BEV is expected to dominate the EVs market with future advancements in battery technology and price reduction. BEV contains only EM which is powered by a battery and does not contain ICE. The electric driving range depends on battery capacity. The main advantage of this type is that it produces no emissions locally, which is very crucial in big cities. There are many configurations of

this type [29] depending on different companies' designs but the basic configuration and principal components are shown in Figure 1c. The battery is charged from the distribution network by different kinds of EV chargers. Moreover, during deceleration and braking of EV the motor operates as a generator and produces electric energy that charges the battery. This is called regenerative braking which is used in other public electrified transportation like metros and trams too. To enable regenerative braking bidirectional DC/AC converter is used. It allows electric energy to flow from battery to AC motor in the driving mode and from the AC motor to the battery in braking mode [29,30]. The main challenge for BEV is the long charging period and limitation of public charging infrastructure. This issue will be tackled in the near future with the advancement in battery technology which will extend the electric range that the EV can drive before the need to be recharged. Furthermore, the large-scale deployment of public fast charging stations will reduce EV owners range anxiety. Moreover, in the future, BEV manufacturers may enable battery swapping. This means replacing the empty battery with a fully charged one in battery swapping station which needs a very short time. More details about battery swapping are given in Section 2.4.3. This type of EVs has the largest battery capacity and its charging can result in a severe negative impact on the power system if not managed properly. Additionally, its ability to provide electrical services is higher than other EV types.

#### 2.2.4. Fuel Cell Electric Vehicle (FCEV)

FCEV is powered by EM like BEV, but it uses a fuel cell instead of a battery. The vehicle is refueled with hydrogen and the fuel cell converts hydrogen gas chemical energy to electric energy which powers EV motor. Hydrogen can be produced from fossil fuels like natural gas or water electrolysis. FCEV has short refueling time like ICEVs [29,30]. The basic configuration of FCEV is shown in Figure 1d. It can also work with a combination of battery and supercapacitors. If no battery is used, this vehicle type will have no impact on the power system because it does not need electric charging from distribution system.

#### *2.3. Benefits and Challenges of Electric Vehicles*

It is essential to introduce the advantages and disadvantages of conventional vehicles which are dominant in the market now. Table 1 shows the advantages and disadvantages of ICEVs. The disadvantages of ICEVs might be a big problem in the future of the transportation sector. This was the reason to look for some alternatives. EVs appeared as a promising solution to some of these disadvantages.


**Table 1.** Advantages and disadvantages of internal combustion engine vehicles (ICEVs).

EVs show many benefits compared with ICEVs but few challenges must be faced to increase EVs adoption. Table 2 shows both the benefits and challenges of EVs [29,31]. Because both traditional vehicles and EVs have their strengths and weaknesses, hybrid vehicles (i.e., HEVs and PHEVs) with several configurations appeared, which try to combine the strengths of both types and avoid weaknesses.


**Table 2.** Benefits and challenges of EVs.

#### *2.4. Electric Vehicles Charging Technologies*

EVs charging can be classified into three main technologies: conductive charging, wireless (i.e., contactless) charging (WC), and battery swapping. Conductive charging is the simplest and the currently used charging method. For conductive charging, there is physical contact (i.e., cable) between the power supply and battery but for WC there is no physical contact. WC and battery swapping are not widely used like conductive charging and still under study and development. The following subsections will provide more details about these technologies. Figure 2 presents classification of different charging technologies.

**Figure 2.** Classification of EVs charging technologies.

#### 2.4.1. Conductive Charging

EV battery chargers have a significant responsibility in the advancement of EVs because the EVs adoption and social acceptance depends on the effortless access to charging stations or street chargers. Several topologies were presented for single phase and three phase EV chargers [32,33]. It consists of AC/DC converter, power factor correction elements, and DC/DC converter as shown in Figure 3. Charger systems are classified to on-board (i.e., inside vehicle for slow charging) and off-board (i.e., outside vehicle for fast charging). Moreover, they can be classified to unidirectional or

bidirectional chargers [34]. Unidirectional charging has simple charging hardware and allows power flow from grid to EV only. Bidirectional charging allows power flow from the grid to EV and can inject power from the EV battery to the grid, building, or home.

**Figure 3.** Onboard EV charger components.

Availability and advancement of EV charging infrastructure can decrease the required onboard energy storage and decrease the range anxiety of EV owners. According to the Society of Automotive Engineers (SAE) Standard J1772 [35], there are three charging levels. Most of the EV owners are expected to charge at home overnight according to the Electric Power Research Institute (EPRI) so Level 1 and Level 2 chargers will be the primary option [36], while level 3 will be used for commercial charging stations.
