978-1-4799-6251-8/14/$31.00 2014 IEEE

Literature Review: Potential Impacts of Plug-In Electric Vehicles on Electric Power Systems

S. Lpez, J. Caicedo, M. Maman, A. A. Romero, G. Ratt Instituto de Energa Elctrica

Universidad Nacional de San Juan CONICET San Juan, Argentina

aromero@iee.unsj.edu.ar

Abstract The air in most large Latin American cities is polluted, primarily because their automotive fleet is composed of internal-combustion engine vehicles. This also implies a strong fossil-fuel dependency. Petroleum fuels are nonrenewable and, as a consequence, the current mobility model becomes unsustainable. Great expectations have been put in electric vehicles. Such vehicles are already penetrating the automotive market and, soon, will be part of the alternatives of mobility in countries of Latin America. Therefore, research and development efforts must be conducted in order to facilitate the transition between the old technology, of internal-combustion engine vehicles, to the new e-mobility technology, in an efficient and effective way. At the Instituto de Energa Elctrica, a three-year research project has begun. The scope of the project includes the analysis of potential impacts of massive penetration of electric vehicles on both: the power system and the way in which it is traditionally operated. In this paper, a partial result of the natural first step in a scientific research is presented, that is a literature/bibliographic review. The paper focuses on three main issues: the impacts on the network planning, on the power quality and on the tariff schemes. After a brief introduction and a quantitative analysis of the available literature on the subject, each one of the issues is separately presented. Finally, the conclusions of the paper are given.

Index Terms Network planning, Plug-in electric vehicle, Plug-in hybrid electric vehicle, Power Quality, Reliability, Smart Grid, Tariff, Time of use.

I. INTRODUCTION On December 11 in Mexico, at the 2010 United Nations

Climate Change Conference, the Cancun Agreements were reached and signed. On that occasion, industrialized countries agreed to help developing nations deal with climate change. Several main objectives were declared, however, the one concerned with this paper is mitigation. I. e., textually: Establish clear goals and a timely schedule for reducing human-generated greenhouse gas emissions over time to keep the global average temperature rise below two degrees [1]. In reference [2], the International Agency of Energy states that to reach the 2C goal requires the long-term concentration of greenhouse gases in the atmosphere to be limited to about 450 parts per million of carbon-dioxide equivalent (ppm CO2-eq). Despite this laudable objective, scientific community estimates that even if global emission rates are stabilized at present-day levels and with zero emissions after 2030, there is a 25%

probability that warming exceeds 2C. Moreover, as was concluded in [3], every year of delayed action increases the chances of exceeding 2C warming.

Furthermore, and from an economic point of view, from contemplating the different forecasted scenarios for future oil prices, a common characteristic appears: increasing prices [2].

These two highlighted environmental and economic facts (together other extremely important technological issues) are some of the key aspects to understand why if early antecedents of e-mobility first appeared in the late 1830s, electric vehicles have only been considered now, to be part of the scheme of mobility in the future society. In fact, plug-in electric vehicles and plug-in hybrid electric vehicles, and in what follows Electric Vehicles (EVs), are set to be introduced into the mass market as a near-term technology to contribute to reduce both oil dependence and greenhouse gas emissions from the transportation sector.

EVs obtain their fuel from the grid, when they are plugged into a standard electric power outlet (either at home, at work, or at parking facilities) to charge their batteries. Therefore, surely the massive connection of EVs will impact on several aspects related to traditional power systems, e.g., on the way in which the electric power infrastructure is operated and its expansion is planned, on its reliability, on the business model, etc. Then, e-mobility technology brings several techno-economic challenges for power engineers.

Many studies regarding EVs are being performed around the world. However, much less has been done in developing countries and even less particularly on Latin America. Therefore, important research and development efforts must be conducted in order to facilitate the transition between the old technology, of internal-combustion engine vehicles, to the new e-mobility technology, in an efficient and effective way, especially in developing nations.

At the Instituto de Energa Elctrica, a three-year research project, on the subject, has started. This project is funded by the Agencia Nacional de Promocin Cientfica y Tecnolgica (ANPCyT), in Argentina. In this paper, partial results of the natural first step of any scientific research are presented, that is a literature/bibliographic review. The paper focuses on three main issues: the impacts on the network planning, on the power quality and on the tariff schemes. By giving an overview on these topics, the paper demonstrates possible future directions

of research in power systems and its strong relationship with the e-mobility; thus providing a suitable framework of analysis to the researchers interested in dealing with this new technology and its related problems.

II. BASE OF KNOWLEDGE: ELECTRIC VEHICLE The literature review is an essential step in any research

project. It must ensure obtaining the most relevant information in the field of study, from a base of knowledge, which is usually extensive. For instance, in this article a set of search rules have been defined and used in the extensive catalog of citations and abstracts from journals and conferences: Elseviers Scopus [4]. The number of references found by each one of the rules, and filtering only documents belonging to the engineering field, is reported in Table I.

TABLE I NUMBER OF REFERENCES OBTAINED BY SEARCHING WITH DIFFERENT RULES

IN THE BASE OF KNOWLEDGE IN SCOPUS

Search rule Type of document

Total Book Article Review Conf. Paper Other

(*) electric vehicle 24 5,154 219 10,260 1,180 16,837

electric vehicle AND planning 0 180 12 318 64 574

electric vehicle AND power quality 0 84 3 188 15 290

electric vehicle AND harmonic 0 101 3 170 25 299

electric vehicle AND tariff 0 21 0 26 2 49

(*) Other: Book Chapter, Conference Review, Note, Short Survey, Business Article, Report, Letter, Abstract Report, Editorial, Erratum, Article in Press. Survey data: 21/02/2014.

Fig. 1. Temporal evolution of the document production on the subject electric vehicle.

By searching the keywords electric vehicle, 16,837 results were found. This indicates that there is a huge base of knowledge on this topic. However, only hundreds of references are related to the specific impacts that are dealt with in this paper. In Fig.1, a plot of the temporal evolution by type of documents is presented. It is notable how e-mobility is becoming a fertile field for exploration, in which research is increasing exponentially. However, as shown in Fig. 2, most of the research, currently developed, is concentrated in industrialized countries, mainly in United States and China,

and only few documents come from Latin American countries, i.e., 174 references were found.

Fig. 2. Scientific production by country.

In this context, some of the main references found through the different search rules are analyzed in what follows.

III. ELECTRIC VEHICLE AND PLANNING The large-scale deployment of EVs on the power system

implies significant impacts on the electricity demand, and therefore, in the generation, transmission and distribution system planning. However, the current trend of the consulted references, in relation to the expansion, shows that major impacts will affect the distribution networks. One of the reasons of why there is prolific research in the distribution area can be reflected in [5], by the research group of the Department of Energy of Pacific Northwest National Laboratory, from US. This group has determined that there is sufficient power capacity installed at transmission and generation levels, but the situation is not the same for the power infrastructure in distribution networks.

In this sense, in [6] a quantitative study of the incorporation of EVs on the distribution network of Santiago de Chile is developed. This methodology allows scheduling the expansion of the network, estimating an increase of the load from the current characteristics of automotive fleet that exists in every area of the city. Moreover, various scenarios for the inclusion of EVs are presented through a percentage of the current vehicle fleet for a period of analysis that contemplates 2020 as deadline. As a conclusion of this work, a large increase in the total demand, for the assessed scenarios of EVs penetration, is rejected. This result is clear since a large penetration of EVs in Santiago city is not expected until 2020. However, the disaggregated demand could affect some local points of the distribution network even in scenarios of low penetration. I.e., these affectations are mainly due to the number of vehicles connected to the feeders, the superposition of effects when the EVs power consumption is at the peak of the demand, etc.

In [7], Mahalik et al., carried out a study of impacts on the electric system of the State of Illinois, located in the Midwest of the US. This work is evaluated in a similar way to [6], assessing the future impacts due to additional load that represent the connection of EVs. The presented methodology simulates the behavior of the system in different scenarios of EV penetration, in order to compare the impact which could be

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incurred in each case from a base of EVs, baseline scenario. As in [6], the scenarios consider the inclusion of this type of vehicles in the system taking into account a horizon of planning, in which the penetration is estimated through a percentage of the vehicle fleet of the city. As a result of the methodology, the increase in demand in the power system for the analyzed period is 0.8%, which represents a slight increase in demand of power and, which must be provided through additional generation.

In [8], Poch et al., applied a similar methodology to the one presented in [7], but in the West of US. Moreover, the obtained results on the impacts of the connection of EVs are lightly different. First of all, the study shows that those generation units that provide base load in the system are not able to supply the additional load that will represent EVs. Furthermore, it evaluates the possible impact of the transmission line congestion on the dispatch of generators. This last impact is associated with the constraints of the generators that can lead to a more costly system operation.

In [9], McCarthy and Wolfs performed some work that assessed the impact of EVs connection on the power system of the city of Perth, Australia. The planning horizon is ten years (2010-2019). In this case, studies of the impacts on generators, underground cables, overhead lines and power transformers, are performed. In that work, the authors determined that for any charging scenario of EVs the electric power may be supplied with the current generation fleet and the assets of the transmission system, since they are underutilized most of the time. Distribution level was not analyzed in [9].

Reference [10] is a work of similar characteristics to those already presented, but the developed methodology is applied to two distribution areas. An area consists of 6,000 residential customers connected to the low-voltage level while the remaining area is an industrial and residential area with more than 61,000 customers. The presented methodology first deals with knowledge of the actual areas of distribution. Then, scenarios of penetration are generated by means of a load evaluator for EVs connected in peak and off-peak of load. Finally, the model based on the concept of optimally adapted network is employed for planning. It identifies where it is convenient to make investments. The results in this paper show quantitatively how the investment costs are dependent on the charging strategies of EVs analyzed in each penetration scenario.

In [11], a study applied to a distribution system is presented. It proposes a multi-year and multi-objective planning algorithm that allows adapting scenarios of high penetration of EVs, with the injection of renewable distributed generation. It minimizes associated cost, such as reinforcement of the system and energy losses. That work, on one hand, solves the problem of planning that determines the optimal level of penetration of EVs on a system of 38 buses. A meta-heuristic tool of genetic algorithms is employed to define the optimal level of renewable distributed generation that will enter into the system at all of its stages, such as location, size and year of installation of the units. This planning problem is defined in terms of non-linear mixed integer multi-objective programming.

The relevant contributions of [11] are: The development of a multi-objective scheduling algorithm that integrates the penetration of EVs altogether with distributed generation through a generic mathematical formulation. The development of a probabilistic annual model of the energy consumption for a fleet of EVs based on Monte Carlo simulations. The investigation of the interaction of renewable distributed generation units and requirements of EVs load.

The work presented in [12] by ElNozahy and Salama constitutes an original contribution to assess the impact of EVs on distribution systems, since it gives a framework based on Monte Carlo simulations to treat various uncertainties that influence the process of charge, such as individual driving habits, the level and type of EV to be connected, and uncertainties associated with the load level. This study gives an idea of the impacts foreseen by its probability of occurrence and also identifies the more vulnerable physical assets of the distribution system.

IV. ELECTRIC VEHICLE AND POWER QUALITY The integration of EVs within the power system could

affect the quality of supply causing technical problems in distribution networks, such as increased losses, harmonics, imbalance, voltage sags, etc. [13]. Being chargers for EVs based on power electronics (nonlinear loads), they could cause Electromagnetic Compatibility (EMC) problems that must be analyzed through different studies: transient analysis, identification and power flow calculation of harmonic loads, and so on. According to the bibliography reviewed by the authors, most studies of power quality related to the connection of EVs are concerned with harmonics and voltage imbalance. Other disturbances such as voltage drops (including voltage sags) and noise have also been studied. Reviews of applicable standards and solutions to power quality disturbances were also found in bibliography consulted.

A. Harmonics Harmonics are components of voltage or current

waveforms, integer multiples of the fundamental frequency (usually 50 Hz or 60 Hz) [14]. Such harmonic components, which generate distortion of the waveform, are produced by nonlinear loads, e.g., EV battery chargers. Orr et al., studied in [15] the harmonic currents, voltage distortion and power of different EV battery chargers. This reference is one of the pioneer works related to the impact of EVs on the power system, which dates from 1984. Documentation of five types of EV battery chargers is presented, including battery chargers both with & without smoothing current inductor, and with & without controlled rectifier. Moreover, the expected harmonic current, active power, reactive power and hourly power factor values were determined. In addition, in [15] the effect of the random location of each type of charger along a typical distribution network was considered. This allows a study closer to reality, but not taking into account other random parameters such as the initial state of charge of batteries and the initial time of charging.

In 1998, Staats et al. presented a statistical method in [16] to predict the effect of EV batter charging on the harmonic voltage levels. Load modeling as sources of harmonic currents and the distribution network modeling for each hour of interest and each harmonic frequency are proposed. The method was validated through a case study on a specific distribution network and results were presented. Summarizing, the method allows for obtaining the Total Harmonic Distortion in Voltage (THDV) in the system nodes, caused by the injection of harmonic currents, allowing evaluating if THDV limits set by standards are exceeded.

In 2011, J. Trovo et al. described the power quality characterization of battery chargers for two EVs in [17]: a forklift and a car for private use. Voltage, current and active and reactive power were monitored during the battery charging, from which the Total Harmonic Distortion in Current (THDI) and the THDV were determined for each case. Finally, reconfiguration of the network is presented as a solution, and the concept of load leveling is introduced as an alternative to minimize the impact of EV chargers on distribution networks.

In 2013, Jiang, et al., developed a comprehensive probabilistic approach based on Monte Carlo simulation to evaluate the impact of EV battery chargers in the distribution network in [18], which considers randomness in parameters such as the charging time, charging duration and location of vehicles. Moreover, a simple forecast model of EV penetration based on consumer trends is presented to study future scenarios. Furthermore, the electric model of a typical distribution network, the harmonic model of an EV battery charger and other nonlinear load models were developed. The method was proven through some case studies, demonstrating the effect of harmonics produced by EVs, and the increasing of the neutral voltage caused by the connection of EVs.

B. Voltage imbalance The condition of imbalance occurs in a three-phase system

when voltages differ in magnitude and/or do not have 120 degrees of phase difference [14]. The number of EVs in residential distribution networks is expected to increase in the coming years, however, the level of penetration and connection points are uncertain. EV chargers are single-phase loads in residential networks; therefore they could cause voltage imbalance in the three-phase system. This impact is studied in papers such as [19], where Shahnia et al., present a sensitivity analysis of voltage imbalance and a stochastic evaluation to determine the location and levels of charging and discharging of EV batteries, this study found that voltage imbalance levels are higher at the end of the distribution feeder.

Another contribution to the study of voltage imbalance is [20], where Meyer et al., describe the EV battery charging behavior of five different chargers and present actual measurements of voltage imbalance in a distribution network. Moreover, a statistical analysis of the impact on voltage imbalance is also included.

In [21], Garca and Jimnez propose an algorithm to carry out power flow calculations, taking into account the connection of EVs in distribution networks. A model that simulates charging and discharging of EVs was developed. The

algorithm was proven in [21] through a case study applied to the IEEE 13-node system. Results of simulation allow analyzing the impact of EVs on voltage unbalance in the distribution network.

C. Other disturbances Connecting EVs cause other disturbances such as voltage

sags and noise, among others. In [22], voltage drops caused by small wind turbines and EVs are studied from a statistical model which considers randomness of loads, wind speed and the charge level of EV batteries. An important contribution of the methodology proposed in [22] is that it allows for swift analysis of the daily probability of voltage violations in a distribution network, thus providing effective solutions in regulation, by short-term and medium-term planning.

Voltage sags are short reductions of nominal voltage in 10% to 90%, with duration from half cycle to one minute [14]. In [23], Lee et al. carried out a study of sags and voltage unbalance caused by EVs in distribution networks. A model of battery charger for EVs in ATP was developed and simulations in a distribution network were performed, considering parameters such as penetration of EVs and the amount connected each hour a day. EVs are considered loads and charging points are concentrated, therefore, the connection of several EVs can cause voltage sags in distribution feeders. This is assessed through a case study in [23].

In [24], simple models are proposed to study emissions of electromagnetic noise produced by traction of EVs and noise coupling channels are determined. The models are validated through laboratory tests. It is concluded that the main cause of noise in EVs is the high frequency switching of power electronic devices in the electrical traction system.

D. Solutions and standards By now, some solutions to the problems that cause the

connection of EVs on power quality have been proposed, such as [25], where a method to mitigate harmonics in a smart grid context was proposed, and optimal dispatch of tap changers considering shunt capacitor installing was carried out. Additional achievements are presented in [26], where the design of an EV battery charger with power quality compensation is described. This charger is designed based on static converters (IGBTs) and controlled with Pulse-width Modulation (PWM), allowing compensation of reactive power and active unbalanced currents.

The integration of EVs and its charging infrastructure require considering operational and safety aspects to ensure quality of services related to EVs and other customers [27]. Hence, it is essential to establish a regulatory framework to bring the guidelines to implement these new technologies, so that the power quality parameters remain within limits. In [28], adaptation of emerging concepts of microgrids and EVs to power quality standards is investigated, where it is concluded that regulations related to EVs should be considered from the EMC viewpoint, since EVs are both receptors and source of disturbances. High penetration of EVs scenarios require special consideration to establish the regulatory framework, because the current power quality limits for power distribution networks will probably be violated.

V. ELECTRIC VEHICLE AND TARIFF With the imminent massive penetration of EVs to the

network, some aspects should be studied carefully, such as the economic impact model development and methodologies for pricing implementation, considering the technological potential of smart grids. In the literature reviewed for this article, the highlighted thematic areas are: load profile smoothing and energy resource optimization. The massive connection of EVs to the power system is primarily a load increase; this increase may contribute to reach the peak of power demand if the control load techniques are not considered. Summarizing, technical and economic impacts in the network depend on the adopted strategies for charging or discharging EV batteries.

In [29], the possibility of including residential customers in programs of power demand response is studied. Such programs may help to reduce the negative impacts of large scale EVs connection. Financial incentives to the EV owner are proposed based on existing rates of time of use (TOU) in the service territory of Madison Gas & Electric. Discounts and additional incentives were also used to encourage participation of owners of EVs. The parameter used to determine the effectiveness of the various discounts and incentives was the ratio of vehicle purchase premium to fuel cost savings over ten years of vehicle life. At each considered case in this analysis, the initial savings were more effective than savings due to time of energy use; the latter was insufficient to convince the owners of EVs to sacrifice comfort for financial savings.

Because of the potential economic impacts of EVs, the consumers must take decisions based on a cost-benefit analysis. In this context, [30] shows that with increasing load due to EV, the mean and standard deviation of Locational Marginal Prices (LMP) would increase substantially and, therefore, EV charging would be much more expensive than nowadays, considering a short-term horizon. Furthermore, the impact of charging EVs at home and charging with a battery supplier is compared. The battery supplier is not only able to charge batteries at a much lower price than the cost incurred by charging at home, but also to stop increasing the mean of LMP and to reduce the standard deviation of LMP, avoiding congestion effects of the electricity transmission.

As expressed in [31], through an on-line energy management (EM), costs for using different energy sources can be minimized. This approach is applicable to a dynamic energy market. The EM system minimizes fuel equivalent of EVs considering the condition of plug-in, and run an optimization algorithm on-line, using knowledge of energy prices taken from the network by EVs, and price of electrical energy stored in its battery. The greater the differences in prices of the electricity grid in time intervals of a future horizon, the greater the benefits that can be achieved. Hence, if it is seen that penetration of renewable energies increased, the value of the available energy is an attractive economic potential.

The network constraints are a problem that must be faced in a massive penetration of EVs scenario. Constraints can limit the increasing of EVs penetration, if additional measures are not considered. In this context, two different approaches are implemented to address this problem in [31]: 1) a simple double-rate approach, where economic incentives are provided

to owners of EVs to change their vehicles charging for off-peak hours; 2) adopting an approach of active load management, where battery power is distributed throughout peak hours. The policy of double tariffs proved to be more effective, improving the ability to integrate the network up to 14%. However, this result was obtained by using the policy framework of double current rate of Portugal. As a conclusion of [31], it is likely that the policy framework of double current rate can be improved if a double dynamic tariff is created and dedicated to EVs. This methodology is also able to monitor the operating conditions of the network at any time, allowing more efficient use of available resources.

Given the scenario of massive penetration of EVs, the dispatch center requires demand management policies for these loads. The state of charge and the charging cost should be optimized, maintaining them at a safe limit to avoid overloading. Management of charging tariff requires data acquired from a center, followed up by the processing step; however, when the problem scale is greater, the central focus may suffer individual node/link and scalability failures. For avoiding this, an approach for cooperative distributed charging of EVs using local prices as control signal is presented in [33]. The charging stations work as local energy distributors, selling power to the connected vehicle, while the price is coordinated with their neighbors, in response to the offered price, the smart vehicle charger adjusts the charging current to maximize user utility. This process is repeated until the convergence to the global optimum is obtained. Thus, it is possible to remove the control/coordination center, and to avoid link/single node failures.

VI. CONCLUSIONS E-mobility in Latin America is a timely, vital topic to be

considered because of the several possibilities for sustainable development. However, in Latin America, research in this field is quite limited when compared to the intensive work developed in industrialized countries. In effect, the e-mobility research subject is a potent fertile field for exploration.

In particular, three main subjects related to the power systems, which will be impacted by the massive penetration of EVs, have been reviewed in this paper, i.e., planning, power quality and tariffs. Regarding planning, the current trend of the consulted references, in relation to the expansion, shows that major impacts will affect the distribution networks. However, some studies have also shown that impacts could even be transferred to the transmission and generation levels, especially when congestion of overhead lines occurs. Regarding power quality, harmonics and voltage imbalance are the most studied topics. Other aspects have been studied, such as voltage drops and increased losses. Most of these studies show that the impact on power quality will be significant for future scenarios, only considering a high penetration of EVs, mainly affecting the distribution network. Finally, regarding tariffs, the trend topics are: load profile smoothing and energy resources optimization. In fact, the load increase due to EVs may contribute to reach the peak of power demand if control techniques are not considered. Summarizing, technical and

economic impacts in the network depend on the adopted strategies for charging or discharging EV batteries.

ACKNOWLEDGMENT This work was supported by the Agencia Nacional de

Promocin Cientfica y Tecnolgica (ANPCyT) and the Consejo Nacional de Investigaciones Cientficas y Tcnicas (CONICET), Repblica Argentina.

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