Contents
1.1. EVs role in the smart grid technology
EV charging systems are of two types. One is the unidirectional charging system (grid to vehicle) and the other is bidirectional charging system (vehicle to grid). Energy flows from grid to vehicle in the unidirectional charging system. In this type of charging system, EVs utilize the total energy for transportation purpose so that EVs perform only the valley filling of the grid load profile. In the bidirectional charging system, energy flows from grid to vehicle as well as from vehicle to grid. In this type of charging system, EVs utilize energy for transportation purpose as well as to support the grid during peak period. Therefore, EVs perform both the valley filling and peak shaving of the grid load profile. Also, EVs charge from the grid with low tariff in off-peak period and discharge to the grid with high tariff during peak period. A fleet of EVs utilization at the distribution grid level provides the economic benefit to both, the grid and EV's owner. Bidirectional charging system suffices the grid as well as EVs.
1. 1.1 Grid to Vehicle
The plug-in EVs have gained popularity due to their clean performance in urban regions.
These vehicles charge instantaneously when they are connected to an ordinary plug/standard outlet at home/at other locations. This type of EVs charging is known as the uncoordinated charging system and it may cause the voltage fluctuations in the grid. Therefore, Kristen et al [9] proposed the coordinated charging of EVs for improving power loss and grid load factor.
They computed an optimal charging profile for EVs. The rise of plug-in EVs charging may lead to the potential stress, performance degradation and overloads to the distribution grid.
Deilmai et al [10] proposed the load management solution for the coordinated charging of multiple plug-in EVs in the smart grid system. Also, they developed real time integration of plug-in EV charging system to reduce the total generating cost plus the losses associated with the grid. Kejun et al [11] proposed the modeling of the load demand in distribution system with EV charging. They also observed the different types of uncontrolled charging of EVs and their effects in different situations. In [12], control of EV charging loads with respect to time of use price has been proposed. This work has focused on the optimized charging pattern of EVs that benefit cost reduction and flattering the load curve [12]. In [13], the unidirectional vehicle to grid regulation with the help of an aggregator has been proposed.
They have tested the optimal algorithm on ten thousand EVs, while reducing system load impacts and consumer costs [13].
Medhi et al [14] proposed the electric service station, which is similar to that of oil-filling station for travelling longer distances with EVs. Here, the fast charging of batteries is needed over small period of time. This work investigates the effect of fast charging EVs on distribution grid. Progress in the research of unidirectional charging system can be observed from the above literature and some other discussions are given in [15]-[21]. Both, the charging and discharging performance of EVs should be required to reduce the stress on the grid during peak period.
1. 1.2 Vehicle to Grid
The interaction of EVs with the grid for maximum time period creates the possibility to manage the peak load power demand. The charging/discharging of EVs from/to the grid in off-peak/peak period flattens the load profile of the grid. The increased EVs and their random arrivals create difficulty in the scheduled charging. Optimal scheduling schemes are needed to overcome such difficulty. Therefore, Yifeng et al [22] proposed a global and local optimal scheduling scheme to charge and discharge the EVs. They optimized charging power to reduce the total cost of all EVs. This work is useful for the dynamic arrivals of EVs and scalable to large EV population. Yuchao et al [23] described a model of storage system using EVs and coordinated with a power system. They decided battery energy deployment based on the time, vehicle charging requirements and electricity prices.
The discussion about vehicle to grid support, regulation of active power, load balancing and filtering the current harmonics are given in [24]. Also, they proposed three elements for successful vehicle to grid operation such as: power connection to the grid, intelligent metering and the communication between the grid and vehicles. Success of vehicle to grid concept relies on the battery technology and smart scheduling schemes, standardization of requirements and infrastructure decisions [25]. In [26], a review of the energy storage for transport and grid applications has been presented. This work focuses on storage and the power converters used in storage technologies for peak shaving. In [27], the vehicle fleet evaluation and vehicle energy demand simulations combined with the transportation simulation has been proposed. They determined the daily behavior of EVs with battery
Mukesh et al [28] proposed various possibilities and their impacts for implementing vehicle to grid technology in Indian scenario. They proposed the grid coordinated EVs for improving the voltage profile and to minimize the power transmission losses [29]. Also, they modeled the EVs charging station that can fulfill different demands of EVs owner such as: state of charge (SOC)/ charging rate (c-rate)/ power management of battery [30]. A multi charging station which can handle a large fleet of EVs without affecting the grid performance has been proposed in [31]-[32]. In [33], the controlled charging and discharging performance of a fleet of EVs depending on their battery energy status and grid condition has been proposed.
Utility Grid
P+
P- Charging station
Controller Aggregator Grid status
EVs net energy status P±
individual EV energy status
n± P Standard outlet
Converter unit-1
Converter unit-2
EV1 EV2 EVn
Inductive charging system
P +n
P -n
Power flow Control signal
Fig. 1.3 Bidirectional power flow between the grid and EVs
Fig. 1.3 shows the conceptual representation of the bidirectional power flow between the grid and EVs. A fleet of EVs (EV1, EV2, --- EVn) have been connected to the charging station to charge/discharge energy from/to the grid. Charging station is connected to the grid via converter unit-1. A controller is present at the charging station to control the energy between the grid and EVs. This controller works based on two inputs and one output. The inputs are:
the grid energy status and EVs net energy status. Output is P±(P+ is the power flow from the grid to EVs and P- is the power flow from EVs to the grid). Controller sends output signal to converter unit-1 as well as to the aggregator. The aggregator works based on two inputs and two outputs. Inputs are: P± and the individual EV energy status. One output is the net energy status of EVs (this signal goes to the controller as input) and the other is Pn±
(power to/from the individual EV) (this signal goes to the converter unit-2). A fleet of EVs have been connected to the standard outlet via converter unit-2. EVs receive power from the standard outlet via inductive charging system as shown in Fig. 1.3. Also, a controller should be required to manage the EVs in and out timings at the charging station. This controller should coordinate with the main controller.
Above mentioned literature represents a progressive research on the vehicle to grid concept. The grid integrated EVs shows the significant benefits from both, the environmental and economic aspects. The expensive price and the battery maintenance of EVs are the main hurdles for their growth. This creates less interest to the common citizen towards EV utilization. Also, a group of EVs occupy more space on the streets of the city to perform low mass transportation. The public transportation systems such as tram, trolleybus and the supercapacitor based buses etc… are the possible options to limit such difficulty. The public transportation system occupies less space on the streets of the city to perform maximum mass transportation. The passenger’s price sharing is the base for public transportation system.
Therefore, it creates the possibility for every citizen to take part in the clean energy based transportation system. Also, this system is economical to the common passenger to travel end to end of the city. The public transportation systems such as trolley bus systems and tram systems were operated in the middle of the 8th decade of 19th century [36]. These systems are the base for enhancement in public transportation system technology and also, brought a revolution in the transportation sector. The different types of public transportation systems and their technological growth are given in the following sub-section.