Novel aggregated controller of wind and PV based grid connected charging station for electric vehicle

ABSTRACT


INTRODUCTION
Electric vehicles (EVs) are becoming popular day by day and attracting many people throughout the world due to its benefits both economically as well as reducing pollution.Due to increasing the number of EVs on the road every day, establishment of many charging stations is required.The charging process of the EV through the charging station is a very important aspect while it is connected to the utility grid.Because of the existence of power converters in charging stations/electric vehicles, many harmonics will be injected into the utility grid which results causes' poor power quality at load buses where other loads are connected [1]- [3].Therefore, a proper controller is required in the charging station to reduce this effect.Further, there will be a peak load every day on the utility grid.During this peak load, the generation of all units will be suffered and this leads to increasing the cost of electricity production.Therefore, energy storage devices such as batteries which lead to high cost and maintenance should be maintained in some key places.On the other hand, many people are using EVs for going to offices, institutions, and industries, where they can keep charging for a long time till they return [1].During this time period, sufficient time will be available to get charging and discharging of batteries, hence the utility grid can take some power from batteries in EVs during the peak load demand.This technology can satisfy local load demand at load bus then again batteries can recharge during low demand.Then there will be no effect or problem for operators of EVs since batteries can get full recharge.Hence this kind of system can provide quality and reliable power to load buses as well as reduce peak load demand on the utility grid with very less cost [1], [3].
The charging stations receive power from the utility grid when it is required.However, there may be an isolated mode where the charging station is disconnected from the utility grid for a number of reasons including daily power cuts, and faults.during this situation, the EVs connected in charging stations cannot get charging.To avoid this problem and to make the system more eco-friendly, locally placed hybrid photovoltaic (PV)wind based power generation systems are integrated to charging stations [4]- [6].Actually, this configuration will be more effective since sufficient space is available for installation of PV-wind based electricity generation systems.However, due to absence of internal batteries in the charging station, the loading must be performed during islanded mode of operation.During a normal scenario, the converter interfaced between grid and charging station must maintain power balance among PV, wind, batteries in EVs and utility grid through proper energy management system [7], [8].An aggregated controller is designed in this paper to fulfill all the above tasks and the objectives of this research work are listed below: i) Integrate PV-wind based power generation system to establish a charging station; ii) Operate loading mode during islanding mode; iii) Design an aggregated controller for managing power bidirectional power flow between charging stationbatteries-utility grid; iv) Design a hardware-in the loop (HIL) for analyzing the performance of the system.

SYSTEM DESCRIPTION
The layout of the utility grid connected two charging stations is shown in Figure 1.The system consists of two charging stations where one is having integration of renewable energy sources and other is a simple charging station without integration of any alternative sources.There will be an option to transfer power from batteries to the utility grid.Hence different directions of power flows are depicted in Figure 1.When selecting the bidirectional flow (where EVs are connected in charging stations for a long time), the batteries used in EVs can able to supply power to the utility grid during peak load and again charge the batteries during light load conditions.In the same manner, the hybrid PV and wind generation will be used for charging the batteries which are connected in charging stations, further the excess power will be fed to the utility grid.Each renewable energy source is having its own maximum power point tracking devices for their best utilization.Each charging station is employed with its own proposed aggregated controller to manage the direction of power flow between grid and charging station depending on requirement.Usually, a bidirectional converter is employed in charging stations (1 and 2).Hence the converter controller should work for reducing harmonics injected to the utility grid like an active filter [2].

ENERGY MANAGEMENT SYSTEMS AT CHARGING STATIONS
Every charging station has its own energy management system [9], however the bidirectional power flow-based energy management system can deliver the best performance where vehicles are keeping charging for more hours [1].Charging stations employed with wind-PV systems can provide reliable supply to vehicles during all conditions [9], [10].These kinds of charging stations are commonly established in office premises, institutions, and universities, where employees can keep charging for a long time.The batteries used in electric vehicles can be used to produce power for maintaining peak load demand on grid when it is required and again it can be charged due to availability of sufficient time.Hence, bidirectional power flow of battery controller as well as converter controller of charging station should be developed to maintain aggregated control.Effective controllers of both battery converter and converter in charging stations are designed to maintain an aggregated energy management system for charging stations in this paper.Hence, two controllers are implemented in this The controller depicted in Figure 2 is able to minimize the nonlinear load effect on the utility grid due to the AC to DC converter which is used in charging stations to charge electric vehicles.Once achieved stable DC link voltage, the proper converter is required to make charging and discharging of the battery which is used to drive EV(s).The bidirectional DC to DC converter is proposed in this paper along with its controller which is shown in Figures 3(a) and 3(b) respectively.The "2ω" oscillations will be presented in DC-Link voltage due to nonlinear load on the grid.The "2ω" oscillations can be tracked by subtracting DC link voltage with pure DC voltage by low pass filter (LPF).These oscillations need to be circulated in DC-to-DC converter, and then the effect of nonlinear load can be minimized on the grid.This is achieved by comparing the "2ω" component to its reference value 'zero' and the error signal further given to the PI controller for generating equivalent oscillations which can allow DC to DC bidirectional converter as shown in Figure 3.The reference current of the battery-bank is obtained through a PI controller to decide the direction of power flow.Required switching pulses are generated by comparing reference and actual battery current.In order to consider the life of the battery, the SoC of the battery is incorporated with a controller of DC-to-DC converter to prevent it from over charging and discharging situations.Switches Q1 and Q2 are used in the converter for managing charging and discharging respectively.Hence the converter can allow power flow in a bidirectional way by controlling dclink voltage at its reference value.The aggregated energy management system will decide the direction of power flow and can set the voltage reference value for different EVs which can be connected for charging in the same charging station.

CONTROL OF MOTOR IN ELECTRICAL VEHICLE
Generally multiple vehicles will be charged from a charging station and there is a chance of using different kinds of motor drives in vehicles [11].Some vehicles are operating with induction motors, and some are with BLDC motors.However, the most popular drive is the BLDC motor used recently [12], [13].Hence, the controller to drive the electric vehicle is considered in this paper is BLDC motor.For an effective operation, a MRAC based sliding mode controller of the BLDC motor is implemented [14]- [17] for testing the performance of the drive.However, while charging, the vehicle will be in standalone mode and while running there is no chance of charging.The MRAC model for speed estimation is shown in Figure 4 for the basic and implemented model on BLDC motor [18]- [25].
similarly, back EMFs are   = −  and   = 0, therefore, the back EMFs of BLDC motor are proportional to the electric speed of the rotor because of the rotor is having permanent magnet and it is denoted by ( 6)- (9).
In steady state, Here, the back EMF's coefficient is denoted by 'ke'.The 'DQ' reference frame equations of BLDC motor can be expressed as: =     +     +   (10)   =     +     (11)   =     +     +   (12)   =     +     (13) The reference speed signal will be compared with motor actual speed which is estimated by MRAC model and error will be given to sliding mode control for generating reference dc-current signal as shown in Figure 5. Once obtained the required reference dc-current from the SMC, required switching pulses for the converter to drive BLDC motor can be easily generated by using control method as shown in Figure 6.

RESULTS AND DISCUSSIONS
The real time simulator (RTS) is able to deliver a real-time operation by simulating dynamic equations of the system to be fast enough to deliver the output similar to the real network.Due to the acceptance of the RTS process worldwide for testing and development of power system control schemes, the OPAL-RT technologies based RTS configuration has been considered in this paper to obtain the results.In order to implement HIL setup, researchers have used two RTS modules designed by OPAL-RT technologies connected back-to-back in a loop.The system is divided into two parts as plant and controller.The unit-1 consists of the complete plant model and another unit is used for dumping the controllers with the help of MATLAB.Both the units are facilitated with analog and digital cards for their interconnection to form a loop.The analog signals are going from plant to control and digital signals are coming from control to plant.The MATLAB installed laptop is used to get extensive results for presenting in this section.The model setup of HIL with the help of two OPAL RT modules is shown in Figure 7.The proposed model shown in Figure 1 is divided into plant and controller.Further the results are presented in the following situations.The proposed model is now tested for changing the number of EVs connected at charging-stations.Assumes many numbers of electric vehicles are connected suddenly at t=2.0 sec as shown in Figure 9(a).During this operation, the SoC of all the vehicles (represented in average) will be suddenly increasing as it is depicted in Figure 9(b).However, there is not much change or effect on DC-link voltage which is quickly stabilized as shown in Figure 9(c) with the help of proposed controllers.The controller of the DC-to-DC converter is working effectively and plays a key role in stabilizing the dc-link at its reference value.The electric vehicle is tested now under running condition to enhance the performance of BLDC motor drive.The reference speed of the motor is reduced from 1800 to 700 rpm at t = 3 sec.However, to achieve realistic performance, the change is considered with slow dynamics not as a sudden step, because of sudden step change cannot be possible in the running condition of the electric vehicle.The MRAC of the motor can also estimate the speed of the BLDC motor.However, the controllers are having their own delay in response time and respective response(s) on the motor speed is depicted in Figure 10.Due to faster tracking response by SMC than PI, the response with the SMC is having the significant priority to use in electric vehicles.The system is tested under grid off mode (isolated mode).During no power from the grid, ordinary charging stations (charging station-1 in Figure 1) will fail to provide charging.However, the proposed system (charging station-1 in Figure 1) has the facility of integration of renewable energy sources.The changes in total power generated by PV plus wind and load are considered as per the figure shown in Figure 11

CONCLUSION
The aggregated energy management system with novel control configuration of charging stations is presented in this paper.Sliding mode controller-based BLDC motor drive is included for testing the electric vehicle during running operation.The renewable energy sources are integrated to the system for power generation during grid off mode.The hybrid system can provide reliable power during all the conditions.Extensive HIL based results are examined in this paper to validate the proposed method.From the results, it is concluded that the performance of the proposed system is quite satisfactory under various conditions.

Figure 1 .
Figure 1.Layout of utility grid connected charging stations with aggregated controllers Int J Pow Elec & Dri Syst ISSN: 2088-8694  Novel aggregated controller of wind and pv based grid connected charging station … (Karanam Deepak) 2321 paper as shown in Figure 2 is for the controller used in the charging station and controller showing Figure 3 is for controller used in vehicles to manage charging/discharging of the battery.Figure 2 is implemented for AC to DC converter to manage flow of active power from both the directions (i.e., vehicle to grid and grid to vehicle) based on requirement of power flow.It can be possible by comparing the direct axis component of voltage with per unit i.e., '1'.At the same time the controller can help to compensating for reactive power demanded by local loads and also mitigate the nonlinear effect on the grid.Hence the reactive component of voltage (i.e., vq) is compared with zero reference signals.The error is given to proportional and integral controllers (PI controllers) to generate reference signals of currents.The required PWM signals are generated by a PWM generator as shown in Figure 2. The Vabc represents grid voltages (Vg) & Iabc refers to grid currents required by the charging station.

Figure 2 .Figure 3 .
Figure 2. AC to DC converter controller used in charging station

Figure 9 .
Figure 9. Response of (a) power, (b) SoC, and (c) voltage at DC-link in charging station

Figure 11 .
Figure 11.Proposed system performance in isolated mode: (a) various powers, (b) voltage at dc-link, and (c) 3-phase RMS voltages