Modelling Of Electric Vehicle Charging Sys- tem

The Electric Vehicle (EV) or Plug-in Electric Vehicle (PEV) can be categorized into two types as Battery Electric Vehicle (BEV) and Hybrid Electric Vehicle (HEV).

The BEVs depends solely on their battery pack to meet all the power requirements.

The HEVs use combination of both conventional power and battery power, the HEVs are of two type, Plug-in Hybrid Electric Vehicle (PHEV) and Extended Range Electric Vehicle (EREV). The PHEV and EREV contains both conventional engine and a battery pack to meet the power requirement, the basic difference between the two is that the PHEV depends mainly on conventional combustion engine whereas the the EREVs first exhaust its battery pack then switches to the conventional fuel.

Of all these types, the BEVs has the highest electrical range for an electrical vehicle.

In this work, the electrical vehicle load is designed as per the specification of PEV.

Appropriate modelling of electric vehicle charger is necessary for accurate as- sessment of the effects of EV charging infrastructure on distribution system. Accord- ing to the Society of Automotive Engineers (SAE) Standard J1772 [111], there are three charging levels that provide the ability to charge the EVs faster by increasing the charging voltage, as shown in Table 3.2. Most of the electric vehicles can be plugged into an outlet at home for Level 1 charging (slow charging). Level 2 charging which requires 240 V outlets is typically described as the primary method for both private and public facilities. Future developments focus on fast Level 3 charging (AC or DC). Hence, considering these factors AC level 2 EV charging system is selected for work in this thesis.

The charger considered in this work is an AC single phase level 2, bidirectional charger. The charging system modeled contains an AC-DC converter and a DC- DC converter; both are capable of bi-directional operation. Fig. 3.3 shows the configuration of the charging system. The topology consists of six IGBT switches,

Table 3.2: Rating of EVs charger based on SAEJ1772 standard

Charging Level Nominal Supply Voltage Max. Current Input Power

1-phase AC Level 1 120 V 12 A 1.4 kW

16A 1.92 kW

1-phase AC Level 2 208 to 240 V

17 A 4 kW

32A 8 kW

80A 19.2 kW

3-phase AC Level 3 208 to 600 V 400 A >7.68 kW

DC Charging 600 V maximum 400 A <240 kW

four switches used in AC-DC converter stage and two in the DC-DC stage [112]. The charger is designed to deliver 6.6 kW power at 240 V to the battery for charging purpose. The following section gives detail about the battery charger.

Figure 3.3: Configuration of PEV battery charger

3.5.1 Bidirectional AC-DC Converter

During the charging of battery power is transferred from grid to vehicle as shown in Fig. 3.4, the direction of current from grid to converter is taken as positive.

Figure 3.4: Representations of Grid and Charger The sinusoidal grid voltage is taken as,

v_grid(t) =√

2V_gridsin(wt) (3.1)

Where, v_grid(t) is instantaneous grid voltage and V_grid is RMS (root mean square) value of grid voltage. The fundamental component of converter voltage is

v_conv(t) = √

2V_convsin(wt−δ) (3.2)

Where, v_conv(t) is instantaneous voltage of converter and V_conv is the RMS value, δ denotes angle between grid voltage and the converter voltage. The grid current is given as

i_grid(t) = √

2I_gridsin(wt−θ) (3.3)

Where, θ is the angle betweeni_grid(t) andv_conv(t)

Active power flows from the grid when v_conv(t) lags v_grid(t) , and it flows to the grid when v_grid(t) lags v_conv(t) . Phase angle of determines the direction of flow of reactive power. For a positive value of θ the reactive power flows from converter to grid and for negativeθ the reactive power flow from grid to converter. The converter is capable of meeting the requirement of bidirectional power flow. The fundamental value of the converter voltage is related to dc link voltage as

V_conv= mV_dc

√2 (3.4)

Here, mis the modulation index taken as 0.9 andV_dc is the voltage of dc link which is set at 380 V. The fundamental component of converter voltage is related to grid voltage by the relation given as

V_conv = q

V_grid² + (I_grid² ×X_l²) (3.5)

Here, X_l is the inductive reactance. The rating of capacitor in dc bus is given as,

C_dc= I_dc

2×w×V_dcripple (3.6)

Where,I_dcis the DC link current,wis the angular frequency andV_dcrippleis 5% ofV_dc. The output of AC-DC converter is the dc link voltageV_dc and it is the input to the DC-DC converter. For control of bidirectional AC-DC converter a unipolar switching scheme is used, in which a fixed frequency triangular carrier wave is compared with the positive reference signal and the negative reference signal. The output dc link voltage varies between ±V_dc. For controlling the dc link voltage a PI controller is used. This PI voltage controller compares the reference voltage to the sensed dc link voltage to calculate the error as the difference between the two. The PI voltage controller then generates the control signal to minimize the error so that the dc link voltage closely tracks the reference voltage. The output of this voltage PI controller is the reference current to the PI current controller, which closely tracks the reference current by minimizing the error between the sensed current the reference current.

The switching signals for triggering the switches of the AC–DC converter at different instants are generated by comparing the amplified control signal to a fixed frequency sinusoidal wave.

3.5.2 Bidirectional DC-DC Converter

The working of bidirectional DC-DC converter is divided into two modes. One is charging and other is discharging mode. Switch S₅ in Fig. 3.3 will be turned

on during charging mode and converter will work as a buck converter to charge the battery. During the discharging mode switchS₆ will be turned on and the converter will function as a boost converter. While boosting the DC-DC converter will boost up the initial voltage to 380 V. For designing the DC-DC converter the inductor value is calculated from the relationship between switching frequency f, inductance L_c given as:

f = 1

2×P ×L_c

1 1 V_dc + 1

V_b

(3.7)

Where, P is the power delivered from grid to battery, V_dc is DC link voltage and V_b is the battery voltage and f is the switching frequency and its value is 50 kHz.

PWM technique is applied for controlling the battery charging and discharging using bi-directional converter. A PI controller controls the input current (I_b) of battery.

The PI controller compares the dc link current to the reference current and error is minimized with the help of control signal generated by the controller. The switching signals for the IGBTs of the DC-DC converter is generated by comparing the con- troller’s output to a fixed frequency saw-tooth carrier waveform [113]. The above control strategy is applicable for both buck and boost mode of operation.

The battery used in the PEV load is modeled as lead-acid battery, imple- mented in SIMULINK using the model parameters given in [114]. The performance of charger is tested by simulation for 2 sec; charging is done for 1 sec and for an- other 1 sec discharging is done. Simulation results obtained are shown in Fig. 3.5.

Fig. 3.5(a) shows, that the voltage across the DC link bus is maintained at 380 V during both buck (charging) and boost (discharging) mode by the converter. The current delivered to the battery for charging and current provided by battery while discharging maintains a magnitude of 30 A as shown in Fig. 3.5(b), the change in polarity of current while giving energy back to grid shows that the supplied current is in phase opposition of the grid voltage. The battery voltage profile while operat- ing in both buck and boost mode are shown in Fig. 3.5(c). During the one second of buck mode of operation battery voltage level increases from 202.6 V to 207.9 V

Figure 3.5: Charging system performance under normal operating conditions and during boost mode it discharges back to 200.1 V. In all these profiles there is change in operating mode at 1 sec i.e. from charging of battery to discharging of battery. The simulated results validate the performance of charger which is used for the integration of PEVs to the grid.