Modeling and Control of Contactless based Smart Charging Station in V2G Scenario
3.3 Modeling of Smart Charging Station
3.3.2 Synchronization Mechanism
When EVs arrive at the CS, the BCCS unit will get connected with the DN and when it leaves the CS, the BCCS unit will get disconnected. However, in the cases of charging/discharging operation of EVs, the initial synchronization process with the grid is necessary without which the real power transfer in any direction cannot be controlled. The importance of synchronization of any power converters connected to the grid is explained in [163] and synchronization of BCCS unit has been described in detail in references [96]. The synchronization between the DN and the BCCS unit can be described by a simple system consisting of two interconnected sources as shown in Fig. 3.6.
The Vprimrepresents the transformer terminal voltage of BCCS unit, RT is the total resistance and XT represents total reactance of the system which includes transformer reactance, grid impedance and a third order LCL filter. The LCL filter is designed based on the standard level determined by
Bidirectional power flow
Vnode Vprim
XT RT
Figure 3.6: Synchronization mechanism between DN and BCCS unit.
IEEE519 for harmonic limits, which considers the level of current harmonics injected into the grid network [164]. The power transferred between the terminals of BCCS unit and DN is given in Eq.
(3.2).
Pbn=VnodeVprim
XT
sin (δ) (3.2)
Here,δdenotes power angle between Vnode and Vprim. This power is equal to the distributed power from the CS aggregator. The real power control between EV batteries and the DN is regulated by controlling theδ. The value ofδcan be derived from Eq. (3.2).
δ=sin−1 PbnXT
VnodeVprim
!
(3.3)
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08
−1
−0.8
−0.6
−0.4
−0.2 0 0.2 0.4 0.6 0.8 1
Time (Sec)
V node and V prim
Vprim
CB Inphase
voltage
Vprim leads Vnode at an angle δ Floating condition
Vnode
Figure 3.7: Synchronization waveform.
For the discussion of power flow, voltage waveform for Vnode and Vprim is shown in Fig. 3.7.
For example, let us consider that the Root-Mean-Square (RMS) value of the Vnode and Vprimis 440V, the Pb1 is 9.8077kW and reactance (XT) of the system is given in Table 3.8. Therefore,δ = −8.43◦
required to transfer the power (Pb1) from BCCS unit to DN. In Appendix D, the detail calculation of power angle has been mentioned.
The BCCS unit and the DN will be in floating condition till the system gets synchronized with the DN. When the system gets synchronized with the DN, the circuit breaker (CB) closes and the angleδ determines the power flow between the BCCS unit and the DN.
To achieve this synchronization operation and to control the real power, load/power angle control have been used in Fig. 3.8.
conversion p.u.
SPWM wt
converter LCL filter
Transformer
Distribution Node
Circuit Breaker
L C
L
P Freq
sin_cos sin_cos
3 phase PLL Freq
Power estimator
wt wt
dq0 to abc transformation 3 phase PLL
abc to dq0 transformation Freq
sin_cos sin_cos
Control signal Synchronization
condition check Synchronization
parameters 1 or 0
LA − FLC
Carrier signal
Gate Pulse
Vnode
Pmea Vprim Iprim Vprim
Vdq0
Vabc δ
Vdq0
Vnode
Vnode
δ Pbn
Er
δ
Figure 3.8: Load angle control.
The load angle control has used enhanced fuzzy based phase locked loop (PLL) based synchro- nization technique, which takes DN voltage as reference and estimates the frequency and phase se- quence components of the grid to synchronize the BCCS unit with the DN. Besides, the converter voltage (Vprim) and current (Iprim) of BCCS unit is given as input to the power estimator block and Pmeas is obtained. Based on Pmeasand Pbnfrom aggregator, the Load Angle FLC (LA-FLC) generates the required δ as shown in Fig. 3.8. The amount and direction of real power transfer takes place, when the voltage produced by the BCCS unit leads or lags the voltage of the DN. A leading phase of BCCS voltage constitutes power transfer from V2G and lagging phase enables power transfer from
G2V. The brief overview of the load angle controller is given below:
• The three-phase phase locked loop (PLL) estimate the unit vectors (sinθand cosθ), frequency and angular time of the BCCS unit and grid.
• The measured power has been calculated from the power estimator unit based on the BCCS unit three-phase voltage and current, unit vectors and frequency of the system.
• The fuzzy based load angle controller decide the power angle based on the Pmea and the Pbn which is obtained from the CS aggregtor.
• The park transformation has been used to convert the three-phase (abc) to two phase (dq0) transformation.
• The required power angle has been added with the two phase quantities and then it converted into three-phase transformation.
• The three-phase voltage is the reference signal for sine pulse width modulation (SPWM).
Fig. 3.8 mainly consists of discrete three-phase PLL units, which extract the synchronization pa- rameters (frequency and phase sequence) of the node and converter voltage, three-phase to two phase transformation and vice versa, FLC based load angle controller (LAC-FLC) and power estimator.
Three-phases Vnode is the input for abc−dq0 transformation unit and discrete three-phases PLL. The abc−dq0 transformation unit converts the three-phase (abc) stationary components of the Vnode to two phase (dq0) reference frame [99]. The three-phase DN voltage can be represented by Eq. (3.4).
Vnode = Vabc =Vm
sin (ωt) sin
ωt− 2π3 sin
ωt+ 2π3
T
(3.4) where Vm is the peak voltage. The three-phase to two phase transformation is given in Eq. (3.5) [165].
Vd Vq V0
= 2 3
cosωt cos
ωt− 2π3 cos
ωt+ 2π3 sin (ωt) sin
ωt− 2π3 sin
ωt+ 2π3
1 2
1 2
1 2
Va Vb Vc
(3.5)
The three-phase discrete PLL extracts the frequency and the unit vector components of Vnode and feeds to abc−dq0 transformation unit and dq0−abc transformation unit (phase shift (30o) between primary and secondary side voltage of the transformer also considered). The frequency, unit vector components (sin cos) and converter measured voltage (Vconv) and current (Iconv) are input for power estimator which calculate the actual bidirectional power flow between BCCS unit to DN. The LAC- FLC decide the load angle (δ) between DN and converter voltage. The range of theδ for LAC-FLC has chosen based on the impedance of the BCCS unit and DN. Theδcan be either positive or negative, if the positive values of theδrepresents the Vconvlagging with respect to Vgridand negative value of the δdenotes the Vconv leading with respect to Vgrid. The inverse transformation Vdq0−Vabc is performed to generate a reference voltages (Vabc∗ ) delayed with δ, which is given in Eq. (3.6) [165]. The Vabc∗ compared with the carrier signal to generate the gate pulse for the converter.
Va Vb Vc
= 2 3
cos
ωt+ π6 ±δ
sin
ωt+ π6 ±δ 1 cos
ωt− π2 ±δ
sin
ωt− π2 ±δ 1 cos
ωt+ 5π6 ±δ sin
ωt+ 5π6 ±δ 1
Vd Vq V0
(3.6) Further the SPWM generates switching pulses for the three-phase convertor unit considering the regenerated sinusoidal wave Vabc∗ as a reference signal. The converter work as a inverter for V2G and rectifier for G2V operating mode, respectively. The operation of the synchronization controller can be explained by considering the switching operation of CB.
NB NS Z PS PB
−1 −0.5 0 0.5 1
0 1
(a)
−90 −45 0 45 90
NL Z PL
NH PH
(b) 1
0
Er δ
Figure 3.9: Fuzzy membership function for LA-FLC control (a) input: Er(b) output: δ.
The input and output membership function of LA-FLC is given in Fig. 3.9. Table 3.3 shows the rule base for LA-FLC. The control parameters used in the BCCS unit are shown in Table 3.4.
Table 3.3: Rule base for LA-FLC.
Er NB NS Z PS PB
δ VS S M B VB
For example, let us suppose that error is -0.198 p.u. This means, the measured power (Pmea) is
grater than the reference power (Pbn). The load angle controller will decides the required power angle to transfer the power between BCCS unit and gird. The LA-FLC decide the power angle is 10.25◦but requiredδto transfer the power is 8.43◦which is theoretically calculated from Eq. (3.3). This power angle information will be updated every instant of time and the LA-FLC minimize the error. If the error is minimum, then the Pmeafollow the Pbn which is distributed from the CS aggregator.