Modeling and Control of Contactless based Smart Charging Station in V2G Scenario
3.2 Modeling of Multi-Point Smart Charging Stations
The EVs are consuming comparatively high power from the grid during charging. Therefore, uncoordinated charging of a large number of EVs gives an adverse impact in the grid [26,32]. One best possible solution can be to ramp-up the power generation but this will lead to significant infrastructure cost of the power plants. As an alternative cost-effective solution, grid allows EVs to coordinate their charging or discharging operations. This coordination amongst EVs facilitates avoiding grid overload and balancing supply/demand potential of the grid. Therefore, in this Chapter multi-point SCS has been modeled for coordinating multiple EVs present in the CS.
3.2.1 Distribution Network Model
Sub feeder 5 11kV/440V
6.3 6.2 Subsystem 6.1
Main Feeder
33 kV
5.3 5.2
5.1 5.4
Sub feeder 4 11kV/440V
11kV/440V
4.4 4.3 4.2 4.1
Sub feeder 3 3.5 3.4 3.3 3.2 3.1 11kV/440V
Sub feeder 2
Sub feeder 1 11kV/440V
2.2
2.1 2.3 2.4
SCS
33/11kV
Figure 3.2: Radial distribution system of Guwahati city.
The practical grid data of typical network of Guwahati city has been taken in this work, which is shown in Fig. 3.2 [74]. This network is a reduced system of substation which consists of 33/11kV, 5MVA as the main feeder and the entire radial sub feeders have 11kV/440V, 500kVA transformers.
The off-peak hour load was assumed to be 60% of peak hour load. The resistance and reactance of the lines are 0.0027p.u and 0.0024p.u, respectively. The location of SCS considered for test condition
is connected in the distribution network at the node 5.3, as shown in Fig. 3.2. Table 3.1 shows the existing peak hour load of substation at different nodes.
Table 3.1: Existing load profile of the substation.
Nodes P (p.u.) Q (p.u.) Nodes P (p.u.) Q (p.u.)
2.1 0.50 0.22 4.2 0.63 0.38
2.2 0.47 0.23 4.3 0.67 0.23
2.3 1.13 0.64 4.4 0.53 0.37
2.4 0.27 0.15 5.1 0.45 0.39
3.1 0.42 0.29 5.2 0.23 0.13
3.2 0.94 0.43 5.3 0.84 0.46
3.3 0.13 0.09 5.4 - -
3.4 - - 6.1 0.37 0.18
3.5 0.25 0.17 6.2 0.23 0.13
4.1 0.23 0.13 6.3 0.73 0.45
3.2.2 Multi-Point Smart Charging Station(SCS)
The layout of contactless based multi-point SCS is shown in Fig. 3.3. The proposed SCS has multi-charging points, where EVs’ batteries of different ratings are connected. The individual charg- ing points are contactless, bidirectional and it is connected to the DN of the grid via ac bus. Hence, it is called as BCCS. The complete architecture of SCS is divided into three units: the central control unit, the CS aggregator and the multi-point BCCS unit.
Energy calculation distribution
Power Central
Control Unit (CCU)
Primary Side (PS)
Primary Side (PS)
. . .
EV1
EVn
.
ac bus
Node (DN) Distribution
. . . .
BCCS unit n BCCS unit 1
.
Primary control Secondary control Contactless point
(SS) Secondary Side
Primary control Secondary control Contactless point
(SS) Secondary Side
CS Aggregator D
Pbn Pb1
Ebn Eb1
Pgrid Vnode
ET
ET Eb1 Ebn
Figure 3.3: Layout of multi-point smart charging station.
The CCU decides the net power flow between the DN of the grid and the CS. The CS aggregator distributes the net power among the EVs’ batteries. The CCU takes decisions of power flow (Pgrid)
based on three inputs: DN Voltage (Vnode) of the grid, total energy availability (ET) of SCS and duration in hours (D) to support the grid. The direction of Pgrid can be either positive or negative.
Positive power implies the CS will charge the EVs’ batteries and negative power implies the CS has to support the grid. The Pgrid obtained from CCU has to be distributed among the EVs by the CS aggregator. The CS aggregator takes individual energy available (Eb1, Eb2,...Ebn) from each EVs’
batteries, ET of the SCS as well as Pgrid and distributes the power among individual BCCS unit (Pb1, Pb2...Pbn). The distributed power can be either positive or negative depending upon the energy state of the batteries and Pgridas decided by CCU. The positive power allocated to the battery will get charged and the batteries with negative power will get discharged. The BCCS unit has two parts: the primary side and the secondary side (pick-up), which is separated by an air-gap and is magnetically coupled to each other. The power is transferred from the primary to the pick-up through weak magnetic coupling.
Primary and secondary controllers are employed on either side of contactless coil, to control the power flow from G2V and V2G. Based on the information of power received from the CS aggregator, the controllers of the BCCS unit takes necessary action to control the Crateof the battery. By this complete SCS arrangement, an EVs’ batteries will be able to charge by absorbing power from the DN of the grid and supply the excess stored energy of EVs into the DN of the grid. The controllers present at the multi-point BCCS unit controls the power flow between individual EVs’ batteries and DN of the grid.
3.2.3 EV Battery Model
Electric equivalent circuit (EEC) based battery model is used in this work for representing the real-time EVs’ batteries, which is given in Fig. 2.3. The electrical parameters of EEC are repre- sented by polynomial equation explained in Chapter 2 Section 2.2. The terminal voltage for charg- ing/discharging scenario is given in Eq. (2.5) and Eq. (2.6). The EVs’ batteries come to the SCS for both charging/ discharging operation. The batteries with excess energy (Eavail) would discharge and support the grid and EVs with less energy would charge (Estor) by taking energy from the grid. The Estor and Eavailare calculated from Eq. (2.13) and Eq. (2.14). The total energy (ET) of SCS depends on the sum of stored energy into the battery and available energy to support the grid which is given in
Eq. (2.17).