in battery charger for idle vehicles. In the first case, the secondary coil moves along a primary coil and captures the horizontal flux component. In the second case, both primary and secondary windings are stationary and captures the vertical flux component [36,42]. Thus, CPT systems are used to transmit several kilowatts of power without any mechanical stress or aging. By this technology, trailing cables in the EVs can be removed and therefore cable breaks can be avoided. Other advantages are no metal oxidation of the electric plug, no wear and tear on the electrical contacts, no contact resistance, no sparking and no non-protected voltage-carrying contacts [41].
Grid
charging
circuit system
EV battery PCC
Figure 1.6: Block diagram of contactless charging system
A common feature of all applications for which CPT systems are used up to now is the relatively small air gap in a range of several millimeters between the primary and secondary system [31, 36, 41–
50]. Therefore, such magnetic assemblies can be accurately described by means of commonly known magnetic equations. However, in applications such as automatic battery charging station require a large air gap of several centimeters [31, 36, 42]. In order to achieve efficient power transfer through this large air gap between the windings of the contactless transformer, principle of resonance is used.
To increase the power transfer capability and to reduce the VA rating of the contactless system, capacitive compensation is used in both primary and secondary windings of the CPT systems [31,36].
The extension of contactless system to longer distance would open new application field for this technology.
1.4.1 Bidirectional charging systems
EVs are primarily considered as a method of clean transport but they can also be used as a potential source of energy by supplying power back to the grid. With bidirectional converters in the charging system, EVs’ batteries will be able to transfer power between the grid and the EVs [51]. This process is coined as Vehicle-to-grid (V2G) and Grid-to-vehicle (G2V) technology, where G2V implies charging
1.4 Contactless charging and electric vehicles
the EVs batteries from the grid and V2G means delivering the stored energy back to the grid [52]. The V2G and G2V services are provided through bidirectional charging systems with bidirectional power flow functionality [51, 53, 54]. This re-electrification process can be achieved through bidirectional converters i.e., the power converters are made of bidirectional switches to facilitate power transfer on both the directions. This makes EVs an ideal candidate to assist the power system operation. Figure 1.7 shows the basic schematics of bidirectional charging system connected to an EV battery.
ac−dc
dc−ac dc−ac
contactlesscoils ac−dc
Main controller
Driver board Driver board
− + +
−
Figure 1.7: Bidirectional contactless charging system
1.4.2 Vehicle-to-grid (V2G) and Grid-to-vehicle (G2V) technology
As explained earlier, EVs provide economic and environmental benefits; they can also be used as a potential source of energy storage which is valuable to the electric power grid [52, 55, 56]. With the additional bidirectional arrangement of converters in the charging system; EVs can charge its battery from the grid and discharge the stored energy back to the grid. Thus, EVs can become an ideal candidate to assist the power system operation such as voltage regulation, frequency regulation, power quality improvement, peak load shaving and other grid ancillary services by increasing the security and reliability of the power system [52]. By incorporating bidirectional switches in the converters of the contactless system this process can be achieved. Hence, EV’s batteries are able communicate the grid through coil coupling induction principle with high safety and low repair rate features [51,53,54]. The term G2V implies charging the EVs battery from the grid [52]. V2G describes a system in which EVs communicate with the power grid to sell demand response services by either delivering electricity into the grid or by throttling their charging rate [57,58]. Since most vehicles are typically driven only a few TH-1345_TPERJOY
hours per day and are parked the rest of the time (during the night or work) their batteries could be used to let electricity flow from the vehicle to the power lines and back. This ability to exchange energy with the grid gives the opportunity to furnish several services to the grid while optimizing the charging and discharging operations [57–60]. The service from G2V and V2G is done through EV charging system. However, charging or discharging of individual EV battery cannot provide any meaningful service to the grid, as it is usually in tens of kWh level. This is far below the base requirement for making transaction in electricity market, which is usually in MWh level. Therefore, the EVs should be synchronized with many vehicle. These problems could be addressed by introducing the concept of aggregator in the architecture of V2G system [55, 60, 61]. Rather than individual EVs batteries, effective impact on grid service provision can be obtained by grouping together a large number of vehicles through this aggregator. A sufficiently large number of aggregated parked EVs could provide several important services to the grid such as regulation, peak power, spinning reserves and other ancillary services [55, 60, 61].
While, two chapter of this thesis has dealt with the modeling and simulation details of power electronic converters and control requirements for bidirectional charging systems; the rest of the two chapters focus on mutual inductance computation and compensations requirements of contactless coils.
Experimental set-ups are built in the laboratory to validate the MI computation and compensation circuits. While the hardware prototype for bidirectional charging systems have not been done.