Design of an Efficiently Aligned Capacitive-coupled Electric Vehicle

Most of the compensation topologies of the capacitive power transmission system, which are reported in the literature, are sensitive to the misalignments between the transmitting and the receiving plates which have not yet been addressed. Furthermore, in the existing optimization of the capacitive power transmission system methods, only limited system parameters are optimized.

Introduction

Development of Electric Vehicles (EV)
Electric Vehicle Charging Technology

Conductive Charging
Inductive Charging

Literature Review

Compensation Topologies of CPT Sytems
Optimization of the CPT systems

Motivations of this Research
Research Objectives
Thesis orientation

Any misalignment between the primary and secondary side affects the resonance of the circuit and reduces the efficiency of the system. The main advantage of the double-sided LCLC compensation topology is that the system power is proportional to the coupling coefficient, and the power can be increased by choosing the right values of parameters.

Figure 1.1: Typical structure of a WPT system. Figure reproduced from [31].

System Model

Capacitive Coupler Structure
Circuit Topology
Circuit Working Principle
Effect of Misalignment on Mutual Capacitance and EfficiencyEfficiency
Parameter Optimization of CPT Systems

Two sinusoidal voltage sources are used instead of the square wave sources V1 and V2 shown in fig. Due to the linear nature of the method, the superposition theorem is used to split the two-source circuit into two single-source linear circuits, as shown in Fig. If the receiver is in an angular position relative to the position of the transmitter is called "rotational or angular displacement" as depicted in fig.

It can be seen from fig. 2.9 that if there is any misalignment between the transmitter and the receiver, the mutual capacitance, CM, decreases, which disturbs the resonances in the circuit and causes degradation of the system output power and overall efficiency. Due to the symmetrical structure of the coupler plates, the x and y displacement performances are the same.CM is maximum when there are zero displacements and highest efficiency is. a) Axial displacement (b) Rotational or angular displacement Figure 2.8: Possible cases of misalignment. However, many of the challenges mentioned in Chapter 1 must be solved to realize CPT systems in real applications.

The mutual capacitance increases as the size of the connecting plates increases, but the size of the coupler is limited by the vehicle chassis. The capacitive and inductive reactance of the compensation components, which determines the compensation parameters, is directly affected by the frequency selection. Due to the risk of human safety, the leakage electric field should be less than 614 V/m, according to the IEEE standard [103].

Figure 2.1: Structure and dimensions of the plates. Figure reproduced from [73].

Proposed Transmitter Positioning System

Design of the Transmitter Positioning System
Formulation of the Algorithm
The transmitter shifts in the XY plane using stepper motors along the X and Y axis from its’ initial location in response to sensor input, as shown in Table 3.1
When the receiving plate is detected by all four sensors, the status of sensor 5 is read

Validation of the Algorithm
Hardware Implementation
Prototype Development
Results and Discussion

After that, the transmitter is positioned along the Y-axis in the direction of the limit switch, as illustrated in Figure 3.2. When the transmitter comes into contact with this limit switch, the transmitter starts moving along the X-axis in the direction of the limit switch. Roller bearings are attached to the top of the static base for smooth rolling motion as shown in Fig.

The gear mechanism associated with the front shaft of the stepper motor and which controls the rotational motion is also shown in the figure. Non-conductive material will be used to install the transmitter plates on top of the rotation base. Two of the four axles are attached to the outer frame using clamps.

Five IR proximity sensors are placed on the rotating base to detect the position of the receiver. The accuracy of the proposed system is tested experimentally under the same misalignment cases, which are taken into account in the software validation and a comparison between. Therefore, the encoder and the motor for the rotating axis, with the same resolution, could be used to obtain the best accuracy of the proposed system.

Figure 3.1: Solidworks design of the proposed system (without transmitter); (a) 3D view, (b) 2D view.

Optimization of the CPT system

Equivalent Circuit Model
Circuit Analysis
Objective Functions Formulation
Decision Variables and Constraints
Optimization Algorithm

Single objective optimization (SOO)
Multi-objective Optimization (MOO)

Due to the symmetry of the plate dimensions and position, C14 is equal to C23 and C34 is equal to C12. VC1 and V0 are in the same phase and the excitation voltage reduction is achieved. The series equivalent resistance (ESR) of the inductances and capacitances is ignored in the current analysis because their resistance is much lower than their reactance.

A higher frequency, on the other hand, will lead to greater EMI, control problems of the high frequency components and more sensitive to the system parameters. The current limits and the voltage limits of the inductances are limited by the Litz wire current limits and the inter-turn voltage, respectively. Most real-world problems can be formulated as mathematical equations, which come with some restrictions.

Thus, the primary goal of the optimization technique is to find the most appropriate solutions without violating system constraints. The flowchart of ACOR-G algorithm based on non-dominant classification, called MOACOR-GA, is shown in Figure 4.5, and the process of using the algorithm in solving the multi-objective optimization problem is described in the following steps. Then the population is further classified by calculating the distance of the set of solutions and ranking them accordingly.

Figure 4.1: Schematic diagram of a four plate CPT system with L compensation

Results and Discussion

Multi-objective Optimization

Since the THD is less than 1%, the efficiency is the decisive factor to select the solution set from Pareto optimal front depending on the system specifications. The maximum efficiency obtained by the proposed algorithm is 0.7985, while that of NSGA II is 0.797. Moreover, there are many available solutions beyond 0.797 in the Pareto front of the proposed algorithm, i.e.

The iterations required to reach the final Pareto and the execution time are shown in Table 5.2. It is observed that both the hybrid algorithm and NSGA II reach the optimal Pareto front, where the number of solutions is equal to the initial population. However, the minimum iterations required to reach the final Pareto optimal front for the hybrid algorithm are less compared to NSGA II.

This is due to the fact that instead of feeding random population to ACORGA, sorted population based on non-dominated and pressure distance is fed to the main algorithm. Also, the updated population is again passed through the same process of non-domination and push distance and ranked until the best solution is found. Moreover, the total time required to complete the optimization process is the same or slightly less for the proposed algorithm compared to NSGA II.

Figure 5.1: Efficiency vs -THD employing MOACO R GA algorithm.

Simulation Results

Thus, global convergence is achieved faster and diversity between solutions and better research, as can be seen in the figure. Thus, the system operates in a fully resonant state and a uniform power factor is achieved. However, it is slightly smaller than the optimization results, since the ESR of inductance and capacitance are not taken into account in the current calculation.

The THD of the inverter current is measured using the FFT analysis tool in Matlab/Simulink. It is found to be 0.93% which is slightly higher than the optimization result since all higher order harmonics are not considered as visible from (4.17). The electric field emissions around the plates can be analyzed by Ansoft Maxwell using the voltage stresses in Table 5.5 and the simulation result is shown in Fig.

According to the IEEE standard, the electric field strength must be lower than 614 V/m for human safety [103]. The safe distance in the system is measured at 900 mm from the panels.

Table 5.4: Parameters values considered in the simulation Parameter Value Parameter Value

Conclusions and future works

Conclusion

The proposed system can maintain a well-matched mutual capacitance in case of mismatch up to the maximum tolerance limit of the proposed system which is 450 mm. The optimization takes into account all system constraints, such as the voltage load on the passive components and connection boards, the voltage and current limitation of the inductors, the maximum excitation voltage and the feasible inductor size. A multi-objective optimization algorithm is formulated using a non-dominated sorting technique with a single-objective hybrid algorithm that combines a genetic algorithm (GA) with an ant colony optimization (ACO) algorithm.

The proposed algorithm is written in M AT L AB and is used to optimize the CPT systems. Then the performance of the proposed multi-objective algorithm is compared with NSGA II algorithm. It shows better performance in terms of the cost of the objective functions and requires less iterations and time compared to NSGA II algorithm.

A solution sets are selected from the Pareto front and other parameters are obtained according to the optimization analysis. Then the entire system is built under M AT L AB/S imul and the simulation results are compared with the optimization results. This research serves as a stepping stone to design a highly efficient capacitive charging system for electric vehicles in case of misalignments and can provide designers with a wide range of parameter values to choose from depending on the charging infrastructure.

Future Research Directions

Efficiency and THD are considered as objective functions, while the input DC voltage, operating frequency and transmission distance are considered as decision variables. Also, electric field emissions around the metal plates are simulated in Anso f t Maxwell and the safe distance to the coupling is shown. An analytical relationship can be developed between the cross-coupling capacity and the coupling structure, which can be used to optimize the size of the coupling plates.

Therefore, an optimization methodology could be formulated to determine the optimal number of LC networks to maximize system efficiency and output power.

Publications from This Research

Bibliography

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