Keywords: electric vehicle, wireless power transfer, bidirectional charging, vehicle-to-grid (V2G), compensation topology, control strategy, efficiency optimization, interoperability
1. Introduction
The rapid growth in the number of electric vehicles (EVs) globally, with over 12 million pure EVs in China alone by June 2023 , has intensified the need for efficient bidirectional power interfaces to enable seamless vehicle-to-grid (V2G) interactions. Bidirectional wireless power transfer (BWPT) systems offer significant advantages over traditional conductive charging, including non-contact operation, environmental resilience, and enhanced automation, making them pivotal for future smart grids . This review explores the key technologies, research advancements, and emerging trends in BWPT systems for EVs, emphasizing their structural components, control methodologies, and practical challenges.
2. System Architecture and Research Progress
A typical BWPT system for EVs comprises grid-side AC-DC conversion, high-frequency inversion, resonant coupling, and on-board power conditioning. Power flows from the grid to the EV battery (forward direction) or vice versa (reverse direction) using fully controlled power devices and resonant networks –.
2.1 Key Components
- Grid-Side Converter: Rectifies AC to DC and performs power factor correction (PFC).
- High-Frequency Inverter/Rectifier: Converts DC to high-frequency AC (or vice versa) for wireless transmission.
- Resonant Network: Comprises coupling coils and compensation components (capacitors/inductors) to enhance power transfer efficiency.
- On-Board Converter: Regulates power to match battery requirements.
2.2 Research Milestones
Early work by MADAWALA et al. (2011) introduced the first BWPT topology for V2G applications . Since then, notable advancements include:
- 20 kW BWPT system by Oak Ridge National Laboratory (2020) with 93% forward and 89% reverse efficiency .
- LCC-S compensated systems achieving high efficiency (92.5–96%) across various coupling conditions .
- Matrix converter-based single-stage topologies for compact designs, though with higher control complexity .
Table 1: Representative BWPT Systems and Their Performance
| Research Institution | Year | Topology | Power (kW) | Efficiency (%) | Key Feature |
|---|---|---|---|---|---|
| Honda R&D Americas | 2018 | SS + Full Bridge | 3.3 | 92–96 | Circular coils, single-phase PFC |
| Oak Ridge National Lab | 2020 | LCC-LCC + Three-Phase PFC | 20 | 93/89 | Medium-duty EVs, high power density |
| Huazhong University | 2021 | LCC-S + Buck-Boost | 6.6 | 93–94 | Wide voltage range, ZVS operation |
| Chinese Academy of Sciences | 2022 | Bilateral LCC | 3.3 | 94.2 | High interoperability |
3. Main Power Topologies
Power topologies in BWPT systems are categorized into two-stage (AC-DC-AC) and single-stage (AC-AC) configurations, each with distinct advantages and challenges.
3.1 Two-Stage Power Conversion
The two-stage topology is the most widely adopted, featuring separate PFC and high-frequency inversion stages.
- Advantages: High power factor, simplified control, and compatibility with voltage-source inverters.
- Disadvantages: Bulkier design due to input inductors and DC-link capacitors, limited reliability due to capacitor lifespan .
Subtypes:
- Voltage-Source Inverters (VSIs): Dominant in BWPT systems, using full-bridge or half-bridge configurations. Full-bridge converters offer higher power handling and soft-switching capabilities but require complex control .
- Current-Source Inverters (CSIs): Less common due to large series inductors, though they offer short-circuit protection and low current stress .
Formula 1: Power Transfer in Two-Stage Systems\(P = \frac{V_p V_s}{\omega M} \sin\delta\) where \(V_p, V_s\) are the RMS voltages of primary and secondary converters, \(\omega\) is the angular frequency, M is the mutual inductance, and \(\delta\) is the phase difference between converters .
3.2 Single-Stage Power Conversion
Single-stage topologies (e.g., matrix converters) directly convert AC to high-frequency AC, eliminating the DC-link.
- Advantages: Compact design, high power density.
- Disadvantages: Complex commutation algorithms, hard-switching losses, and limited THD performance .
Table 2: Two-Stage vs. Single-Stage Topologies
| Parameter | Two-Stage (AC-DC-AC) | Single-Stage (AC-AC) |
|---|---|---|
| Power Factor | High (≥0.95) | Moderate (0.8–0.95) |
| Efficiency | 85–96% | 80–90% |
| Complexity | Low (decoupled stages) | High (integrated control) |
| Applications | High-power EVs | Low-to-medium power EVs |
4. Compensation Networks and Interoperability
Compensation networks are critical for maximizing power transfer efficiency in loosely coupled systems (coupling coefficient \(k = 0.2–0.4\)) .
4.1 Types of Compensation Topologies
- Series-Series (SS): Simple structure, offers constant current output but sensitive to misalignment .
- LCL/LCC: T-type networks with inductive and capacitive components, providing stable current output and better misalignment tolerance .
- LCC-S: Asymmetric topology for constant voltage output, often paired with DC-DC converters for wide voltage ranges .
Table 3: Compensation Topology Characteristics
| Topology | Output Type | Max Power | Efficiency (%) | Misalignment Tolerance | Safety (Short Circuit) |
|---|---|---|---|---|---|
| SS | Constant current | High | 85–95 | Low | Safe |
| Bilateral LCL | Constant current | Low | 83–93 | High | Safe |
| Bilateral LCC | Constant current | Medium | 84–94 | High | Safe |
| LCC-S | Constant voltage | High | 82–92 | Moderate | Unsafe |
4.2 Interoperability
Interoperability ensures compatibility between different EV models and charging infrastructure, covering:
- Magnetic Interoperability: Requires matching coil types (e.g., DD coils for horizontal flux) to avoid zero coupling at alignment .
- Electrical Interoperability: Quantified by port impedance \(Z_{GA}\) and \(Z_{VA}\), which must lie within compatible ranges for power transfer :\(Z_{GA} = j\omega L_{GA} + \frac{\omega^2 M^2}{Z_{VA} + j\omega L_{VA}}\) Adjustable components (e.g., tunable capacitors) and active control (e.g., Buck-Boost converters) enhance interoperability –.
5. Control Strategies and Efficiency Optimization
Effective control in BWPT systems must manage power flow, efficiency, and synchronization between grid and vehicle.
5.1 System Modeling
- Steady-State Models: Use coupled-mode theory or circuit mutual inductance to analyze power transfer under stable conditions .
- Dynamic Models: Employ techniques like Generalized State Space Averaging (GSSA) to linearize switching dynamics for controller design –.
5.2 Control Methods
- Phase-Shift Control: Adjusts the phase difference (\(\delta\)) between converters to regulate power flow. Single-phase-shift, dual-phase-shift, and three-phase-shift strategies exist, with three-phase-shift offering the broadest ZVS range –.
- Frequency Control: Modulates switching frequency to track resonance, though limited by frequency splitting in high-order networks .
- Pulse Density Control: Uses intermittent voltage pulses for soft switching, suitable for low-power applications .
Formula 2: Efficiency Optimization in Phase-Shift Control\(\eta = \frac{P_{out}}{P_{in}} = \frac{V_2 I_2}{V_1 I_1} = \frac{\sin\delta}{\sin\delta + \frac{R_{loss}}{V_p V_s / (\omega M)}}\) where \(R_{loss}\) includes conduction and switching losses .
5.3 Synchronization and Communication
- Phase Synchronization: Critical for maintaining constant power flow, achieved via:
- Auxiliary coils to detect phase differences .
- Active power/reactive power (P&Q) monitoring .
- Current-based synchronization (e.g., resonant current detection) .
- Simultaneous Wireless Power and Data Transfer (SWPDT): Uses high-frequency carriers for data transmission, minimizing interference with power transfer .
6. Challenges and Future Trends
Despite progress, BWPT systems face challenges in cost, efficiency across wide power ranges, and interoperability. Future research should focus on:
- Multi-Power Level Interoperability: Developing scalable magnetic couplers and modular converters for different EV classes (e.g., cars vs. trucks) .
- Wide-Range Efficiency Optimization: Combining variable compensation (e.g., switch-controlled capacitors) with adaptive control to maintain high efficiency from low to full load –.
- Dual-Side Coordination: Advanced control algorithms to manage bidirectional power flow while integrating grid stability requirements –.
- Cost Reduction: Miniaturizing passive components and adopting GaN/SiC devices to improve power density and reliability .
Table 4: Future Research Directions
| Area | Objective | Key Techniques |
|---|---|---|
| Magnetic Design | High coupling, low EMI | Multicoil structures, magnetic shielding |
| Control | Real-time efficiency optimization | Model predictive control (MPC), AI-driven algorithms |
| Hardware | Compact, high-reliability converters | Integrated power stages, planar magnetics |
| Standards | Global interoperability protocols | Unified impedance matching criteria |
7. Conclusion
Bidirectional wireless power transfer systems are pivotal for enabling efficient V2G interactions in electric vehicles. While two-stage topologies dominate current applications, single-stage designs and advanced compensation networks (e.g., LCC) offer promising solutions for compact, high-efficiency systems. Future advancements in control strategies, magnetic design, and standardization will accelerate the deployment of BWPT, driving the transition toward smarter, more sustainable EV ecosystems.
This review highlights the need for interdisciplinary research to address technical challenges, emphasizing the importance of collaboration between power electronics, control systems, and grid integration experts to realize the full potential of BWPT in the electric vehicle sector.