1. Research Background
With the growing concerns over environmental pollution and energy sustainability, electric vehicles (EVs) and renewable energy sources have gained significant attention and are rapidly evolving. The integrated power station for EV charging and discharging combines the advantages of traditional charging stations with bidirectional power systems, enabling efficient energy exchange between electric vehicles and the grid. This system offers a promising solution to the challenges of large-scale integration of renewable energy and EVs, enhancing grid stability and energy efficiency.
The focus of this paper is on the Parallel Power Regulation System (PCS) within the integrated power station that manages both charging and discharging processes. This system comprises multiple bidirectional converter modules connected in parallel, operating under a centralized control framework. Based on commands from the Energy Management System (EMS), the PCS can switch between various operational modes. The paper provides a comprehensive analysis of the control strategies involved in the seamless transition between independent operation, grid-connected operation, and other scenarios in the high-power bidirectional regulation system.
Structure of the Parallel Power Regulation System in an Integrated Charging and Discharging Station
2. Operating Modes and Control Strategies
V2G Mode and Control Strategy
In V2G mode, the PCS uses a dual-loop control structure: an inner loop for inductor current on the inverter side and an outer loop for grid-side inductor current. The EMS issues active and reactive power commands, allowing the system to maintain a high power factor and suppress grid current resonance. During charging, the converter operates as a voltage-type PWM rectifier, while during discharging, it functions as an inverter. The four-quadrant operation capability of the high-frequency PWM converter enables smooth transitions between charge and discharge states. In this mode, the PCS can absorb or supply power to the distribution network, maintaining stable tidal currents at the point of common coupling (PCC) and acting as a controllable unit relative to the grid.
Independent Mode and Control Strategy
In the independent mode, the PCS employs a dual-loop control scheme based on the inverter-side inductor current inner loop and the capacitor voltage outer loop. This approach ensures high-quality output voltage waveforms and improved dynamic performance. The system operates as a voltage source, and key challenges include synchronization and current sharing among parallel modules. A digital synchronization method is used, where the centralized controller generates a power frequency square wave signal, which is optically converted and sent to each module. To ensure equal current distribution, a power-sharing control strategy is implemented, based on the output power and impedance of each PCS module.
Seamless Switching Mode and Control Strategy
The seamless switching control system consists of a voltage control unit and a current control unit. During the transition between grid-connected and independent operation, the inductor current inner loop is maintained, while the outer loop switches between the grid-side inductor current loop and the filter capacitor voltage loop. When transitioning to grid-connected mode, the bidirectional converter detects the voltage and phase of the distribution network and adjusts its own output accordingly. Once the grid conditions are met, the static switch activates, and the control system smoothly transitions from the capacitor voltage outer loop to the grid-side inductor current outer loop. This ensures fast and accurate grid state detection and phase-locked control, minimizing disturbances during mode changes. Similarly, when a fault or maintenance is detected, the system can switch back to independent mode without interruption.
3. Simulation and Experimental Verification
Using the TMS320F2812 microprocessor as the core controller, two 500kVA PCS parallel test systems were developed. In V2G mode, the system operated as a current-mode converter, with the centralized controller issuing grid power commands. Experimental results showed that the system produced high-quality input current waveforms and achieved a high power factor. In the independent mode, the PCS functioned as a voltage-source converter, demonstrating excellent voltage waveform quality and quick response to load variations. During abrupt load changes, the system maintained stable current distribution and good dynamic performance.
Experimental Waveform of V2G to Independent Mode Transition
Experimental Waveform of Independent to V2G Mode Transition
4. Conclusion
The power regulation system is a crucial component in the integrated EV charging, discharging, and storage power station. It manages vehicle charging, battery energy storage, and peak shaving, playing a vital role in grid stability and energy efficiency. This paper presents a detailed analysis of the control strategies for V2G, independent, and seamless switching operations. A double-loop control strategy was proposed, incorporating inverter-side inductor current inner loops, filter capacitor voltage outer loops, and grid-side inductor current outer loops. The research demonstrates that the control strategy significantly enhances the reliability and economic efficiency of the integrated power station, ensuring stable and smooth operation under various conditions.
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