Inverter Frequency Freezing for Stable Grid-Off-Grid Switching
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Solution Overview
Problem
Current energy storage systems experience long switching times and voltage/frequency distortions during on-grid/off-grid mode transitions, affecting power supply stability due to mechanical and transmission delays in inverter control.
Innovation Solution
The system freezes the output frequency of the inverter and controls the output voltage by managing the output current based on association relationships between voltage and apparent power, allowing the inverter to switch to a voltage source mode only when the voltage meets a preset range, thereby improving control efficiency and stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the inverter directly controls the output voltage during mode switching, then the voltage control response is slow due to mechanical and transmission delays, but the control system is simple
Solution Approach 1:
The inverter pre-freezes the output frequency to a preset value before the mode switching occurs. This preliminary action prepares the system in advance, eliminating frequency distortion during switching and reducing the overall switching time while maintaining voltage stability.
Solution Approach 2:
The patent replaces direct voltage control with current control during the switching process. By controlling the output current instead of directly controlling voltage, the system avoids mechanical delays in voltage regulation, achieves faster response, and then transitions to voltage source mode only when voltage stabilizes within the preset range.
2Loss of time
If the inverter switches modes immediately upon receiving a switching instruction, then the switching time is reduced, but voltage distortion and frequency distortion occur during switching
Solution Approach 1:
The system performs preliminary frequency freezing before mode switching occurs. By setting the output frequency to a preset value in advance, the system prevents frequency distortion during switching while maintaining rapid response, thus reducing overall switching time without causing harmful distortions.
Solution Approach 2:
The inverter continuously monitors the output voltage and only switches to voltage source mode when the voltage stabilizes within the preset range. This feedback mechanism ensures that mode switching occurs at the optimal moment, preventing voltage distortion while minimizing switching time.
3Productivity
If the inverter controls output voltage directly, then the control process is straightforward, but the control efficiency is low due to delays
Solution Approach 1:
The patent substitutes direct voltage control with current control during the switching process. This replacement eliminates the mechanical delays inherent in direct voltage control, significantly improving control efficiency. The control process remains manageable through automated feedback mechanisms that monitor voltage stability.
Solution Approach 2:
The system pre-freezes the frequency and prepositions the control mechanism before switching occurs. This preliminary preparation eliminates delays during the actual switching event, improving control efficiency without requiring complex real-time adjustments during the transition.
Data Source
AI summary
This application provides an energy storage system, including a controller, an inverter, an on-grid/off-grid mode switch, and a load. The inverter is configured to: in response to determining that, the energy storage system is in an islanding state, freeze an output frequency of the inverter based on a preset frequency, and control an output current of the inverter based on an association relationship that an output voltage of the inverter is negatively correlated with apparent power of the inverter. The controller is configured to send a disconnection instruction to the on-grid/off-grid mode switch. The controller is further configured to send a switching instruction to the inverter in response to receiving a disconnection complete message from the on-grid/off-grid mode switch. The inverter is further configured to switch to a voltage source mode in response to determining that the output voltage of the inverter meets the preset range of the rated voltage.