Inverter Mode Switching for Stable Grid-Tied and Islanded Operation
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Current energy storage systems experience long switching times and voltage/frequency distortions during on-grid/off-grid mode transitions due to mechanical delays and inverter control inefficiencies, affecting power supply stability.
Innovation Solution
The system freezes the inverter's output frequency and controls the output current to maintain a stable voltage within a preset range, using association relationships between voltage and apparent power to optimize switching to a voltage source mode.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of time
If the inverter directly controls the output voltage during mode switching, then the voltage control is straightforward, but the switching time is long and voltage distortion occurs
Solution Approach 1:
The inverter pre-freezes the output frequency to a preset value before the mode switching occurs. This preliminary action prepares the inverter for rapid switching by establishing the frequency in advance, eliminating the need for frequency adjustment during the switching process and thereby reducing switching time while preventing frequency distortion
Solution Approach 2:
The control method changes from directly controlling output voltage to controlling output current based on the negative correlation between output voltage and apparent power. By changing the controlled parameter from voltage to current, the system achieves faster response during switching while maintaining voltage within the rated range, thus reducing switching time and preventing voltage distortion
2Device complexity
If the mechanical switch is used for on-grid/off-grid mode switching, then the switching mechanism is simple, but mechanical delay occurs during switching
Solution Approach 1:
The patent replaces the mechanical switching mechanism with an electronic control approach. The inverter uses electronic switching based on control signals to transition between on-grid and off-grid modes, eliminating mechanical delays and contact wear while maintaining simple overall system structure. The electronic switching occurs much faster than mechanical switching, resolving the time loss issue
3Productivity
If the inverter switches to voltage source mode immediately after disconnection, then the mode transition is rapid, but voltage distortion occurs due to control inefficiency
Solution Approach 1:
The inverter changes the controlled parameter from output voltage to output current during the switching process. By controlling output current based on the negative correlation with apparent power, the system achieves rapid mode transition while maintaining voltage within the rated range through current regulation, thus improving both switching speed and voltage quality
Solution Approach 2:
The control method incorporates feedback by continuously monitoring the output voltage and adjusting the output current accordingly. The inverter uses the relationship between output voltage and apparent power to regulate current, ensuring voltage remains within the rated range during and after mode switching, thereby preventing voltage distortion while maintaining fast switching
Data Source
Figure 1~2
Figure 3
Figure 4~5
AI summary
Embodiments of this application provide 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, send an islanding state message to the controller, 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 when receiving the switching instruction and determining that the output voltage of the inverter meets the preset range of the rated voltage. In embodiments of this application, frequency distortion and voltage distortion events can be avoided, control efficiency of the inverter is improved, and operation stability of the load can be improved.