Grid-Connected Inverter Control for Weak Grid Mode Switching
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Solution Overview
Problem
Existing control solutions for grid-connected inverters fail to adapt to complex and changeable grid conditions, leading to instability issues due to fluctuations in short circuit ratio (SCR) and series compensation, which affects the stability of new energy generation systems.
Innovation Solution
A multi-mode control method for grid-connected inverters that includes control parameter perturbation, grid condition estimation, and adaptive control mode switching between current control, voltage control, and added-damping-based current control modes, allowing the inverter to adjust based on estimated SCR and series compensation degree without additional measurement equipment.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If current control mode is used, then power generation efficiency is high, but stability deteriorates in weak grid conditions
Solution Approach 1:
The patent implements dynamic switching between current control mode and voltage control mode based on real-time grid impedance detection. When grid impedance is low, current control mode is used for high efficiency; when grid impedance increases indicating weak grid conditions, the system switches to voltage control mode to maintain stability. This dynamic adaptation resolves the contradiction between efficiency and stability.
Solution Approach 2:
The system changes control parameters (control mode) based on grid conditions. By detecting grid impedance and comparing it with threshold values, the system adjusts the control strategy from current control to voltage control, thereby adapting to changing grid conditions and maintaining both efficiency and stability.
2Reliability
If voltage control mode is used, then stability margin improves in weak grid conditions, but instability risk increases in strong grid conditions
Solution Approach 1:
The system dynamically selects control mode based on real-time grid impedance assessment. In weak grid conditions (high impedance), voltage control mode provides enhanced stability margin. In strong grid conditions (low impedance), the system switches to current control mode to avoid instability risks, thus achieving adaptability across different grid conditions.
Solution Approach 2:
The control parameter (mode) is changed according to grid impedance levels. The system compares detected impedance with predefined thresholds to determine whether to use voltage control or current control, enabling the inverter to adapt to both weak and strong grid conditions optimally.
3Productivity
If series compensation device is added, then transmission capacity improves, but grid impedance uncertainty increases
Solution Approach 1:
The system continuously detects grid impedance and uses this feedback to adjust control strategy. When series compensation devices cause impedance changes, the feedback mechanism detects these changes and triggers appropriate control mode switching, thereby maintaining stability despite impedance uncertainty.
Solution Approach 2:
The system changes control parameters in response to impedance variations caused by series compensation. By detecting impedance changes and switching between control modes, the system compensates for the instability introduced by series compensation devices, maintaining reliable operation.
4Measurement precision
If grid impedance detection equipment is added, then measurement precision improves, but device complexity increases
Solution Approach 1:
The system uses its own operating parameters (current and voltage measurements already taken for control purposes) to calculate and detect grid impedance. This self-service approach eliminates the need for separate measurement equipment, achieving accurate impedance detection without increasing device complexity.
Solution Approach 2:
The system uses existing current and voltage sensors as intermediaries to indirectly measure grid impedance. Instead of adding dedicated impedance measurement equipment, the system calculates impedance from readily available electrical parameters, achieving the same measurement precision with simpler equipment.
Data Source
AI summary
A multi-mode control method for a grid-connected inverter includes: continuously calculating an effective value VHarRms of a voltage harmonic of a filtering capacitor; perturbing a control parameter to excite the voltage harmonic of the filtering capacitor; estimating a grid condition based on the perturbed control parameter and the effective value VHarRms of the voltage harmonic of the filtering capacitor; and switching a control mode of the grid-connected inverter based on the estimated grid condition. The multi-mode control method provides a grid condition detection method, which excites the voltage harmonic by perturbing the control parameter and determines the grid condition by the relationship between the control parameter and the effective value of the voltage harmonic. The multi-mode control method eliminates the need for additional grid impedance measurement equipment, and adapts to complex grid conditions with significant changes in the system short-circuit ratio (SCR) and the system series compensation degree.