Inverter Control Strategy for Photovoltaic Stability
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Solution Overview
Problem
Inverters operating with a five-level topology structure experience output current distortion when the floating capacitor voltage is not within a preset range, affecting the stability of photovoltaic systems, especially when the direct current side voltage exceeds the operation voltage threshold.
Innovation Solution
A method and device that dynamically adjust the inverter's operation mode by using a maximum power tracking algorithm to adjust the direct current side voltage and floating capacitor voltage, switching between five-level and seven-level control strategies based on whether the floating capacitor voltage is within a preset range, to prevent output current distortion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the inverter operates in five level operation mode with floating capacitor voltage at U dc /4, then the output voltage has five levels and normal operation is achieved, but when direct current side voltage is 1500V and floating capacitor voltage is pre-charged to preset value, the inverter must output seven different levels causing severe output current distortion
Solution Approach 1:
The patent applies dynamics by making the control strategy adaptive and changeable based on operating conditions. The controller dynamically switches between five-level control strategy and seven-level control strategy according to the direct current side voltage level. When voltage is normal, five-level strategy is used; when voltage exceeds threshold (1500V), seven-level strategy is activated, resolving the contradiction between maintaining stable operation and avoiding current distortion under varying voltage conditions
Solution Approach 2:
The patent changes the control parameter (control strategy) based on the voltage parameter. By monitoring the direct current side voltage and comparing it with the threshold, the system adjusts the control strategy from five-level to seven-level operation, thereby adapting to parameter changes and preventing output current distortion while maintaining operation stability
2Strength
If the floating capacitor voltage is pre-charged to preset value (450V) to avoid voltage stresses exceeding withstanding range, then switching transistor safety is improved, but the inverter must operate in seven level mode causing output current distortion
Solution Approach 1:
The patent applies preliminary action by pre-charging the floating capacitor voltage to a preset value (450V) before normal operation to prevent voltage stress from exceeding the withstanding range of switching transistors. This preliminary preparation ensures transistor safety while the control strategy adapts to maintain acceptable output current characteristics
Solution Approach 2:
The system dynamically adjusts the control strategy based on the pre-charged capacitor voltage condition. When the floating capacitor voltage is at preset value and direct current side voltage is 1500V, the controller switches to seven-level control strategy, enabling the inverter to operate safely with adapted output characteristics that prevent severe current distortion
3Ease of operation
If the inverter is controlled to operate in five level operation mode with pre-charged floating capacitor voltage, then the control simplicity is maintained, but the output current becomes severely distorted affecting system stability
Solution Approach 1:
The patent maintains control simplicity through automated dynamic adaptation. The controller automatically detects the operating condition (direct current side voltage level) and switches between control strategies without requiring manual intervention. This dynamic control approach ensures system stability by selecting the appropriate strategy (five-level or seven-level) while maintaining ease of operation through automated decision-making
Data Source
Figure 1
Figure 2(a)~2(b)
Figure 2(c)~2(d)
AI summary
Provided are a method and a device for controlling an operation of an inverter. The method includes: determining whether a direct current side voltage of the inverter is greater than an operation voltage setting threshold; and if no, controlling the inverter to operate according to a five level control strategy; and if yes: adjusting the direct current side voltage by using a maximum power tracking algorithm; adjusting linearly a floating capacitor voltage of the inverter based on the adjusted direct current side voltage; determining whether the adjusted floating capacitor voltage is in a preset range; and if yes, controlling the inverter to operate according to a five level control strategy; and if no, controlling the inverter to operate according to a seven level control strategy.