Wind Turbine Converter Control for Grid Overvoltage Ride-Through
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Solution Overview
Problem
Wind turbines face challenges in increasing output voltage without causing grid pollution and reducing the lifetime of switching elements due to high DC-link voltages, especially during overvoltage situations.
Innovation Solution
A method for operating wind turbines that involves controlling the DC-link voltage and modulation index to achieve desired output voltages, using a first voltage level for dynamic control and a second voltage level for overvoltage conditions, with concurrent adjustments to minimize the duration of high DC-link voltage and reduce grid pollution.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If the DC-link voltage is increased to raise the output voltage amplitude of the power converter, then the output voltage capability is improved, but the lifetime of the IGBT transistors is reduced due to cosmic ray impact
Solution Approach 1:
The patent applies dynamics by making the DC-link voltage adjustable rather than fixed. The control system dynamically switches between a first DC-link voltage level (lower, for normal operation) and a second DC-link voltage level (higher, for overvoltage situations), optimizing both IGBT lifetime and output voltage capability based on real-time grid conditions
Solution Approach 2:
The patent changes the DC-link voltage parameter between two discrete levels. By switching between a first voltage level (Udc1) and a second voltage level (Udc2), the system adapts the voltage parameter to match operational requirements, reducing IGBT stress during normal operation while ensuring sufficient output voltage during overvoltage events
2Power
If over-modulation is used to increase the output voltage during overvoltage events, then the voltage amplitude is improved, but grid pollution occurs due to harmonic injection
Solution Approach 1:
The patent applies preliminary action by pre-charging the DC-link capacitor to a higher voltage level (Udc2) before the overvoltage event fully develops. This allows the power converter to immediately respond to grid overvoltage conditions without needing to use over-modulation, thereby avoiding harmonic injection while still providing the necessary voltage boost
Solution Approach 2:
The patent introduces the DC-link voltage as an intermediary variable between the grid voltage and the power converter output. By adjusting the DC-link voltage level, the system mediates the relationship between grid conditions and converter output, enabling voltage regulation without direct over-modulation of the converter switches
3Reliability
If the DC-link voltage is kept low to extend IGBT lifetime, then the switching element reliability is improved, but the output voltage amplitude cannot be increased when needed
Solution Approach 1:
The patent makes the DC-link voltage dynamic by implementing control that switches between two voltage levels based on grid conditions. This dynamic adjustment allows the system to maintain low voltage (extending IGBT life) during normal operation while being capable of high voltage (providing adaptability) when grid overvoltage events occur
Solution Approach 2:
The patent achieves multi-functionality by using the same DC-link capacitor and power converter hardware to serve two different voltage level requirements. The single DC-link system universally handles both low-voltage normal operation and high-voltage overvoltage events, eliminating the need for separate voltage regulation systems
Data Source
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Figure 3A~3B
AI summary
The invention relates to a method for operating a wind turbine. The method comprises monitoring a wind turbine signal for detection of an over voltage of the grid and upon detection of the over voltage, initiate an over-modulation mode wherein the grid side converter is operated with a modulation index being increased in an over-modulation range at least during a sub-period of the over-modulation mode, and upon the detection of the operational condition, initiate a DC-voltage adjustment mode wherein the DC voltage of the DC link is decreased from a second voltage level towards a first voltage level at least during a sub-period of the DC-voltage adjustment mode.