Inverter Control for Wind Turbine Grid Faults
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Solution Overview
Problem
Inverters used in wind turbines face challenges in rapidly responding to grid faults, leading to high current transients that can exceed instantaneous over-current thresholds, potentially causing the inverter to trip and disconnect from the grid, and existing current limiting techniques result in excessive thermal stress on power semi-conductor devices.
Innovation Solution
A method and control arrangement that dynamically adjusts current thresholds oscillating with the alternating current phase, allowing a switch from pulse width modulation to sliding mode control when over-currents are detected, and uses sinusoidal thresholds to minimize thermal stress on power switches, with the ability to recalibrate thresholds based on current demand signals and limit switching frequency to prevent excessive power dissipation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional current limiting techniques are used during grid faults, then over-current protection is achieved, but excessive thermal stress is imposed on power semi-conductor devices
Solution Approach 1:
The patent applies dynamics by transitioning from fixed current thresholds to dynamically oscillating thresholds that follow the AC current waveform. The thresholds are continuously adjusted based on the instantaneous phase and amplitude of the AC current, allowing the system to adapt to changing operating conditions during grid faults while minimizing thermal stress on power switches.
Solution Approach 2:
The invention changes the parameter of current thresholds from static values to dynamic values that oscillate with the AC current. By making the thresholds time-varying and phase-synchronized with the AC waveform, the system achieves better current control that reduces both over-current excursions and thermal stress on semiconductor devices.
2Productivity
If pulse width modulation is used for inverter control, then efficient power conversion is achieved, but the inverter cannot rapidly respond to grid faults
Solution Approach 1:
The system dynamically switches between pulse width modulation and sliding mode control based on grid conditions. During normal operation, PWM provides efficient power conversion. During grid faults, the system transitions to sliding mode control which offers faster response and inherent over-current protection, achieving both efficiency and rapid response capability.
Solution Approach 2:
The control strategy changes by switching between two different control modes (PWM and sliding mode) depending on the operating conditions. This parameter change in control methodology allows the system to optimize for efficiency during normal operation and for rapid response during fault conditions.
3Power
If current thresholds are increased to allow higher current during grid faults, then power transmission is maintained, but the risk of trip conditions increases
Solution Approach 1:
The patent uses dynamic thresholds that oscillate with the AC current waveform to maintain power transmission during grid faults while avoiding trip conditions. The thresholds adapt to the instantaneous current phase and amplitude, allowing maximum safe current flow without exceeding device ratings, thus maintaining power transmission while ensuring reliability.
Data Source
AI summary
A method of operating an inverter for converting a DC power into AC power by use of a pulse width modulation switching scheme is provided is disclosed. The inverter is controlled by use of the pulse width modulation switching scheme to provide an alternating current based on a current demand signal defining an alternating current provided by the inverter. An upper current threshold and a lower current threshold are provided. An instantaneous value of the alternating current is measured. When the instantaneous value of the alternating current overshoots the upper current threshold or undershoots the lower current threshold, the pulse width modulation switching scheme is replaced by an amended switching scheme which controls the instantaneous value of the alternating current to be between the upper current threshold and the lower current threshold. The upper current threshold and the lower current threshold oscillate with at least one alternating phase.