Variable Speed Wind Turbine Grid Isolation
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Solution Overview
Problem
Variable speed wind turbines with doubly fed induction generators face issues such as harmonic distortion, reactive power consumption, and mechanical stress due to power fluctuations, which affect power quality and equipment longevity, especially in weak grids and during synchronization with the grid.
Innovation Solution
A variable speed wind turbine system with a doubly fed induction generator where power is delivered only through the stator, using an exciter machine and active power electronic converters isolated from the grid, employing Grid Flux Orientation control for accurate power management, and enabling electric braking to prevent mechanical stress.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If power electronic converters are connected to the grid in variable speed wind turbines, then the turbine can operate at variable speeds to maximize efficiency, but harmonic distortion is injected into the grid causing power quality degradation
Solution Approach 1:
The patent extracts the power electronic converters from direct grid connection by introducing an intermediate DC link. The converters remain connected to the turbine but interface with the grid through a controlled rectifier and DC capacitor, isolating the harmonic-generating switching operations from the grid while maintaining variable speed operation capability.
Solution Approach 2:
The patent introduces an intermediate DC link with capacitor as a mediator between the power electronic converters and the grid. This intermediate stage allows energy buffering and decouples the AC grid frequency from the converter switching frequency, preventing harmonic injection while enabling variable speed operation.
2Reliability
If reactive energy compensation elements are used in fixed speed wind turbines, then starting current limitation is achieved, but the system complexity increases and power quality issues persist
Solution Approach 1:
The patent replaces traditional mechanical/reactive compensation elements with electronic control of the rectifier bridge. The firing angle control of the thyristors provides starting current limitation and reactive power compensation through electronic means, eliminating the need for separate mechanical compensation devices and reducing overall system complexity.
Solution Approach 2:
The rectifier bridge serves multiple functions simultaneously: it performs AC to DC conversion, provides starting current limitation through firing angle control, supplies reactive power compensation, and enables variable speed operation. This multi-functionality eliminates the need for separate dedicated components for each function.
3Device complexity
If the generator is directly connected to the grid in fixed speed turbines, then the structure is simple, but torque variations from wind turbulence are directly transmitted to the drive train causing mechanical stress
Solution Approach 1:
The patent introduces variable speed capability that allows the rotor to dynamically adjust its rotational speed in response to wind turbulence. This dynamic response decouples the direct mechanical coupling between wind torque variations and the drive train, reducing mechanical stress while the power electronic converters provide electrical decoupling.
4Productivity
If variable speed operation is implemented with grid-connected converters, then maximum efficiency is achieved, but the converters must be sized to handle full power causing high cost and complexity
Solution Approach 1:
The patent implements partial power conversion where only a portion of the total generator power goes through the full-power converters. The intermediate DC link allows the converters to handle reactive power and control functions while the majority of active power is transmitted directly to the grid through the rectifier, reducing converter size and cost.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration improves power quality by eliminating harmonic distortion, reduces reactive power consumption, and extends equipment life by avoiding mechanical stress, while maintaining constant power output above rated speed and allowing efficient operation in weak grids.
Implementation Method 1
a variable speed wind turbine with a doubly fed induction generator
Implementation Method 2
an exciter machine and active power electronic converters isolated from the grid
Data Source
AI summary
A variable speed wind turbine having a doubly fed induction generator (DFIG), includes an exciter machine mechanically coupled to the DFIG and a power converter placed between a rotor of the DFIG and the exciter machine. Thus, the power converter is not directly connected to the grid avoiding the introduction of undesired harmonic distortion and achieving a better power quality fed into the utility grid. Moreover, the variable speed wind turbine includes a power control and a pitch regulation.