Controlled Impedance Protection for DFIG Grid Faults
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Solution Overview
Problem
Double fed induction generators (DFIGs) are sensitive to grid faults, leading to severe transients in air-gap torque and shaft torque, which can reduce system reliability and gearbox life, and existing solutions are either ineffective in reducing shaft stress or expensive.
Innovation Solution
A protection system with controlled impedance devices is introduced between the stator windings and the grid side converter, using semiconductor devices to couple or decouple impedance devices based on grid voltage and current thresholds, regulating air-gap torque and stator current during faults to maintain stability and reduce transient stress.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If anti-parallel thyristors are used in the stator circuit to achieve quick disconnection and remagnetization, then the disconnection speed is improved (within 10 milliseconds), but the shaft stress is not reduced and the system complexity increases
Solution Approach 1:
The patent introduces a controlled impedance device as an intermediary component in the stator circuit. This device mediates between the generator and the grid fault, controlling the flow of current and torque during transient conditions. The controlled impedance device acts as a buffer that protects the system without requiring complex switching operations, thereby reducing shaft stress while maintaining operational simplicity.
2Reliability
If a static switch is used in the rotor circuit to break short circuit current, then the rotor protection is improved, but the shaft stress reduction is ineffective and the device complexity increases
Solution Approach 1:
The controlled impedance device serves as an intermediary that protects both the rotor and shaft simultaneously. By positioning the impedance control in the stator circuit, the system indirectly protects the rotor from short circuit currents while also reducing shaft stress through torque control, eliminating the need for separate rotor switches.
3Stability of the object's composition
If a DVR (Dynamic Voltage Restorer) is used to isolate the DFIG system from voltage sags, then the system stability is improved, but the cost increases significantly
Solution Approach 1:
The patent implements a self-service protection mechanism where the controlled impedance device automatically adjusts its impedance based on detected grid conditions. The system monitors voltage sags and transient conditions, then autonomously controls the impedance to maintain stability without requiring expensive external DVR equipment. This self-regulating approach achieves stability at lower cost.
4Reliability
If the wind turbine generator is disconnected from the grid during voltage sags, then the generator protection is improved, but the power production interruption increases and grid stability contribution decreases
Solution Approach 1:
The controlled impedance device acts as a protective intermediary that allows the generator to remain connected to the grid during voltage sags. By controlling the impedance, the device protects the generator from harmful transients while maintaining the electrical connection, thus preventing power production interruptions and enabling continued grid stability contribution.
5Device complexity
If conventional control is used during grid faults, then the control simplicity is maintained, but severe transients in air-gap torque and shaft torque occur reducing system reliability
Solution Approach 1:
The patent implements a feedback control mechanism where the controller continuously monitors grid voltage and current conditions. Based on this feedback, the controller dynamically adjusts the controlled impedance device to maintain optimal torque levels during grid faults. This feedback loop prevents severe transients while maintaining relatively simple control architecture.
Data Source
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AI summary
A protection system and a protection method are provided for protecting a double fed induction generator (16) and a gearbox (24) during a grid fault. The protection system includes a plurality of controlled impedance devices (44). Each of the controlled impedance devices is coupled between a respective phase of a stator winding of the double fed induction generator and a respective phase of a grid side converter (26). The protection system also includes a controller (32) configured for coupling and decoupling impedance in one or more of the plurality of controlled impedance devices in response to changes in at least one of a utility grid voltage and a utility grid current.