DFIG Rotor Modulation Index Control via Reactive Current
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Solution Overview
Problem
Wind turbines with doubly-fed induction generator (DFIG) systems face instability due to over-modulation index during high-voltage ride through and sub-synchronous resonance, as existing reactive current control methods rely on stator voltage magnitude and fail to prevent over-modulation at the rotor converter.
Innovation Solution
A method that calculates the maximum reactive current based on generator speed, voltage conditions, and maximum rotor modulation index to prevent the actual modulation index of the power converter from exceeding a predetermined threshold, using a controller to monitor and adjust the rotor speed, stator voltage, and DC link voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If reactive current control is used to reduce rotor voltage during high-slip conditions, then stator voltage magnitude is reduced, but over-modulation at the rotor converter occurs and stability is lost
Solution Approach 1:
The patent changes the control parameter from stator voltage magnitude to maximum reactive current calculation. By calculating the maximum reactive current as a function of rotor speed, slip, and power factor, the system directly controls the reactive current component rather than indirectly controlling it through stator voltage magnitude, thereby preventing over-modulation while maintaining stability during high-slip conditions
Solution Approach 2:
The patent implements feedback control by continuously monitoring rotor speed, slip, and power factor to calculate the maximum reactive current. The controller adjusts the reactive current command based on the calculated maximum value, creating a closed-loop control system that prevents over-modulation by adapting to changing operating conditions in real-time
2Productivity
If DFIG operates at wider speed range to increase annual energy production, then energy production increases, but rotor modulation index exceeds 1.0 and stable control is lost during grid events
Solution Approach 1:
The patent applies preliminary action by calculating and limiting the maximum reactive current before grid events occur. By pre-establishing the maximum reactive current threshold as a function of operating conditions, the system is prepared to maintain stable control during high-voltage ride through and sub-synchronous resonance events, preventing modulation index from exceeding 1.0 even during wider speed range operation
Data Source
AI summary
The present subject matter is directed to a system and method for operating an electrical power circuit connected to a power grid. The power circuit includes a power converter electrically coupled to a generator. The method includes monitoring at least one speed condition of the generator during operation of the power circuit. Another step includes determining one or more voltage conditions of the power circuit. The method also includes calculating a maximum reactive current for the generator as a function of at least one of the speed condition or the one or more voltage conditions. Thus, the method also includes operating the generator based on the maximum reactive current so as to prevent an actual modulation index of the power converter from exceeding a predetermined threshold.