Transformer Inrush Current Detection via Waveform Symmetry
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Solution Overview
Problem
Magnetizing inrush currents when a transformer is switched on with no load can erroneously trigger differential protection, leading to unnecessary tripping due to their high amplitude and DC component, which existing methods struggle to accurately distinguish from fault currents.
Innovation Solution
A method and device that detect magnetizing inrush currents by calculating waveform symmetry parameters (Ax and Bx) based on current differences across three AC phases, filtering out DC components, and using a waveform symmetry recognition algorithm to determine asymmetry, thereby preventing erroneous activation of differential protection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If second harmonic restraint is used for inrush current detection, then inrush current restraint is achieved, but the method requires dual-function configuration and complex hardware implementation
Solution Approach 1:
The patent changes the detection parameter from second harmonic content to waveform symmetry characteristics. Instead of analyzing harmonic composition, the method calculates symmetry factors by comparing current values at corresponding points in opposite half-cycles. This parameter transformation simplifies the protection system by eliminating the need for complex harmonic analysis hardware while maintaining effective inrush current restraint through symmetry-based detection.
2Device complexity
If waveform symmetry principle is used for inrush current detection, then hardware requirements are reduced and structure is simplified, but accurate distinction from fault current must be maintained
Solution Approach 1:
The patent implements feedback by continuously monitoring the symmetry factor and comparing it against predetermined thresholds. The protection system calculates symmetry factors in real-time based on current waveform data, and when the symmetry factor exceeds the threshold indicating significant asymmetry, the system blocks the protection action. This feedback mechanism ensures accurate distinction between inrush current (asymmetric, blocks protection) and fault current (symmetric, allows protection) while maintaining a simple structure.
Data Source
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AI summary
The present invention proposes a magnetizing inrush current detection method and device for a transformer. The method comprises: obtaining (S210) a difference current ix(n) for each AC phase of the transformer; calculating (S230), for each AC phase, at least two factors (Ax(n), Bx(n)) for waveform symmetry recognition, on the basis of the difference currents obtained within one period; for each type of factor, obtaining (S240) the maximum value of the three factors corresponding to the three AC phases, to serve as a maximum phase factor of that factor; based on the maximum phase factor obtained, using a waveform symmetry recognition algorithm (S250) to calculate a corresponding waveform symmetry parameter kmax; and if the waveform symmetry parameter kmax meets a requirement for waveform asymmetry (S260), then determining that a magnetizing inrush current has occurred.