Earth Fault Protection Using Second Harmonic Analysis
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Solution Overview
Problem
Existing earth protection devices in three-phase electrical systems face issues with false residual current detection due to non-linear operation of current sensors, leading to potential nuisance tripping and inhibition of true fault protection, especially during transformer switching and intermittent faults.
Innovation Solution
An earth protection device and method that detect single-phase fault currents by analyzing the ratio of second harmonic to fundamental component signals, activating or blocking second harmonic restraint based on predetermined thresholds to prevent false tripping and ensure accurate fault detection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If current sensors operate in non-linear parts of their curve, then measurement range is extended, but false residual currents appear
Solution Approach 1:
The patent converts the harmful effect of second harmonics (which indicate sensor non-linearity) into a useful diagnostic tool. By calculating the second harmonic ratio Ih2/Ih1 for each phase and comparing it against a threshold, the system identifies which phase is causing the non-linear operation. This allows the protection to either block false tripping for that specific phase or adjust the measurement approach, thereby maintaining measurement range while improving residual current accuracy.
Data Source
Figure 1~2
Figure 3A~3B
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AI summary
The device has a processing unit (12) including an analyzing unit providing signals (H1) representing fundamental frequency of current and providing signals (H2) representing a second harmonic of current. A retaining unit (16) blocks detection of earth fault when a rate of the second-harmonic is reached. Detectors (17, 17A, 17B, 17C) detect single-phase current having the individual second harmonic rate greater than high threshold for enabling detection of earth fault and/or blocking actions of the retaining unit. An independent claim is also included for a method for earth-fault protection of a three-phase electrical apparatus.