Countermeasure to Prevent the Incorrect Blocking of Differential Protection Applied to Converter Transformers

Harmonic blocking is usually applied by converter transformer differential protection to prevent protection mal-operation during an inrush condition, but it may incorrectly block the protection during an internal fault. This paper first makes an in-depth analysis on factors that lead to differential...

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Bibliographic Details
Published in:IEEE transactions on power delivery 2020-02, Vol.35 (1), p.95-105
Main Authors: Zhao, Yucong, Crossley, Peter
Format: Article
Language:English
Subjects:
Blocking
Circuit faults
Computer simulation
Converter transformer protection
Converters
Criteria
CT saturation
Current transformers
Failure analysis
Fault currents
Fault detection
Harmonic analysis
inrush identification
internal fault
Mathematical morphology
Saturation
Simulation
Surge protection
Surges
Transformers
Valves
Waveforms
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Summary:Harmonic blocking is usually applied by converter transformer differential protection to prevent protection mal-operation during an inrush condition, but it may incorrectly block the protection during an internal fault. This paper first makes an in-depth analysis on factors that lead to differential protection failure to trip for converter transformer internal faults. The main factor responsible for the protection failure to trip is half-cycle saturation of the converter transformer core, which results from the dc component in the fault current. This suggests that harmonic blocking may not be suitable for differential protection of converter transformers to identify inrush conditions. In this regard, this paper proposes a novel method using mathematical morphology to effectively discriminate between the conditions caused by inrush and internal faults. This method implements the waveform symmetry criterion and improved morphological gradient criterion to distinguish between these two conditions. The effectiveness of the proposed method is evaluated based on extensive simulation, and the presented results show that the proposed method can ensure operation for all types of internal faults. Additionally, this method improves the accuracy of inrush identification, even in extreme cases, such as sympathetic inrush and inrush with current transformer saturation.
ISSN:0885-8977
1937-4208
DOI:10.1109/TPWRD.2019.2901056