The Development of a d–q Axis WPT-Based Digital Protection for Power Transformers

This paper presents the analysis and development of a new digital differential protection for three phase (3φ ) power transformers. The proposed digital protection is based on extracting high frequency sub-band contents present in thed-qaxis components of the differential currents in order to identify internal faults. The desired high frequency sub-band contents are extracted using a single-stage wavelet packet transform (WPT), which offers parameterizing the energy in the high frequency sub-band contents using a finite number of coefficients. The WPT coefficients contain information that offer accurate, fast, and reliable detection and classification of transient disturbances, including magnetizing inrush and internal fault currents that may be experienced by 3φ power transformers. Thed-qWPT-based digital differential protection is implemented for off-line performance evaluation using differential currents collected from a 5 kVA, 3φ, laboratory power transformer. Off-line test results demonstrate accurate, fast, and reliable detection, classification, and response to different types of fault and non-fault currents. These encouraging performances of thed-qWPT-based digital differential protection are obtained with simple implementation, reduced computational burdens, and small memory requirements.