This paper presents the analysis and development of a new digital differential protection for three phase (3 ) power transformers. The proposed digital protection is based on extracting high frequency sub-band contents present in the axis 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 3 power transformers. The WPT-based digital differential protection is implemented for off-line performance evaluation using differential currents collected from a 5 kVA, 3 , 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 the WPT-based digital differential protection are obtained with simple implementation, reduced computational burdens, and small memory requirements.Index Terms-Digital differential protection, digital filters, internal fault currents, magnetizing inrush current, synchronous rotating reference frame ( axis), three phase power transformers, wavelet analysis.
NOMENCLATURE-point circular convolution operation. Continuous frequency.Discrete frequency. Scaling function.Wavelet function associated with . Set of real numbers. Inception angle. Set of integer numbers. Approximation signal at scale and shift .