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 .
A successful development and implementation of the dq-axis components and the wavelet packet transform (WPT)-based hybrid technique for power transformer protection is introduced in this paper. In this approach, the high-frequency subband contents of the dq-axis components of the differential current is extracted using the WPT. This characterization helps to provide enough information to efficiently detect and discriminate internal faults from inrush currents in power transformers. This hybrid method provides accurate information with only one level of the WPT of the dq-axis components of the differential current for power transformer protection. A real-time experiment is carried out for different normal and abnormal operating conditions, such as inrush and internal faults for different cases of loading, to test the efficacy of the proposed algorithm. The experimental results show fast, accurate, and reliable responses to all different types of disturbances that may occur in power transformers.Index Terms-Digital differential protection, magnetizing inrush current, power transformers, synchronously rotating reference frame dq, wavelet packet transform (WPT).
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