Digital Differential Protection for $3\phi$ Solid-State Transformers

This article presents the development, implementation, and testing of a digital differential protection for three-phase (3φ) solid-state transformers (SSTs). The developed digital differential protection is designed to implement the ANSI 87 T protection function. The differential currents used by the developed protection are created by the dq0 components of the currents flowing in the high and low voltage sides of a 3φ SST. Internal faults are detected and identified using the energy contents of high-frequency subbands present in the dq0 differential currents. The desired high-frequency subbands are extracted using the phaselet transform that processes signals without sensitivity to nonstationary changes in their frequency and/or phases. Energy contents in the extracted high-frequency subbands allow accurate, fast, and reliable detection and identification of faults in any part of a 3φ SST. The proposed digital differential protection is implemented for performance testing using a laboratory 25-kVA 3φ SST. Performance results demonstrate accurate, fast, and reliable response to different fault and nonfault events. Furthermore, responses of the developed differential protection are found to be consistent regardless of the fault type, fault location, and/or loading level.