123. Evaluation of Differential Protection Relay Performance During Transformer Inrush Current & CT Saturation Period

There are varieties of Protective relays using different measuring techniques to provide reliable & secure transformer protection. This includes electromechanical, static & modern multi-functional numerical relays. Within each group, various algorithms exists. The magnetizing current upsets the balance between currents at transformer terminals & is, therefore, experienced by the differential relay as a “false” differential current. The however, must remain stable during inrush conditions. The paper describes a new approach for Transformer differential Protection that ensures stability for inrush, over excitation & CT saturation period and provides dependability for internal faults. This approach combines harmonic restraint and blocking methods with wave shape recognition technique. We then analyze the inrush current, over excitation & current transformer saturation phenomena's cause wrong operation of relays. The paper also deals with new adaptive approach to differential protection of Transformer for fault cases with CT saturation due to DC component are presented. Improved stabilization for external faults is achieved by appropriate adjustment of the differential slope curve for time period of CT saturation. All necessary algorithms needed to implement the adaptation, i.e., Assessment of CT saturation time & determination of needed adaptation grade are described.