Current differential principle is a well-known principle used for protection of transformers, motors, generators, buses, and any other type of power equipment with input and output current measurements. Further the principle is used in developing percent differential protection which can be programmed to the desired sensitivity for detecting in-zone faults, and security during external faults. This protection dependability is usually achieved by modeling a differential-retraining characteristic with two regions, operating and non-operating, and tracking the real differential restraint ratio during faults. Some external faults with high DC offset and high X/R system time constant would easily saturate the installed current transformers (CTs), which in return would cause high differential/restraint ratio above the preset characteristic into the operating region. In such cases the differential protection would operate and cause unwanted transformer trip. This paper focuses on a number of enhancements applied to the differential principle of the main differential protection, and defines guidance on how to setup the protection for better sensitivity and security. The paper is supported by fault cases, showing the improved security and dependability during internal/external faults with and without CT saturation.
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