TRANSFORMER DIFFERENTIAL PROTECTION

Transformer Differential Protection
✅ Purpose: Transformer differential protection protects the transformer against internal faults.
Basic Principle: It compares the current entering the transformer with the current leaving the transformer.
✅ Current Balance: Under normal conditions and external faults the compensated differential currents are approximately equal, so the relay does not trip.
✅ Internal Fault: During an internal fault, the currents become unbalanced and a differential current (operate current) flows through the relay.
✅ Operating Principle:
Differential Current = |I1 - I2|
where I1 and I2 are the properly ratio- and phase-compensated currents from the two sides.
✅ CTs: Current Transformers (CTs) are installed on different sides of the transformer to provide current signals to the differential relay.
✅ CT Ratio Matching: CT ratios must be selected or compensated so that the relay can correctly compare currents from the HV and LV sides.
✅ Vector Group Compensation: The transformer phase displacement, such as Dyn11, YNd1, must be compensated by the relay.
✅ Percentage Bias/Restraint: Modern differential relays use a biased characteristic to prevent unwanted tripping during heavy external faults and CT saturation.
✅ Two Important Quantities:
Operate/Differential Current Id Causes the relay to operate.
Restraint/Bias Current Ib Provides stability against unwanted operation.
✅ Internal vs External Fault:
Internal fault $\rightarrow$ Differential current increases $\rightarrow$ Relay trips.
External fault $\rightarrow$ Through current increases, but differential current remains low $\rightarrow$ Relay remains stable.
✅ Inrush Current: Transformer energization produces a high magnetizing inrush current, which can look like a differential current.
Inrush Restraint: To avoid unwanted tripping during energization, differential relays commonly use 2nd-harmonic restraint/blocking or advanced inrush detection.
✅ CT Saturation: CT saturation during heavy external faults can create false differential current. The biased differential characteristic helps maintain stability.
✅ Trip Command: When the differential current exceeds the relay's operating characteristic, the relay sends a trip command to the transformer circuit