Working Principle of Transformer Differential Protection
The current entering the transformer = The current leaving it (in normal conditions).
When a fault occurs inside the transformer, this balance is disrupted, and the protective system acts fast to isolate the transformer, preventing further damage.
Essential Equipment in the System
1). Current Transformers (CTs)
CT1 and CT2 measure the current entering and leaving the transformer.
2). Differential Relay
Compares the currents measured by CT1 and CT2.
If the difference exceeds a set threshold, it trips the circuit breaker.
Working Conditions
1). Normal Condition
Current in CT1 = Current in CT2
No action needed.
Ex: I₁ = I₂ = 10 A (Balanced currents).
2). Fault Condition
Internal fault disrupts current balance.
The relay detects the difference and trips the breaker.
Ex: I₁ = 10 A, I₂ = 5 A (Unbalanced currents).
Why Is This Important?
- Ensures fast & reliable isolation of transformer faults.
- Protects the transformer from severe damage.
- Maintains system stability & prevents cascading failures.
Visualization
CT1 & CT2 monitor the current on both sides of the transformer winding.
Differential Relay acts when I₁ ≠ I₂, protecting the system.
Differential protection isn’t limited to transformers and also widely used to protect
- Generators,
- Motors, &
- Busbars in power systems.