Circulating Current Differential Protection
Principle of Operation
Two (or) more current transformers (CTs) are fitted at each end of the protected part.
The secondary CT windings are linked in parallel in a circulating current arrangement via a relay.
Under normal circumstances or during external faults, the CTs create equal and opposing currents, resulting to zero resultant current in relay (relay stays idle).
During an internal failure, the CTs create an imbalanced current, causing a differential current which activates the relay & trips the circuit breaker.
Current Circulation Scheme
CTs are linked in such a manner that the current flows between the CTs under normal circumstances.
A differential relay is attached across the midway of the circulating circuit to check any imbalance.
Applications for Circulating Current Differential Protection
1). Transformer Differential Protection
Protects in opposition to internal short circuits in transformer windings.
Requires ratio matching, phase shift correction, and zero-sequence filtering to eliminate false trips.
Helps to identify defects like as
- Inter-turn faults &
- Winding insulation failures.
2). Generator Differential Protection
Protects generators against stator winding failures such phase-to-phase and phase-to-ground.
Large generators must have inbuilt short circuit protection to avoid catastrophic damage.
3). Busbar Differential Protection
It safeguards busbars from internal short circuits.
A high-impedance differential protection method is often employed to minimize CT saturation problems.
Ensures quick fault resolution, avoiding cascade failures.
4). Transmission Line Differential Protection
Used for short-distance transmission lines and cable protection.
For longer lines, distance (or) line current differential protection is sometimes used alternatively.
Ensures quick tripping of internal issues.
5). Motor Differential Protection.
Used in big motors to identify stator winding issues.
Allows for the early identification of phase-to-phase & phase-to-ground defects in the stator.
Advantages of Circulating Current Differential Protection
- Only internal defects are recognized, preventing false alarms from exterior issues.
- Identifies low-level internal issues fast.
- External disturbances do not cause incorrect functioning.
- Can be used with a variety of equipment, including transformers, generators, busbars, & lines.
Challenges & Mitigations
- Issues with CT saturation may be mitigated by using high-impedance differential protection & stabilizing resistors.
- Numerical relays may correct for CT mismatches.
- In transformers, zero-sequence filtering may reduce undesirable currents.