1. Definition
Inrush current is a high-magnitude transient current that flows into a transformer when it is first energized (switched on) or after a power outage recovery. It occurs due to core saturation and can reach 5–15 times the transformer's rated current for a short duration (typically a few cycles to seconds).
2. Causes of Inrush Current
Core Saturation:
At zero-voltage switching, the magnetic flux in the core can reach up to twice the normal flux, driving the core into saturation.
Saturation reduces inductance, causing a sudden surge of current.
Remanent Flux (Residual Magnetism):
If the transformer was previously energized, residual flux in the core can worsen saturation when re-energized.
Point-on-Wave Switching:
Worst case: Switching at voltage zero-crossing maximizes flux asymmetry.
3. Characteristics
High Peak Value (5–15× rated current).
DC Offset & Harmonic Content (mainly 2nd harmonic).
Decays Rapidly (within 0.1–2 seconds).
Asymmetrical Waveform (unidirectional pulses).
4. Effects of Inrush Current
Mechanical Stress: Can damage windings due to electromagnetic forces.
Protection Relay Misoperation: May falsely trigger overcurrent/differential relays.
Voltage Dips: Causes temporary voltage drops in the power system.
5. Mitigation Methods
Soft Starting (Sequential Phase Energization)
Pre-insertion Resistors (limits initial current)
Controlled Switching (Point-on-Wave Closing)
Differential Relay Harmonic Restraint (blocks tripping on 2nd harmonic)
6. Standards & References
IEEE C57.12.00 (Transformer Inrush Current Limits)
IEC 60076-1 (Power Transformer Testing)
Key Takeaway:
Inrush current is a normal but potentially harmful transient during transformer energization. Proper protection and switching techniques are essential to minimize risks.
