- Transformer Inrush Current Definition: Transformer inrush current is defined as the high transient current drawn by a transformer when it is energized.
- Initial Flux and Voltage: At startup, there is no initial flux, and the flux wave starts from zero, following the voltage waveform.
- Peak Flux and Core Saturation: The flux can peak at double the steady-state maximum value, leading to core saturation and high inrush current.
- Transient Nature of Inrush Current: Inrush current is transient, lasting only a few milliseconds, but can be up to 10 times the normal rated current.
- Impact on Circuit Operation: High inrush current can cause fuse or breaker interruptions, component failures, and introduce noise and distortion into the electrical system.
When an unloaded transformer is energised, its primary draws the current needed to establish core flux. In sinusoidal steady state, the ideal flux waveform is 90 electrical degrees from the applied voltage.
Flux is the time integral of winding voltage. Its steady-state maximum therefore occurs one-quarter cycle from the voltage maximum. At a voltage zero crossing, the corresponding steady-state flux is at either its positive or negative peak, depending on the direction of the crossing.
The core does not always start with zero flux. It can retain residual, or remanent, flux from previous operation. The transient at energisation depends on how that initial flux compares with the steady-state flux required at the closing instant.
The flux waveform must satisfy both the applied-voltage integral and its initial residual-flux condition. Any mismatch produces an offset transient. Its magnitude and decay depend on the switching angle, residual flux, core characteristic, transformer resistance and system source impedance.
According to Faraday’s law of electromagnetic induction, winding voltage is proportional to the number of turns and the rate of change of flux. Flux is therefore obtained by integrating voltage with the correct initial condition, including any residual flux. The idealised sinusoidal relation is shown below.
If the transformer is switched at a voltage zero crossing and residual flux is assumed to be zero, the first half-cycle can drive a one-direction flux change approaching twice the normal steady-state peak. The ideal calculation is shown below.
Here, φm is the maximum steady-state flux in the simplified model. Energising at a voltage zero crossing can drive the total flux towards twice that value when residual flux is zero. Residual flux of the same polarity can increase it further. Whether the core saturates, and by how much, depends on its magnetising characteristic.
When the core enters the steep saturated region of its magnetising curve, a small additional increase in flux requires a large magnetising current. The resulting transient primary current is called transformer inrush current or magnetizing inrush current.
Magnetising inrush is a transient excitation current associated with energisation, voltage recovery or interaction with another transformer. Its peak and decay are not fixed. The initial peak can be several times rated current, while the offset may persist for many cycles or longer in some transformer and system conditions.
Inrush is not an internal fault in transformer, but protection devices can see its current as an apparent fault. A high inrush current in power transformer can also cause a temporary voltage dip and stress switching equipment. Protection must remain secure for inrush while still tripping quickly for a genuine internal fault during energisation.
Possible effects include nuisance operation of fuses or breakers, contact wear, voltage distortion and current-transformer saturation. Device selection should use the transformer’s expected inrush envelope and the protective device’s time-current response. Controlled point-on-wave switching can reduce inrush in suitable high-voltage applications by accounting for residual flux and closing angle.
