Modeling transformer magnetizing inrush current for a simple simulation

Hi,

I am looking to simulate the magnetizing inrush current of a transformer during a black start. The context of this simulation was originally to determine how large of an inverter is necessary to handle the inrush current, but I've simplified it for now to just an AC voltage source and a transformer to try to observe the inrush current. Here is a screen shot of my model, and the results I am getting.

My questions are:

1. Given that the secondary winding is open, why is the magnetizing inrush current not decaying to 0 after an initial magnetizing inrush?
2. Why is the magnetizing inrush only positive (large initial values are only in positive direction)?

Some more information that could be useful:

AC voltage source: 120V

Breaker: Closes at 1 second

Saturable transformer:

The multimeter is measuring the magnetizing inrush current, which matches the ammeter, which makes sense.

Thanks so much for any help, I really appreciate it!

Answer 1

[Sameer]

Hi @Loic

When a transformer is energized (especially if the breaker closes near the voltage zero‑crossing), the core flux can be driven far from its steady‑state value. This produces a DC flux offset that drives the core deep into saturation for one polarity, giving a large unidirectional inrush.

Why the current doesn’t decay to zero:

• Even with the secondary open, the transformer still draws magnetizing current to sustain flux.
• In your model, the core loss resistance (Rm = 100 pu) is very large (low losses), so there’s almost no damping. The DC offset decays very slowly, so you keep seeing a persistent no‑load current.

Why it’s mostly positive:

• The DC flux bias means one half‑cycle saturates heavily (large spike) while the opposite half‑cycle is much smaller.

To make the simulation more realistic:

• Use a realistic core loss resistance so the DC offset can decay.
• Add realistic winding resistance/source impedance for damping.
• Set initial flux (phi₀) explicitly if needed.
• Use a robust solver (e.g., Backward Euler) for stable no‑load simulations.

Hope this helps!