You're on the right track with your understanding of transformer inrush current. Your question touches on a key nuance related to remanent flux and the duration of the power interruption. Here's a breakdown

Understanding the Basics (As you already seem to):
Transformer Magnetization

When a transformer is energized, it needs to establish a magnetic flux in its core. The core's magnetic domains need to align themselves with the applied voltage. This process takes current – often much more than the normal operating current.

Inrush Current

The initial current surge during transformer energization, significantly higher than the steady-state current. It's caused by the core attempting to quickly reach its operating flux level. The magnitude and duration depend on factors like the transformer's size, impedance, the point on the voltage waveform when it's energized, and the residual flux in the core.

Remanent Flux (Residual Magnetism)

When a transformer is de-energized, the core doesn't completely demagnetize. A certain amount of magnetic flux remains trapped in the core. This is the remanent flux. Its magnitude and polarity depend on the last operating conditions and the transformer's core material.

The Key to the "No Inrush After Flicker" Scenario

The idea that a transformer might not experience a significant inrush current after a brief power interruption hinges on these factors:1.

Short Duration of Interruption

If the power interruption is very short (e.g., a few cycles or less), the remanent flux in the core has little time to significantly decay. It essentially "remembers" its magnetic state.2.

Polarity of Reapplied Voltage

The critical factor. Favorable Scenario: If the voltage is reapplied with the same polarity as it had just before the interruption, and the core is already magnetized in that direction (due to remanent flux), the core is "happy." It doesn't need a large current surge to quickly reach the desired flux level. The initial flux condition is already close to what's needed for normal operation. Unfavorable Scenario (Worst Case): If the voltage is reapplied with the opposite polarity, the situation is far worse than a cold start. The core now has to be driven from its remanent magnetization in one direction, through zero, and then up to the operating point in the opposite direction. This can result in even higher inrush currents than a standard energization. This is called a double inrush.3.

Load Characteristics

The type of load connected to the transformer can also play a role. A load that stores energy (like a motor) can help to maintain some level of flux in the transformer core during a brief outage.

Think of it this way

Imagine pushing a swing.

Inrush

You start the swing from rest. It takes a big push to get it going.

Brief Interruption, Correct Polarity

The swing is already moving in one direction. A small push in the same direction keeps it going smoothly.

Brief Interruption, Wrong Polarity

The swing is already moving in one direction. You push in the opposite direction. You need a huge push to stop it and get it moving the other way.

In summary



Yes, it's possible for a transformer to avoid significant inrush current after a brief power interruption. The key is the combination of a short interruption time and the polarity of the reapplied voltage. If the interruption is brief and the voltage polarity is favorable, the core doesn't need to be drastically re-magnetized. If the interruption is long or the voltage polarity is unfavorable, a significant inrush current can still occur (or be even worse than a normal energization).Practical Implications



Power System Design

Engineers consider these factors when designing power systems with sensitive loads that need uninterrupted power (e.g., using UPS systems).

Transformer Protection

Overcurrent relays and other protective devices need to be able to differentiate between genuine faults and inrush current. "Harmonic restraint" is often used in transformer differential relays to prevent them from falsely tripping during inrush.

Motor Starting: Motor starting inrush current, while different in nature, shares a common feature: a higher starting current that subsides to a normal running current. The concepts are parallel in terms of the challenge to grid stability and protection systems.Keep asking questions like these! It shows a deep understanding of the underlying principles. Flag for review