The Worst Transformer Inrush Current Occurs When…

Possibly 40 times the full-load current

When a transformer is first energized or reenergized after a short interruption, the transformer may draw inrush current from the system due to the core magnetization being out of sync with the voltage. The inrush current may approach short-circuit levels, as much as 40 times the transformer’s full-load current.

Inrush current may cause fuses, reclosers, or relays to falsely operate. It may also falsely operate faulted-circuit indicators or cause sectionalizers to misoperate.

Attempted interruption anywhere here leaves B_r on the core.

The worst inrush occurs with residual flux left on the transformer core.

Consider Figure 1 and Figure 2, which shows the worst-case scenario. A transformer is deenergized near the peak core flux density (B_max), when the voltage is near zero.

The flux decays to about 70% of the maximum and holds there (the residual flux, B_r). Some time later, the transformer is reenergized at a point in time when the flux would have been at its negative peak. The system voltage is crossing through zero and rising positively.

The positive voltage creates positive flux that adds to the residual flux already on the transformer core (remember, flux is the time integral of the voltage!).

This quickly saturates the core. The effective magnetizing branch drops to the air-core impedance of the transformer.

The air core impedance is roughly the same magnitude as the transformer’s leakage impedance. Flux controls the effective impedance, so when the transformer core saturates, the small impedance pulls high-magnitude current from the system. The core saturates in one direction, so the transformer draws pulses of inrush every other half cycle with a heavy dc component. The DC offset introduced by the switching decays away relatively quickly.

Figure 3 shows an example of inrush current following a reclose operation measured at the distribution substation breaker.

4 factors significantly impact inrush current //

1. Closing point

The point where the circuit closes back in determines how close the core flux can get to its theoretical maximum. The worst case is when the flux is near its peak. Fortunately, this is also when the voltage is near zero, and switches tend to engage closer to a voltage peak (an arc tends to jump the gap).

2. Design flux

A transformer that is designed to operate lower on the saturation curve draws less inrush current. Because there is more margin between the saturation point and the normal operating region, the extra flux during switching is less likely to push the core into saturation.

3. Transformer size

Larger transformers draw more inrush, that’s the fact. Their saturated impedances are smaller. But, on a per-unit basis relative to their full-load capability, smaller transformers draw more inrush. The inrush into smaller transformers dies out more quickly.

4. Source impedance

Higher source impedance relative to the transformer size limits the current that the transformer can pull from the system. The peak inrush current with significant source impedance (Westinghouse Electric Corporation, 1950) is defined as:

where //

• i₀ – peak inrush current without source impedance in per unit of the transformer rated current
• X – source impedance in per unit on the transformer kVA base

Other factors have less significance. The load on the transformer does not significantly change the inrush. For most typical loading conditions, the current into the transformer will interrupt at points that still leave about 70% of the peak flux on the core.

While interruptions generally cause the most severe inrush, other voltage disturbances may cause inrush into a transformer. Voltage transients and especially voltage with a DC component can saturate the transformer and cause inrush.

Some examples are:

Voltage sags – Upon recovery from a voltage sag from a nearby fault, the sudden rise in voltage can drive a transformer into saturation.

Sympathetic inrush – Energizing a transformer can cause a nearby transformer to also draw inrush. The inrush into the switched transformer has a significant dc component that causes a dc voltage drop. The DC voltage can push the other transformer into saturation and draw inrush.

Lightning – A flash to the line near the transformer can push the transformer into saturation.

Reference // Electric power distribution equipment and systems / T.A. Short (Purchase at Amazon)