Why does excitation inrush current occur when a transformer is energized with no load?

When a transformer is energized with no load, the excitation current immediately undergoes a transient phase, and its peak value may exceed several times the rated load current. Compared to the normal excitation current, the steady-state no-load current of the transformer is several tens of times larger. This transient current is referred to as excitation inrush current. Excessive inrush current can lead to relay misoperation, preventing the smooth energization of the transformer into the circuit. The magnitude of the inrush current depends on the phase of the line voltage at the moment of transformer energization and the state of the magnetic core residual flux. The no-load closing process mainly manifests as the transitional change of the transformer's magnetic flux.

When the magnetic core has no residual flux, if the voltage at the moment of closing is at its maximum, the magnetic flux leads the voltage by 90 degrees, causing the flux to be zero. In this case, there is no flux in the magnetic core before and after closing, and no transitional process occurs. If the voltage at the moment of closing is zero, the flux reaches its maximum value. To keep the instantaneous flux at zero when closing, a counter-flux (DC component flux) is formed inside the core to cancel out the instantaneous flux (steady-state flux), and its magnitude is equal but opposite in direction. As a result, the synthesized magnetic flux is zero at the moment of closing, but it becomes twice the steady-state flux after half a cycle.

This doubling of the flux value causes significant saturation of the core, leading to a large excitation current due to the non-linear nature of the core magnetization curve. When the core has residual flux φ and it aligns with the direction of the magnetic flux of the first half-cycle, the instantaneous flux will increase to 2×φ+φ. This results in a larger excitation inrush current.

Since the DC component flux decays, the excitation inrush current also decreases and attenuates to the normal steady-state excitation current value within a few cycles. Therefore, the harm to the transformer is not significant. In reality, the likelihood of such a large inrush current is small due to the following reasons: a. The probability of the circuit breaker being energized at the voltage zero-crossing point is low. b. During inrush current flow, the voltage in the external circuit itself decreases. c. The residual flux may not necessarily be in phase with the voltage change direction and may decrease depending on the status of the external circuit.

One phase of the three-phase transformer bank always experiences a transitional phenomenon, as inrush current is inevitable whenever it is energized. Typically, the inner winding of a core-type transformer has a smaller reactance, resulting in a larger inrush current when exciting the inner winding. To prevent relay misoperation, methods such as using interlocking relays for a certain period after transformer energization can be employed.

LuShan, est. 1975, is a Chinese professional manufacturer specializing in power transformers and reactors for 49 years. Leading products are single-phase transformer, three-phase transformers, DC inductors, AC reactors, filtering reactor, expoxy resin high-voltage transformer and intermediate, high-frequency products. Our transformers and reactors are widely used in 10 application areas: rapid transit, construction machinery, renewable energy, intelligent manufacturing, medical equipment, coal mine explosion prevention , excitation system, vacuum sintering, central air conditioning.

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