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Inrush current testing is typically conducted on newly installed or recently overhauled transformers. This test involves energizing the transformer at full voltage under no-load conditions, usually from the primary side, and then switching off. The transformer is switched on and off five times, with the first energization lasting at least 10 minutes (ideally 30 minutes), and subsequent intervals of at least 5 minutes.
Transformers experience an inrush current, or magnetizing surge current, upon energization. This test helps evaluate:
1.Protection Relay Reliability: Ensuring that magnetizing inrush current does not cause a misoperation or unnecessary tripping of protection relays. During no-load energization, the primary side experiences significant magnetizing inrush current, while the secondary side remains open with no current, potentially leading to differential misoperation.
2.Mechanical Strength of the Transformer: Verifying that the transformer can withstand the mechanical stresses generated by the magnetizing inrush current, which can be 4-5 times the rated current, and up to 6-8 times in extreme cases.
3.Insulation Strength: Assessing insulation robustness against operating overvoltages caused by switching, where grounded transformers can experience twice the phase voltage at the terminal, while ungrounded transformers may see up to three times the phase voltage.
The magnitude of inrush current during transformer energization depends on the instantaneous phase angle of the voltage and the residual flux in the core:
Maximum Inrush Current: Occurs when the voltage is at 0° (zero-crossing) at the moment of energization. In this case, the magnetic flux, which lags the voltage by 90°, builds up to its maximum value. Since the core flux cannot change instantaneously, a counteracting direct current (DC) flux develops, decaying over time, leading to flux saturation and high inrush current, reaching 6-8 times the rated current.
Minimum Inrush Current: Occurs when the voltage is at its maximum (90° phase angle) during energization, as this phase does not induce magnetic saturation, hence avoiding high inrush current.
(1).Dependence on Initial Voltage Phase: The inrush current amplitude varies based on the initial phase angle of the voltage at energization. For single-phase transformers, inrush current peaks at a 0° energization angle and is minimized (close to zero) at a 90° angle. In three-phase transformers, the inrush current differs between phases due to the 120° phase difference.
(2).Asymmetric Waveform with DC Component: Inrush current contains a large unidirectional (non-periodic) component, skewing the waveform to one side of the time axis, with shorter interruption angles as inrush increases.
(3).Peaked Waveform with Harmonic Content: Inrush current produces a peaked waveform with substantial harmonic content, primarily second harmonics. Higher inrush currents have smaller interruption angles and less second harmonic content.
(4).Rapid Decay at Initial Stage: The inrush current decays quickly at first, then gradually slows. The decay constant is related to core saturation depth, with more saturation leading to faster decay. Larger transformers exhibit longer inrush decay, though generally slower than short-circuit current decay.
