Transformers are vital components in power systems, and their stable operation is crucial. When energizing a transformer after installation or maintenance, you might sometimes hear a significant humming noise or a loud "bang." This phenomenon, possible in power operations worldwide, often raises concerns among operators: does it indicate a fault or damage to the transformer? This article delves into the root causes, helping you distinguish between a normal physical event and a potential warning sign.

 

1. Main Cause of the Switching Noise: Inrush Current

The most common cause of the loud noise is the inrush current or magnetizing inrush current. This is a transient, high-magnitude current surge—far exceeding the rated current—that flows into the primary winding when a transformer is switched onto the power grid in a no-load state (without connected load).

1.1 How Inrush Current Happens

Transformers work on the principle of electromagnetic induction. During steady-state operation, the magnetic flux (Φ) in the core closely follows the applied voltage (V). However, at the exact moment of switching, this balance is disrupted.

1.1.1 The Transient Process of Flux Establishment

The AC voltage is a sine wave. The instant of switching (the voltage phase angle) is critical for the initial establishment of magnetic flux. According to Faraday's law:

V = N(dΦ/dt)

where V is instantaneous voltage

 N is turns

 dΦ/dt is the rate of change of flux.

 

Le point clé :If the switch closes precisely at the voltage zero-crossing (instantaneous voltage is 0), the required rate of change of flux (dΦ/dt) must also be zero. However, the core flux needs to start from zero. To "catch up" with the steady-state flux waveform corresponding to the voltage, the system creates a large transient flux offset. Establishing this offset requires a very large instantaneous current in the winding—this is the inrush current.

 

1.2 Physical Effects and the Resulting Sound

This instantaneous current, which can be 6-10 times the rated current, triggers strong physical effects inside the transformer:

(1)Intense Electromagnetic Forces:The high current creates a powerful magnetic field around the windings, subjecting the winding conductors and core structural parts to significant mechanical stress, causing tiny, instantaneous displacements or vibrations.

(2)Core Magnetostriction:Transformer cores are made of laminated silicon steel. Under the intense, changing magnetic field, the core material exhibits "magnetostriction," meaning its physical dimensions change minutely with magnetic field strength.

(3)Génération sonore :The structural vibrations from electromagnetic forces combine with vibrations from core magnetostriction. These vibrations, with rich frequency content, are amplified by the tank, radiators, etc., and are released as an audible "bang," "boom," or humming noise.

 

Caractéristique	Valeur typique / Description
Peak Magnitude	Up to 6-10 times rated current
Durée	Typically several power cycles (tens to hundreds of milliseconds)
Taux de décroissance	Fast decay to steady-state no-load current (depends on system impedance & transformer design)
Main Physical Effects	Electromagnetic mechanical forces, Core magnetostriction
Son typique	A single or short series of "bang," "boom," or "thud" sounds

 

Table 1: Transformer Inrush Current Characteristics & Impact

 

2. Distinguishing Normal Inrush Noise from Fault Indicators

While most loud noises are due to normal transformer inrush current, certain abnormal sounds can signal problems. Knowing how to differentiate is key.

 

2.1 Characteristics of Normal Inrush Noise

(1)Timing Relatif (RT)Occurs strictly at the moment of switching on. The sound is brief (usually ends within 1-2 seconds).

(2)La nature:A single "boom" or a short series of "bangs" that quickly subsides. Afterward, the transformer runs with a uniform, steady hum.

(3)Aucun autre symptôme :After the sound, there is no persistent abnormal noise, no unusual smoke or gas emission, and no protective device operation (e.g., relays do not trip).

 

2.2 Characteristics of Abnormal Sounds Indicating Potential Faults

If the following sound patterns occur during or after switching, it may indicate an issue:

2.2.1 Sounds from Internal Electrical Faults

e.g., inter-turn winding shorts, severe internal discharge. Sounds may mix with inrush noise but have differences:

(1)Sharp cracking or continuous arcing/discharge sounds:May accompany internal insulation breakdown.

(2)Accompanied by protection trips:Often causes rapid operation of overcurrent, differential, or gas (Buchholz) relays, tripping the circuit.

(3)Persistance:The fault and associated sound/vibration typically do not quickly subside like inrush effects.

 

2.2.2 Sounds from Mechanical Issues

e.g., poor core grounding, loose clamping structures excited by the large inrush forces.

(1)Persistent, rhythmic vibrating or knocking sounds:Abnormal, rhythmic "ticking," "clicking," or uneven "buzzing" persists during steady-state operation after switching.

(2)Localized sound:Sometimes the sound source can be traced by listening near different parts of the transformer, indicating a local mechanical problem.

 

Aspect	Normal Inrush Sound	Suspected Fault Sound
Timing	Strictly at switch-on moment	May occur at switch-on, but more often persists or recurs afterward
Qualité sonore	Low-pitched "boom," brief	Sharp crackle, continuous arcing sound, rhythmic knocking/vibration
Durée	Disappears completely within seconds	Persists or occurs intermittently
Accompanying Electrical Signs	No protection trip; meters return to normal	Often protective trips, sustained abnormal meter readings, sudden changes in online DGA data
Opération ultérieure	Sound immediately returns to uniform steady hum	Operational noise remains abnormally loud or unstable

Table 2: Guide to Identifying Transformer Switching Sounds

 

3. How to Manage & Mitigate Inrush Current and Its Effects

Even if the noise is normal, high inrush current poses potential stress to the grid and the transformer itself (e.g., voltage dips, risk of relay misoperation). The following internationally adopted measures can help manage it.

3.1 Operational Measure: Controlled Switching (Phase Control)

(1)Principe:As explained, inrush magnitude depends directly on the voltage phase angle at switch-on. Theoretically, switching at the voltage peak results in the smallest inrush.

(2)Action:Use synchronous (point-on-wave) controllers or smart circuit breakers to precisely close the contacts at a specific point on the voltage waveform (near the peak).

(3)Effet:Can reduce inrush current peaks by over 50%, significantly dampening the noise and electromagnetic stress.

 

3.2 Device-Level Measures: Remanence Management or Series Resistor

3.2.1Remanence Management:

(1)Principe:After de-energization, the core may retain some magnetic flux (remanence). If the "target flux" from the switching voltage is opposite to the remanence direction, they add up, worsening the flux offset and inrush. Actively managing remanence (e.g., bringing it to zero or knowing its state before switching) aids optimal switching strategy.

(2)Effet:Reduces unpredictability and, combined with phase control, optimizes switching.

 

3.2.2Pre-insertion Resistors:

(1)Principe:Before the main breaker contacts close, auxiliary contacts with a series resistor close first. The resistor limits the initial current, allowing flux to build up smoothly. After a brief delay, the main contacts close, shorting the resistor.

(2)Effet:Physically limits the initial current peak; a very effective method for large HV transformers.

 

3.3 System Design Measures: Inrush Current Limiters & Protection Logic

(1)Inrush Current Limiters:Specially designed inductive or non-linear resistive devices temporarily connected to the circuit to suppress the peak.

(2)Protection Relay Setting Optimization:Adjusting the harmonic restraint settings in differential or overcurrent relays helps them better distinguish between inrush current (rich in 2nd harmonic) and genuine fault current, preventing nuisance trips.

 

Conclusion et recommandation

Hearing a loud bang when switching on a transformer is no immediate cause for panic. It is most likely due to inrush current—a strong but generally normal physical phenomenon. However, a responsible operator must have the ability to discriminate. Monitor for the warning signs listed above. If in doubt, or if abnormal symptoms persist, consult a qualified professional for inspection. Proper understanding and management of transformer energization ensure both safety and reliability.

LuShan, HNE.1975, est un fabricant professionnel chinois spécialisé dans les transformateurs de puissance et les réacteurs pour50années. Les produits phares sonttransformateur monophasé, triphaséseultransformateurs, transformateur électrique,transformateur de distribution, transformateur abaisseur et élévateur, transformateur basse tension, transformateur haute tension, transformateur de contrôle, transformateur toroïdal, transformateur à noyau R ;Inductances CC, réacteurs CA, réacteurs filtrants, réacteurs de ligne et de charge, selfs, réacteurs filtrants et produits intermédiaires à haute fréquence.

 

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