After the dry-type transformer is energized, an alternating magnetic flux is generated in the iron core. This alternating magnetic flux passes through the iron core, continuously emitting a “buzzing” electromagnetic sound. The electromagnetic sound varies proportionally with the magnitude of the current. The larger the current, the louder the sound. If the current suddenly increases, the sound also suddenly intensifies; if the current is interrupted, the sound disappears. Therefore, based on the change in the transformer’s sound, combined with visual inspection, it is entirely possible to determine the nature and location of internal faults in the transformer.

1) Normal and phase loss. When the transformer is powered on, the normal situation is as follows: when the first phase is energized, there is no sound from the transformer because no current flows through it; when the second phase is energized, the transformer emits a small, uniform electromagnetic “buzzing” sound because the current circuit on the high-voltage side is already connected; when the third phase is energized, the electromagnetic “buzzing” sound increases because three-phase current flows through. If there is an abnormality, such as when the second phase is not connected, there will be no sound when the second phase is energized, and only when the third phase is energized will there be sound; if the third phase is not connected, the sound will not change, just like when only two phases are connected. There are roughly three reasons for a phase loss: ① the power supply lacks one phase; ② one phase of the transformer’s high-voltage fuse is blown; ③ due to careless transportation of the transformer, coupled with the thin high-voltage leads, vibration causes the wires to break (but not connected to the shell).

2) The tap changer is not properly connected or has poor contact. When the transformer is put into operation, if the tap changer is not properly connected, a loud “chirping” sound will be emitted. In severe cases, it may cause the high-voltage fuse to melt and the fuse to fall off. If the tap changer has poor contact, a slight “squeaking” sound of spark discharge will be generated. Once the load increases, the tap changer will be burned out.

3) Foreign objects falling in and loose through-bolts. When the through-bolts that clamp the core of the transformer become loose, there are nuts left on the core, or small metal objects fall into the transformer, the transformer will emit a “ding-ding-dang-dang” hammering sound or a “hoo…hoo…” blowing sound, as well as a “sizzling” sound like a magnet attracting a small gasket, while the voltage, current, and temperature of the transformer remain normal.

4) Dirt and damage to the transformer’s high-voltage bushing. When the high-voltage bushing of the transformer becomes dirty, with surface enamel peeling off or damaged, surface flashover may occur, accompanied by audible “hissing” or “sizzling” sounds, and sparks can be seen at night.

5) The transformer core grounding wire is disconnected. When the transformer core grounding wire is disconnected, the transformer will produce a slight discharge sound of “beep beep beep”.

6) Internal discharge. When power is turned on and you hear a crisp “crackling” sound, it is the discharge of the conductive lead through the air to the transformer casing. This is often caused by oil leakage from the cooling pipes below the transformer tank cover or from the bottom of the tank shell, resulting in a lack of oil in the upper part of the transformer. If you hear a muffled “crackling” sound through the liquid, it is the discharge of the conductor through the transformer oil to the casing. The reason is that the distance between the conductor and the tank shell is not sufficient or the insulating oil contains moisture.

7) Disconnection or short circuit of external lines. When the line breaks at the connection or T-connection point of the conductor, it will be intermittently disconnected during windy weather, generating arc light when in contact. At this time, the transformer emits a “chirr-chirr-chirr” sound like a frog; when there is a grounding or short circuit fault on the low-voltage line, the transformer emits a “boom-boom” sound; if the short circuit is close, the transformer will emit a heavy roar.

8) Overload of transformer. When the transformer is severely overloaded, it will emit a low-pitched “buzzing” sound.

9) Short circuit occurs in the winding. When a layer-to-layer or turn-to-turn short circuit occurs in the transformer winding, the transformer emits a “gudugudu” sound, akin to boiling water.

During the operation of a special transformer, the sound becomes noticeably louder but remains relatively uniform. This abnormal noise is generally caused by excessive load. It can be determined by measuring the load or by touching the temperature of the distribution transformer’s casing. Once the cause of the overload is identified, the load exceeding the allowable range should be reduced. If the overload is severe, the possibility of insulation deterioration and damage to the special transformer should also be considered, and further necessary tests should be conducted.

2. The distribution transformer experiences impact loads, such as frequent starting of large motors and intermittent operation of electric welding machines, which cause the sound to increase and decrease abruptly and irregularly. By measuring the load, a clear judgment can be made. As long as the allowable load standard and voltage fluctuation are not exceeded, no action is required; if they are exceeded, the responsibility should be clarified before taking action. If the user’s equipment is approved for use, the power supply department should consider increasing the capacity of the distribution transformer; if the user’s equipment is used without approval, not only should it be immediately stopped, but also necessary penalties should be imposed.

3. When the system experiences a remote short circuit or grounding, and the fuse fails to blow in a timely manner, intense and uniform noise will occur due to the severe imbalance in the magnetic circuit caused by the magnetic flux under the influence of the short-circuit current.

4. If the internal iron core or clamping parts of the distribution transformer become loose, the sound will increase and there will be intermittent noise. After testing, if the load current is not high, the temperature is not elevated, and the secondary no-load voltage is basically balanced, the operation can be left unmonitored. However, if the sound continues to increase, consideration should be given to replacing or repairing it.