How to determine if a dry-type transformer has malfunctioned？

I. Sensory Intuitive Judgment (No need for professional instruments, key points of daily inspection)
1. Listen to the sound: Normal operation vs. Fault signals
Normal state: A Dry Transformer emits a uniform, low-pitched "buzzing" electromagnetic sound (caused by the magnetic expansion of the iron core), without noise or significant fluctuations.
Fault signals and corresponding problems:
Sound suddenly becomes louder and sharper: This may indicate overload (excessive current), loose iron core (loose bolts causing increased vibration of the iron core), or winding deformation (abnormal electromagnetic coupling).
"Sparkling" or "chattering" discharge sounds: This is likely due to insulation damage (winding inter-turn short circuit, lead wire discharging with the casing, poor grounding of the iron core causing suspended potential discharge), and in severe cases, may be accompanied by local sparks.
Irregular sound and "clicking" noise: This may be due to loose components (such as connection terminals, iron core clamps), or high temperature causing expansion and contraction of the insulation material.
2. Observe the appearance: Visible fault marks
Winding (coil) appearance:
Normal: The winding surface is clean and uniform (epoxy cast type is light yellow / brown, without discoloration or cracking).
Fault: The winding surface is discolored (blackened, burnt brown), cracked, deformed, or脱落 (broken off)， possibly due to overheating， insulation aging， or short circuit； if there is white powder (silicone grease precipitation)， it may be due to local overheating causing insulation decomposition.
Iron core and casing：
Exposed parts of the iron core are rusted or have oil stains (possibly due to poor sealing， water ingress， or insulation oil leakage (if equipped with a cooling system))；
Casing deformation， loose bolts (caused by transportation or operation vibration， which may cause internal components to shift)；
Loose grounding terminals， rusted (causing poor grounding of the iron core， resulting in discharge).
Lead wires and terminals：
Connection terminals are heated and discolored (copper busbars blackened， bolts oxidized)， or have traces of sparking (burned areas around the terminals)， possibly due to excessive contact resistance (causing local overheating).
Cooling system (forced air cooling type)：
Fan does not rotate， abnormal rotation speed， or overheated fan shell， or strange odor (fan failure leading to poor heat dissipation， causing The Transformer to overheat).
3. Smell： "Early Warning" of abnormal insulation materials
Normal： No obvious odor.
Fault signals：
Foul smell， plastic / epoxy burning smell： Core danger signal！ This indicates overheating and carbonization of the winding insulation (epoxy， insulation paper)， or local short circuit and fire (even though there is no oil in a dry transformer， the combustion of insulation materials will produce a pungent smell).
Moldy smell： Possibly due to damp environment or transformer moisture (decreased insulation performance， prone to discharge faults).
4. Touch the temperature： Abnormal surface temperature judgment (need to power off or use infrared temperature measurement， avoid burns)
Normal： The temperature of the dry transformer casing is generally not more than 80℃ (brief contact with the hand is possible， not hot)， different insulation grades (F grade， H grade) allow different temperature rises (F grade winding temperature rise ≤ 105K， H grade ≤ 125K).
Fault signals：
Local overheating of the casing (a certain point is hot to the touch， while the rest of the temperature is normal)： This may indicate winding inter-turn short circuit， poor contact of the lead wire， or local short circuit of the iron core.
Severe increase in overall temperature (far exceeding normal operating temperature)： This may be due to overload， cooling system failure， or poor ventilation (environmentally sealed， dust blocking the cooling channels).