Common faults and abnormalities of transformers

Transformer faults can be divided into internal faults and external faults.

Internal faults refer to faults that occur inside the box, including phase-to-phase short-circuit faults in windings, inter-turn short-circuit faults in one-phase windings, short-circuit faults between windings and iron cores, winding disconnection faults, etc.
External faults refer to various phase-to-phase short circuit faults between the external lead wires of the transformer, and single-phase ground faults that occur through the box shell due to flashover of the lead wire insulation bushings.
Transformer failure is very harmful. Especially when an internal fault occurs, the high-temperature arc generated by the short-circuit current will not only burn the insulation and iron core of the transformer winding, but also cause the transformer oil to thermally decompose to produce a large amount of gas, causing the transformer shell to deform or even explode. Therefore, the transformer must be removed when it fails.
Abnormal conditions of the transformer mainly include overload, reduced oil level, overcurrent caused by external short circuit, excessive oil temperature of the transformer during operation, excessive winding temperature, excessive transformer pressure, and cooling system failure, etc. When the transformer is in abnormal operation, an alarm signal should be given. Configuration of transformer protection
Main protection for short circuit faults: mainly longitudinal difference protection, heavy gas protection, etc.
Backup protection for short circuit faults: mainly including composite voltage blocking overcurrent protection, zero sequence (directional) overcurrent protection, low impedance protection, etc.
Abnormal operation protection: mainly includes overload protection, over-excitation protection, light gas protection, neutral point gap protection, temperature oil level and cooling system fault protection, etc. Non-power protection
Transformer protection that utilizes non-electrical quantities such as oil, gas, and temperature of the transformer is called non-electrical protection. Mainly include gas protection, pressure protection, temperature protection, oil level protection and cooler full stop protection. Non-electricity protection operates in tripping or signaling according to on-site needs.
1. Gas protection
When a fault occurs inside the transformer, due to the action of short-circuit current and short-circuit point arc, a large amount of gas will be generated inside the transformer, and the oil flow of the transformer will accelerate. The protection achieved by using gas and oil flow is called gas protection.
(1) Light gas protection: When a minor fault or abnormality occurs inside the transformer, the fault point is locally overheated, causing part of the oil to expand. The gas in the oil forms bubbles and enters the gas relay. The light gas protection operates and sends out a light gas signal.
(2) Heavy gas protection: When a serious fault occurs in the transformer oil tank, the fault current is large, and the arc causes the transformer oil to decompose in large quantities, generating a large amount of gas and oil flow. The impact of the baffle causes the heavy gas relay protection to act, sending out a heavy gas signal and The outlet tripped and the transformer was cut off.
(3) Heavy gas protection is the main protection for internal faults in the oil tank. It can reflect various faults inside the transformer. When a few inter-turn short circuits occur in the transformer, although the fault current is large, the differential current generated in the differential protection may not be large, and the differential protection may refuse to operate. Therefore, for internal faults in the transformer, it is necessary to rely on heavy gas protection to remove the fault.
2. Pressure protection
Pressure protection is also the main protection for internal faults in the transformer tank. Contains pressure relief and pressure sudden change protection to react to transformer oil pressure.
3. Temperature and oil level protection
When the temperature of the transformer rises and reaches the pre-warning value, the temperature protection sends an alarm signal and starts the backup cooler.
When the transformer leaks oil or the oil level drops due to other reasons, the oil level protection will act and send an alarm signal.
4. Cooler full stop protection
When the operating transformer cooler is completely stopped, the temperature of the transformer will rise. If not handled in time, the insulation of the transformer winding may be damaged. Therefore, when the cooler is completely stopped during the operation of the transformer, the protection will send out an alarm signal and cut off the transformer after a long delay.