Analysis and judgment methods for common faults in transformers

Staff can judge the operation status of transformers at any time by observing changes in sound, vibration, odor, color change, temperature, and other phenomena, and analyze the causes, locations, and severity of accidents. Based on the acquired information, a comprehensive analysis is conducted, and various detection results are combined to make a final judgment on the operating status of the transformer.

（1） Intuitive judgment

1. Sound

During normal operation, due to the alternating current passing through the transformer winding, periodic alternating magnetic flux is generated in the iron core, causing magnetostriction of the electrical steel sheet. The magnetic force between the joints and layers of the iron core, as well as the electromagnetic force between the wires of the winding, cause vibration and produce an average "buzzing" sound. If uneven or other noises are produced, they are considered abnormal phenomena.

(1) If the sound becomes larger and more uniform than usual, one possibility is that there is overvoltage in the power grid, and the other possibility is that the transformer is overloaded. In high-power equipment (such as large ones), the load changes greatly, and due to the action of the fifth harmonic, a "wow" sound is instantly emitted inside the transformer. At this point, by referring to the voltage and circuit table, the nature of the fault can be determined. Then, according to the specific situation, change the operation mode of the power grid and reduce the load on transformers, or stop the operation of transformers.

(2) When the sound is loud and noisy, it may be a problem with the transformer core. For example, when the screws of the clamp or clamping iron core are loose, the indication of the instrument is generally normal, and there is no significant change in the color, temperature, and oil level of the insulation oil. At this time, the operation of the transformer should be stopped for inspection.

(3) When there is a squeaking sound of discharge in the audio system, it may be due to surface partial discharge in the transformer or bushing. If it is a problem with the casing, corona glow or small blue or purple sparks can be seen in harsh weather or at night. At this time, the surface of the casing should be cleaned of dirt and then coated with coatings such as silicone oil or grease. If it is a problem with the transformer body, placing your ear close to the transformer's oil tank will result in a "squeaking" or "crackling" sound inside the transformer due to local or poor electrical contact, and this sound will vary depending on the distance from the faulty location. If you stand in front of the transformer and hear a "crackling" sound, it may be due to poor grounding or static discharge of ungrounded metal parts. At this point, the transformer should be stopped and the distance between the grounding of the iron core and the ground of each live part should be checked to ensure that it meets the requirements.

(4) If there is a boiling sound of water in the sound system, it may be due to a serious fault in the winding, causing nearby parts to overheat severely. Poor contact of the tap changer and severe overheating at certain points will inevitably result in this sound. At this point, the operation of the transformer should be immediately stopped for maintenance.

(5) When there is a loud and uneven burst sound in the audio system, it may be due to insulation breakdown in the transformer body. At this point, the operation of the transformer should also be immediately stopped for maintenance.

(6) When there is continuous and regular collision or friction sound in the audio system, it may be due to mechanical contact between certain components of the transformer caused by core vibration. If it occurs at the oil pipe or wire on the outer wall of the fuel tank, it can be solved by increasing its spacing or enhancing fixation. In addition, the bearings of the cooling fan and oil pump are worn and the ball bearings have cracks, which can also cause mechanical friction sounds.



2. Odor, color

Internal faults in transformers and overheating of various components will cause a series of changes in odor and color.

(1) The fastening part of the ceramic sleeve terminal is loose, and the surface contact is overheated and oxidized, which can cause discoloration and abnormal odor.

(2) The poor demagnetization ability and uneven distribution of magnetic field caused by transformer leakage can lead to eddy currents and cause local overheating of various parts of the oil tank, resulting in discoloration of the paint.

(3) Porcelain casing stains and produces corona, which will emit a strange odor. When the cooling fan or oil pump is burned, it will emit a burnt smell.

(4) The discoloration of the hygrometer is caused by excessive moisture absorption, damaged gaskets, and excessive water entering its oil chamber.

3. Temperature

(1) Many faults in transformers are accompanied by rapid temperature rise. For transformers in operation, it is necessary to regularly check whether the connection between the terminals of the bushing and the busbar or cable is tight and whether there are any signs of heating.

(2) Overloading, environmental temperature exceeding specified values, cooling fans and oil pumps malfunctioning, radiator valves forgetting to open, oil leakage causing insufficient oil volume, thermometer damage, and internal faults in transformers can cause the readings on the thermometer to exceed the allowable temperature specified in the operating standards.

The above mentioned criteria for judging transformer accidents based on sound, vibration, odor, discoloration, and temperature phenomena can only serve as a preliminary on-site intuitive judgment. Because internal faults in transformers are not only a direct reflection of a single aspect, but also involve many factors, sometimes even leading to false appearances. Therefore, it is necessary to conduct measurements and comprehensive analysis in order to accurately and reliably identify the cause of the fault, determine the nature of the accident, and propose more comprehensive and reasonable handling methods.

（2） Analysis and judgment

1. Damaged iron core

Damage to the insulation between silicon steel sheets in the transformer core can increase eddy current losses in the core. The increase in eddy current loss is directly proportional to the square of the thickness of the silicon steel. If the insulation between silicon steel sheets is damaged, doubling the thickness of the silicon steel sheets will increase eddy current losses by four times, and heating will further damage the insulation of adjacent iron cores. At the same time, it will cause an increase in oil temperature and accelerate the deterioration of oil quality. In severe cases, gas action may occur.

2. Cracked iron core grounding plate

During operation, the internal metal components of a transformer will generate a floating potential due to induction. If the grounding is poor or the grounding piece is broken, intermittent discharge will occur. When the voltage rises, there may be a slight cracking sound inside. In severe cases, it may cause the gas relay to activate, and the oil color spectrum analysis result is unqualified. The reason may be that the grounding plate is not tightly inserted. For this, a hanging core inspection of the grounding plate can be carried out, and the damaged grounding plate can be replaced.

3. Short circuit between winding turns

When there is a short circuit between turns, the gas in the general gas relay is gray white or blue, and the tripping circuit acts. When the fault is severe, differential protection will act. The overcurrent protection action installed on the side causes the high-voltage fuse to melt. If a short circuit between turns cannot be detected in a timely manner, it will cause melted copper (lead) particles to scatter and affect adjacent windings. The reason for short circuit between windings is: poor heat dissipation or long-term overload causing damage to the insulation between turns; Due to a short circuit or other fault in the transformer, the winding is subject to vibration and deformation caused by the impact of the short circuit current, resulting in damage to the inter turn insulation. The decrease in oil level causes the winding to expose the oil level and the inter turn insulation to break through; During lightning strikes, atmospheric overvoltage invades and damages the inter turn insulation; No defects were found during winding, or the winding arrangement and transposition were incorrect, resulting in damage to the inter turn insulation.

4. Short circuit in the grounding part during winding

After the transformer oil is damp, the insulation strength decreases, the oil level decreases, or the insulation ages; Lightning atmospheric voltage and operating overvoltage, deformation of the winding caused by short-circuit current impact, aging, rupture, and breakage of the main insulation, and other defects; Accidents such as debris falling into the winding can occur. In the event of an accident, the gas relay usually activates and the explosion-proof tube sprays oil. If the center point of the transformer is grounded, the differential and DC protection will also activate. In general, measuring the insulation resistance of the winding to ground can reveal the presence of winding grounding.

5. Winding and lead wire breakage

Wire breakage due to poor connection or short circuit stress; Poor internal welding of the wire, inter turn short circuit causing the wire turns to burn out, and the occurrence of an arc at the broken wire can cause winding grounding and inter phase short circuit. Oil decomposition promotes the action of the gas relay. During processing, core suspension inspection can be carried out, and the DC resistance of the winding can be measured using an electric bridge to determine the faulty phase and rewind the winding.

6. Short circuit between winding phases

When there is a turn to turn short circuit or ground fault in the winding, the accident may spread and expand due to the arc and the scattered splashing of melted copper (lead) particles, which may develop into a phase to phase short circuit. When a phase to phase short circuit occurs, a strong short-circuit current will generate a violent arc. At this time, the gas relay, differential protection, and overcurrent protection will all activate. The explosion-proof tube is severely sprayed with oil, causing a sharp increase in oil temperature. By measuring the insulation resistance and the DC resistance and transformer ratio of the winding, the damage to the winding can be determined.

7. Broken casing or loose outlet connection

When the surface of the transformer bushing is dirty and foggy, rainy, or cloudy, it can cause corona discharge and produce a "squeaking" sound. The bushing is damaged and produces corona, and flashover can cause a strange odor; Loose connection of the conduit outlet, overheating of the surface contact surface, and oxidation can cause discoloration and abnormal odor; When there is ground breakdown or discharge between the sleeves, the external protective device operates.

8. Gas relay

When the gas relay is activated and gas accumulates inside, it does not prove that there is a fault inside the transformer, such as incomplete degassing of insulation oil or non vacuum oil injection. If the cooling system is not tight, it will cause air to enter the gas relay and activate it, sending a signal. After the air is discharged, the transformer can still continue to operate. However, when there is a fault inside the transformer, the situation is different. At this time, although gas accumulates inside the gas relay, it is not ordinary air. Therefore, these gases should be collected, their quantity, flammability color, and chemical composition should be analyzed to determine the nature of the fault. In areas where chromatographic analysis cannot be performed temporarily, the following methods can be used for identification:

(1) Colorless and non combustible is air.

(2) The yellow and flammable gases are generated by wood faults.

(3) The light yellow, flammable, and odorous gas produced by paper malfunctions.

(4) The gray black and flammable gas is produced by the decomposition of insulation oil due to iron faults.

In summary, when a transformer malfunctions, as long as a scientific and detailed analysis is carried out based on the state of the fault, the cause of the fault can be accurately determined, providing accurate basis for fault handling and ensuring the restoration of operation in the shortest possible time. At the same time, strengthening the inspection of transformer status in daily operation can also prevent the occurrence of faults and improve the reliability of power supply.