Transformer is a very important equipment in the power system. Its stable operation is directly related to the smooth transmission of electric energy. Based on the operating characteristics of the transformer, this article introduces several methods for judging and analyzing its operating faults from the appearance and corresponding measures.
The operating conditions of transformers are generally relatively simple. They are placed in the open air and have to withstand the test of high temperature, severe cold, wind and sun. Especially during use in plateau areas, accidents such as breakdown and explosion often occur. The current failure phenomenon is , the causes and preventive measures are analyzed as follows:
1. Abnormal sound
1. When the sound is loud and noisy, it may be a problem with the transformer core. For example, when the clamp or the screw holding down the iron core is loose, the indication of the instrument is generally normal, and the color, temperature and oil level of the insulating oil do not change significantly. At this time, the operation of the transformer should be stopped and inspected.
2. If there is a boiling sound of water in the speaker and a "gurgling" sound of bubbles overflowing, it may be that the winding has a serious fault, which causes the nearby parts to become severely heated and the oil vaporizes. This sound will be emitted due to poor contact of the tap changer, severe overheating at local points or short circuit between turns of the transformer. at this time. The transformer should be stopped immediately and inspected.
3. When there is an explosion sound in the audio, which is loud and uneven, it may be that the insulation of the transformer body has broken down. at this time. The transformer should be stopped and inspected.
4. When there is continuous and regular impact or friction sound in the audio, it may be that some parts of the transformer are in mechanical contact due to iron core vibration, or it is abnormal sound caused by electrostatic discharge, and various measuring meters indicate There is no response to the temperature and temperature. Although this kind of noise is abnormal, it does not cause great harm to the operation. It is not necessary to stop the operation immediately. It can be eliminated during planned maintenance.
2. Abnormal temperature
Abnormal temperature: When the load, heat dissipation conditions, and ambient temperature remain unchanged, the temperature of the transformer is higher than the original temperature under the same conditions, and has a rising trend. This is also an abnormal rise in the temperature of the transformer, which is the same as the rise in super-limit temperature. Transformer failure symbol. Reasons for abnormal temperature rise include:
(1) Short circuit between turns, layers and strands of transformer
(2) Partial short circuit in the transformer core
(3) Fuel tanks, tank covers, etc. heat up due to magnetic leakage or eddy currents
(4) Long-term overload operation and accidental overload
(5) Deterioration of heat dissipation conditions, etc.
Preventive measures: When the load, heat dissipation conditions and ambient temperature of the operating transformer do not change much, the temperature rises abnormally and the changes are unbalanced. The operating temperature can be measured with a thermometer and other instruments to be close to or exceed the allowable value. At this time, the operating sound of the transformer is also abnormal, indicating that there is an obvious fault in the transformer. The operation should be stopped immediately and maintenance personnel should be arranged to deal with it.
3. Short circuit fault
1. Inter-turn short circuit: refers to the insulation damage of the wire itself. An inter-turn short circuit fault occurs.
Cause analysis: The transformer overheats and the oil temperature increases; the current on the power supply side increases slightly; sometimes there are squeaking and gurgling bubbling sounds in the oil: in severe cases, the oil pillow sprays oil. Long-term overload causes damage to the inter-turn insulation. Due to a short circuit at the transformer outlet or other faults, the winding is impacted by the short-circuit current and vibrates and deforms, thereby damaging the inter-turn insulation. The oil level decreases, causing the winding to expose the oil level. Inter-turn insulation breakdown: lightning strike Atmospheric overvoltage intrusion damages the turn insulation, the winding winding is unqualified or the turn arrangement and transposition, the winding press-fitting is incorrect, etc., causing the inter-turn insulation to be damaged.
Solutions and preventive measures: Check the hanging core, the insulation at the short circuit between turns is black and burnt; measure the DC resistance of the winding, the three-phase resistance is unbalanced; the short-circuit phase resistance is small; apply 10% to 20% of the rated voltage test on the low-voltage side winding , the damaged point will emit smoke. Generally, there are fewer inter-turn short circuits during operation the day after tomorrow. Therefore, for newly put into operation transformers, sound, voltage and current inspections should be strengthened to detect and deal with them early.
2. Winding grounding: refers to a short circuit between the winding and the grounding part.
Cause analysis: deterioration of oil quality, prolonged grounding fault, lightning atmospheric overvoltage and operating overvoltage, causing the windings to be deformed by the impact of short-circuit current, and the main insulation to age, crack, and break; the insulation strength of the transformer oil is reduced when it becomes damp, and the oil level Decline or aging of insulation.
Solution and preventive measures: Use a megger to measure the insulation resistance of the winding to ground. If the resistance is zero or close to zero, it is the ground phase. Lift out the body and check whether there are any debris, whether the insulating sleeves and insulating cardboard of the winding and core are damaged, and whether the winding is deformed. Test whether the breakdown voltage value of the oil is qualified. The fuse capacity and grounding resistance should be accurate and the sealing bolts should be tightened regularly.
3. Fuel injection explosion
The cause of the oil injection explosion was that the fault short-circuit current and high-temperature arc inside the transformer caused the transformer oil to rapidly age, and the relay protection device failed to cut off the power supply in time, causing the fault to persist for a long time. The internal pressure of the box continues to increase, and high-pressure oil and gas eject from the explosion-proof pipe or other weak points in the box, causing an accident.
4. Serious oil leakage
Leakage and oil leakage are common in transformers during operation. If the oil level is within the specified range, the transformer can still continue to operate or arrange planned maintenance. However, the transformer oil leaks seriously, or continuously overflows from the damaged area, so that the oil level can no longer be seen on the oil level gauge. The causes of oil leakage in transformers include cracking of welds or failure of seals, shock and external force impact during operation, and severe corrosion and damage to the oil tank.
Preventive measures: At this time, the transformer should be stopped immediately, the leakage should be repaired and the oil should be refueled. If the oil level of the transformer oil is too low, the bushing leads and tap-changer will be exposed to the air, and the insulation level will be greatly reduced, so it is easy to cause breakdown discharge.
5. Transformer oil gets damp
If not handled in time, the insulation level of the transformer will drop sharply, causing serious consequences such as inter-turn short circuit and coil burnout. A professional team should be asked to eliminate the moisture in the transformer oil, and the transformer oil should be replaced when the discharge is severe.
The main insulation is aging and the insulation level is reduced; the breakdown voltage of the transformer oil is low; or it is caused by the expansion of other faults, such as when the winding has an inter-turn short circuit or a ground fault, due to the arc and melted copper (aluminum) particles scattering, causing the accident Spread and expand, developing into a phase-to-phase short circuit. Or it may be due to poor internal welding of the wire, fusion due to overheating or blowout due to inter-turn short circuit, or winding breakage caused by short circuit stress.
The above judgment of distribution transformer failure based on the transformer's sound, temperature, oil level, appearance and other phenomena can only be used as an intuitive preliminary judgment on site. Because the internal fault of the transformer is not only an intuitive reflection of a single aspect, it involves many factors and sometimes even appears as an illusion. When necessary, transformer characteristic testing and comprehensive analysis must be conducted to accurately and reliably find out the cause of the fault, determine the nature of the accident, and propose a more complete and reasonable handling method.
Transformers are important equipment in the power system. In order to ensure their safe and stable operation, we must pay attention to various abnormal phenomena that occur during daily maintenance and find the cause of the fault through analysis, thereby improving reliability for the operation of the power system.