Analysis of Common Faults in Mining Transformers
2021-04-29
During the use of transformers, there are various types of faults that occur due to differences in the duration of use, the degree of damage caused by overvoltage and overcurrent, and the different situations of use and maintenance. Therefore, in order to detect faults in a timely manner, analyze and eliminate them, and improve the ability and accuracy of fault detection and diagnosis, it is necessary to fully utilize various diagnostic techniques and integrate various historical data of equipment operation. Here are some opinions on the common fault diagnosis of transformers.1、 Inspection of Transformer Faults1. Look. Carefully inspect every part of the transformer from the outside to the inside by detecting any abnormal phenomena such as color when a fault occurs Leakage of oil. During the operation of transformers, oil leakage is a common phenomenon that can be easily identified by humans on the outer body of the equipment. Small transformers are installed in some shells, and attention should be paid to the bottom plate on or below the shell during inspection. Oil leakage varies greatly, mainly due to poor sealing between the oil tank and components, defects in welding or casting parts, equipment overload, or impact vibration. In addition, internal faults in transformers during use can also cause an increase in transformer oil temperature, volume expansion, and oil leakage Due to temperature, humidity, ultraviolet radiation, or the presence of acids, salts, etc. in the surrounding air, the surface paint film of the box may crack, bubble, or peel off. Overvoltage can cause surface cracks and discharge marks on porcelain parts, porcelain sleeves, and porcelain bottles. Loose screws in the fastening part of the porcelain bottle terminal wiring can cause surface contact surface peroxidation and discoloration.The discoloration of the moisture absorbent inside the dehumidifier is caused by excessive moisture absorption, damaged gaskets, and excessive water volume in the turbid room. Typically, the material of the moisture absorbent is alumina, silicone, etc., and it appears blue. If it turns pink, it needs to be regenerated.2. Listen. During normal operation, the transformer emits a uniform buzzing sound. If uneven or other noises are produced, it is considered an abnormal phenomenon.① The sound is louder and sharper than usual, partly due to overvoltage in the power grid, and partly due to transformer overload.② If a "chirping" sound is made and the high-voltage fuse is blown, it is because the tap changer is not closed properly. If a slight "squeaking" sound is produced, it is a discharge sound and a poor contact of the tap changer.③ If the transformer emits a "whooshing" sound and a "creaking" sound, and the sound is loud and chaotic, it may be a problem with the transformer's iron core.④ There is a hissing sound of discharge in the sound, and corona or blue sparks can be seen at night, which may be due to partial discharge of the transformer's wiring porcelain bottle.⑤ If you hear a "crackling" sound while standing in front of the transformer, it may indicate poor grounding or static discharge from ungrounded metal parts. At this time, the operation of the transformer should be stopped for maintenance.⑥ During operation, there is a loud and uneven cracking sound, which may be due to insulation breakdown of the transformer. If the dull "crackling" sound is the discharge of the conductor through the transformer oil to the shell.⑦ When there is a "gurgling" sound of water boiling in the transformer, it may be due to interlayer or turn to turn short circuit in the winding of the transformer. At this time, the operation of the transformer must be stopped for maintenance.3. Testing. The diagnosis of transformer faults based on sound, color, and other phenomena can only be used as a preliminary analysis and judgment on site, because the condition of any equipment cannot be directly reflected by a single aspect. There are many factors that can sometimes create false impressions. Therefore, it is necessary to measure and conduct comprehensive analysis in order to accurately identify the cause of the fault and the nature of the accident, and propose a more complete and economically reasonable solution.① Measurement of insulation. Mainly used to determine the insulation condition of windings, this is a relatively simple and effective method. Insulation resistance measurement usually uses an insulation resistance meter, and high-voltage transformers above 3KV generally use a 2500V insulation resistance meter. Measurement item: Measure the insulation resistance of the winding. Three items should be measured: high voltage winding to ground, low voltage winding to ground, and high voltage winding to low voltage winding. Here, "ground" refers to the metal shell of the transformer.② Measurement of DC resistance. When a transformer is subjected to short-circuit impact, it often causes winding distortion and deformation. In addition, insulation damage to the transformer winding can lead to turn to turn short circuits, and even phase to phase short circuits. When the winding fails, the distance between turns changes due to overall or local stretching and compression, and the resistance between turns also changes. When measuring, the DC resistance values of the high and low voltage windings of the transformer should be measured separately. According to the relevant regulations on the manufacturing of power transformers, the DC resistance difference between the windings of each phase of transformers above 630KVA should not exceed 2% of the average value of the three phases. For transformers below 630KVA, the phase difference should not exceed 4% of the average value of the three phases, and the line difference should not exceed 2% of the average value of the three phases.③ Measurement of absorption ratio. The insulation condition and moisture content of transformer windings can be determined by measuring the absorption ratio. When measuring, use a stopwatch to measure the time. When the insulation resistance meter reaches the rated speed of 120 revolutions per minute, connect the insulation resistance meter and start timing. Read one value at 15 seconds and another at 60 seconds. The ratio of the two values is the absorption ratio, which should be greater than 1.3. Otherwise, it indicates that the transformer has been affected by moisture or has insulation defects, and a detailed inspection is required.2、 Common faults and causes of high temperature rise in transformersDuring the operation of transformers, their losses mainly include hysteresis and eddy current losses of the iron core, as well as resistance losses of the windings. These losses generate a large amount of heat, which is normally transmitted to the outside through the heat dissipation device, while also causing the temperature of the transformer to rise to a certain extent. After a certain period of time, the transformer should reach a stable temperature rise. If the temperature rise is too high or too fast, or if the temperature rise is significantly higher compared to transformers of the same type and load, it should be considered as a fault. The common causes of high temperature rise in transformers are as follows:1. Local short circuit in the iron core.2. Poor grounding of the iron core.3. Iron yoke screw grounding.4. The power supply voltage is too high.5. Caused by poor contact, looseness, burning out, etc. of the lead part.6. The tap changer has malfunctioned.7. Caused by poor quality in coil manufacturing and maintenance.8. The large burrs and uneven gaps around the core laminations are caused.9. The transformer load is too high.10. Caused by unqualified primary and secondary wiring or poor contact of transformers.3、 Common faults and causes of abnormal output voltage in transformers. Under normal circumstances, the output voltage of transformers should be maintained within a certain range. Low or high voltage is considered a fault phenomenon and should be promptly eliminated. Firstly, the power supply voltage should be checked to see if it is caused by abnormal power supply voltage. If the power supply voltage is high, it can be measured through a transformer. In addition to power supply reasons, there are several common causes of malfunctions, including:1. Caused by short circuit between turns of the winding. This type of fault is often caused by transformer overload or insulation reduction. When a turn to turn short circuit occurs, the transformer will exhibit abnormal output voltage, sometimes accompanied by overheating and abnormal sound.2. It is caused by incorrect gear position of the tap changer. Tap changers are used to regulate voltage. If the gear is incorrect and cannot match the actual power supply voltage, it can cause the output voltage to be too high or too low.3. Caused by unbalanced three-phase load. If the secondary three-phase of the transformer is unbalanced, it will cause changes in the three-phase voltage of the transformer and abnormal output voltage. In severe cases, the phase voltage increases by 1.732 times.4. There is a phase loss on the high-voltage side. Phase loss on the high voltage side can cause serious imbalance in the output voltage on the low voltage side, and even damage to low voltage equipment. There are many reasons for high voltage phase loss, but generally it is caused by inadequate maintenance and other reasons that result in burnt wiring or faults in the old higher-level circuit.In addition, there are also many cases of abnormal secondary voltage output caused by defects in the iron core and winding.
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