Common faults and treatment of global distribution transformer
Writer： Hengfeng you electric Time：2023-03-31 views：times
Common faults and treatment of global distribution transformer
Common faults of transformer, transformer fault treatment, distribution transformer maintenance
In the long-term operation process, the distribution transformer and its auxiliary equipment will be affected by many factors such as electricity, heat, machinery, chemistry and environment. The health status will gradually deteriorate. When its health status drops to a certain extent, it will affect the safe operation of the distribution transformer. Distribution transformer fault can be divided into internal fault and external fault. Internal fault refers to the fault of internal insulation or winding of distribution transformer body, and external fault refers to the fault of auxiliary equipment of distribution transformer. The fault degree is light and heavy. Although the distribution transformer can continue to operate when the fault is light, measures must be taken to monitor the development and change of the fault; When the fault is serious, the operation of the distribution transformer must be stopped immediately to prevent the expansion of the fault. In case of fault of distribution transformer, it is necessary to judge accurately and handle properly. It is not only necessary to prevent the expansion of fault, but also not to stop the operation of distribution transformer rashly. This requires the operators to improve the fault discrimination ability and accumulate operation experience, so as to correctly judge the fault of distribution transformer and deal with several common abnormalities or faults in time.
1、 Abnormal sound of distribution transformer:
1. When single-phase grounding or resonant overvoltage occurs in the power grid, the sound of distribution transformer is sharper than usual;
2. When large capacity power equipment is started, the load changes greatly, which increases the sound of distribution transformer. For example, when the distribution transformer is loaded with electric arc furnace, thyristor rectifier and other loads, due to the harmonic component, the "wow" sound or "cluck" intermittent sound will be emitted instantly in the distribution transformer;
3. Overload makes the distribution transformer emit a high and heavy "buzzing" sound;
4. Some parts are loose, such as the core screw of the iron core is not tightly clamped or there are missing parts on the iron core, and the distribution transformer sends out strong and uneven "noise" or "hammering" and "blowing";
5. The distribution transformer has poor internal contact or insulation breakdown, and the distribution transformer sends out "crackle" or "squeak" sound, which changes with the distance from the fault point;
6. When the system is short circuited or grounded, the distribution transformer will make a "crackling" noise through a large short-circuit current, and there will be a huge roar in serious cases;
7. When ferromagnetic resonance occurs in the system, the distribution transformer produces uneven noise.
2、 Distribution transformer overheating
Overheating is extremely harmful to distribution transformers. The insulation damage of distribution transformer is mostly caused by overheating. The rise of temperature reduces the withstand voltage capacity and mechanical strength of insulating materials. Iec354 guide for operating load of distribution transformer points out that bubbles will be generated in the oil when the hottest spot temperature of distribution transformer reaches 140 ℃, which will reduce the insulation or cause flashover, resulting in damage to distribution transformer. The overheating of distribution transformer also has a great impact on the service life of distribution transformer. The International Electrotechnical Commission (IEC) believes that within the temperature range of 80-140 ℃, for every 6 ℃ increase in temperature, the reduction rate of effective insulation service life of distribution transformer will double, which is the 6 ℃ rule for distribution transformer operation. The national standard GB1094 stipulates that the average temperature rise limit of winding of oil immersed distribution transformer is 65 ℃, the temperature rise of top oil is 55 ℃, and the temperature rise of iron core and oil tank is 80 ℃. IEC also stipulates that the hot spot temperature of the coil shall not exceed 140 ℃ at any time, and 130 ℃ is generally taken as the design value; The causes of abnormal oil temperature rise of distribution transformer may be: ① overload of distribution transformer; ② The cooling device fails (or the cooling device is not fully put into operation); ③ Internal fault of distribution transformer; ④ Misindication of temperature indicating device.
If it is found that the oil temperature of distribution transformer rises abnormally, the above possible causes shall be checked one by one, accurate judgment shall be made, checked and handled in time:
(1) If the operating instrument indicates that the distribution transformer has been overloaded, the indications of three-phase thermometers of single-phase distribution transformer group are basically the same (there may be several degrees of deviation), and there is no fault sign of distribution transformer and cooling device, the oil temperature rise is caused by overload, which shall be treated as overload.
(2) If the cooling device is not fully put into operation or has faults, it shall be handled immediately to eliminate the faults; If the fault cannot be eliminated immediately, the operating load of the distribution transformer must be reduced and operated according to the corresponding value of the cooling performance and load of the corresponding cooling device.
(3) If the remote temperature measuring device sends a temperature alarm signal and the indicated temperature value is very high, but the indication of the on-site thermometer is not high, and there are no other faults in the distribution transformer, it may be a false alarm of the fault of the remote temperature measuring circuit, which can be eliminated at an appropriate time.
(4) If the oil temperature of a phase in the three-phase distribution transformer group rises, which is significantly higher than the operating oil temperature of the phase under the same load and the same cooling conditions in the past, and the cooling device and thermometer are normal, the overheating may be caused by some fault inside the distribution transformer. Professionals should be notified to take oil samples for chromatographic analysis immediately to further find out the fault. If the chromatographic analysis shows that there is an internal fault in the distribution transformer, or the oil temperature of the distribution transformer keeps rising under the same load and cooling conditions, the distribution transformer shall be shut down according to the provisions of the site regulations.
(5) The main causes of abnormal temperature rise of main transformer are:
a) The hair inside is hot
b) The coil has an inter turn short circuit
c) The iron core has short circuit or abnormal eddy current
3、 The color of insulating oil of distribution transformer changes significantly
Insulating oil may come into contact with air during operation and gradually absorb water in the air, so as to reduce the insulation performance. At the same time, insulating oil may also absorb and dissolve a large amount of air. Because the oil often operates at high temperature, the oil contacts with oxygen in the air to generate various oxides, and these oxides are acidic, which is easy to corrode the metal and insulating materials inside the distribution transformer, increase the dielectric loss of the oil, reduce the insulation strength, and cause flashover in the distribution transformer, It is easy to cause breakdown of winding and housing.
4、 Oil injection in conservator or explosion-proof pipe
When the secondary system is suddenly short circuited, the protection refuses to operate, or there is a short circuit fault inside, and the air outlet and explosion-proof pipe are blocked, the internal high temperature and high heat will suddenly eject the distribution transformer oil, reduce the oil level after ejecting, and may cause gas protection action.
5、 The three-phase voltage is unbalanced
1. The neutral point displacement caused by three-phase load imbalance makes the three-phase voltage unbalanced;
2. Ferromagnetic resonance occurs in the system, which makes the three-phase voltage unbalanced;
3. Turn to turn and layer to layer short circuit occurs locally in the winding, resulting in three-phase voltage imbalance.
Common faults of transformer, transformer fault treatment, distribution transformer maintenance
6、 Action of relay protection
Relay protection action generally indicates that there is an internal fault in the distribution transformer. Gas protection is the main protection of distribution transformer. It can monitor some faults in distribution transformer. It often sends a signal first by light gas action, and then removes the gate by heavy gas action.
1. The reasons for light gas action are as follows:
(1) The air enters the distribution transformer due to the lax oil filtering, refueling and cooling system;
(2) The oil level decreases slowly due to temperature drop and oil leakage;
(3) Internal fault of distribution transformer, producing a small amount of gas;
(4) Internal short circuit of distribution transformer;
(5) Secondary circuit fault of protection device.
2. When no abnormality is found in the distribution transformer through external inspection, the nature of the gas in the gas relay shall be found out
(1) If the gas accumulated in the gas relay is nonflammable, colorless and odorless, and the mixed gas is mainly inert gas, the oxygen content is greater than 16%, and the flash point of oil does not decrease, it indicates that air enters the gas relay. At this time, the distribution transformer can continue to operate.
(2) If the gas is combustible, it indicates that there is an internal fault in the distribution transformer. The nature of the internal fault of the distribution transformer shall be identified according to the nature of the gas accumulated in the gas relay. For example, the color of the gas is:
a. Yellow nonflammable, with carbon monoxide content greater than 1-2%, is wood insulation damage;
b. Gray and black are flammable, and the hydrogen content is less than 30%, with tar smell and reduced flash point, indicating that the oil has been decomposed due to overheating or flashover fault has occurred in the oil;
c. Light gray with strong odor and combustible, which is the insulation damage of paper or paperboard.
(3) If the above analysis can not correctly judge the latent fault in the distribution transformer, gas chromatography can be used to make appropriate judgment.
During gas chromatographic analysis, the internal fault of distribution transformer can be judged from the content change of hydrogen, hydrocarbon, carbon monoxide, carbon dioxide and acetylene. Generally:
a. When the content of hydrogen and hydrocarbon increases sharply, but the content of carbon monoxide and carbon dioxide changes little, it is an overheating fault of bare metal (such as tap changer);
b. When the content of carbon monoxide and carbon dioxide increases sharply, it is the overheating fault of solid insulation (wood, paper and paperboard);
c. When hydrogen and hydrocarbon gas increase, acetylene content is very high, which is a discharge fault such as inter turn short circuit or multi-point grounding of iron core.
7、 Flashover and explosion of insulating porcelain bushing
1. Due to poor quality of sealing rubber gasket, improper installation position, loose nut pressure and other reasons, the casing is not tightly sealed, and the insulation is damaged due to moisture caused by water or moisture immersion;
2. There is internal free discharge in the insulation delamination gap of capacitive bushing;
3. Serious scale on the casing surface and large fragments and cracks on the casing will cause casing flashover and explosion accidents.
8、 Failure of tap changer
There is a "squeaking" discharge sound on the oil tank of the distribution transformer, the ammeter swings with the sound, the gas protection may send a signal, and the flash point of the oil decreases. These may be phenomena caused by tap changer failure.
1. The causes of tap changer failure are as follows:
(1) Insufficient contact spring pressure and uneven contact roller pressure of the tap changer will reduce the effective contact area, and serious wear due to insufficient mechanical strength of the silver plating layer will cause the tap changer to burn out;
(2) The tap changer has poor contact and cannot withstand the impact of short-circuit current;
(3) When reversing the tap changer, the switch is burnt out due to the wrong position switching of the tap changer;
(4) The phase to phase distance is not enough, or the performance of insulating materials is reduced, resulting in short circuit under the action of overvoltage.
In case of any change in current, voltage, temperature, oil level, oil color and sound, take oil samples for gas chromatography analysis immediately. When it is identified as a switch fault, the tap changer shall be switched to the intact gear for operation immediately.
2. During operation, some contacts of the switch may be worn, and some unused contacts may be immersed in oil for a long time, resulting in a layer of oxide film due to oxidation, resulting in poor contact of the tap. Therefore, in order to prevent tap changer failure, the DC resistance of each tap must be measured during switching. If the three-phase resistance is found to be unbalanced, the difference value shall not exceed 2%.
3. When reversing the tap, check the actual connection between the tap switch indicator outside the oil tank and the internal connector to ensure correct wiring. In addition, each time the tap changer is reversed, the handle of the tap changer shall be turned more than 10 times to eliminate the oxide film and oil dirt on the contact part, and then adjusted to a new position.
9、 Cooling device failure
The cooling device helps the winding and iron core to dissipate heat through the distribution transformer oil. Whether the cooling device is normal or not is an important condition for the normal operation of distribution transformer. When the cooling equipment fails or the cooling efficiency fails to meet the design requirements, the distribution transformer should not operate at full load, let alone overload. It should be noted that in the process of oil temperature rise, the temperature of winding and iron core rises quickly, while the oil temperature rises slowly. On the surface, the oil temperature does not rise much, but the temperature of iron core and winding is already very high. Therefore, when there is a fault in the cooling device, not only observe the oil temperature, but also pay attention to other changes in the operation of distribution transformer, so as to comprehensively judge the operation status of distribution transformer.
Common faults and treatment methods of cooling device are as follows.
(1) Cooling unit power failure. Common faults of cooling devices are power supply faults, such as fuse fusing, poor contact or disconnection of wires, etc. When it is found that the whole group of cooling device is shut down or individual fans are shut down and the submersible pump is shut down, check the power supply, find the fault point and deal with it quickly. If the power supply has returned to normal and the fan or submersible pump still cannot operate, press the reset button of thermal relay for a try. If there is no time to recover from power failure and the load of distribution transformer is large, temporary power supply can be used to make the cooling device run first, and then check and deal with power failure.
(2) Mechanical failure. Mechanical faults of the cooling device include motor bearing damage, motor winding damage, fan blade deformation, submersible pump bearing damage, etc. At this time, it needs to be replaced or repaired as soon as possible.
(3) Control circuit failure. When the components in the small control circuit are damaged, the lead wire is in poor contact or broken, and the contact is in poor contact, the cause shall be found out and handled quickly.
