Repair and maintenance of abnormal phenomena in transformer operation

According to the investigation, the application failure rate of transformer has seriously affected the security and stability of the power system, and will reduce the national economic growth rate in the long run. Therefore, relevant departments must pay attention to the implementation of measures to reduce the transformer failure rate, and improve the power application level in China on the basis of ensuring the normal operation of the transformer.

1、 Inspection points during transformer operation

1.1 Inspect the appearance of oil-immersed transformer

① Oil leakage. When there is black liquid or glittering on the outside of the transformer, first suspect whether there is oil leakage. Some large and medium-sized transformers have oil level gauges, so it is possible to observe the level line of the oil level and observe whether it has decreased. If the level line decreases, it indicates that there is oil leakage.

② Check the temperature of transformer oil.

③ Observe whether the hygroscopic agent of the respirator changes color. When the hygroscopic agent changes color seriously, the main reason is that the hygroscopic agent absorbs more moisture; The gasket is damaged; The respirator was damaged to varying degrees; Too much oil enters the oil cup. The commonly used hygroscopic agent is alumina, which requires high activity, or use silica gel to color it to make it blue. When the hygroscopic amount exceeds 20~25% of the hygroscopic agent's gravity, the hygroscopic agent will change from blue to pink. At this time, regeneration treatment is required.

1.2 Electrical detection device

1.2.1 Differential relay

When the current transformer is installed on the primary side and secondary side of the transformer respectively, the selection of the current transformer shall be determined according to the number of turns of the transformer, and the difference in the current of the short-circuit accident between the transformers shall be used to form the relay protection action. Therefore, once the transformer is in actual operation, the differential relay protection action occurs, which is usually caused by inter-turn short circuit.

1.2.2 Overcurrent relay

Overcurrent relay is mainly a relay that can protect the line when there is a short circuit accident in the power equipment and the line, or when the load in the line is high. If there is no inter-phase short circuit or overload in the external line of the equipment, it can be considered whether there is a short circuit in the transformer. In addition, when detecting the internal fault of the transformer, the gas composition dissolved in the oil can also be analyzed, and the indication of the thermometer can be observed to see whether there is any abnormality, and it can also be judged by listening to the sound.

2、 Abnormal phenomena in the operation of power transformer

2.1 Project cases

Technical parameters and operation parameters of main transformer in a 110kV substation. The main transformer is SZ11 type 110kV, with rated capacity of 100000kVA, rated voltage of (230 ± 8)/35kV, no-load current of 0.2% and short-circuit impedance of 12.95%. The main transformer is three-phase operation, with the rated current of 1650A at the low voltage side and 2520A at the high voltage side. The no-load loss during operation is 62.5kW and the load loss is 280kW. The main transformer is installed outdoors, with insulation grade of A, main body weight of 62t, oil weight of 35t and oil tank weight of 8.5t.

2.2 Cause and treatment of failure

2.2.1 Fault phenomenon

Since the main transformer in this power plant operates for a long time, it is necessary to clean the outside of the No. 1 main transformer after the power failure treatment. After the completion of the work, it is necessary to charge the No. 1 main transformer. The charging work is realized by the 110kV high-voltage side circuit breaker. When the high-voltage side circuit breaker is closed, the "10kV single-phase grounding" signal appears on the photon board of the signal panel in the central control room of the power plant. After a few minutes, the signal "No. 1 main transformer light gas action" appears again. The duty personnel immediately checked the 10kV section bus voltage, and the result is that the phase B voltage is zero, A C The two-phase phase voltage is normal, and the voltage of each phase of 110kV bus is also normal. The operator also inspected the main transformer body and found that there was an abnormal noise like water boiling at the excitation tap changer of the transformer. The operator on duty immediately stopped the transformer and reported to the leader.

2.2.2 Treatment process

The troubleshooting personnel should analyze the gas in the gas relay and find that the gas in the relay is yellow and has a flammable characteristic, which indicates that there is a fault in the transformer. After checking the tap switch of the main transformer, when the position of the tap switch changes, the position of the previous adjustment is at+1, while the actual position is between+1 and+2. Then the withstand voltage and chromatographic analysis tests of insulation resistance, DC resistance and insulating oil were carried out for the main transformer. Through comparison, two test results were unqualified: ① the contact resistance test of tap changer at+1 position failed; ② After chromatographic analysis of transformer insulation oil, the contents of acetylene (C2, H2), hydrogen (H2) and total hydrocarbon in the insulation oil exceed the specified value. According to the provisions of the Preventive Test Code for Electric Power Equipment, attention should be paid when C2H2>5PPm, H,>150PPm, and total hydrocarbon>150PPm. After the maintenance personnel emptied the oil in the transformer, they entered the transformer to inspect the tap changer and found that there were traces of discharge and slight burns on the two contact surfaces of the tap changer+1 gear. The surface was polished and the contact resistance was retested after the positioning pin was installed. The contact resistance of+1 gear was AO=1.229, BO=1.227, CO=1228 (the test temperature was 14 ℃), and the DC resistance met the specification requirements. Inject the insulating oil into the transformer, use the vacuum oil filter to filter the insulating oil automatically, and conduct the chromatographic analysis test of the insulating oil again. At this time, the values of acetylene, hydrogen and total hydrocarbon in the oil meet the standard. After the transformer is put into operation, everything is normal.

2.2.3 Reason summary

After analyzing the accident, it was found that the main reason for the accident was that the position of the tap changer had changed. After adjusting the gear of the previous tap changer, the bolt for the limit locking of the tap changer could not be fixed, and the dirt outside the transformer was removed in time, and the tap changer disc was adjusted properly. After the disc was rotated under the action of external force, it would drive the tap changer and rotate at the same time, The gear of the tap changer has been changed. After the power supply is reconnected, the connector of the tap changer will have poor contact. Therefore, partial discharge will occur on the surface of the two contacts, which will cause the content of acetylene, hydrogen and total hydrocarbon in the insulating oil to exceed the standard.

3、 Discussion on preventive measures for transformer failure

3.1 Preventive measures

After the analysis of the accident, the following points should be taken into consideration when tapping the tap changer of the no-load tap changer:

(1) Before and after switching the tap changer, measure the DC resistance of the line. However, when the tap changer has contact value, local melting damage will occur, and the following phenomena will occur on the tap changer surface that has not been used for a long time: ① oxidation; ② The contact is not clean enough; ③ The coating has peeled off; ④ Poor elasticity; ⑤ The problem of falling off in welding leads to the poor contact of the tap, the contact resistance gradually increases, and the problem of overheating occurs in 12 parts, which threatens the safe operation of the transformer. The transformer will be burned, and even lead to the rapid deterioration of the insulating oil in serious cases.

(2) When switching the no-load voltage regulating tap changer, ensure that it is in the state of power failure. During the switching process, increase the number of turns of the handle, and remove the oil stain on the contact surface to ensure that the switch is in place. When the switch is in place, align the pin.

3.2 Technical measures

① In order to effectively ensure the correctness of the action and avoid damaging the transformer due to the non-operation protection in case of transformer failure, it is necessary to pay attention to the annual inspection of the transformer and the setting value of the relay protection and strengthen the management of the protection strap.

② Do a good job in technical supervision, prohibit the equipment from over-cycle operation, and timely clean the indoor bus and porcelain bottles to ensure that the equipment has good insulation performance.

③ Do a good job in plugging the cable structure. Avoid mixing small animals in the switch room, and prevent single-phase grounding of small animals, resulting in short circuit at the transformer outlet. In the fully enclosed switch room, some exhaust fans are installed to keep the switch room dry.

3.3 Management measures

Constantly strengthen the authority of the power department to manage the whole power system and improve the operation environment of the transformer. It is not only necessary to improve the quality of its quality inspection, but also to improve the professional management level and equipment operation environment of the professional main transformer and distribution substation as soon as possible.

4、 Conclusion

To sum up, more attention should be paid to ensure the normal operation of the transformer. Only in this way can the security and stability of the power system be improved. Reasonable control, improvement of maintenance and guarantee of transformer function are important daily work of power department staff. Only by ensuring work efficiency can the development of power industry be well maintained.