The power transformer is one of the core equipment of the power grid, so its stable and reliable operation will play a very important role in the security of the power system. However, due to the limitations of design and manufacturing technology, technology and operation and maintenance level, transformer faults still occur from time to time, especially in recent years, the near zone or outlet short circuit (hereinafter referred to as outlet short circuit) fault of transformer has gradually attracted attention, which greatly affects the safe and stable operation of the power system.

Statistics show that more than 80% of the damage to the transformer is caused by the high current impact of the transformer's outlet short circuit. Therefore, strengthening the operation and maintenance of the transformer and taking practical and effective measures to prevent the short circuit of the transformer outlet are of great significance to ensure the safe and stable operation of the transformer.

Ⅰ. Installation of power transformers

1. Preparation of equipment and materials

The transformer shall be fitted with a nameplate. The nameplate should indicate the manufacturer's name, rated capacity, primary and secondary rated capacity, primary and secondary rated voltage, current, impedance, voltage and wiring group and other technical data. The capacity, specification and model of the transformer must meet the design requirements. The accessories and spare parts should be complete, and there should be factory certificates and technical documents. Section steel: section steel of various specifications should meet the design requirements and have no obvious rust. Bolts: except for anchor bolts and shockproof device bolts, galvanized bolts should be used, and corresponding flat washers and spring washers should be used. Other materials: welding rod, anti-rust paint, blending paint, etc. shall meet the design requirements and have product qualification certificates.

2. Main machinery and tools

Handling and hoisting equipment: lorry-mounted crane, automobile, hoist, hoisting chain, three-step lift, road wood, steel wire rope, belt rope, roller; installation equipment: bench drill, grinding wheel, electric welding machine, gas welding tool, electric hammer, bench vise, adjustable wrench, hammer head, threading board; test equipment: steel tape measure, steel ruler, spirit level, line weight, shaking table, multimeter, electric bridge and testing equipment.

3. Working conditions

The construction drawings and technical data should be complete and correct; the basic construction of the civil engineering has been completed.

4. Operation technology

Equipment inspection: the equipment inspection should be carried out by the installation unit, the supplier unit, and the representatives of the construction unit together, and they should make a record. Check whether the specifications and models of the transformer body and accessories and spare parts meet the requirements of the design drawings according to the equipment list, construction drawings and equipment technical documents. And judge whether it is complete, whether it has lost some parts or is damaged. In termss of the appearance inspection of the transformer body, it should be free of damage and deformation, and the paint is intact and free of damage, and the insulation porcelain and epoxy resin castings should be free of damage, defects and cracks. The secondary transportation of the transformer should be carried out by the lifting workers and the electrician together. It is best to use a lorry-mounted crane, or a chain hoist. When workers are transporting the transformers, care should be taken to protect the porcelain bottles. It is best to cover the high and low voltage porcelain bottles with cloth boxes or cartons to prevent damage. During the transportation of the transformer, there should be no shock or severe vibration. When the mechanical traction is used, the traction force should be below the center of gravity of the transformer to prevent tilting. The transport tilt angle should not exceed 15° to prevent deformation of the internal structure. Before handling or loading and unloading large transformers, the direction of the high and low voltage sides should be checked to avoid difficulty in changing directions during installation.

Stable installation of the transformer: the transformer can be directly thrown into the transformer room by a lorry-mounted crane when it is in place. Or a temporary track can be set up with road wood, and it is hoisted to the temporary track with three steps and a sling, and then pulled into a suitable position indoors with a sling. When the transformer is in place, it should be noted that its orientation and the distance from the wall should be consistent with the drawings, and the allowable error is ±25mm. When the drawings are not marked, the vertical distance should be in line with the track, and the horizontal distance from the wall should not be less than 800mm, and the distance from the door should not be less than 1000mm.

Installation of accessories; contents of the commissioning test of transformer: measure the DC resistance of the coil together with the bushing; check the transformation ratio of the transformer of all taps; check the wiring group of the three-phase transformer and the polarity of the single-phase transformer lead wire; measure the insulation resistance of the coil together with the bushing; AC withstand voltage test is carried out for the coil together with the bushing. Inspection before power transmission of the transformer: a comprehensive inspection shall be done before the test operation of the transformer, and it can be put into operation only when it is confirmed that the test operation conditions are met. Before the test run of the transformer, it must be checked and qualified by the quality supervision department.

The inspection contents before the test run of the transformer: all kinds of documents of the commissioning test are complete, and the data meets the requirements; the transformer should be cleaned and wiped clean, there is no debris left on the top cover, and the body and accessories are not damaged; the transformer primary and secondary leads are in correct phase and have good insulation; the grounding wire is good; the ventilation facilities are installed and working normally; the signs are hung and the doors are locked.

Ⅱ. The commissioning and operation of transformer

1. Inspection before commissioning and operation of transformers

Check whether the various documents of the commissioning test are complete, the phase and color of the primary and secondary leads of the transformer are correct, and the isobaric contact of the ground wire is good. The transformer should be cleaned and wiped clean, with no debris left on the top cover, no damage to the body and appendages, and no oil. The ventilation facilities have been installed and work normally, the emergency oil discharge facilities are in good condition, and the fire protection facilities are complete. The gate of the oil system of the oil-immersed transformer should be opened, the throttle indication is correct, and the oil level is normal. The voltage switching position of the oil-immersed transformer is in the normal voltage gear. The setting value of the protection device meets the specified requirements, and the operation and linkage test are normal.

2. Commissioning and operation of power transmission of transformer

(1) The no-load input impact test of transformer. When the transformer is put into operation for the first time, the full-voltage switching on can be carried out. Generally, start from the high-voltage side during the process. After the transformer is powered for the first time, the duration should not be less than 10min, and there is no abnormal situation.

(2) The inspection method of no-load operation of transformer is mainly to listen to the sound. When it is normal, there is a buzzing sound, but when it is abnormal, the following situations occur: when the sound is relatively loud and uniform, the applied voltage may be relatively high; when the sound is relatively loud and noisy, the core may be loose; when there is a squeaking discharge sound, there may be flashover between core and bushing surface; when there is a popping sound, it may be the core breakdown phenomenon.

(3) Commissioning and operation of transformer. After the no-load impact test, it can run at no-load for 24 to 28 hours. If it is confirmed that there is no abnormality, it can be run with half-load. After the half-load power-on commissioning operation of the transformer meets the safe operation regulations, the full-load commissioning operation can be carried out. The transformer should be commissioned and run at full load for 48 hours, and the temperature rise of the transformer, oil level, oil leakage, and the operation of the cooler should be checked again. After it passes the full load test, the handover procedure can be completed before it can be put into operation.

3. Transformer acceptance

Acceptance plan: after the transformer starts to be electrified, there is no abnormality within 24 hours, the acceptance procedures should be handled; the following materials and documents should be handed over during acceptance: design change certificate; product specification, test report, certificate of conformity and installation drawings and other technical documents; installation inspection and adjustment records.

The test adjustment and body inspection results of the power transformer and its accessories must comply with the construction specifications. Defects such as crack damage and enamel damage are strictly prohibited on the surface of high and low voltage porcelain parts. The installation position of the transformer should be accurate, the surface of the body should be clean, and the paint should be complete. The connection between the transformer and the line shall meet the following requirements:

(1) The connection is tight, the locking device of the connecting bolt is complete, and the porcelain bushing is not subject to external force.

(2) The zero line is connected down to the grounding device along the body, and it is firmly fixed.

(3) The connecting wires between the accessories of the body have protective tubes, the protective tubes and junction boxes are firmly fixed, and the box covers are complete.

(4) The busbar leading to the transformer and its support, wire protection tube and zero wire should be easy to disassemble and not hinder the movement of the transformer during maintenance. The bolt threads used for each connection leak out 2 to 3 nuts, the color of the protection tube is the same, and the bracket is completely anti-corrosion.

(5) The bushing of the transformer and its accessories and other non-live metal parts should be grounded and meet the relevant requirements.

4. Conclusion

Power transformers play an extremely important role in the power system. The quality of their operation, overhaul and maintenance is directly related to the economic benefits of power supply enterprises, social image and the quality of power consumption of users, as well as the power quality of the majority of users and the security of the entire system. Therefore, it is highly concerned by the power system. In the actual on-site operation and maintenance process, due to the limitations of using conditions, operating environment, operating cycle, and the responsibility and skill level of maintenance personnel, the actually reflected operating level varies greatly.