At present, most of China's dry-type power transformers are three-phase solid forming SC series, such as: SCB9 series three-phase winding transformer, SCB10 series three-phase foil transformer, SCB9 series three-phase foil transformer. Its voltage level is generally in the range of 6-35KV, and the maximum capacity can reach 25MVA. Do you want to know more about dry-type transformers? The following small series will give you a detailed introduction to the structural form, cooling method, and type of dry-type transformer, and what are the advantages of oil-immersed transformer? Installation and commissioning, how to judge the fault from the sound, noise caused by external structures and solutions, and selection points. Let's take a look!

■ Structural form of dry-type transformer

1. Open type: It is a commonly used form, its body is in direct contact with the atmosphere, suitable for a relatively dry and clean room, (when the ambient temperature is 20 degrees, the relative humidity should not exceed 85%), there are generally two cooling methods: air self-cooling and air cooling.

2. Closed type: the vessel body is in a closed shell and is not in direct contact with the atmosphere (due to poor sealing and heat dissipation conditions, it is mainly used for mining and belongs to the explosion-proof type).

3. Pouring type: Epoxy resin or other resin casting as the main insulation, it has a simple structure and small size, and is suitable for transformers with smaller capacity.

■ Cooling method of dry-type transformer

Dry-type transformer cooling methods are divided into free air cooling (AN) and forced air cooling (AF). When naturally air-cooled, the transformer can operate continuously for a long time at the rated capacity. When forced air cooling, the output capacity of the transformer can be increased by 50%. It is suitable for intermittent overload operation, or emergency overload operation; Because the load loss and impedance voltage increase greatly during overload, it is in a non-economic operating state, so it should not be in long-term continuous overload operation.

■ Types of dry-type transformers

1. Impregnated air-insulated dry-type transformer: rarely used at present. The insulation and insulation structure materials of winding wires are made of insulating materials with different heat resistance grades according to the needs of B-class, F-class and H-class insulated dry-type transformers.

2. Epoxy resin cast dry-type transformer: the insulating materials used are polyester resin and epoxy resin. At present, epoxy resin is mostly used in cast insulated dry-type power transformers.

3. Wrapped insulated dry-type transformer: Wrapped insulated dry-type transformer is also a kind of resin insulation, and there are few manufacturers at present.

4. Composite insulated dry-type transformer:

(1) The high-voltage winding adopts pouring type, and the low-voltage winding adopts impregnated insulation;
(2) The high pressure adopts the pouring type, and the low pressure adopts the foil winding made of copper foil and aluminum foil.

■ What are the advantages of dry-type transformers and oil-immersed transformers?

1. Dry-type power transformers can avoid the danger of fire and explosion of transformer oil due to faults during operation. Since the insulation materials of dry-type transformers are all flame-retardant materials, even if the transformer fails during operation and causes a fire or has an external fire source, the fire damage will not be expanded.

2. Dry-type power transformers will not have the problem of oil leakage like oil-immersed transformers, and there are no problems such as transformer oil aging, and usually the operation and maintenance and overhaul workload of dry-type power transformers is greatly reduced, and even maintenance-free.

3. The dry-type power transformer is generally an indoor device, and it can also be made into an outdoor type for places with special requirements, and it can be installed in the same room as the switch cabinet to reduce the installation area.

4. Due to the lack of oil, the dry-type power transformer has few accessories, no oil storage cabinet, safety airway and a large number of valves and other components, and no sealing problems.

■ Installation and commissioning of dry-type transformer

1. Unpack and check before installation

Check whether the packaging is in good condition, after the transformer is unpacked, check whether the transformer nameplate data meets the design requirements, whether the factory documents are complete, whether the transformer is in good condition, whether there are signs of external damage, whether the parts are displaced and damaged, whether the electrical supports or connecting wires are damaged, and finally check whether the spare parts are damaged and in shortage.

2. Transformer installation

First of all, check the foundation of the transformer to check whether the embedded steel plate is level, and there should be no cavities under the steel plate to ensure that the foundation of the transformer has good seismic and sound-absorbing properties, otherwise the noise of the transformer after installation will increase. Then, the transformer is moved to the installation position with a roller, the roller bar is removed, and the transformer is accurately adjusted to the design position in place, and the installation levelness error meets the design requirements. Finally, four short channel steels are welded on the four corners of the transformer base, that is, the embedded steel plate, so that the transformer does not move in the process of use.

3. Transformer wiring

The minimum distance between the charged bodies and from the live bodies to the ground should be ensured during wiring, especially the distance between the cable and the high-voltage coil. When the current is greater than 1000A (such as the 2000A low-voltage bus used in this project), there must be a soft connection between the bus and the transformer terminal to compensate for the thermal expansion and contraction of the conductor and isolate the vibration of the bus and the transformer. The necessary contact pressure must be maintained at the electrical connections at each wiring, an elastic element (such as a disc ring or spring washer) should be used, and a torque wrench should be used when tightening the connecting bolts, as shown in Table 1

4. Transformer grounding

The grounding point of the transformer is at the base of the low-voltage side and leads to a special grounding bolt marked with a grounding center, through which the grounding of the transformer must be reliably connected to the protective grounding system. When the transformer has a shell, the shell should be reliably connected to the grounding system, and when the low-voltage side adopts the three-phase four-wire system, the neutral wire should be reliably connected to the grounding system.

5. Check the transformer before operation

Check whether all fasteners are loose, whether the electrical connection is correct and reliable, whether the insulation distance between the charged bodies and the charged bodies to the ground is in accordance with the regulations, there should be no foreign objects near the transformer, and the surface of the coil should be cleaned.

6. Debugging before transformer operation

(1) Check the transformation ratio and connection group of the transformer, measure the DC resistance of the high and low voltage windings, and compare the results with the factory test data provided by the manufacturer.

(2) Check the insulation resistance between the coils and the coil to the ground, if the insulation resistance is significantly lower than the factory measurement data of the equipment, it indicates that the transformer is damp. When the insulation resistance is lower than 1000Ω/V (operating voltage), the transformer must be dried.

(3) The test voltage of the withstand voltage test should meet the regulations, and the temperature sensor TP100 should be taken out when doing the low-voltage withstand voltage test, and the sensor should be returned to its position in time after the test is completed.

(4) When the transformer is equipped with a fan, the fan should be energized and ensured to operate normally.

7. Test run

After careful inspection before commissioning, the transformer can be energized for trial operation. During the commissioning, special attention must be paid to checking the following points. There are no abnormal sounds, noises and vibrations. There is no abnormal odor such as burnt odor. Whether there is discoloration due to local overheating. Whether the ventilation is good. In addition, the following points should be noted.

First, although the dry-type transformer has strong moisture resistance, it is still susceptible to moisture because it is generally an open structure, especially the insulation level of the dry-type transformer produced in China is low (lower insulation grade). Therefore, dry-type transformers can only be operated with a relative humidity of 70% in order to achieve high reliability. Dry-type transformers should also avoid long-term outages to avoid severe moisture. When the insulation resistance value is lower than 1000/V (operating voltage), it means that the transformer is seriously damp and should stop the trial operation.

Second, the dry-type transformer used for step-up of power station is different from oil-immersed transformer, and it is forbidden to operate open-circuit on the low-voltage side, so as to avoid the insulation breakdown of the dry-type transformer due to overvoltage on the grid side or lightning strike on the line. In order to prevent the hazard of transmission overvoltage, a set of overvoltage protection arresters (such as Y5CS zinc oxide arrester) should be installed on the voltage bus side of the dry-type transformer.

■ How to judge the fault from the sound of the dry-type transformer

1. The sound when there is a lack of phase

When the transformer is out of phase, if the second phase is not connected, the second phase is still silent, and the third phase is sent before the sound is heard; If the third phase is not connected, the sound does not change, as in the second phase. There are three reasons for the lack of phase:
(1) the power supply lacks one phase of electricity;
(2) The transformer high-voltage fuse blows one phase;
(3) Due to careless transportation and thin high-voltage leads, the transformer is vibrated and disconnected (but not grounded).

2. The voltage regulating tap-changer is not in place or the contact is poor

When the transformer is put into operation, if the tap changer is not in place, it will make a loud "chirping" sound, which will cause the high-voltage fuse to blow in serious cases; If the tap-changer is not in good contact, a slight "squeak" spark discharge sound is generated, and if the load increases, the tap-changer contacts may be burned out. In this case, it is necessary to repair the power outage in time.

3. Falling into foreign objects and loosening of the piercing screw

When the core screw of the transformer clamping the core is loose, and there are nut parts left on the core or small metal objects fall into the transformer, the transformer will emit a "clanging" knocking sound or "whew". Whew...", and the sound of a small gasket like a magnet pulling on a small gasket, while the voltage, current, and temperature of the transformer are normal. This kind of situation generally does not affect the normal operation of the transformer, and can wait until the power failure to deal with it.

4. The high-voltage bushing of the transformer is dirty and cracked

When the transformer's high-voltage bushing is dirty, and the surface enamel is peeled off or cracked, a surface flashover will occur, a "hiss" or "whoosh" sound can be heard, and sparks can be seen at night.

5. The core of the transformer is grounded and disconnected

When the core of the transformer is grounded and disconnected, the transformer will produce a slight discharge sound of "beep and beep".

6. Internal discharge

When the power is transmitted, the crisp sound of "crackling" is the discharge sound of the conductive lead through the air to the transformer shell; If a dull "crackling" sound is heard through the liquid, it is the discharge sound of the conductor passing through the transformer oil facing the housing. If the insulation distance is not enough, the power should be cut off to check the heart, strengthen the insulation or add an insulating partition.

7. The external line is broken or short-circuited

When the line is broken at the junction of the wire or at the T junction, it is connected and broken when the wind blows, and arc light or sparks occur when it is in contact, then the transformer will make a sound like a frog's "wow, wow, wow"; When the low-voltage line is grounded or there is a short circuit accident, the transformer will make a "boom" sound; If the short-circuit point is closer, the transformer will make a tiger-like roar.

8. The transformer is overloaded

When the transformer is seriously overloaded, it emits a muffled "hum" sound like that of a heavy-duty aircraft.

9. The voltage is too high

When the power supply voltage is too high, the transformer will be overexcited, and the sound will be louder and sharper.

10. A short circuit occurs in the winding

When the transformer windings are burned out due to interlayer or inter-turn short circuits, the transformer will emit a "gurgling" boiling sound.

■ Noise caused by the external structure of dry-type transformer and its solution

(1) Dry-type transformers generally have a fan cooling system, and the abnormal noise of dry-type transformers is often caused by the failure of the fan system. There are three main types of fault phenomena in fans:

(1) When the fan is put into use, it makes a "crackling" metal impact sound. This means that there is a foreign object in the fan, and it is necessary to clean the foreign matter.

(2) When the fan is just started, the sound of friction is continuous, which is the quality problem of the fan itself, and the fan must be replaced to ensure the normal operation of the fan system.

(2) The transformer with protection level of IP20 or IP40 has a shell device, which will also be the source of transformer noise, the transformer will produce vibration during operation, if the shell is not fixed, it will cause the vibration of the shell, thus producing noise, so when installing the shell, it is best to add a rubber pad between the shell and the ground and the shell and the transformer base to reduce the transmission of vibration sound.

(3) After entering the electric room, the "humming" sound of high and low can be heard in a certain direction of the transformer, which is the result of superposition of the sound wave generated by the vibration of the transformer through the reflection of the wall, this situation is more special, and the space of the electric room and the position of the transformer are related, at this time, the position of the transformer can be adjusted to reduce the sound, and some sound-absorbing materials can also be appropriately installed on the wall in the electric room.

(4) The installation position of the transformer is not good, the floor or bracket will aggravate the vibration of the transformer and increase the noise of the transformer. Some transformers are placed on the ground is not solid, then you will find that the ground will vibrate, stand next to it will feel the vibration, serious will see the ground cracks, if so, the transformer should be placed to reinforce, so as to reduce noise.

■ Key points for the selection of dry-type transformers

1. Select the transformer according to the load situation:

1. When there are a large number of primary or secondary loads, it is advisable to install two or more transformers, and when any one of the transformers is disconnected, the capacity of the remaining transformers can meet the electricity consumption of primary and secondary loads. The primary and secondary loads should be concentrated as much as possible and should not be too dispersed.

2. When the seasonal load capacity is large, it is advisable to install a special transformer. For example, large civil S4270D27-29 27 2005.7.29, 3:24AM building air conditioning and refrigerator load, heating electricity and heating load, etc.

3. When the concentrated load is large, it is advisable to install a special transformer. Such as large-scale heating equipment, large-scale X-ray machines, electric arc furnaces, etc.

4. When the lighting load is large or the power and lighting use of a common transformer seriously affect the lighting quality and bulb life, a special transformer for lighting can be set.

2. Select the transformer according to the use environment:

1. Under normal medium conditions, oil-immersed transformers or dry-type transformers can be selected according to the situation, such as independent or attached substations of industrial and mining enterprises, agriculture, and independent substations of residential areas. The transformers available are S8, S9, S10, SC(B)9, SC(B)10, etc.

2. In multi-storey or high-rise main buildings, non-combustible or flame-retardant transformers should be selected, such as SC (B) 9, SC (B) 10, SCZ (B) 9, SCZ (B) 10, etc.

3. In places where dusty or corrosive gases seriously affect the safe operation of transformers, closed or sealed transformers should be selected, such as BS 9, S9-, S10-, SH12-M, etc.

4. The high and low distribution devices without flammable oil and the non-oil-immersed distribution transformer can be installed in the same room, and the transformer should be equipped with IP2X protective shell for safety.

■ Conclusion

The above is the structure of the dry-type transformer, the cooling method, the type, and the advantages of the oil-immersed transformer? Installation and commissioning, how to judge the fault from the sound, noise caused by external structures and solutions, and selection points. I hope this knowledge can be helpful to everyone! As one of the important power equipment in transmission and distribution lines, dry-type transformers are more and more popular with users for their high dielectric strength, strong short-circuit resistance, environmental protection, fire prevention, explosion-proof, maintenance-free and other advantages. Therefore, for the installation technicians, we must use professional science and technology to do every preparation, solve and summarize the problems in the installation process in a timely manner, and ensure the safe operation of the equipment. Of course, the dry-type transformer market is uneven, the price is even more chaotic, and the same transformer may be very different from different manufacturers. Many manufacturers are fighting price wars, forcing excellent products to have no market, and they have to cut corners. Of course, the quality of many brands of transformers is still guaranteed. In this case, as a procurement, you should consider all factors and choose a product that meets your needs.