The Dry-type transformer refers to a power transformer in which the core and windings are not immersed in insulating oil and are naturally cooled or air-cooled. As a newly emerging power transformation and distribution equipment, it has been widely used in power transmission and transformation systems in factory workshops, high-rise buildings, commercial centers, airports, docks, subways, oil platforms, and other places, which can be used together with switch cabinets to form compact complete sets of substations.

Currently, most dry-type power transformers in my country are three-phase solid molded SC series, such as SCB9 series three-phase wound transformers, SCB10 series three-phase foil transformers, and SCB9 series three-phase foil transformers. Its voltage level is generally in the range of 6-35KV, and the maximum capacity can reach 25MVA.

■ Structural form of dry-type transformer

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

2. Closed type: The device 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 in mining and is explosion-proof).

3. Casting type: The main insulation is epoxy resin or other resin casting. It has a simple structure and small volume and is suitable for smaller-capacity transformers.

■ Cooling method of dry-type transformer

Dry-type transformer cooling methods are divided into natural air cooling (AN) and forced air cooling (AF).

When naturally air-cooled, the transformer can operate continuously for a long time at rated capacity.

When forced air cooling is used, the output capacity of the transformer can be increased by 50%. Suitable for intermittent overload operation or emergency overload operation. Since the load loss and impedance voltage increase greatly during overload, it is in an uneconomical operating state, so it should not be placed in continuous overload operation for a long time.

■ Installation and commissioning of dry-type transformers

Check whether the packaging is intact. After unpacking the transformer, check whether the data on the nameplate of the transformer meets the design requirements, whether the factory documents are complete, whether the transformer is intact, whether there are signs of external damage, whether parts have been displaced and damaged, whether electrical supports or connecting wires are If there is damage, finally check whether there is any damage or shortage of spare parts.

Transformer installation

First, check the foundation of the transformer to see if the embedded steel plate is level. There should be no holes under the steel plate to ensure that the foundation of the transformer has good seismic resistance and sound-absorbing properties. Otherwise, the noise of the installed transformer will increase.

Then, use the rolling bar to move the transformer to the installation position, remove the rolling bar, and accurately adjust the transformer to the designed position. The installation level error meets the design requirements.

Finally, four pieces of short channel steel are welded on the four corners of the embedded steel plate close to the transformer base to prevent the transformer from moving during use.

Transformer wiring

When wiring, the minimum distance between live objects and between live bodies and ground should be ensured, especially from the cable to the high-voltage coil. The high-current low-voltage busbar should be supported independently and cannot be directly connected to the transformer terminals to cause excessive mechanical tension and torque. When the current is greater than 1000A (such as the 2000A low-voltage busbar used in this project), the busbar and transformer There must be a soft connection between the terminals to compensate for the thermal expansion and contraction of the conductor and to isolate the vibration of the busbar and transformer.

The electrical connections at each connection must maintain the necessary contact pressure, and elastic components (such as disc-shaped plastic rings or spring washers) should be used. When tightening the connecting bolts, a torque wrench should be used.

Transformer grounding

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

Inspection before operation of transformer

Check whether all fasteners are loose, whether the electrical connections are correct and reliable, and whether the insulation distance between live parts and between live parts and ground meets the regulations. No foreign matter should be near the transformer, and the coil surface should be kept clean.

Debugging before the operation of the transformer

(1) Check the transformer's transformation ratio and connection group, 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 coils and coils with ground. If the insulation resistance is significantly lower than the factory measurement data of the equipment, it means that the transformer is damp. The transformer must be dried when the insulation resistance is lower than 1000Ω/V (operating voltage).

(3) The test voltage of the withstand voltage test should comply with the regulations. The temperature sensor TP100 should be brought out after the low-voltage withstand voltage test. After the test, the sensor should be returned to its original position in time.

(4) When the transformer is equipped with a fan, the fan should be powered on and ensure its normal operation.