Dry-Type Transformer Structure, Cooling Methods, Installation, and Commissioning

Dry-Type Transformer Structure, Cooling Methods, Installation, and Commissioning

Introduction: Dry-type transformers are power transformers whose core and windings are not immersed in insulating oil and instead utilize natural or air cooling. As a relatively new type of power distribution equipment, dry-type transformers are widely used in power transmission and transformation systems in factories, high-rise buildings, commercial centers, airports, docks, subways, oil platforms, and other locations. They can be combined with switchgear to form compact substations.

Currently, most dry-type power transformers in my country are three-phase solid-molded SC series transformers, such as the SCB9 series three-phase wound-type transformers, the SCB10 series three-phase foil-type transformers, and the SCB9 series three-phase foil-type transformers. Their voltage ratings generally range from 6-35 kV, with a maximum capacity of up to 25 MVA. Want to learn more about dry-type transformers? Below, our editors from Xianji.com will provide a detailed introduction to dry-type transformers’ structure, cooling methods, types, advantages over oil-immersed transformers, installation and commissioning, fault diagnosis by sound, noise caused by external structures and solutions, and selection considerations. Let’s take a look!

Dry-Type Transformers

■ Dry-Type Transformer Structures

1. Open Type: This is a commonly used type. The transformer body is in direct contact with the atmosphere and is suitable for relatively dry and clean indoor environments (relative humidity should not exceed 85% at an ambient temperature of 20°C). It generally has two cooling methods: natural air cooling and air cooling.

2. Enclosed Type: The transformer body is enclosed in a sealed casing 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. Cast Type: Epoxy resin or other resins are cast as the main insulation. It has a simple structure and small size, making it suitable for smaller capacity transformers.

■ Dry-Type Transformer Cooling Methods

Dry-type transformer cooling methods are divided into natural air cooling (AN) and forced air cooling (AF). With natural air cooling, the transformer can operate continuously at rated capacity for long periods of time. With forced air cooling, the transformer output capacity can be increased by 50%. Suitable for intermittent overload operation or emergency overload operation; due to the large load loss and impedance voltage increase during overload, it is in a non-economic operation state, so it should not be kept in long-term continuous overload operation.

■ Types of dry-type transformers

1. Impregnated air-insulated dry-type transformer: Currently rarely used. Winding conductor insulation and insulation structure materials are selected from insulation materials of different heat-resistant grades according to needs to make Class B, Class F and Class H insulation dry-type transformers.

2. Epoxy resin cast dry-type transformer: The insulation materials used are polyester resin and epoxy resin. Currently, epoxy resin is mostly used for cast insulation dry-type power transformers.

3. Wrapped insulation dry-type transformer: Wrapped insulation dry-type transformer is also a type of resin insulation. Currently, there are few manufacturers.

4. Composite insulation dry-type transformer:

(1) High-voltage winding adopts cast insulation, and low-voltage winding adopts impregnated insulation;

(2) High voltage adopts cast insulation, and low voltage adopts foil winding wound with copper foil or aluminum foil.

■ What are the advantages of dry-type power transformers compared to oil-immersed transformers?

1. Dry-type power transformers eliminate the risk of transformer oil fires and explosions caused by faults during operation. Because dry-type transformer insulation materials are flame-retardant, even if a transformer fault or external ignition source causes a fire during operation, the fire will not escalate.

2. Dry-type power transformers are immune to oil leakage and oil aging, unlike oil-immersed transformers. Operational maintenance and overhaul workloads for dry-type power transformers are typically significantly reduced, and may even be maintenance-free.

3. Dry-type power transformers are generally indoor installations, but they can also be manufactured for outdoor installations for specialized applications. They can be installed in the same room as switchgear, reducing installation space.

4. Because dry-type power transformers are oil-free, they require fewer accessories. There are no oil storage tanks, safety air ducts, or numerous valves, eliminating sealing issues.

■ Dry-Type Transformer Installation and Commissioning

1. Pre-Installation Unpacking Inspection

Check the packaging for integrity. After unpacking the transformer, verify that the transformer nameplate data meets the design requirements, that all factory documentation is complete, and that the transformer is in good condition. Check for signs of external damage, displacement, or damage to components, or damage to electrical supports or cables. Finally, check for damage or missing spare parts.

2. Transformer Installation

First, inspect the transformer foundation and the embedded steel plate for levelness. Ensure there are no voids beneath the steel plate to ensure the transformer foundation has good seismic and sound absorption properties. Failure to do so will increase transformer noise after installation. Then, use rollers to move the transformer to the installation location. Remove the rollers and precisely adjust the transformer to the designed position, ensuring the installation level tolerance meets the design requirements. Finally, weld four short steel channels to the embedded steel plate at the four corners of the transformer base to prevent the transformer from shifting during operation.

3. Transformer Wiring

When wiring, maintain the minimum distance between live parts and between live parts and ground, especially the distance between the cable and the high-voltage coil. High-current low-voltage busbars should be supported separately and should not be directly crimped onto transformer terminals, as this would generate excessive mechanical tension and torque. When the current exceeds 1000A (for example, a 2000A low-voltage busbar was used in this project), a flexible connection must be provided between the busbar and the transformer terminals to compensate for thermal expansion and contraction of the conductors and isolate vibration between the busbar and the transformer. Electrical connections at all points must maintain the required contact pressure. Flexible elements (such as disc-shaped plastic rings or spring washers) should be used. A torque wrench should be used when tightening connecting bolts. Manufacturer-provided torque reference values ​​are shown in Table 1.

Torque

4. Transformer Grounding

The transformer grounding point is located on the low-voltage side of the chassis. A dedicated grounding bolt with a grounding center mark extends outward. The transformer ground must be reliably connected to the protective grounding system through this point. If the transformer has a housing, the housing must be reliably connected to the grounding system. If the low-voltage side uses a three-phase, four-wire system, the neutral conductor must be reliably connected to the grounding system.

5. Inspection before transformer operation

Check whether all fasteners are loose, whether the electrical connections are correct and reliable, whether the insulation distance between live parts and between live parts and ground meets the requirements, there should be no foreign objects near the transformer, and the coil surface should be clean.

6. Debugging before transformer operation

(1) Check the transformer 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 the coils and between the coils and 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 requirements. When performing the low voltage withstand voltage test, the temperature sensor TP100 should be removed. 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 that it operates normally. 7. Trial Run

After a thorough pre-operation inspection, the transformer can be powered on for a trial run. During the trial run, special attention must be paid to the following: Check for abnormal sounds, noise, and vibration. Check for unusual odors such as burning. Check for discoloration caused by localized overheating. Ensure adequate ventilation. Additionally, the following points should be noted.

First, although dry-type transformers are highly moisture-resistant, they are generally open-type and susceptible to moisture. This is especially true for dry-type transformers produced in my country, which have lower insulation levels (lower insulation grades). Therefore, dry-type transformers must operate below 70% relative humidity for maximum reliability. Dry-type transformers should also avoid prolonged outages to prevent them from becoming severely damp. If the insulation resistance falls below 1000/V (operating voltage), the transformer is severely damp and the trial run should be discontinued.

Second, unlike oil-immersed transformers, dry-type transformers used for step-up power stations must not be operated with the low-voltage side open circuit to prevent overvoltage on the grid side or lightning strikes on the lines, which could cause overvoltage to break down the dry-type transformer’s insulation. In order to prevent the transmission of overvoltage hazards, a set of overvoltage protection arresters (such as Y5CS zinc oxide arresters) should be installed on the voltage bus side of the dry-type transformer.