Discussion on Transformer VPD Technology

Summary

Vacuum drying is a critical step in the transformer installation or inspection process, and the technical quality of the vacuum drying directly impacts the transformer's overall health.

If the drying process is not thorough enough, residual moisture can remain within the internal insulation materials. Under the high electric field of the transformer, this moisture can generate electrodes with a large number of charges, which can easily lead to partial discharge. This phenomenon can cause serious damage to the internal insulation.

Vapor-phase drying is widely used for large power transformers to ensure a thorough drying process. This is because the internal insulating medium of oil-immersed power transformers is typically paperboard, which inevitably contains a small amount of moisture. Before a transformer is put into operation, this insulation material must be vacuum-treated to prevent moisture molecules from discharging under a high electric field, which would cause insulation damage.

Inadequate vacuum drying can accelerate the aging process of the insulating materials during the transformer's normal operation. This not only creates a safety hazard but also significantly shortens the transformer's lifespan.

Conventional Vacuum Drying Technology

Water molecules are typically adsorbed deep within the transformer insulation materials. The drying process is carried out by first heating the insulation to turn the liquid water into a gas (vapor). Then, a high vacuum creates a pressure differential between the inside of the insulation and the outside environment, forcing the vaporized water out. The higher the temperature under the same vacuum conditions, the faster the transformer drying process will take.

Traditional Vacuum Drying Method

  • Process: Under standard atmospheric pressure, the transformer's internal temperature is heated to 105°C using hot air. Then, a vacuum pump is used to evacuate the air, along with the moisture.

  • Disadvantages: Air has a slow heat transfer rate, which leads to uneven heating both inside and outside the transformer. This can easily result in incomplete drying.

Hot Oil Spraying/Circulating under Vacuum Method

  • Process: Hot oil is sprayed onto the transformer's core and coils while simultaneously applying a vacuum to the tank. The vacuum removes the moisture that has been drawn to the surface by the hot oil. 

  • Disadvantages: The drying process takes a long time.

VPD Technology

As large power transformers have larger internal spaces and thicker insulation pressboard, conventional vacuum drying methods would result in excessively long drying times, incomplete drying, and higher investment costs. In contrast, vapor-phase drying can achieve high-vacuum and high-temperature, providing a relatively fast and effective drying process. The main medium used in transformer vapor-phase drying is kerosene.

1. Preparation :

  • A vacuum pump is used to evacuate the transformer, reducing its internal pressure to below 700 Pa.

  • Simultaneously, the vacuum tank is heated and insulated to maintain temperature.

  • The kerosene evaporator is preheated to raise the temperature of the kerosene vapor at the outlet from 98°C to 130°C.

2. Heating (Vapor Injection and Circulation):

  • Only one vacuum maintenance pump is required to keep the system running.

  • Kerosene vapor is injected into the transformer under high pressure for heat exchange.

  • When the vapor encounters the cooler insulation materials or tank walls, it condenses to liquid, which then flows back into the kerosene collection tank and returns to the evaporator, forming a continuous circulation system.

3. Pressure Reduction (Low-Vacuum Treatment):

  • Stop the injection of kerosene vapor into the evaporator, and the gas in the vacuum tank is continuously extracted.

  • The high-temperature inside the transformer re-evaporates the residual kerosene on the insulation paperboard, which is then condensed and recovered.

4. High-Vacuum Phase:

  • The main vacuum system is activated to continue evacuating the vacuum tank, bringing the final pressure down to below 10 Pa.

  • The drying process is considered complete only when the transformer body's temperature reaches the specified value and the moisture removal rate from the internal insulation materials drops to below 10 g/t·h (grams per ton per hour).

Advantages of VPD

  1. Uniform Temperature: This method make use of the heat released from kerosene vapor condensation. Lower temperature spots will occur more intense condensation and release more heat. This self-regulating process ensures that the transformer's body is evenly heated.

  2. Higher Temperature: During the kerosene vapor-phase drying process, the transformer's body remains under a constant vacuum. This significantly slows down the aging of insulation materials. Compared to hot-air circulation vacuum drying, this method can raise the heating temperature by about 30°C, reaching to 130°C.

  3. Faster Drying Speed: The transformer remains in a vacuum throughout the entire process, allowing for continuous moisture removal.

  4. Internal Cleaning Effect: When the kerosene vapor condenses into liquid, it also has a cleaning effect on the inside and outside of the transformer. This is particularly beneficial when processing refurbished transformers, as the process leaves the surface of the transformer body clean and free of oil and other stains.

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