Company Profile: JINSHANMEN TECHNOLOGY CO., LTD mainly produces oil immersed power transformers, dry-type power transformers, oil immersed three-dimensional coiled power transformers, dry-type three-dimensional coiled power transformers, mining explosion-proof dry-type transformers, mining explosion-proof mobile substations, amorphous alloy power transformers, on load capacity regulating power transformers, locomotive dry-type transformers, as well as prefabricated substations, modular substations, wind energy box type substations, high and low voltage switchgear and other transmission and distribution equipment. Our products cover various specifications including 1000 kva power transformer and customized oil filled power transformer solutions for global power transmission and distribution projects.
Ⅰ. Overview of Transformer Body Moisture Absorption
After the vacuum vapor phase drying process, the transformer body needs to go through a series of procedures including body finishing, tank entering, internal wiring and tap changer installation before vacuum oil injection. During the entire open manufacturing process, the transformer's insulating materials are continuously exposed to the atmospheric environment, inevitably absorbing moisture, which negatively impacts the insulation performance and service life of the oil filled power transformer.
In general, power transformers with larger capacity and higher voltage levels feature more complex structures and more insulating materials, leading to longer assembly time and more severe moisture absorption. Even mainstream products such as the1000 kva power transformer are prone to obvious moisture accumulation during prolonged open assembly, which is a common key problem in transformer manufacturing.
Ⅱ. Principle of Insulation Material Moisture Absorption
The transformer body that meets the drying standard has an extremely low water content (0.5%–1%, maximum no more than 1%) after vapor phase drying. When placed in the atmospheric environment, a significant water vapor partial pressure difference forms between the dry insulating material and the ambient air. The water vapor partial pressure of the dried insulation is far lower than that of the external atmosphere, driving the migration of environmental moisture into the insulating material.
For example, take a 40000kVA/110kV transformer: after being out of the drying furnace for 3 hours with an insulation temperature of 65℃, its water vapor partial pressure is 590Pa (checked via Piper Curve). When the ambient temperature is 25℃ with 65% humidity, the environmental water vapor partial pressure reaches 2050Pa, 1460Pa higher than that of the transformer insulation. This stable pressure difference causes continuous moisture absorption.
The moisture absorption process follows a clear rule: initially, moisture only accumulates on the insulation surface with a surface peak value. Over time, moisture diffuses from the surface to the inner layer of the insulation. Meanwhile, the transformer body temperature gradually drops, further expanding the partial pressure difference and aggravating moisture absorption. As the insulation water content increases, its internal water vapor partial pressure rises gradually until it is balanced with the ambient atmosphere, and moisture absorption stops completely.
Ⅲ. Hazards of Uncontrolled Moisture Absorption
Uncontrolled moisture absorption in the transformer body will cause deviations in core operating parameters. On-site installation and commissioning often show obvious differences in insulation resistance, absorption ratio, and dielectric loss compared with factory test values, and even lead to power frequency withstand test failure. Most low-capacity models including the 1000 kva power transformer have compact insulation structures, and subtle moisture accumulation will directly damage insulation stability, affecting the overall quality of the oil filled power transformer.
It is worth noting that when the insulation water content is less than 2%, the deterioration of insulation strength is not obvious in the short term, which is easy to ignore. However, secondary moisture absorption will occur during on-site core inspection (the body is exposed to atmospheric environment at room temperature), further increasing water content and eventually causing long-term operational hidden dangers such as insulation aging and partial discharge.
Ⅳ. Effective Prevention & Control Methods for Moisture Absorption
To suppress transformer body moisture absorption during manufacturing and assembly, the industry adopts four mature and efficient control measures, which are widely applicable to all types of power transformers:
1. Shorten Assembly Time and Maintain High Body Temperature
Optimize pre-production preparation, standardize operating procedures, and arrange professional personnel for rapid finishing and tank entering operations. Shortening the open exposure time of the insulation can effectively reduce the pressure difference moisture absorption probability, which is the most basic and cost-effective control method for all transformer products.
2. Reheat and Preserve Heat in Drying Furnace
For transformers with complex structures that cannot complete assembly in a short time, adopt the working mode of daytime finishing and night furnace heat preservation. Reheating increases the transformer body temperature, making the internal water vapor partial pressure of insulation higher than the ambient pressure, so as to completely block external moisture invasion.
3. Vacuum Oil Impregnation Before Body Finishing
Complete vacuum oil impregnation after vapor phase drying. Transformer oil penetrates into the insulation fiber gaps, forming a protective barrier to slow down moisture absorption. This process is especially suitable for high-voltage and large-capacity oil filled power transformer products, effectively extending the safe assembly time.
4. Dry Air Positive Pressure Protection
Build a closed operation workshop and fill it with dry air with a dew point below -40℃ to form a continuous positive pressure environment and isolate humid air. For long-term internal wiring and assembly operations inside the fuel tank, continuously fill dry air into the tank to eliminate moisture and isolate the humidity exhaled by operators, which is very suitable for on-site maintenance and renovation of various transformers including the 1000 kva power transformer.
Ⅴ. Professional Dewetting Treatment Technology for Insulation Surface
Despite strict moisture prevention measures, slight surface moisture absorption is inevitable due to atmospheric humidity. Thorough vacuum dewetting treatment is required to restore insulation performance, which is divided into normal temperature vacuum dewetting and heating vacuum dewetting:
1. Normal Temperature Vacuum Dewetting
Reduce the internal pressure of the transformer tank through vacuuming. When the ambient pressure around the insulation is lower than the internal water vapor partial pressure of the insulation, internal moisture will migrate outward continuously. The higher the vacuum degree and the longer the pumping time, the faster the moisture diffusion and evaporation speed. When the vacuum degree is stabilized below 133.3Pa, the water vapor partial pressure is controlled within 4–5Pa, and the paper insulation water content can be reduced to 0.5%, meeting the standard operational requirement.
2. Heating Vacuum Dewetting
For transformers with severe surface moisture absorption and high water content, adopt drying furnace heating + high vacuum dewetting composite process. Heating increases the molecular activity of internal moisture, cooperating with high vacuum pumping to realize rapid and thorough dehumidification, completely eliminating hidden dangers of insulation moisture.
Ⅵ. Conclusion
Transformer body moisture absorption is an unavoidable physical phenomenon in the manufacturing process. The core of quality control is to adhere to the principle of "prevention first, treatment supplemented". Manufacturers need to optimize production processes, shorten open assembly time, maintain stable high temperature of the transformer body, and match scientific vacuum dewetting technology. As a professional manufacturer, JINSHANMEN TECHNOLOGY CO., LTD strictly implements standardized drying, assembly and dewetting processes for all customized oil filled power transformer and 1000 kva power transformer products to ensure stable insulation performance and long service life of equipment.