In the manufacturing and maintenance of critical electrical equipment—such as power transformers, high-voltage instrument transformers, cable terminations, and GIS (Gas-Insulated Switchgear)—the purity and dielectric strength of the insulating medium directly determine long-term operational safety and service life. Conventional oil-filling methods performed under atmospheric pressure struggle to fully remove trace moisture and gases trapped deep within insulation materials or dissolved in the insulating oil itself, creating latent risks for partial discharge.
Vacuum Oil Injection (VOI), a mature and essential specialized process, provides the industry-standard solution to this persistent challenge. By establishing and maintaining a high-vacuum environment, VOI enables deep drying of insulation structures and bubble-free impregnation—ensuring optimal dielectric integrity.
Technical Principle: Why Must Oil Be Injected Under Vacuum?
The VOI process is grounded in fundamental physics: under extremely low pressure (high vacuum), the boiling point of liquids decreases, and the solubility of gases—including water vapor—in both solids and liquids drops dramatically, making them far easier to extract.
The process typically consists of two critical phases:
Deep Vacuum Drying & Degassing Phase:
The fully assembled equipment is placed under high vacuum. During sustained evacuation, moisture adsorbed within solid insulation materials (e.g., pressboard, laminated wood) and dissolved air, moisture, and volatile gases in the oil are efficiently released at temperatures well below their normal boiling points and removed by the vacuum system.
Vacuum Oil Injection & Impregnation Phase:
While maintaining high vacuum inside the equipment, pre-treated, ultra-clean, and dry insulating oil—already degassed and dehydrated in a separate unit—is introduced from the bottom. Driven by differential pressure, the oil rises smoothly, filling every insulation void and main dielectric path without trapping air or forming bubbles, achieving complete and uniform impregnation.
Process Advantages: Revolutionary Performance Beyond Conventional Methods
Compared to atmospheric oil filling, professional Vacuum Oil Injection delivers transformative quality improvements:
Maximized Insulation Strength:
Complete elimination of bubbles and moisture in oil-paper insulation systems elevates the Partial Discharge Inception Voltage (PDIV) and power-frequency withstand voltage to theoretical maximums, drastically reducing in-service insulation failure risks.
Significantly Extended Equipment Lifespan:
Moisture and oxygen are primary accelerators of cellulose aging and oil oxidation. VOI suppresses their levels to ultra-low thresholds (e.g., moisture ≤10 ppm, gas content ≤0.5%), enabling equipment to exceed its designed multi-decade service life.
Guaranteed Process Consistency & Reliability:
Critical parameters—vacuum level, oil injection rate, oil temperature, and soaking time—are automatically controlled via Programmable Logic Controllers (PLCs), ensuring every unit meets identical, highest-quality standards and eliminating human-induced variability.
Reduced Manufacturing Cycle Time:
When combined with hot oil circulation, drying and impregnation under vacuum proceed far faster than traditional hot-air drying methods, significantly shortening overall assembly timelines.
Core System Components: A Complete Vacuum Oil Injection Solution
An industrial-grade VOI system is far more than just a “pump + tank.” It is a highly integrated, intelligent platform typically comprising:
High-Vacuum Generation & Maintenance Unit:
A multi-stage vacuum pump system (e.g., roots pumps, rotary vane pumps, or dry screw pumps) capable of rapidly achieving and sustaining high vacuum (often below 1 Pa) even in large-volume vessels.
Insulating Oil Conditioning & Degassing Unit:
Integrated with high-efficiency oil filters (for particle removal), coalescing separators (for demulsification and dehydration), and a vacuum degassing tower—ensuring injected oil meets stringent specifications: cleanliness (e.g., NAS Class 6), moisture ≤3 ppm, and gas content ≤0.1%.
Precision Oil Injection & Temperature Control Unit:
Features variable-frequency oil transfer pumps for stepless flow control (from a few L/min to hundreds of L/min) and closed-loop heating systems (electric or steam) to maintain oil within the optimal process temperature range (typically 40–60°C).
Intelligent Central Control System:
Built around an industrial PLC and HMI interface, it stores standard and custom process recipes, enables one-button operation, provides full data logging and traceability, and includes fault alarms and safety interlocks.
Key Application Areas
Vacuum Oil Injection is a mandatory, non-negotiable process in the production and overhaul of high-reliability electrical equipment, including:
Power Transformers: For new manufacturing and post-overhaul oil filling and full impregnation.
HV/EHV Instrument Transformers: To ensure complete impregnation of capacitor cores or windings for accurate voltage/current sensing.
HV Cable Terminations & Joints: For bubble-free filling inside epoxy bushings or stress cones in both pre-molded and field-assembled terminations.
Gas-Insulated Switchgear (GIS): As a supplementary process for oil-insulated subunits or bushings.
Reactors, Large Motors, and other oil-immersed equipment requiring premium insulation quality.
Success Story: Empowering a Global Transformer Leader
We delivered a fully customized Vacuum Oil Injection and Hot Oil Circulation system to a Fortune Global 500 transformer manufacturer for the production of 750kV+ extra-high-voltage (EHV) transformers. Achieving ultimate vacuum levels below 5 Pa and precise oil temperature control (±0.5°C), our system enabled the client to consistently maintain factory partial discharge levels below the industry-leading benchmark of 50 pC—and successfully pass multiple international certifications.
Client Testimonial:
“This Vacuum Oil Injection system is the ‘guardian of quality’ for our premium product line. Its stability and process repeatability have earned us trust in global markets.”