What is VPI transformers
Vacuum Pressure Impregnated (VPI) transformers are vacuum pressure impregnated with a high-temperature polyester varnish, are designed to meet most commercial and industrial applications while providing excellent mechanical and short-circuit strength, no danger of fire or explosion, and no liquids to leak. They require careful consideration for their environmental and economic solutions, as they minimize fire hazards due to low combustible content, reduce the risk of environmental pollution, ensure stable operation under low and adverse atmospheric conditions, and enhance fire safety. Our VPI transformers use NOMEX insulation material to enable excellent dielectric properties and heat resistance between electrical windings, reducing voltage stress and improving overall safety and flood resilience. Regardless of application in schools, hospitals, industrial plants, or commercial buildings, VPI dry-type transformers can provide stable and economic power solutions, boasting excellent design and material quality to suit any safe and demanding application, making them reliable power solutions that offer unmatched performance and sustainability.
Construction
Manufacturing
Enclosure
The casing of the VPI dry-type transformer is fabricated from high quality sheet steel, with removable panels at both the front and rear to allow easy access to the core and coils for maintenance or inspection. It is equipped with ventilated openings that meet NEMA, IEEE and other required standards to ensure effective air circulation. The design also integrates features for lifting, rolling, and skidding, making it easier to move during installation and maintenance.
The exterior is coated with an electrostatic powder finish using UL-approved outdoor-grade polyurethane paint, which provides excellent corrosion resistance and durability. For indoor use, the enclosure meets NEMA 1, Category “C” standards. All surfaces are cleaned of contaminants before painting, and the standard color is ANSI 61 (light gray). If additional protection from environmental factors is needed for outdoor installations, optional enclosures meeting NEMA 3R or NEMA 4 standards are available.
Core
The iron core of the VPI dry-type transformer is made of high-quality, non-aging silicon steel, optimized to achieve maximum magnetic permeability. It uses cold-rolled, grain-oriented laminations, which reduce energy wastage and enable efficient operation below saturation levels. These laminations are meticulously manufactured to eliminate burrs and gaps, ensuring a snug fit that enhances performance. The core features a sturdy clamping structure that distributes pressure evenly, providing stability and support for the coil assembly. Depending on project requirements, it can be constructed with either a standard butt-lap assembly or a mitered configuration. Each unit is fitted with vibration-dampening pads that effectively isolate the core from external disturbances, further stabilizing operation. To guard against environmental factors, the core is coated with a protective varnish, ensuring long service life and resilience in various operating conditions.
Winding
The windings of VPI dry-type transformers are tailored to voltage classes and capacity requirements. For low-voltage (LV) operations (below 2,400 volts), they typically employ multi-conductor barrel or sheet conductor structures. This selection is cost-efficient for medium to small kVA scenarios, as the current levels and axial short-circuit forces in such applications are relatively modest. For high-voltage (HV) scenarios (2,400 volts or higher), designs like single-section barrel, multi-section barrel, or disk constructions are more commonly adopted. In well-ventilated dry-type coils, round, oval, or rectangular designs are typical for applications around 2,000 kVA. Yet, for transformers with a capacity exceeding 2,000 kVA, round windings are generally favored unless specific conditions (such as dimensional restrictions) call for a different approach. These insulated coil assemblies must pass all standard tests, including impulse tests conducted prior to coil encapsulation. Additionally, the vacuum pressure impregnation process allows resin to fully penetrate the windings, boosting insulation performance and mechanical strength, which in turn ensures reliable operation under diverse conditions.
Application
Powers equipment in factories (automotive, machinery, chemical, etc.), suitable for medium-to-small kVA loads. It resists short-circuit interference, withstands dust and temperature fluctuations, and prevents production halts caused by equipment failures.
Used in malls, hotels, hospitals, and schools. It has no flammable oil or fire hazards, and stably supports critical loads like lighting, HVAC systems, and medical equipment (e.g., ICU devices).
Serves airports, subways, and high-speed rail stations. It adapts to load fluctuations during peak passenger flow, withstands dust and humidity, and ensures continuous power for baggage systems, signaling equipment, and passenger facilities.
Paired with photovoltaic (PV) plants and wind farms, it uses NEMA 3R/4 outdoor enclosures to resist rain, snow, and UV radiation for voltage step-up and grid connection. It is also compatible with charge-discharge power conversion in energy storage systems.
Applied in coastal areas (salt spray corrosion-resistant), mines (dust and moisture-proof), and cold chain warehouses (low-temperature resistant). It maintains performance through special coatings and insulation designs.
Integrated into residential and neighborhood substations, it meets NEMA 1 indoor standards, has no leakage risk, and can be installed close to residential areas to improve the safety of urban power distribution.
V. Unveiling the VPI Process: The Art of Manufacturing in Steady Steps
The VPI process is far from a simple “soaking”; it is a physical process completed under precise control, mainly consisting of the following key steps:
1. Pre-baking and drying: First, the assembled transformer windings are placed in an oven and baked for a long time under vacuum and heating conditions. The purpose of this step is to thoroughly remove all moisture and volatile substances that have been absorbed in the insulation materials (such as paper, cloth, film) and in the gaps of the windings, preparing for the subsequent impregnation.
2. Vacuum impregnation: The fully dried windings are transferred to a large, sealable VPI treatment tank. Then, a powerful vacuum system is activated to draw the tank to a high vacuum state. Under high vacuum, the residual air and trace moisture inside the windings are forcibly extracted, creating a “negative pressure” state in the micro-gaps, which greatly promotes the subsequent penetration of insulating varnish.
3. Painting and pressurization: After maintaining a vacuum or releasing it, specially treated insulating paint (usually epoxy resin, polyester resin or silicone resin) that has been pre-degassed is injected into the tank until it completely submerges the windings. Then comes the core step of the process – applying several atmospheres of positive pressure (such as 3 to 6 atmospheres) to the tank. This pressure forces the insulating paint to be pressed into and fill every tiny gap previously occupied by air, achieving deep and thorough impregnation.
4. Dipping and curing: After the impregnation is completed, the excess insulating varnish is drained off, and the windings are taken out and dripped with varnish at a specific workstation to ensure a uniform surface. Finally, the windings are sent into the curing oven and heated according to a strictly set temperature-time curve. During this process, the insulating varnish undergoes a polymerization reaction, transforming from a liquid to a solid state, and ultimately curing the entire winding into a rock-solid insulating unit.
Advantages of VPI Transformers
The Vacuum Pressure Impregnation (VPI) process fundamentally enhances transformer performance and reliability. Its core advantages can be summarized in the following six points:
- Superior Electrical Insulation Performance
The VPI process forces insulating resin into every microscopic void within the winding insulation under vacuum and pressure. This eliminates the root cause of internal partial discharges, significantly increasing the dielectric strength and service life of the insulation system, allowing the transformer to withstand higher electrical stress and various overvoltage impulses. - Exceptional Environmental Resistance and Protection
The process creates a dense, complete resin encapsulation layer that effectively blocks moisture, salt spray, dust, chemical contaminants, and mold. This makes VPI transformers particularly suitable for harsh environments like those with high humidity, high pollution, and coastal areas (e.g., offshore wind power, chemical plants, mining), where they demonstrate outstanding stability. - Enhanced Thermal Management and Heat Dissipation
The thermal conductivity of the impregnating resin is far superior to that of the air it replaces. By filling the voids, the resin creates superior thermal conduction paths inside the winding, allowing heat to be transferred more efficiently to the external cooling surfaces. This not only reduces operating temperatures but also enables higher loading capacity or a more compact design. - Robust Mechanical Structure and Short-Circuit Withstand Capability
After impregnation and curing, the windings, insulating materials, and resin are solidified into a single, rigid solid. This structure gives the windings immense mechanical strength, enabling them to withstand powerful electromagnetic forces (e.g., short-circuit currents), severe vibration, and frequent thermal cycling, thereby greatly enhancing the equipment’s mechanical stability and short-circuit resistance. - Extended Service Life and High Reliability
By combining excellent electrical, mechanical, and anti-aging properties, the aging process of the internal insulation materials in VPI transformers is significantly slowed. This directly translates into a longer design service life, lower failure rates, and higher operational reliability, making them ideal for critical applications with stringent demands for power supply continuity (e.g., data centers, rail transportation). - Optimized Total Cost of Ownership (TCO)
Although the initial manufacturing cost of a VPI transformer may be higher, its superior reliability means significantly reduced expenses related to repairs, outages, and component replacements over its entire service life. In the long run, its higher operational efficiency and minimal maintenance requirements result in a more favorable Total Cost of Ownership.
Features of SCOTECH VPI dry type transformer
I. Standard Features
1. Compliance: Meets GB, IEC, CSA, ANSI/IEEE, UL, and NEMA standards to ensure universality and safety.
2.Voltage Performance:
- Maximum Voltage Capacity: 35kV
- Low-Voltage Output: Configurations available such as 0.208, 0.24, 0.48, 2.4, and 4.16kV
- Basic Insulation Level: Up to 125kV (tested), ensuring high insulation standards (a core advantage of the VPI process)
3. Power Range: up to 20MVA, suitable for various application scenarios.
4. Frequency Compatibility: Supports 50Hz or 60Hz.
5. Insulation & Windings:
- Insulation System: 220°C high-temperature insulation system (VPI process enhances insulation stability)
- Winding Materials: Aluminum or copper options; Winding temperature rise options: 80°C, 115°C, 150°C
- Tap Design: ±5% taps on the High-Voltage (H.T.) side, with each step of 2.5% (realized via off-circuit tap links/switches)
6. Structure & Protection:
- Enclosure Options: NEMA Types 1, 12, 3R, 4, & 4X, with adaptable IP ratings tailored to specific needs; NEMA 1-rated enclosures support indoor ventilation for cooling
- Coating: Durable ANSI 61 paint for corrosion resistance
- Auxiliary Design: Includes lifting provisions, removable panels (for easy maintenance), and base-mounted vibration isolation pads (reduces noise and wear)
7. Safety & Identification:
- Grounding System: Equipped with ANSI-standard grounding pads and core straps
- Identification: Features nameplates with parameters and wiring diagrams for easy reference
- Seismic Resistance: OSHPD certification for earthquake-prone areas
8. Monitoring & Termination:
- Winding temperature indicator (with marshalling box)
- Cable end box/bus duct termination
9. Winding Connection Groups: Supports Dyn 11, Dyn1, Ddo, Dyn5, or as per customer requirements.
II. Optional Features & Accessories
1. Heat Dissipation Enhancement: Fans for 133% FA kVA rating; future fan wiring and control upgrade provisions.
2. Connection & Protection:
- Full-length copper ground bus
- Impact indicator: A mechanical device for monitoring equipment status and maintenance needs
- Polyurethane overcoat for long-lasting corrosion protection
- Screened ventilation openings
3. Control & Monitoring:
- Remote and Automatic Control Panel: Integrated with RTCC and AVR for improved automation
- Space heaters and matching thermostats
- Temperature monitor/fan controller
4. Structure & Components:
- Hinged panels and knockdown enclosures
- Electrostatic shield
- Flex leads
- Unidirectional rollers
- Off-circuit tap switch
5. Special Components: Reactors, water-cooled reactors, arresters; supports customization for special application scenarios.
Test
- Winding Resistance Measurement: Checks for proper conductivity and connections.
- Voltage Ratio and Phase Displacement Verification: Confirms correct turns ratio and vector group.
- Short-Circuit Impedance and Load Loss Measurement: Determines key parameters for load performance.
- Polarity and Phase Relation Check: Ensures proper winding connections.
- No-Load Loss and Current Measurement: Evaluates core performance at 90% and 110% of rated voltage.
- Applied Voltage Withstand Test: Verifies the main insulation strength between windings and ground.
- Induced Voltage Withstand Test: Checks inter-turn and layer insulation integrity.
Type Tests
- Temperature Rise Test: Validates the transformer’s ability to dissipate heat under full load.
- Sound Level Measurement: Measures the core and winding noise during operation.
- Short-Circuit Withstand Test: Demonstrates mechanical and thermal strength against external short-circuits.
- Lightning Impulse Test: Verifies the winding’s ability to withstand high-voltage surges.
Special Tests
- Partial Discharge Measurement: Detects and quantifies internal insulation imperfections.
- Accessories Functional Test: Ensures proper operation of cooling fans, temperature controllers, etc.
Key Factors for Selecting a VPI Dry-Type Transformer
1. Insulation System & Impregnation Quality
- VPI Process: The transformer is impregnated with a Class H (typically silicone or epoxy) insulating varnish under vacuum and pressure. This ensures complete penetration, eliminating air pockets and creating a moisture-resistant, sealed winding.
- Insulation Class: Standard VPI transformers are Class F (155°C) but use Class H (180°C) materials. This provides a significant thermal margin, enhancing longevity and overload capacity.
- Environmental Protection: The VPI seal offers excellent protection against moisture, chemicals, dust, and salt spray, making it ideal for harsh industrial, marine, or coastal applications (typically rated IP20-IP23 standard, with enclosures up to IP56).
2. Thermal Performance and Cooling
- Temperature Rise: Choose the temperature rise rating based on the application’s load profile. A lower temperature rise (e.g., 80°C or 115°C) indicates a larger core and conductor, offering higher efficiency and longer life, ideal for continuous heavy loads.
- Cooling Method: Standard is AN (air natural). For higher ratings or confined spaces, AF (air forced) with fans can be specified to increase capacity, but this adds complexity and a dependency on the fan system.
- Overload Capability: The robust Class H insulation system allows for better short-term overload performance compared to non-impregnated or resin-cast transformers.
3. Electrical Performance & Robustness
- Impulse Level (BIL): The VPI process results in a very consistent insulation wall. Ensure the Basic Impulse Level (BIL) is adequate for your system’s surge and lightning arrestor rating.
- Short-Circuit Withstand: Verify that the transformer has been designed and type-tested to withstand the mechanical and thermal stresses of a fault current for the duration specified by your system protection.
- Harmonic Handling: In applications with significant non-linear loads (e.g., VFDs, IT equipment), specify a K-Factor or Harmonic Mitigating design with appropriately sized neutral conductors and possibly reduced flux density in the core.
4. Efficiency and Losses
- No-Load Loss (Core Loss): A fixed loss dependent on core material and design. Lower no-load loss is critical for applications where the transformer is energized 24/7.
- Load Loss (Copper Loss): A variable loss dependent on winding resistance. Lower load loss is important for applications with constant high loading.
- Energy Efficiency Standards: Select a transformer that meets or exceeds relevant regional efficiency standards (e.g., DOE 2016, NRCan, EU Ecodesign).
5. Environmental and Safety Considerations
- Fire Safety: A primary advantage. VPI transformers use no flammable liquids, making them safe for installation inside buildings, near loads, in high-rise buildings, and in tunnels with minimal fire risk.
- Environmental Impact: No oil means no risk of soil or water contamination. They are also fully recyclable at end-of-life.
- Sound Level: Verify the sound level (dB(A)) meets the requirements for the installation location (e.g., office buildings, hospitals).
6. Physical & Installation Factors
- Size and Weight: VPI transformers are generally more compact and lighter than cast resin transformers of the same rating, easing installation and saving space.
- Enclosure Type: Select the Ingress Protection (IP) code based on the environment. Options range from open (IP00) for built-in switchgear to totally enclosed (IP54) for outdoor or harsh industrial sites.
- Connections: Ensure the terminal arrangement (e.g., front-access, cable box size) is suitable for your cabling.
7. Reliability, Standards, and Service
- Manufacturer’s Reputation & Warranty: Choose a manufacturer with a proven track record and a strong warranty.
- Compliance with Standards: The transformer must be designed and tested in accordance with international standards (e.g., IEEE C57.12.01, IEC 60076).
- Service and Support: Consider the availability of technical support, spare parts, and after-sales service.
