Dry Type Transformers

Overview

The 1970s were the formative years for Virginia Transformer, where our product offering was primarily specialty harmonic load transformers. In the 1980s, we invested in industrial solutions for mills, transit, and rail. By the 1990s, we diversified to utility generation, transmission, and distribution. In the 21st century, we added large power transformer capability to our portfolio to support the Renewable Energy Generation sector where we currently hold the largest market share.

Virginia Transformer takes pride in being a US-based power transformer OEM with the mission to single-handedly reduce US power transformer imports and bring power transformer manufacturing jobs back to the US. To support our mission, we expanded our manufacturing capacity by adding 5 additional plants in 1995, 2003, 2013, 2014, and 2022, while also adding management and technical resources in the US, Mexico, and India. We have established strategic sourcing partnerships with vendors in the US, India, and China to ensure a robust supply chain

VTC’s Dry-Type Transformers

Virginia Transformer first began producing dry-type
transformers in the 1970s. Over the years, we have refined
our design and technology enabling us to manufacture
dry-type transformers from 300 kVA to 20 MVA up to 35
kV class. Applications for dry-type transformers range
from commercial buildings, airports, hotels, industrial
plants, mines, and data centers. Virginia Transformer
dry-type transformers are built to withstand the toughest
environments. We use aluminum conductors, temperature controlled winding rooms, superior insulation, and special
harmonics handling to produce reliable transformers with the
lowest core losses.

We offer the following for your dry-type transformer needs:

Shortest lead times
Proven technology since 1971
Designs for any industrial and commercial applications
Choice of coil processing
Enclosure for indoor or outdoor applications
Units designed for high-humidity or corrosive environments
UNIClad a premium dry-type transformer exclusively from Virginia Transformer

TRANSFORMER FEATURES

RANGE

LOADING

SERVICE

BASIC IMPULSE LEVEL (BIL)

COILS

COIL SEALING

INSULATION

ENCLOSURE

PAINT

NAMEPLATE

4 PAINT SYSTEMS AVAILABLE

STAINLESS STEEL GROUND PADS

12/18 month standard warranty. Options:

OTHER TYPICAL APPLICATIONS INCLUDING MAIN USE IN DATA CENTERS

Subway
Rapid transit
Hospitals
Hotels
Schools
Utilities & power plants
Chemical Plants
Mining operations

Paper & steel mills
Oil & gas refineries
Office & shopping complexes
Manufacturing plants
Airport terminals
Water treatment plants
Research facilities

MARKETS

Switchgear – Siemens, Cutler Hammer, Square D, GE
Transit – NYCTA, WMATA, DART, LIR, LA Metro, San Diego Metro, Houston Metro, RTD (Denver)
Metals – US Steel, Nucor Steel, ALCOA, California Steel, N American Stainless, IPSCO Steel
Chemical – PPG Industries, Dominion Cove Point, DH Compounding, Occidental Chemical
Government – US Dept of Energy, NASA, US Navy, IRS, Fermi Lab

APPLICABLE STANDARDS

ANSI - American National Standards Institute
IEEE - Institute of Electrical and Electronic Engineers
C57.12.01 - General Requirements for dry-type transformers
C57.12.51- Requirements for ventilated dry-type power transformers 501kVA and larger, three phase, with high voltage 601 to 34,500 volts, low voltage 208Y/120 to 4160 volts
C57.12.52 - Requirements for sealed dry-type power transformer 501kVA and larger, three phase, with high voltage 601 to 34,500 volts, low voltage 208Y/120 to 4160 volts
C57.12.91 - Test Code for dry-type distribution and power transformers
C57.94 - Practice for installation, application, operation, and maintenance of dry-type general purpose distribution and power transformers
C57.96 - Guide for loading dry-type distribution and power transformers

Processing for Coil Protection

VPI: The coils are dried and then sealed with thermoset polyester varnish under vacuum conditions. The process requires proprietary processing of vacuum temperature and pressure, and results in a completely sealed coil.

VPI End Cap: The ends of the coils are sealed with special formulations of resins to provide rigid support to withstand axial impact found in industrial applications.

Unidip: A proprietary process used to seal the coils with a thick, resilient coating. This coating is impervious to moisture and will keep it from diffusing into the coils, which extends the life of the transformer.

UNIClad: The coils are filled with a resin mixture under vacuum pressure. The result is void-free coils in which the turns are supported to withstand significant mechanical impacts. UNIClad coils are designed to last for 40 years.

COIL OPTIONS

SHAPE

WINDING

CONFIGURATIONS

SEALING

CONDUCTORS

ENCLOSURES

NEMA 101: Clean indoor applications

NEMA 103R: Outdoor applications per ANSI standards C57.12.55

NEMA 103 R: Coastal Applications per ANSI Standard C57.12.59

TENV: Totally Enclosed Non Ventilated – Indoor/Outdoor Application where corrosive gasses, high dust and other contaminants are present.

TOTAL COIL TEMPERATURE

Average Temp. Rise by resistance	150°C	115°C
Ambient Temp. (Maximum)	40°C	40°C
Hotspot Temp. Allowance (est.)	30°C	30°C
Total Temp. of Coil	220°C	185°C

Unidip Processing for Dry-Type Transformers

Unidip is a special process for dry-type transformer core and coil sealing. It was developed by Virginia Transformer Corp. for industrial applications where:

Coil sealing is recommended for process applications such as capital and consumer goods manufacturing of large and medium scale.

The Process

Coils are wound with insulation processed in a controlled environment to reduce moisture content to a minimum.
Finished coils are dried in a hot-air oven above 230ºF
Hot coils are placed in a vacuum chamber at less than 15 Torr (mm of mercury absolute pressure) to remove all remaining moisture and gasses. Water will vaporize at this pressure at less than 25ºC.
Thermoset polyester (or other specified resin) is added under vacuum to cover the coils completely. Thermoset polyester is a 220ºC-rated resin when used with 100 percent solid insulation and no solvents.
Coils are soaked in this resin to seal the insulation porosity against all industrial contamination and gases to ensure full life of the insulation in actual service.
Resin fills all voids in the coil to ensure a solid insulation system.
Resin is pressed into the coil with greater than 5 atmosphere pressure (80 PSI) to ensure full penetration into the coil and insulation to make a solid coil — conductor, insulation, and duct sticks.
Resin is withdrawn from the chamber.
The coil is placed in a preheated oven to cure the resin at the required temperature and time for complete cure.
The coils are assembled with the core. Connections are made to the HV and LV bus/cable; tap connections are made to the terminals/cable. The assembly is tested to verify design performance.
The complete assembly is heated in a hot-air oven above 230ºF
The hot core and coil assembly complete with all wiring and bus connections are processed in a resin mixture containing thermoset polyester resin rated at 220ºC, inorganic filling compound rated for operation at greater than 1000ºC to assure a high continuous-use temperature, and an inorganic compound to provide a barrier against diffusion of water and other chemically similar molecules encountered in industrial processes and manufacturing.

The hot assembly is covered with the resin-compound mixture in a vessel equipped to maintain the mixture ratios and allow for removal of excess resin to clear all cooling passages and ensure long life of the insulation system.
The resin starts to cure and forms a thick (.005 Inch/ >125 microns) cover that seals all parts of the coil, connections, and the mechanical hardware to protect against vibration, shock, and rusting in industrial manufacturing environment.
The resin is cured by placing the assembly in an air-circulating oven at the recommended temperature and time.

Unidip Advantages

The core and coil assembly is sealed against industrial gas, vapor, and moisture contamination.
The insulation system is protected from industrial environment and moisture to deliver full design life of more than 20 years when operated within the designed temperature and dielectric stresses.
Unidip coating will withstand 95 percent relative humidity and subsequently pass all ANSI standard dielectric field tests.
The mechanical system of the core and coil assembly is protected from loosening due to industrial vibrations and shocks.
The mechanical parts of the core and coil assembly and the electrical connections will not rust in humidity encountered in industrial manufacturing.

Unidip-processed, dry-type transformers from Virginia Transformer Corp. of Roanoke, VA, USA, are equal to or better than cast resin or other sealed encapsulated dry-type transformers in their overall performance in the capital and consumer goods manufacturing environment. Additional environmental protection can be achieved by using optional accessories offered by VTC for long-term storage.

Unidip vs. Cast Oil – The Choice is Clear

VTC developed Unidip to operate under demanding applications in harsh environments. Because it is designed to withstand industrial fumes and vapors, fibrous and particle contamination, high temperature and humidity, short-circuit forces, and electrical voltage impulses, Unidip is the cost-effective alternative to liquid-filled or cast-coil transformers. Unidip is the perfect solution for power, rectifier, drive isolation and generator exciters up to 15,000kVA, 35kV class.

Desired Features	Unidip	Cast Coil
High thermal rating	Excellent: 220°C	Acceptable: 150°C to 185°C
Overload capacity	Very Good: 15% overload without fans	Limited: fans necessary for overloading
Seal integrity	Yes: flexible insulation and cladding system eliminates coil cracking	No: Coil cracking because of rigid casting and high thermal coefficient of epoxy expansion
Void-free coil	Void density is low because of low varnish viscosity and vacuum/pressure process	Void density is higher because of high viscosity of epoxy resin
Superior short-circuit test performance	Yes: performed with 0% impedance change in a 4-duration test, which is 60 times longer than required per ANSI standards	No: passes standard ANSI test of 4 tests of 4 cycles (60Hz) each, with less than 2% impedance change
High percent tensile	>20%	<5%
Flexible designs	Yes: All Unidip units are designed to customer specifications	No: must choose from existing molds
Design tested	Internal and external are self-extinguishing; vibration shock of 30G	Not available
Quick delivery	Yes: typically 10 to 12 weeks	Limited: typically 12 months
Competitive pricing	Yes: never need to upsize; Unidip units are designed exactly for customer's application	Sometimes: choosing from pre-set molds can result in a 10% to 15% premium, because any non-standard design may force customer to order larger-sized unit than they actually need