Dry-Type Transformers & Their Advantages in Industrial Applications

by Mike Evans
Jul 2, 2019 10:20:54 AM

Topics: Dry-Type Transformer


Dry-type transformers play an important role in an electrical circuit. The design of any electrical distribution system begins with the transformer as they step-up or step-down the voltage and current of a circuit.

The most conventional type of transformer is the oil-filled transformer, which is encased in a vault containing combustible cooling oil that provides both insulation and heat transfer. Because the oil may pose a fire hazard, they are usually found outdoors as either a standard type in substations or pad mounted outside of a building.Designed with a smaller profile, they can handle higher load requirements of greater than 5MVA.

Fire-resistant or cast resin dry-type transformers are subdivided into three types: vacuum pressure impregnated (VPI), vacuum pressure encapsulated (VPE), and cast coil. These transformers offer several advantages over traditional open-core designs. One clear benefit is the physical protection that the encapsulate, or outer casing, provides to the fragile windings, core, and lead connections - as well as protection from environmental threats. They can be customized according to kVA sizes, voltage ratings, features, and accessories - like NEMA enclosures for reliable protection from corrosive atmospheres, hazardous gases, dust and moisture. 

VPI Dry-Type Transformers

Vacuum Pressure Impregnated Transformers are designed with minimally inflammable insulation. The windings are placed in a continuous layer, in the form of strips. Higher voltages require that disc windings be used, based on power rating and voltage levels. VPI transformer coils are vacuum impregnated in a complete submersion of high-temperature polyester varnish and cured.

VPE Dry-Type Transformers

VPE transformer coils, as well as core and clamping devices, are vacuum pressure encapsulated in high-temperature silicone varnish and oven cured - which gives a smooth and durable coating, superior dielectric strength, and excellent environmental protection.

Cast Coil Dry-Type Transformers

A transformer with coils solidly casts or locks the windings in a super-strong, high dielectric epoxy resin under vacuum in a mold. Cast coil transformers offer long service life, lower insurance expenses, reduced cost of ownership, and maintenance-free operation.

Insulation Types

The safe and reliable operation of dry-type transformer depends largely on the insulation safety of the transformer winding. One of the most important factors that can damage the insulation of dry-type transformer is that winding coils' temperature exceeds the temperature tolerance of insulating material.

Dry-type transformers are designed to last longer and to be more efficient to operate. Insulation is key to reliable operation. These transformers are usually made in the following insulation classes: Class R (220oC), Class H (180oC), Class F (155oC), Class B (130oC), and Class A (105oC). The insulation material is speced based on its mechanical ruggedness, dialectical strength, and resistance to the thermal shock expected in the electrical circuit.

Dry-type transformers have a larger profile, because they need to be designed with robust CSA or UL-recognized insulation materials - and with air circulation or ventilation systems to handle the generated heat. They can also be installed indoors, closer to the targeted area, because there is less of a fire-related risk. They also reduce secondary line losses.

Windings and Core

Copper and aluminum are the usual winding materials for the coil inside the transformer, due to their excellent conduction along with their other electrical properties. The core material is selected based on the low hysteresis loss and high permeability. The cores are typically manufactured from thin silicon steel lamination assembled together to provide the required magnetic path with the minimum of magnetic losses.

Dry-type transformers are designed to withstand higher temperature gradients when operating with overload conditions, which contributes to their high life expectancy. Their operational efficiency is superior, because each winding is designed with optimum leakage reactance and winding resistance, for better voltage regulation.

Acutran Lower TSO

Dry-Type Transformers for Industry

Dry-type transformers require little to no maintenance throughout their 15- to 20- year service life. Connections need to be kept clean and tight, with no impedance to air flow by dust or dirt, to avoid overheating.

Dry-type transformers are the ideal choice for lower-rated applications due to their lower cost, higher safety features, and greater operational efficiency. Medium-voltage dry-types are found in indoor applications such as high-rise buildings, underground tunnels, schools, and hospitals - where weight, safety, and environmental concerns are high priorities.

Given their mechanical ruggedness, low-cost to high-safety ratio, high efficiency, and performance, dry-type transformers are ideal in industrial applications such as steel factories, paper mills, refineries, chemical plants, mines, water treatment facilities, and manufacturing plants.

Dry-Type Transformers for Mining

Mine-duty dry-type transformers are used extensively in surface and underground mining for power distribution. They are a cost-effective, low-maintenance solution that’s nearly impervious to the harsh environmental dust, dirt, chemicals and other contaminants. They are used in equipment such as load and power centers, switchgear, longwall controls, and distribution boxes.

Dry-Type Transformers for Steelmaking

Dry-type transformers are used in steel mills for a wide variety of purposes. Electric arc furnaces, induction furnaces, and smelters all require a high delivery of current over a specified range of voltages. They bear the extreme dielectric, mechanical, and thermal constraints of a steel burning furnace.

Dry-Type Transformers for Heat Treating

Heat treating furnaces have diverse transformer requirements for various metalworking processes, such as annealing, case hardening, and quenching. Dry-type transformers have eligible properties, structure and multi tapping voltages to withstand the extremely high currents of heat-treating furnace environments.

Dry-Type Transformers for Pulp and Paper Mills

Energy expenses are a huge part of the production costs of pulp and paper mills. Typical paper mills use the majority of their energy in the pulping and drying processes. Dry-type transformers deliver enhanced safety, reduced downtime, and enhanced performance.

Types of Dry-Type Transformers for Industrial Applications

Control Power Transformers

Control power transformers, or industrial isolation transformers, provide excellent voltage regulation. They are typically used to produce high secondary voltages at times of current inrush in electromagnetic devices which require a steady voltage with a low-voltage ampere reading. A control power transformer is used as an isolation transformer for reaching sufficient voltage regulation in the circuit. These transformers are designed to give a high degree of secondary voltage stability, and thus are well known for their higher voltage regulation capacity. This type of transformer can withstand brief periods of overload conditions, as they provide higher inrush current capacity.

General Purpose Isolation Transformers

General purpose isolation transformers are primarily used in electronics testing, protecting the tester from severe electric shock. With this transformer, there is no connectivity between the transformer’s secondary and the earth. General purpose isolation transformers allow you to connect any device to an AC power source, without directly connecting the powered device to the source. This type of transformer is mostly used for safety reasons, as they are able to both reduce power surges and to help the equipment to function normally - even in the case of a malfunction or a power surge.

K-Factor Rated Transformers

The dry-type transformers which use a K-factor value are called as K-factor rated transformers. They are used wherever there are non-linear loads - primarily, in high harmonic content areas. When you need a driving transformer that reduces the heating effects of harmonic currents in the circuit, then a K-factor transformer is the best option.  The K-factor rating of an individual transformer indicates its ability to withstand the effects of the harmonic current while operating within the set temperature limits for the insulating system.

Drive Isolation Transformers

Drive isolation transformers are used to bear the mechanical stresses, voltage disruptions, and harmonics associated with silicon-controlled rectifiers. They are extensively used in motor drives and appliances that are energy efficient. Drive isolation transformers are, in many respects, similar to line reactors, and are placed into a circuit to add inductive impedance, to slow down the rate of current change, and to reduce both the current and the voltage distortion.

Custom Low- and Medium-Voltage Transformers

These transformers are designed for converting from high voltage to either low or medium voltage. They are primarily used to operate the switching components of various kinds of industrial machinery. They are used in relays, contactors, motor starter buttons, industrial machinery indicator lamps, and electrical distribution systems. A medium power transformer is useful in an industrial power supply and is primarily used for stepping down the circuit voltage to the medium range of end-user voltage levels. Those that are classified as extra-low voltage transformers normally operate at 50 volts. Those classified as low-voltage transformers typically operate at 120 volts, while a medium-voltage transformer operates at between 240 and 360 volts.

Zig-Zag Transformers

These are special dry-type transformers used in power systems with interlinked start winding. They are mostly used as grounding transformers, power electronic converters, and earthing transformers. They are also used in photovoltaic plants. This type of transformer is not useful for transforming power, but rather can be used for combining various features of star and delta-type winding connections.

Let’s review the advantages of dry-type transformers:

  • They offer high overload capacity.
  • They operate with low load loss, and with low partial discharge.
  • They are safe to operate, due to their noninflammable winding insulation.
  • They are fire resistant.
  • They can be used both indoors and outdoors.
  • They require zero oil usage.
  • They are eco-friendly and safe.
  • They are low maintenance.
  • They allow no restrictions on installation site.
  • They are sustainable.

A custom dry-type transformer is an ideal choice for industrial applications. These transformers are available in 50 hertz and 60-hertz designs, 1kVA to 3000kVA, in both copper and aluminum windings, with multiple insulation systems up to 240oC. Primary and secondary voltages can be designed, per your requirements, with fuse protection and optional Faraday shields. They can be customized according to kVA sizes, voltage ratings, features, and accessories - like NEMA enclosures for reliable protection from corrosive atmospheres, hazardous gases, dust and moisture. 

These are all the reasons and more on why dry-type transformers are reliable components of any industrial electrical infrastructure. Routine maintenance is the best way to protect this long-term investment.