DRY TYPE TRANSFORMERS

What is a dry-type transformer?

Dry-type transformers, as opposed to a liquid-filled transformer, need no medium for dissipating excess heat or meeting temperature requirements since they use gas or dry insulation instead of fluid.

Common Dry-Type Transformers

General-Purpose Ventilated Dry-Type Transformers

General-purpose ventilated transformers come in different designs and shapes. Some are dry while others have oil cooling (liquid transformers). The general purpose ventilated transformers are meant for use in distribution systems. All the transformers have a sine wave voltage and frequency rating of 50 or 60 Hz as per the country’s electrical code.

Enclosed Non-Ventilated Dry-Type Transformers

Non-ventilated transformers operate as a vacuum and contain no ventilation pathways for the build-up of particles that might cause damage to the transformer. The surface area of this enclosure provides a large amount of heat dissipation. Typical enclosed non-ventilated dry-type transformers are appropriate for varying power outputs in the 15-300 KVA range, and primary voltages of 380-575V. They operate within a temperature rise of 150°C to 80°C at 60 or 50Hz. Enclosures typically receive NEMA ratings of 3R, 4X, or 12.

Mini-Power Center Dry-Type Transformers

Mini-power centers close the gap in a modern distribution system by combining an encapsulated transformer, interior, primary and secondary main circuit breakers into a single assembly. Mini-power center transformers can be 3 – 30 kVA, with 380 – 575 V primary voltages and 120/240 V secondary voltage (single-phase) or 120/208 V secondary voltage (three-phase). These solutions are ideally suited for applications where 120 volts are required at a remote location, such as parking lots, workbenches, and temporary power on construction sites.

Vacuum Pressure Encapsulated Transformers

A high voltage dry-type vacuum pressure encapsulated transformer (Vacuum Pressure Encapsulated Transformer or VPET) consists of an inner tank of glass or porcelain containing a vacuum and an outer steel tank. The inner vacuum is created by pumping out the air with a vacuum pump.

Types of windings for dry-type transformers


Copper Or Aluminum Windings

When choosing transformers for your project, you will often have a choice of whether to use copper or aluminum windings.

The raw material used for windings of the dry-type transformer is chosen depending on the required voltage level. Copper is generally utilized for higher-rated systems whereas aluminum has found extensive applications in medium to low voltage systems.

Aluminum Dry Type Transformers

These transformers have the advantage of being lightweight and can be easily installed in hard to reach locations

Primary and Secondary Windings For Dry Type Transformers

This will depend on the amount of voltage you require as well as the power rating of the transformer.

Specialty Dry-Type Transformers 

Specialty dry-type transformers are a type of transformer that is used in places where there is potential for moisture. Dry-type transformers can have vents that allow for the passage of air to help cool the coils and prevent damage to them from excess moisture, but they cannot be submerged in water. This makes them perfect for applications like dust hauling or recycling yards, where there is a lot of airborne particulate matter present.

Motor Drive Isolation Transformers

The motor drive isolation transformer is an electric power transformer used to isolate motor or other loads from the power supply. The design of a motor drive isolation transformer enables it to be connected either in series with the load for normal service or shunt across the load for backup protection. The isolation transformer can also be configured to serve other types of motor loads: open-drip, induction motors, etc.

Some advantages of a motor drive isolation transformer are:

  • Voltage regulation – In the event of an outage or power fluctuation, voltage regulation automatically kicks in and stabilizes the output voltage to correct the problem. This stops any possible damage caused by imbalanced supply and load voltages.

  • Protection: By continuously monitoring for fault conditions such as overcurrent, overvoltage, and Undervoltage, motor drive isolation transformers offer protection against damage caused by system faults.

  • Energy efficiency: By eliminating power loss through small load impedance and reducing the need for inefficient voltage regulation, motor drive isolation power transformers deliver energy savings of up to 20% compared with conventional designs.  

Buck-boost Transformers

Buck-boost transformers are used when the voltage ratio is the same but the phase between input and output is lagging or leading (the line voltages are not in quadrature). Numerous manufacturers offer a wide variety of designs with voltage ratios ranging from 2:1 to 60:1, rated output powers ranging from 0.5 W to 20 kW, and maximum input voltages ranging from 16 to 36 kV.

Buck-boost transformers are encapsulated single-phase transformers with three voltage combinations:

  • 120 V x 240–12/24 V
  • 120 V x 240–16/32 V
  • 240 x 480 V–24/48 V

Buck-boost transformers are applied as autotransformers and therefore typically need less energy to keep them operating. Buck-boost transformers only range in size from 0.05 kVA through 7.5 kVA but can be applied to boost or buck the voltage to loads as large as hundreds of kVA.

These transformers can be used at their nameplate ratings for low voltage interior or landscape lighting.

K-Factor Rate Dry Type Transformers

K-factor transformers are specially designed to withstand the harmful effects caused by harmonics generated by nonlinear (non-sinusoidal) loads. Is suggested that K-factor transformers be proportional to the load in watts of the devices they will service. One of the advantages of K-factor transformers is that they have lower induction levels and therefore weaker stray losses. K-factor transformers are not reactive to harmonic levels because they were specifically designed to withstand the damaging effects of nonlinear loads. K-factor transformers are typically oversized (200% rated) with neutrals and shielding.

Harmonic Mitigation Dry Type Transformers

In complex electrical systems, power flow can occur not only from line-to-ground and between opposite phases but also from phase-to-phase. This power flow is known as harmonics, the third order of components in a waveform. Harmonics are usually caused by nonlinear loads such as rectifiers and industrial processes. Since energy flows to the ground via the neutral conductor, harmonic currents in a system can introduce voltage imbalance among the phases and cause large magnitude current flowing through transformers, leading to overheating that could result in failure or fire. Harmonic mitigation dry-type transformers are designed to address this problem.

Breaker Integrated Dry Type Transformers

Breaker Integrated Transformers (BITs) provide a simple, cost-effective approach to reducing arc flash risk and increasing flexibility for mounting secondary panels. They are designed to mount on an existing breaker bar using the breaker’s integral mounting flanges. They incorporate a BT-R type dry type transformer and panel with up to four bus bars for control, power, or grounding connections.

BITs provide flexibility in three areas: location of secondary circuit breakers, number, and location of taps, and protection against arc flash hazards.

This flexibility provides opportunities to reduce the rigidity of secondary panel design by allowing multiple breaker positions, ground or power bus locations, and many types of connections: 12 kV to 480Y/277 V to 600 V 4-wire, power distribution; 24 kV to 480Y/277 V to 500 V 3-wire, control; 480Y/277 V to 600 V 3-wire, control, and power.

Class 1, Division 2 Hazardous Area Dry Type Transformers

Dry-type transformers and similar equipment can be used in Class 1, Division 2 hazardous areas. The equipment is considered safe when it conforms to the requirements given in this article. Design, construction, testing, classifying, and marking of dry-type transformers and similar equipment must comply with IEC 60300-1:2012.

Transformers for Class I, Division 2, Groups A, B, C, and D meet the stringent standards set forth by NEC 501 with recommended installation procedures for dry-type transformers rated under 600V.

Marine-duty Dry Type Transformers

Marine duty dry-type transformers are available in power ratings ranging from 2 to 20 kVA.

These units have features that make them suitable for seagoing and inland marine use:

– Hermetically sealed transformer core, dry type insulating system and metallic tank construction

– Waterproof conduit entries with terminal clamps located above water level

– Severe duty terminal box for wire entry with bottom terminals above water level

– Available for voltages up to 220 V (one of the widest ratings available)

– IP56 rated enclosures suitable for outdoor use – Submersible and explosion-proof models are also available.

Enclosure options for dry-type transformers

There are several different types of enclosures to meet the environmental challenges of dry-type transformers, for various applications. The degree of protection from electrostatic discharge, permanent or temporary latch-up and ingress by particulates (e.g., dirt) is provided by the appropriate enclosure’s rating for environmental protection.

Type 1 Enclosure

This enclosure is rated IP5X, and provides protection against electrostatic discharge. Ingress of foreign material (dust, dirt) prevents the latch-up of the transformer.

Type 2 Enclosure

This enclosure is rated IP4X, and provides protection against electrostatic discharge and foreign material ingress. The enclosure is suitable for indoor and outdoor use.

Type 3 Enclosure

This enclosure meets the requirements for protection against particulate matter, and the latch-up of the transformer is prevented. This enclosure is suitable for indoor and outdoor use.

Type 4 Enclosure

This enclosure is rated IP66, and provides protection against ingress of dust or water (submersion to 1 meter for 30 minutes), and against temporary or permanent latch-up of the transformer. This enclosure is suitable for use in environmentally demanding situations, such as marine, farm, or construction site environments.

Type 5 Enclosure

This enclosure is rated IP68, and provides protection against ingress of dust or water (submersion to 1 meter for 30 minutes), and against temporary or permanent latch-up of the transformer. This enclosure is suitable for use in environmentally demanding situations, such as marine, farm, or construction site environments.

What is difference between dry type and oil type transformer?

There is a huge difference between dry type and oil type transformers. This mainly depends upon the insulation technique used in each which makes them different. Dry-type transformers, as the name, suggests using dry insulations which are of high voltage stability. The insulation of the coils is also different from that used in oil-type transformers.
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