Cooling System use In Transformer

Transformer Cooling Systems: Classifications and Operation Explained897×550 87.7 KB

Heat is one of the leading causes of transformer failure. The main source of heat generation in transformers is caused by copper loss in the windings and core (I²R losses). If heat inside a transformer is not properly dissipated, the temperature will continually rise, potentially damaging the insulation.

A transformer operating at just 10°C above its rating will reduce its life by 50%. It is easy to see why it is imperative to understand how transformers are cooled and how to detect problems in their cooling systems.

Transformers may have multiple load ratings that correspond to multiple stages of cooling. ANSI and IEEE require the cooling class of each transformer to appear on its nameplate. The cooling classification of a transformer, expressed in letters, designates the type of cooling system used.

Dry-Type Transformers

Dry-type transformers feature coils in open air and depend primarily on convection currents created by the transformer’s heat to generate airflow across the coils and maintain cool temperatures.

Louvers or screened openings are used to direct the flow of cool air across the transformer coils. Fans are often used to force the circulation of air through the case.

The kVA rating of a fan-cooled dry-type transformer is increased by as much as 33% compared to that of a self-cooled dry-type transformer of the same design.

It is important to keep dry-type transformer enclosures clean, and the area around them clear. Items placed near or against the transformer will restrict heat transfer around the enclosure.

As dirt accumulates on cooling surfaces, it becomes increasingly difficult for the air around the transformer to remove heat. Consequently, over time, the transformer temperature slowly rises unnoticed, reducing its service life.

Dry Type Transformer Cooling Classification

Cooling classes of dry type transformers are covered by ANSI/IEEE standard C57.12.01. Below is a summary of some common examples:

AA

Ventilated, self-cooled transformers have ventilation ports located in the outside walls of the enclosure. There are no fans to force air into and out of the enclosure, typically with no external fins or radiators. Cooler air enters the lower ports, is heated as it rises past windings, and exits through the upper ventilation ports.

AFA

Self-cooled (A) and additionally cooled by forced circulation of air (FA). These transformers have ventilation ports for fan inlets and outlets only. Normally, there are no additional ventilation ports for natural air circulation.

AA/FA

Ventilated, self-cooled (same as Class AA). These transformers have a fan or fans providing additional forced-air cooling. Fans may be wired to start automatically when the temperature reaches a pre-set value. These transformers generally have a dual load rating, one for AA (self-cooling natural air flow) and a larger load rating for FA (forced air flow).

ANV

Self-cooled (A), non-ventilated (NV). These transformers have no ventilation ports or fans on the enclosure and are not sealed to exclude migration of outside air, but there are no provisions to intentionally allow outside air to enter and exit. Cooling is by natural circulation of air around the enclosure. This transformer may have some type of fins attached outside the enclosure to increase surface area for additional cooling.

GA

Sealed with internal gas (G) and self-cooled (A). These transformers typically have a gas, such as nitrogen, SF6, or freon, to provide high dielectric and efficient heat removal. Cooling occurs by natural circulation of air around the outside of the enclosure. There are no fans to circulate cooling air; however, there may be fins attached to the outside to aid in cooling. The enclosure is hermetically sealed to prevent leakage.

Liquid Cooled Transformers

Liquid-filled transformers have coils immersed in an insulating medium, usually oil, which serves as both an insulator and provides an effective medium for removing excess heat.

As transformer oil deteriorates through aging and moisture ingress, transformer oil samples should be periodically drawn and analyzed in accordance with ASTM insulating oil test methods.

Liquid-cooled transformer cooling classes underwent significant changes in the United States when IEEE Standard C57.12.00-2000 adopted the four-letter designation found on modern power transformers.

Modern Cooling Class Designations (2000 and Later)

First Letter: Internal cooling medium in contact with the windings

• O: Mineral oil or synthetic insulating liquid with fire point < 300°C
• K: Insulating liquid with fire point > 300°C
• L: Insulating liquid with no measurable fire point
• G: Gas insulated

Second Letter: Circulation mechanism for internal cooling medium

• N: Natural convection flow through cooling equipment and windings
• F: Forced circulation through cooling equipment (cooling pumps), natural convection flow in windings (non-direct flow)
• D: Forced circulation through cooling equipment, directed from the cooling equipment into at least the main windings

Third Letter: External cooling medium

• A: Air
• W: Water

Fourth Letter: Mechanism for external cooling medium

• N: Natural convection
• F: Forced convection

The cooling design of a Gas Insulated Transformer is nearly the same as that for an Oil Insulated Transformer

Gas Insulated Transformer Cooling Diagram823×350 56.7 KB