POWER TRANSFORMERS INSULATING LIQUIDS BASIC INFORMATION
What Are The Insulating Liquids Of Power Transformers?
Insulating Liquids
Dielectric liquids of various types are used as an insulating medium as well as a means of cooling liquid-filled transformers. Common insulating liquids include the following:
• Mineral oil. A mineral oil-filled transformer is generally the smallest, lightest, and most economical transformer available. Mineral oil has excellent properties for use in transformers, but it has the inherent weakness of being flammable. Its use, therefore, is restricted to outdoor installations or when the transformer is installed within a vault if used indoors.
• Silicone. A wide variety of synthetic polymer chemicals are referred to by the generic term silicone. Silicone transformer liquids are actually known chemically as polydimethylsiloxane (PDMS). PDMS is a water-clear, odorless, chemically stable, nontoxic liquid.
• High-molecular-weight hydrocarbon (HMWH). HMWH is another high-firepoint dielectric that is widely used as a transformer liquid. It has similar values for dielectric strength and dielectric constant, power factor, and thermal conductivity as mineral oil.
There are no established standards for testing the fire safety of transformers. Factory Mutual Research (FM) and Underwriters Laboratories (UL) both have different criteria for listing transformer liquids. Fire properties of dielectric fluids are typically classified by the following characteristics.
• Flash point: the temperature at which vapors from a liquid surface will ignite in the presence of a flame.
• Fire point: the temperature at the surface of a liquid that will sustain a fire.
• Flame spread: a series of consecutive ignitions.
• Ease of ignition: how readily the liquid will generate and maintain a flammable fuel/vapor mixture at the surface.
• Heat release rate: the product of vaporization rate and the heat of combustion of the fluid. The higher this rate in a large-scale fire, the higher the degree of fire hazard.
Selection of the dielectric liquid depends on the transformer application. Normally, the choice is mineral oil if the device is to be located outdoors.
The National Electrical Code (NEC) does, however, specify certain limitations regarding the use of oil filled transformers in particular outdoor locations. The selection of less-flammable liquids (PDMS and HMWH) often depends upon personal preference, the liquid used in other transformers on the site, or the transformer manufacturer's recommendation.
DIFFERENCE BETWEEN DRY TYPE AND LIQUID FILLED TRANSFORMERS BASIC AND TUTORIALS
COMPARISON BETWEEN DRY TYPE AND LIQUID FILLED TRANSFORMERS
What Is The Difference Between Dry Type and Liquid Filled Transformers?
The advantages and disadvantages of dry-type transformers versus liquid-filled units depend upon the application. Dry-type transformers can usually be located closer to the load, resulting in cost savings because of shorter cable runs and lower electrical losses.
A liquid-filled transformer, on the other hand, may require special construction features for the room in which it will be placed because of fire safety considerations. This may dictate a location some distance from the load.
In addition, periodic testing must be conducted on the fluid to determine its dielectric strength, water content, dissolved gases, and other parameters.
In some applications, there is no option to the use of liquid-filled transformers; dry-types are limited in size and voltage handling capability. Liquid-filled types are available in almost limitless kVA and voltage ratings.
Also, if requirements call for a transformer to be located outdoors, it may be less expensive to purchase a liquid filled unit. With oil as the liquid, the cost would be lower than for a dry-type of equivalent rating; with low-firepoint fluids, the cost would probably be comparable to a dry-type.
For liquid-filled transformers, the main cooling/insulating mediums used today are mineral oil, high-molecular-weight hydrocarbon, and silicone fluid. If a leak occurs in the transformer tank, fire safety becomes an important issue.
Because of hazards associated with tank rupture and the possible ignition of the dielectric, a thorough analysis covering fire safety and the possible effects on the environment should be carried out well in advance of device installation.
Some materials are covered under the Federal Resource Conservation and Recovery Act and the Clean Water Act, including requirements for:
• Special handling
• Spill reporting
• Disposal procedures
• Record-keeping
These considerations can have an effect on installation costs, long-term operating expenses, and maintenance procedures.
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