Compositions – Fluent dielectric
Reexamination Certificate
2002-11-25
2004-04-27
Gupta, Yogendra N. (Department: 1751)
Compositions
Fluent dielectric
C252S070000, C252S073000, C252S578000, C252S579000
Reexamination Certificate
active
06726857
ABSTRACT:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
TECHNICAL FIELD OF THE INVENTION
This invention relates generally to equipment utilized in the transmission and distribution of electrical power. More specifically, the invention relates to transformers and other apparatus containing dielectric fluids, particularly dielectric fluids comprising relatively pure blends of compounds selected from the group consisting of aromatic hydrocarbons, polyalphaolefins, polyol esters, and natural vegetable oils. The invention further relates to the methods for preparing and processing such fluids and filling and sealing electrical apparatus with such fluids.
BACKGROUND OF THE INVENTION
Many types of conventional electrical equipment contain a dielectric fluid for dissipating the heat that is generated by energized components, and for insulating those components from the equipment enclosure and from other internal parts and devices. Examples of such equipment include transformers, capacitors, switches, regulators, circuit breakers and reclosers. A transformer is a device that transfers electric power from one circuit to another by electrical magnetic means. Transformers are used extensively in the transmission of electrical power, both at the generating end and the user's end of the power distribution system. A distribution transformer is one that receives electrical power at a first voltage and delivers it at a second, lower voltage.
A distribution transformer consists generally of a core and conductors that are wound about the core so as to form at least two windings. The windings (also referred to as coils) are insulated from each other, and are wound on a common core of magnetically suitable material, such as iron or steel. The primary winding or coil receives energy from an alternating current (AC) source. The secondary winding receives energy by mutual inductance from the primary winding and delivers that energy to a load that is connected to the secondary winding. The core provides a circuit or path for the magnetic lines of force (magnetic flux) which are created by the alternating current flow in the primary winding and which induce the current flow in the secondary winding. The core and windings are typically retained in an enclosure for safety and to protect the core and coil assembly from damage caused by the elements or vandalism.
The transformer windings or coils themselves are typically made of copper or aluminum. The cross section of the conductors forming the coil must be large enough to conduct the intended current without overheating. For small transformers, those rated less than 1 kVA, the coil wire may be insulated with shellac, varnish, enamel, or paper. For larger units, such as transformers rated 5 kVA and more, the conductor forming the coil is typically insulated with oil-impregnated paper. The insulation must provide not only for normal operating voltages and temporary overvoltages, but also must provide the required insulative levels during transient overvoltages as may result from lightning strikes or switching operations.
Distribution transformers used by the electric utilities in the United States operate at a frequency of 60 Hz (cycles per second). In Europe, the operating frequency is typically 50 Hz. Where the size and weight of the transformer are critical, such as in aircraft, transformers are typically designed to operate at a frequency of from 400 to 4,000 cycles per second. These high frequency applications allow the transformer to be made smaller and lighter than the 50 Hz and 60 Hz transformers designed for power distribution by the electric utilities.
The capacity of a transformer to transmit power from one circuit to another is expressed as a rating and is limited by the permissible temperature rise during operation. The rating of a transformer is generally expressed as a product of the voltage and current of one of the windings and is expressed in volt-amperes, or for practical purposes, kVA (kilovolt-amperes). Thus, the kVA rating of a transformer indicates the maximum power for which the transformer is designed to operate with a permissible temperature rise and under normal operating conditions.
Modern transformers are highly efficient, and typically operate with efficiencies in the range of 97-99%. The losses in the transformation process arise from several sources, but all losses manifest themselves as heat. As an example of the heat that is generated by even relatively small, fluid-filled distribution transformers, it is not uncommon for a 15 kVA mineral oil-filled transformer to operate with temperatures inside the transformer enclosure exceeding approximately 90° C. continuously.
A first category of losses in a transformer are losses resulting from the electrical resistance in the conductors that constitute the primary and secondary windings. These losses can be quantified by multiplying the electrical resistance in each winding by the square of the current conducted through the winding (typically referred to as I
2
R losses).
Similarly, the alternating magnetic flux (or lines of force) generates current flow in the core material as the flux cuts through the core. These currents are referred to “eddy currents” and also create heat and thus contribute to the losses in a transformer. Eddy currents are minimized in a transformer by constructing the core of thin laminations and by insulating adjacent laminations with insulative coatings. The laminations and coatings tend to present a high resistance path to eddy currents so as to reduce the current magnitudes, thereby reducing the I
2
R losses.
Heat is also generated in a transformer through an action known as “hysteresis” which is the friction between the magnetic molecular particles in the core material as they reverse their orientation within the core steel which occurs when the AC magnetic field reverses its direction. Hysteresis losses are minimized by using a special grade of heat-treated, grain-orientated silicon steel for the core laminations to afford its molecules the greatest ease in reversing their position as the AC magnetic field reverses direction.
Although conventional transformers operate efficiently at relatively high temperatures, excessive heat is detrimental to transformer life. This is because transformers, like other electrical equipment, contain electrical insulation which is utilized to prevent energized components or conductors from contacting or arcing over to other components, conductors, structural members or other internal circuitry. Heat degrades insulation, causing it to loose its ability to perform its intended insulative function. Further, the higher the temperatures experienced by the insulation, the shorter the life of the insulation. When insulation fails, an internal fault or short circuit may occur. Such occurrences could cause the equipment to fail. Such failures, in turn, typically lead to system outages. On occasion, equipment can fail catastrophically and endanger personnel who may be in the vicinity. Accordingly, it is of utmost importance to maintain temperatures within the transformer to acceptably low levels.
To prevent excessive temperature rise and premature transformer failure, distribution transformers are generally provided with a liquid coolant to dissipate the relatively large quantities of heat generated during normal transformer operation. The coolant also functions to electrically insulate the transformer components and is often therefore referred to as a dielectric coolant. A dielectric coolant must be able to effectively and reliably perform its cooling and insulating functions for the service life of the transformer which, for example, may be up to 20 years or more. The ability of the fluid and the transformer to dissipate heat must be such as to maintain an average temperature rise below a predetermined maximum at the transformer's rated kVA. The cooling system must also prevent hot spots or excessive temperature rises in any portions of the transformer. Generally, this is accomplished by submerg
Gauger Gary A.
Goedde Gary L.
Lapp John
Yerges Alan P.
Cooper Industries Inc.
Fish & Richardson P.C. P.A.
Gupta Yogendra N.
Hamlin Derrick G.
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