Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – Of individual circuit component or element
Reexamination Certificate
2001-05-03
2003-04-15
Oda, Christine (Department: 2858)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
Of individual circuit component or element
C324S726000
Reexamination Certificate
active
06549017
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to energy delivery systems, and more particularly, to a system and method for testing winding and winding connection deformations and/or displacements in a transformer.
BACKGROUND OF THE INVENTION
Electric utilities and other organizations are responsible for supplying an economic, reliable and safe source of electricity. Three major components are employed in an energy delivery system to provide the electricity to the end user, the generator, the transmission line and the transformer.
Generators are rotating machines operated in a manner such that electricity is created when mechanical energy is used to rotate the generator shaft. A generator rotor is coupled to the shaft, and when the shaft is rotated, thereby rotating the rotor, a voltage and current is caused in the generator stator. One typical form of mechanical energy used to generate electricity is steam, which is passed through a turbine that forces the generator shaft to rotate. Steam is often created by boiling water using coal, natural gas or nuclear fission heat sources. Or, steam may be taken directly from naturally occurring geothermal sources. Other sources of mechanical power employed for rotating a generator rotor may also include hydroelectric power or wind power. Since the end user of the electricity is rarely located near a generator, the electricity generated by the generator must be “transported” to the end user.
The second major component employed in an energy delivery system is the transmission line. Transmission lines consist of a grouping of wires which connect the generator to the end user. The “amount” of electricity that a transmission line can carry depends primarily upon the diameter of and number of the conductors (wires) used on the transmission line, and the voltage at which the transmission line is operated at. Typically, transmission lines from the generators employ a relatively high voltage so that a large amount of electricity is economically and reliably transported over long distances to locations where large concentrations of end users are found, such as a city or a large industrial manufacturing plant. Examples of extra high voltage (EHV) and intermediate transmission voltages employed in the industry include, but are not limited to, 500 kilo-volts (kV), 230 kV, 138 kV, 115 kV, 69 kV and 46 kV. Typically, lower transmission line voltages are employed on the transmission line distribution system (such as, but not limited to, 25 kV, 20 kV, 13.8 kV, 12 kV, 4 kV, 480V and 240V) to provide energy to the end user's premises connection point.
The third major component employed in an energy delivery system is the transformer. The transformer is a device that changes voltage. Generally, voltage from the generator is a lower voltage than used by the transmission lines that transmit the electricity to the end user. Furthermore, the voltage used by the end user is much lower than voltage used by the transmission lines. Thus, the transformer couples elements of an energy delivery system that employ different voltages.
For example, two voltages typically found in a home are 240 volts and 120 volts. An EHV 500 kV transmission line may be delivering power to a city that employs a 230 kV transmission line system to deliver energy to a 13.8 kV distribution system. A 500/230 kV transformer changes voltage from 500 kV to 230 kV, thereby allowing two transmission lines having different operating voltages (500 kV and 230 kV) to be coupled together. Such a transformer has at least two terminals, a 500 kV terminal and a 230 kV terminal. Similarly, a 13.8 kV/240V/120V transformer may be used to convert voltage of the 13.8 kV distribution system to a voltage used in the end user's home or office. Thus, transformers allow the various voltage generators, transmission lines and distribution lines to be coupled to a home, office or other facility where the end user will be using the electricity.
Transformers come in many different sizes, shapes and constructions. Typically, transformer size (rating) is specified as the product of the maximum voltage and current, as measured from one side of the transformer, that the transformer is capable of converting at a particular operating condition. Such operating conditions include temperature and/or altitude. For example, a 500/230 kV transformer may be rated at 300 MVA (3,000 kilo-volt-amps) when operating at sea level and at 65° Celsius rise above ambient. Transformers may be constructed as separately insulated winding transformers or auto transformers, and as single phase or multiple phase transformers. The operating voltages, ratings and winding types of transformers employed in the industry, well known to one skilled in the art, are too numerous to describe in detail here other than to the extent necessary to understand the present deficiencies in the prior art.
All transformers, independent of size, rating and operating voltage, have several common characteristics. First, the transformer is constructed from one or more windings, each winding having a plurality of individual coils arranged and connected in an end-to-end fashion. In some transformers, the winding is made by wrapping a wire around a laminated solid member, called a core. Alternatively, there may be no core. However, in all transformers, the individual windings must be electrically isolated from each other. An insulation material is wrapped around the wires such that when the plurality of coils are made, the metal wires of each winding are physically and electrically separated, or insulated, from each other. Insulation materials wrapped around the windings may vary. Paper, impregnated with oil, is often used. Other types of transformers may use only paper, or may use another suitable material such as a polymeric compound.
Maintaining the electrical insulation between the windings is absolutely essential for the proper operation of a transformer. In the event that the electrical insulation is breached, such that electricity passes from one winding coil across the breach to another winding coil, special protective devices will operate to disconnect the transformer from the electrical system. The devices, by removing electricity applied to the transformer, interrupt the undesirable current flow through the insulation breach to minimize damage to the transformer. This condition is commonly referred to in the industry as a transformer fault.
Transformer faults are undesirable for at least two major reasons. First, end users may become separated from the energy delivery system, thereby loosing their electrical service. Second, transformer faults may result in large magnitudes of current flow, known as fault current, across the breach and through the transformer windings. Also, faults occurring on the energy delivery system at locations relatively close to the transformer may result in large fault currents flowing through the transformer. Often, fault current may be orders of magnitude greater than the highest level of normal operating current that the transformer was designed to carry. Such fault currents may cause severe physical damage to the transformer. For example, a fault current may physically bend portions of the transformer winding (winding deformation) and/or move the windings out of their original position in the transformer (winding displacement). Such winding deformation and/or displacement can cause over-voltage stresses on portions of the winding insulation and exacerbate the process of the naturally occurring deterioration of the winding insulation that occurs over a period of time. The fault current may further increase damage to the insulation, or damage insulation of adjacent windings, thereby increasing the magnitude and severity of the fault. In the most extreme cases, the fault current may cause an ignition in the transformer oil, resulting in a breach of the transformer casing and a subsequent fire or explosion.
Therefore, it is desirable to ensure the integrity of the transformer winding insulation. Once a transformer fa
Georgia Tech Research Corporation
Kerveros James
Oda Christine
Thomas Kayden Horstemeyer & Risley LLP
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