Wave transmission lines and networks – Coupling networks – Frequency domain filters utilizing only lumped parameters
Reexamination Certificate
1998-12-30
2001-01-23
Bettendorf, Justin P. (Department: 2817)
Wave transmission lines and networks
Coupling networks
Frequency domain filters utilizing only lumped parameters
C333S177000, C333S185000, C336S070000
Reexamination Certificate
active
06177848
ABSTRACT:
DESCRIPTION
1. Technical Field
The present invention relates generally to the protection of transformers. More particularly, the present invention relates to the protection of transformers in which voltage transients, such as voltage surges created when switching the transformer on and off, are dampened so that the transients do not damage the transformer.
2. Background of the Invention
Power transformers and other wire-wound devices have been known to fail by a phenomena called “switching resonance.” For example, a circuit breaker connecting a power transformer to a power source may go through a state known as multiple re-ignitions as the power transformer is switched on or off. The multiple re-ignitions may last for less than 10 microseconds. During this short period of time, the re-ignition rate of the circuit breaker may be on the order of 10 to 10,000 kilohertz. The rapid re-ignitions cause the coils of the transformer to develop resonance at these frequencies. At these very high frequencies, very high voltages can be induced between the turns of the transformer coils. The large voltages can arise when some type of switching occurs in the network.
One method used to prevent the harmonic effects of voltage transients is to attempt to restrict harmonic currents by the use of low pass filters or high frequency traps. These filters are configured to become increasingly conductive as frequency increases. They shunt high frequency disturbances to ground and dissipate the energy. Further, the switching resonance problem typically occurs deep in the center of the windings where normal means of over-voltage suppression become very difficult and impractical. Although the use of external RC networks have been successfully used to control these events, these devices require a significant economic investment.
Various electrostatic shielding techniques have also been used to control the magnitude of internal voltage oscillations. The shielding consists of a metal foil, and is heavily insulated from the coil and from surrounding structural parts at ground potential. The shielding is electrically connected to the line terminal of the coil. The electrostatic shield adds series capacitance to the circuit, thus minimizing the magnitude of the high frequency oscillations. The resonance of the oscillations, however, is not dampened by the electrostatic shield. In addition, although the electrostatic shield adds series capacitance to the outer layer of turns, no series capacitance is directly added to the inner layers of turns.
SUMMARY OF THE INVENTION
The present invention is directed to a transformer comprising a coil and a resistive element. The coil has an insulated wire formed into a plurality of adjacent turns defining a layer. The resistive element couples one of the plurality of turns with another of the plurality of turns. The resistive element adds both series capacitance and series resistance to the transformer circuit, thereby minimizing the magnitude of the high frequency oscillations as well as dampening the resonance of the oscillations.
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Electrical Transients in Power Systems; Second Edition; Allan Greenwood; Rensselaer Polytechnic Institute; “Modeling Power Apparatus and the Behavior of such Equipment under Transient Conditions” -pp. 322-384, 1991.
Transformer Engineering; A Treatise on the Theory, Operation, and Application of Transformers; The late L.F. Blume, A. Boyajian, G. Camilli, T.C. Lennox, S. Minneci, V.M. Montsinger; All of the Engineering Division of the General Electric Company, Pittsfield, Massachusetts; Second Edition; “Transient Voltage Characteristics of Transformers”; pp. 463-500, 1951.
Hopkinson Philip J.
Rettew Richard R.
Bettendorf Justin P.
Femal Michael J.
Golden Larry I.
Square D Company
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