SUPPRESSION TECHNIQUE OF TRANSIENTS IN ROLLINGS
Description 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 shock waves created when the active and inactive transformer is switched, are damped so that the transients do not damage the transformer. BACKGROUND OF THE INVENTION Power transformers and other wound wire devices have been known to fail by a phenomenon called "switching resonance". For example, a circuit breaker that connects a power transformer to a power source can pass through a state known as multiple re-ignitions when the power transformer is active or inactive. Multiple re-ignitions can last less than 10 microseconds. During this short period of time, the re-ignition rate of the circuit breaker can be of the order of 10 to 10,000 kHz. Rapid re-ignitions cause the transformer coils to develop resonance at these frequencies. At these extremely high frequencies, extremely high voltages can be induced between the turns of the transformer coils. Large voltages can arise when some type of switching occurs in the network. A method used to prevent the harmonic effects of voltage transients is to try to restrict harmonic currents through the use of low pass filters or high frequency traps. These filters are configured to become more and more drivers as the frequency increases. They derive high frequency disturbances to ground and dissipate energy. In addition, the switching resonance problem typically occurs at a site deep in the center of the windings, where normal means of suppression of overvoltage become very difficult and impractical. Although the use of external RC networks has been successful in controlling these events, these devices require a considerable economic investment. Various electrostatic shielding techniques have also been used to control the magnitude of internal voltage oscillations. The shield consists of a sheet or sheet of metal, and is heavily insulated from the coil and from surrounding structures to ground potential. The shield is electrically connected to the line terminal of the coil. The electrostatic shield adds capacitance in series to the circuit, thus minimizing the magnitude of the high frequency oscillations. However, the resonance of the oscillations is not buffered by electrostatic shielding. In addition, although electrostatic shielding adds serial capacitance to the outer spinning layer, serial capacitance is not added directly to the inner spinning layers. SUMMARY OF THE INVENTION The present invention is directed to a transformer comprising a coil and a conductive element. The coil has an isolated wire length formed in a plurality of adjacent turns. The conductive element spirals around the length of insulated wire and couples one turn of the plurality of turns with another turn of the plurality of turns. The conductive element adds capacitance in series to the transformer circuit, thereby minimizing the magnitude of the high frequency oscillations. Brief Description of the Drawings Figure 1 is a perspective view showing the formation of a coil for a transformer according to the present invention. Detailed Description Although this invention is susceptible of being embodied in many different forms, it is shown in the drawings and will be described in detail in the present preferred embodiments of the invention, with the understanding that the present disclosure should considered as an exemplification of the principles of the invention and is not intended to limit the broad aspects of the invention to the illustrated embodiments. Conventional transformers comprise a primary coil and at least one secondary coil. The current through the primary coil produces a magnetic field that induces a voltage through the secondary coil. Both primary and secondary coils have an isolated wire length formed in a plurality of adjacent turns that define a layer. As it is well known, many layers of adjacent turns separated by means of insulation typically form the coils. The coil 10 of a transformer according to the present invention is shown in Figure 1. A resistive element 12 extends along the length of the insulated wire 14 of the coil 10, and forms a spiral around the insulated wire 14. The resistive element 12 has a resistance between adjacent turns 16 of 10 to 1,000 ohms. Preferably, the resistive element 12 comprises a semiconductor paint. In particular, the semi-conductive paint comprises carbon black or metal oxide. The resistive element 12 is of sufficient thickness to ensure that one revolution of the plurality of turns 16 is coupled with another turn of the plurality of turns 16 of the same layer 18. The wire 20 by winding on the coil 10 is insulated by winding one insulating tape 22 on the surface of the wire 20. The resistive element 12 can be applied directly to the insulating tape 22. The tape 22 is preferably one inch wide, and is wrapped around the wire 20 at such a high tilt so that in a turn around the wire 20, the tape 22 becomes semi-bent. Thus, in most areas on the wire surface, there are two thicknesses of tape 22, except for a small free space where there would only be one layer of tape 22. When the insulated wire 14 is wound on a bobbin 10, there are two and four layers of insulation between adjacent turns 16 of the wire 14. The resistive element 12 is placed on the insulating tape 22 before wrapping the wire 20 with the tape 22. Specifically, the resistive element 12 is a semi-conductive coating a along the length of the insulating tape 22. The resistive element 12 can cover a portion of a surface of the insulating tape 22, or it can cover the entire surface of the insulating tape 22. Preferably, the resistive element 12 is painted as a strip 24 running longitudinally along the length of the tape 22. The strip 24 is placed along the edge 24 of the tape 22 so as to wrap around the wire 20, the resistive element 12 would present itself only on the outer surface of the insulated wire 14. In this manner, there would be no resistive element 12 in contact with the wire 20, nor would there be any resistive element 12 between the insulating layers. By winding the insulated wire 14 in a coil 10, the resistive element 12 of a turn 16 of the insulated wire 14 will contact the resistive element 12 of an adjacent turn 16 of the coil 10 and form an electrical connection between the outer surfaces of the isolated wires 14. A small continuous RC network is thus formed between each turn in the coil 10. Specifically, the wire 14 of a turn 16 forms a plate of a first capacitor, the insulating material of that round 16 forms the dielectric for the first capacitor, and the resistive element 12 on the surface of that turn 16 becomes the second plate of the first capacitor. The resistive element 12 also forms a resistor. The resistive element 12 on the surface of an adjacent turn 16 forms a second resistor connected in series. The resistive element 12 of the adjacent turn 16 also forms the first plate for a second capacitor, the insulating material and the wire 14 of the second turn 16 forming the dielectric and the second plate of the second capacitor, respectively. The electrical equivalent of this circuit would be a capacitor, two resistors and a second capacitor, all in series between all the turns 16 of the coil 10. Consequently, the resistive element 12 not only increases the series capacitance of the transformer circuit, but also it also increases the series conductance of the transformer circuit through the layer 18 of the transformer winding. The increase in the series conductance increases the damping of the switching resonance. A conductive element 12 can also be used in the present invention to add capacitance in series without adding series resistance to the transformer circuit. Not only can the currents of the RC network flow perpendicularly through the resistive element 12, as described above, but the current also flows longitudinally along the length of the wire 14. Likewise, the resistive material 1216 can more evenly distribute the dielectric stress within the insulating material. Abrupt changes in dielectric materials having different dielectric constants can have an adverse effect on the dielectric materials in contact with each other as a result of high levels of dielectric stress. The resistive elements 12 will distribute any concentrated levels of stress that may develop in the winding process. At power frequencies, the current flow in any direction through the resistive element 12 would be small due to the capacitance reactance, or impedance, relatively high across the dielectric of the insulation material. However, at high frequencies, the capacitive reactance of the insulation material becomes low and the resistive element 12 becomes connected to each wire 14. This causes the energy of the transient to be absorbed by the resistive element 12 which transforms the energy into heat that dissipates over time. This dissipation of energy dampens the resonant activity of coil 10, preventing high voltages between turns. In this way, the transformer is self-protective. Although specific embodiments have been illustrated and described, numerous modifications come to mind without significantly departing from the spirit of the invention and the scope of protection is limited only by the scope of the accompanying claims.