1. Field of the Invention
The disclosed invention is related to current limiting devices and more particularly to a current limiting device utilizing a vacuum interrupter providing for a transverse magnetic field to be applied to the arc formed across the relatively movable contacts during circuit interruption.
2. Description of the Prior Art
There is a need in the Electric Power industry for a fast acting switch to be an integral part of a current limiting system. The necessity of the current limiting device is brought about by the increased fault current available in power systems. This necessitates an upgrading of present circuit breakers or installing devices to limit fault current. The various requirements have been discussed thoroughly by C. A. Falcon et al., IEEE Symposium on Current Limiting Devices held at California on July 18, 1974 in a paper entitled "Current Limiting Devices -- Need and Application".
Various current limiting devices have been proposed and are under development. In a prior art current limiting device, a vacuum circuit interrupter is opened during the current rise to peak fault current. The arc current is subsequently pulsed to zero by discharging capacitor current in the opposite direction to fault current flow. This opposing current is supplied from a capacitor discharge through a triggered vacuum gap. The vacuum interrupter circuit current then commutates into a current limiting reactor or resistor bank in parallel with the vacuum switch. This system has the disadvantage of requiring two current pulse circuits, one for each arc polarity. A further disadvantage is that the capacitors of the pulse circuit are exposed to the power line potential, and consequently are expensive.
In another prior art current limiting device when fault current is sensed a fast acting SF.sub.6 switch is activated. Activating the SF.sub.6 switch transfers the current to a cross field tube and when the magnetic field in the tube is relaxed the current commutates in the tube either into a reactor or a resistor bank. Disadvantages of this system are the required three stage current transferring device and the use of the cross field tube which at this time is an experimental tube still undergoing research development.
U.S. Pat. No. 3,283,103 teaches the use of an applied magnetic field to a vacuum interrupter during current interruption to confine the arc plasma to the inner electrode gap. In this device interruption occurs at current zero, not before. There is no suggestion that this device can be used for current limiting.
U.S. Pat. No. 3,716,685 disclosed a magnetic field oriented in direction both parallel to and radially perpendicular to the general direction of the current flow at the instant the circuit is interrupted to create instability in the arc thereby forming to cause the arc to move into contact with surrounding baffles. This patent teaches that the axial magnetic field component be stronger than the radial magnetic field component for the desired arc movement. There is no indication in U.S. Pat. No. 3,716,685 of the desirability of using a transverse magnetic field on a vacuum circuit interrupter utilizing current chopping for current limiting.
U.S. Pat. Nos. 3,564,176 and 2,922,926 disclose an arc extinguishing device utilizing magnetic plates to produce arc instability which facilitates arc extinction. In these patents there is no suggestion the plates can be utilized with current chopping in a vacuum interrupter to provide current limiting.
In proceedings of the IEEE, Volume 60, No. 8, August, 1962; a paper entitled "Pulsed Metallic-Plasma Generators" describes a device which utilizes a vacuum arc as the plasma source. The arc is initiated on the surface of a consumable cathode which can be any electrically conductive material. The plasma is ejected in the form of a high velocity highly directional conical plume through a ring shaped anode. One or more magnetic field coils may be used to control the impedance and direction of the plume. It is suggested in this paper that the generator can be used as a very high power switching device. The pulsed metallic plasma generator utilizes fixed electrodes with a ring shaped anode concentric with a cathode igniter assembly. Problems with this device are that it is sensitive to arc polarity, which can present problems when utilized on an alternating current circuit, and the electrodes of the device are fixed so they cannot close to create a circuit therethrough.