US2246303A - Electrical discharge device - Google Patents

Description

June 17, 1941. J HQDNETTE 2,246,303 ELECTRICAL DISCHARGE DEVICE Filed Dec. 23, 1938 WITNESSE$1 INVENTOR C John K. Hoaneffe I BY ATTORNEY Patented June 17, 1941 2,246,303 ELECTRICAL DISCHARGE DEVICE John K. Hodnette, Sharon, Pa., assignor to Westinghouse Electric & Manufacturing Company, East Pittsburgh, Pa., a corporation of Pennsylvania Application December 23, 1938, Serial No. 247,447

3 Claims.

My invention relates to electrical discharge devices and particularly to a self -interrupting space discharge device for draining excess voltage from transmission lines. The invention is adapted for use on the terminal bushings of transformer structures.

Electrical transmission lines are subject to excess voltage surges such as may be caused by lightning or by switching operations on the circuit. These high voltage surges frequently cause severe damage to the transmission line or to apparatus, such as transformers, connected to it. It is customary to protect transmission lines and apparatus connected thereto by various kinds of over-voltage discharge devices for draining the over-voltage from the transmission line. Of the various devices which have heretofore been used, the simple arc gap, set to begin operation or to breakdown at a voltage somewhat in excess of the highest normal operating voltage of the line, are very eflicient.

Although these simple arc gaps are effective in reducing over-voltage on the lines, they introduce disadvantages in that once current flows across the gap due to the over-voltage, thus breaking down the gap, a conducting path is established over which current may continue to flow at normal line voltage. In order to extinguish the are following a discharge through the arc gap, it has been necessary to completely disconnect the line before service could be reestablished. It is highly desirable to maintain continuity of service on electrical systems and, as a consequence, many attempts have been made to provide self-quenching over-voltage drainage devices for replacing the simple space gap discharge device.

In the case of power or distribution transformers used in alternating current electrical systems, a discharge gap having a breakdown voltage less than the dielectric strength of the transformer insulation is employed to protect the transformer and drain ofi the surge voltage. The gap may be between the line conductor leading into the transformer through the transformer terminal bushing and the tank when connected to ground.

It is an object of my invention to provide a self-quenching space discharge device for draining off excess voltage from alternating current transmission lines that is simple in construction and economical in operation.

It is a further object of my invention to provide an improved self-quenching space discharge device that is especially suitable for use with transformers.

Other objects and advantages of my invention will be apparent from the following description of one preferred embodiment of the invention, reference being had to the accompanying drawing in which:

Figure 1 is a sectional view of apparatus comprising one embodiment of the invention, and

Fig. 2 is an enlarged sectional view showing a portion thereof.

In Fig. 1 a porcelain insulator type bushing l is illustrated that is supported within a metal flange 2 by well known means, not shown, and mounted on the cover of a metal tank 3, which may be a transformer tank through which the bushing extends. The flange 2 may be held in place on the tank cover by bolts 4. A stud 5 of conducting material such as copper extends through a hole centrally of the bushing I for connecting the line circuit conductor to the apparatus within the tank 3. The stud may be attached to the bushing by means of a washer I and nut B at its lower end and a nut 8 and a cap 9 at the upper end thereof. The upper electrode to of the electrical discharge device of my invention is attached to the upper end of the stud 5 by means of a nut H. The electrode l0 extends laterally from the stud a suiticient distance that its down turned terminal end i2 and the parts attached thereto clear the porcelain bushing i.

An insulating block [3 of gas evolving material, such as hard fibre, is secured about the lower end it of the electrode Ill. The lower end of the insulating block is formed to provide a cup shaped or conical inner wal1 14 about the path of the arc stream between the electrode end I2 and a lower electrode [5 positioned directly below and connected to ground through the tank wall 3. The electrode l5 may, as illustrated, be provided onthe metal flange 2. A tube l6 of conducting material may be provided about the tube I3 to maintain the breakdown voltage substantially constant. The purpose of this tube is to maintain the total breakdown are gap from the electrode ill to the tube l6, and from the tube l6 to the terminal I 5 substantially constant as the electrode and tube l3 erode with use. The tube l6 may be omitted, in some instances, if desired.

When overvoltage occurs on the system sufficient to bridge the two gaps from the end I! of the electrode III to the tube l6, and from tube I6 to the electrode IS, a path of ionized gas or path causes the arc is established between the electrodes l and IS. The presence of the are passing along this gas evolving body [3 to discharge non-ionized gas from the inner surface ll thereof at substantially right-angles thereto, or in the direction shown by the dotted arrows IS in Fig. 2. The gas thus emitted from the inner wall ll of the gas-evolving body I3 is discharged from all sides of the arc stream la in a direction to cross the path of the arc stream, this discharge being concentrated along the central axis of the conical surface occupied by the arc stream. The swiftly moving gas evolved from the surface of the member I3 is thus concentrated in the region of the arc and carries away the ions produced by the are so rapidly that when the abnormal voltage across the gap disappears, the insulating properties of the arc are restored at the end of the half cycle voltage wave of the transmission line so that normal alternating voltage of the line will not reestablish the arc to maintain the flow of current after the disappearance of the abnormal voltage of the system. Thus a simple and eflicient self-quenching gap for use with transformers is provided.

As the discharge gap operates, the electrode i2 and the insulating tube l3 burn away. In order to correlate the rate of burning of the electrode I2 with the rate of burning of the tube I3, it may be desirable to provide an electrode, in some instances, which erodes more rapidly than solid metal. For this purpose the electrode may be formed of a cellulose acetate material that is heavily graphited to make it conducting. Such material erodes in the presence of an electric arc, in the same manner as the fibre tube or like gas-evolving material of the tube l3. An electrode formed of such material might be desirable in applications where very small amounts of power current are experienced.

Modifications in the structure disclosed within the spirit of my invention will occur to those skilled in the art, and I do not wish to be limited otherwise than by the scope or the appended claims.

I claim as my invention: 7

1, An excessive-voltage protective device for an alternating current system, comprising a pair of spaced electrodes, an insulating body about the path between the electrodes comprising material which emits substantially non-ionized gas in the presence of an electric arc, said body being so disposed forwardly of one electrode as to discharge gas evolved therefrom into the arc stream, and a tubular conducting member about the insulating body and extending forwardly thereof about the arc path for determining the discharge p.

2. An excess-voltage protective device for an alternating current system, comprising a pair of spaced electrodes connected respectively to a line circuit conductor and to ground, an insulating body about the path between the electrodes having a skirt portion extending outwardly from one of the electrodes and formed of a material capable of evolving gas when subjected to an arc,

, and a tubular conducting member about the insulating body and extending forwardly thereof about the arc path for determining the discharge gap.

3. An excess-voltage protective device for an alternating current system, comprising a pair of spaced electrodes connected respectively to a line circuit conductor and to ground, a body of insulating material which emits substantially nonionized gas in the presence of an electric arc, said body formed to provide an inner wall about the path between the electrodes for discharging nonionized gas into the arc stream simultaneously from all sides, and a tubular conducting member about the insulating body and extending forwardlythereof about the arc path for determining the discharge gap.

JOHN K. HODNE'I'I'E.

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