US2703852A - Overvoltage protected induction apparatus - Google Patents

Description

March 8 1955 J. R. MEADOR OVERVOLTAGE PROTECTED INDUCTION APPARATUS Filed Aug. 31, 1951 Inventor: Jack R. Meadow",

His Abba-hag.

have been made by this method, the method "present invention aims to overcome.

acteristics of the winding itself,

' 2,276,855, which explains also the limitations tain limitations in to reduce this wastage 'the different classes and types of equipment they are to United States Patent:

OVERVOLTAGE PROTECTED INDUCTION APPARATUS Jack R. Meador, Pittsfield, Mass,

Electric Company, a corporation of New York Application Augu st 31, 1951, Serial No. 244,541

2 Claims. (Cl. 317-) shields to avoid voltage concentrations and resonance phenomena and external lightning arresters to avoid excessive voltages at the terminals of the winding. Although well protected high voltage transformers can and is not free from various limitations and disadvantages, which the For instance, conthe factor of shielding, adequate electrostatic shielding has been found applicable so far only to limited types of windings, and only in a limited class of circuit connections, so that certain winding arrangements desirable for other purposes have either had to be dispensed with or used with inadequate shielding. Furthermore, the successful performance of a shield depends on the successful adaptation of the shield design to the charrequiring great design skill, and not all of the so-called shielded windings sidering' have necessarily the hoped-for impulse voltage distribution.

It follows that no standardized shield may be applied to any and all high voltage windings and be expected to do well what it is intended to do, regardless of the type of design of thewinding. The nature of the overvoltages and the protection problems involved in such cases are explained in my United States Patent No.

of electrostatic shields as applied to them. That patent discloses a protective arrangement, the main feature of which is the application of a plurality of nonlinear resistance shunts betweentaps along the winding. That arrangement represents the best in the prior art that I know of, and it has been used effectively and successfully in a number of such cases. However, I find that it has certhat the shunt resistors are a constant drain on the current and energy of the circuits. In order to tolerance values, the resistance values must be increased, and this tends to lower the protection which they afford.

Concerning the lightning arrester, although good arresters are available as such, yet by virtue of their design as an independent piece of apparatus, they cannot discrirninate between the different kinds of overvoltages and for whic provide protection. Neither can they take substantial advantage of possible help from the protected apparatus. For example, when a lightning arrester made up of several elements, each comprising a gap In series -'with a current limiting resistor, is connected to a high voltage circuit, the voltage distribution among the elements, in the absence of a discharge current, is determined by their capacitances to each other and toground. This generally tends to be some undesiredfunction, and, therefore, auxiliary means have to be provided to improve this distribution. Even with such auxiliary means, some of the standard arresters are sensitive to changes in their exposure to ground and their protective value may be lowered by unappreciated unfavorable electrostatic exposures in various installations. Under impulse conditions, it is expected of an arrester that it discharge an excessive overvoltage without time lag. for very steep impulses, arrester gaps do lag, and various types ;.a r s a t re ce,

However, have a time utilized in the of auxil'arles are this time lag and resulting overassignor to General Y closely adjacent one end of the winding and plate 26 of metal Patented Mar. 8, 1955 "ice 2 voltage. Nevertheless, in theleading types of the present day arresters with such auxiliaries, the impulse discharge voltage for the standard AIEE impulse wave is about twice as high as the normal frequency spark over voltage of the arrester.

It is, therefore, an object of the present invention, to provide a protective arrangement in which electrostatic shields can be dispensed with altogether, thereby eliminating all. such limitations in winding design or circuit connection, as well as expense of shielding.

It is also an object of the present invention to provide a protective arrangement in which'the lightning arrester can be assisted by the protected apparatus to make the response of positive, improve the protection, and simplify the arrester structure.

The invention will be better vunderstood from the following description when taken in connection with the accompanying drawing and the scope of the invention will be pointed out in the appended claims. In the drawing, Fig. 1 illustrates a side elevation view partly in section of an induction apparatus and an arrangement for the protection thereof in accordance with the invention; Fig. 2 is a schematic diagram illustrating the circuit layout in the arrangement illustrated in Fig. 1 and Fig. 3 is an enlarged cross-sectional view of a protective element shown in Fig. 1.

Referring now mor specifically to the details of the figures, a representative two-legged core-type high-voltage transformer in combination'with the overvoltage protective means of the present'invention is shown mounted in a tank or container 10 filled with an insulating oil or liquid 11. Transformer windings 12 and 13 are arranged in two stacks respectively on the two legs of a magnetic core and are connected in parallel and submerged along with the core in the insulating fluid 11. The numeral 14 identifies the line terminal of the windings while the other terminal 15 is shown as grounded although it may be connected to a line circuit. A plurality of protector an arrester protective device are stacked end to end in electrical contact with each other in a tube 16 of adjacent to windings 12 and 13 with the longitudinal axis of the tube and the common axis of the stack of elements 17 substantially parallel to the axis of the winding. A portion of tube 16 has been shown cut away to expose to view the lightning arrester protector elements which have an over-all length substantially equal to the portion of the winding leg along which the winding is disposed and are so positioned in the tube that the end element at one end of the stack'is disposed opposite and the end element at the other end of the stack is disposed opposite and closely adjacent the other or opposite end of the winding. Relatively short electrical conductors extending through openings in tube 16 electrically connect the end elements of the stack respectively to the winding ends adjacent and opposite thereto while a plurality of similarly short conductors or cross connections 30, also extending through openings in the tube, electrically interconnect the junction between each pair of elements 17, intermediate the ends of the stack, to a corresponding point of each winding located, intermediate the ends of the winding, at approximately the same distance as the junction to which it is connected is located from an end of the winding. Corresponding points here means points that are intended to have'the same potential under operating conditions and the shortness ofthese cross conthat the pair of points so interconnected shall have their potential alike under impulse conditions also. The tube 16 is located in such a position that each of the short conductors 30 is of substantially equal length. As better illustrated by Fig. 3, each of the arrestor elements comprise an arc gap 18 connected electrically in series with a resistor disk 19, preferably of the non-linear type. Gap 18 is definedby'a pair of electrodes 20 and 21, respectively part of metal disks'22 and 23, maintained spaced or separated by a spacer of insulating material such as porcelain ring 24. The resistance and gap in series therewith are hermetically sealed within a chamber or enclosure comprising a glass ring 25 with an end wall conducting material, sealed thereto by arrester discharges, it throws the arrangement, the more nonlinear the fusion. Ametal collar 27 is also sealed by fusion to the opposite 'end of theglas's'ring 25 and another end wall plate 28 of metal conducting material is Welded to metal Element 29 represents resilient spring'of conductivematerial bymeans ofwhich the electrode-resistor assembly are maintained by pressure in good electrical contact with end or terminalplates 26 and 28. The free space-within the element or unit 17 is filled with a and pressure. In-aconventional lightning arrester, generally'freely ventilated or at *leastcapable of breathing,"

the gasdensityat the gaps may vary by as much as 30% due to variations in the atmospheric temperature and pressure, with corresponding changes in the breakdown voltage of the gaps. The submersion of thearrester with age is d1scharged,not-"merely reduced or distributed. Furthermore, in thepresent arrangement there is, normally, no leakage of current'through the protective shunts and, therefore, the resistanceelements can be set for much lower values than in the older schemes involving -rnerely*resista'nce shunts betweentapsalong the winding.

In this connection, I have found permitted by the-olderschemes can be reduced in the ratio asmuch as invention.

be subjected distribution his result-follows from the fact that excessive voltage concentrationacross any portion of the winding, due to theomission ofelectrostatic shields,

ischarge That is, the discharging of one of the arrester elements actsasa trigger to cause the entire arrester to discharge Thus, no particularauxiliaries are required for the gaps to lrnprove their discharge characteristics in this appearsalsoasa voltage-concentration across the corresponding portion of the arrester. As this portion of the overvoltage across the remainder of the arrestenwlnchnow visoverstressed more thus; progressively (though very fast) the entire arrester discharges. It n-raybe-seenthat, in this initial voltage distribution in the windingjthe lower will be the terminal value of the o'vervoltage-whichthe arrester will discharge. That is, in this a'rrangemenn the' arrester is highly responsiveto the specific' needs of any give'n-winding to be procontainer, insulating fluid in said tected, and provides equivalent protection to all, without being designed forthe peculiarities of anyparticular winding with respect to impulse voltages. Because of this action the auxiliary structures provided of the winding, cerned'about because it is only this is nothing to be conan attempted overvoltage or protection by. an arrester element in shunt for that portion of the winding.

A further important beneficial effect of the present arrangement on the performance of the arrester is that in the conventional practice, in which the arrester is mounted external to the-transformer, the arrester is some distance from the protected winding, and the rules of the American Institute of Electrical Engineers permits a separationof as much as feet. basedon somerecent experiences and tests, I amcon' vinced thateven =a-25 foot separation may result in as much as 20% ditference between the terminal impulse overvoltages of the arresterand the winding. In view of the closeproximity of the arrester to the winding and the relatively shortcross connections, such difference is obviously impossiblein the present arrangement.

WhileI have, in accordance with thepatent statutes, shownanddescribedraparticulariembodiment of my invention,

such changes and modificatrons as fall within the true spirit and scope of the invention.

WhatIclaim-as new anddesire to Patent: of the United? States is 1. ElectromagneticIinduction apparatus comprisinga az'magnetic having a pair 'of spaced winding legs submerged in secure by Letters said :fluid, said winding legs having their axes in sub- 50 5 References Cited in thelfile. of this. patent UNITED STATES *PATENTS 745,379 Pearson'et a1. June 13, 19.02 1,300,127 Creighton Apr. 8, 1919 1,809,895 Gay June 16, 1931 2,220,615 Pittman'et al -Nov.5, 1940 2,242,838 Somes -May20, 1-941 2;276,855 Meador "Mar. '17, 1942

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