US3015774A

US3015774A - Corona-testing of electric insulation

Info

Publication number: US3015774A
Application number: US703320A
Authority: US
Inventors: Eigen David
Original assignee: Okonite Co
Current assignee: Okonite Co
Priority date: 1957-12-17
Filing date: 1957-12-17
Publication date: 1962-01-02
Anticipated expiration: 1979-01-02

Description

Jan. 2, D. EIGEN CORONA-TESTING OF ELECTRIC INSULATION 2 Sheets-Sheet 1 Filed Dec. 17, 1957 l//l/ l l/ l OOO mui-Jai( PSE D. EIGEN Jan. 2, 1962 CORONA-TESTING OF ELECTRIC INSULATION 2 sheets-sheet 2 Filed Deo. 17, 1957 INVENTOR.

l V/ f/fff/ 3,015,774 Patented Jan. 1962 3,015,774 .i y CORONA-TESTING F ELECTRIC INSULATION. David Eigen, Passaic, NJ., assignor to The Okonite Com- A pany, Passaic, NJ., -a corporation of New Jersey Filed Dec. 17, 1957, Ser. No. 703,320

7 Claims.vv (Cl. 324-54).

inventiony relates to the;'corona-testingmo'f`- highltension cables and more particularlyfto the detectionA ofy voids in the insulation thereof. Y Y

When an insulated cable is put in to service, a corona discharge will occur under certain conditions wherever there is a gas occlusion within .the insulation. Whether or not such a discharge will occur depends upon a number of factors, such as the position, size, and shapeof fests itself as yevidenced by the signal from the radio or other corona. detector. Also various visual means have been suggested to permit analysis of the signal, such as recorders and oscillographs. VEven such instruments require a h igh degree of judgment and do ,not permit increase of amplification tothe degree required for positive detection of weak coronafsignals, since any attempt atl ysuch amplification ,would lalso Aamplify V,the extraneous noise signals'. e Often the extraneousjnoise is so `great that a 'valid evaluation of the corona-producing properties of thec'able insulation cannot accurately be made,

the occlusion; pressure and composition of the gas; presence of solids or liquids in the occlusion; and the conductivity of the walls of the occlusion.V The corona discharges are the result of the Yionization of the gas within the occlusion. 1 Y

From a cable'manufacturers point of view the above theoretical considerations serve as considerable guides for fabrication. Despite all precautions, however,`occlu sions occur, and what appears to beta perfectly insulated cable may contain occlusions which serve assites for corona discharge while the cable is in servicei' Such discharges have a very'destructive-effect. l 'j' "i In view of the above, itf-"has been'proposedv tov subject the cable insulation to electrical stress in the sarnemanner as it would be stressed inservice, with va possible overstress as a safety factor, and attempt to ldetect any corona which may be generated by such electrical-stress.

-It is known that corona discharges result in the generation of alternating currents of randomrfrequency',"extending over a wide portion of the radio and audio; bands. Thus, an ordinary amplitude modulated radio can pick up the emanations from corona discharge if directly connected to, or Vhaving an antennainthe vicinity of, conductors leading to the vicinity of such discharges. Simplystated, a radio receiverv will emit fromy its speaker a characteristic noise if the receiver is arranged to respond to the electromagnetic radiation from the corona or is connected or coupled to the circuit supplying thevoltage which causes the corona. Thus it would appear sufficient for-testing purposes to stress the insulating"material` as it would be stressed in service or such multiple thereof as may be considered a factor of safety.

However, in the practical applicationy of the `test for corona, where an alternating current voltage is applied in a manner to stress the insulation as it`will be stressed in service, it has been found that a radio yor other corona detector of sufficient sensitivity to respond to corona discharge in cable insulation occlusions will also respond tovatmospheric or man-made amplitude-modulated dis'- turbances in the vicinity and in the lines supplying the stressing voltage and even in those lines supplying the power necessary to operate the detector. Around an industrial plant there are many motors, generators, switching mechanisms, and the like,.likey all offwhich in general create` a certain amount of amplitude-modulated ratio' frequency disturbance. Suppression'of man-made disturbances at their source by proper application of-,condensers and other arrangements'is theoretically possible, but, because of'the great power' requirements of Athe plant machinery, such suppression may not be practical. Even if we assume complete suppression of man-made disturbances, the AM- radio used as a detection device will still pick up the atmospheric disturbances. The

technician conducting the test .may have suflicient aural' discrimination, imagination, and concentration to listen to the signals from the radio or othercorona detector andV decide, as a matter'of judgment, when corona mani- In view'ofall of the above it is a primary object of my'invention Ito, provide a corona-indicating arrangernen'tffor 'testing'cable insulation which isnrelatively little influenced by electrical disturbances in the surroundings or,in the lines .supplying the power necessary to perform thevtest, so`as toppermit amplification of the corona signal'and, hence, increase the sensitivity ofthe apparatus. 'Y Referringn'ow tothe drawings:

i PIG.l l is a showing of a form of apparatus embodying my invention utiliedin one form of testing; and

F'IGQZ lis apparatus substituted'for part of thateshown in FIG. l for use inal diiferentform of test.

Both figures oft'hedrawings are diagrammatic inform. In testing cable insulation for corona-producing voids, the' cable maybe reeled and have its insulation stressed g at tonetirrie by'applying voltage Vbetween'thecable con- 'ductor and theouter surface of the cable insulation, or

the -'cable may be'run through an apparatus which progressively 'subjects the "insulation to a Voltage suicient to result in corona discharge.

Referring ',rst'of all to FIG.v 1, 10 designates a tank into which a reel of the cable to betested is inserted.-

A 'suitableA -liquid,` for instance water, vcovers the cable. The tank 10 is grounded at 14 fora reason which will appear.v 'To stress .the cable insulation, I employ a high alternating-current voltage.' As shown, a transformer 16 has'ovne of its Secondary terminals grounded at 18 and y its other terminal connected to the cable conductor 20. This arrangement, it will be seen, stresses the insulation as it would be stressed in service, as the voltageoverv the entire surface'of the cable insulation is susbtantial-ly that of the ground, and the voltage along the conductor is whatever is generated by the transformer 16. Alternatively, if the cable is of the shielded type, the shield may be connected to ground 14 and the high-voltage lead connected to lconductor 20, to stress-the insulation in a manner similar to that above described vbut Without the use of the tank 10.

For the purpose of detecting corona, an amplifier 22 is provided, lwhich is connected to the cable conductor 20, as shown. A condenser 24 and a small inductor 26 serve as a filter, permitting only high-frequency corona discharge currents to pass into the amplifier, the 60-cycle high voltage passing through condenser 24 being grounded through the inductor 26.`

It will be appreciated that the output of the 22 will consist of. whatever corona discharge is generated in voids in the cable insulation plus disturbances entering the high-voltage line by capacitance between turns in the transformer `and for that, matter, disturbances traveling to the amplifier along'its'power supply. In addition, however, there are the radio-frequency disturbances traveling through fthe air and picked up by the reeled cable 12 and/or the leads connecting the various equipments and the equipments themselves shown in FIG. 1, Ias transformer 16, condenser 24, and inductor 26. The output of the amplifier 22 is lone which only great skill and discrimination caninterpret in terms of corona, ifiany worthwhile interpret-ation at all is possible: I Attempts to create i.' andsuppress corona by turning the'high voltage'off and on do lnot always work, as the turning off and on of the amplifier f nullify'interfe'rence coming in over' the lines and through the air. To achieve this, I provide an antenna v28, which is intended to pick upthe interference which travels through the air. As a' general matter, by observing the operations in the plant and the location of those mechanismsproducing the' disturbances, the orientation posi'- Y tion, size, and shapfej of the antenna can be fairly accu` rarely detennined, the videa beiiig to` install an antenna whichr'eceive's signals from such d'isturban'esfaud iht'rb duces these signals' into amplifier 30, e reel of cable `and the refst of 4the equipment intdue the saine signals to arnplier` 22. The antenna i'nu'st be suliicieritly remote' from the reel of cable I2 that sign-als from corona disturbances emanating from the' cable are attenuated be fore reaching the antenna.

,The Signals picked upgeynie; antennazsj are amplified in an amplifier 3U, similar in all espccts to the amplifier 22. It is preferable ltl'l'al the twof'atplifeisle physically close to each other, and their power leads connected to the power supplyY near-each; other, so that all noise or interference entering amplifier V2 `2 has yits counterpart of the vsame strengthalsoetering the amplilier 30. The output of amplifier whichissolely noise and interference, is utilized Vto buckthe signals fromamplier 22. nFor this purpose I provide a phase inverter 3-2, which may also include the necessary eqlipment ,to balance the two outputs.

The iesiutingsignai win be .that *produced by miglia in the cable the insulation of which is being tested. This signal 'may be amplied vfor operating indicating, means, for example -aspeaker, or `a cathode-ray oscilloscope' such as shown, at 34, or it may be amplied and/oi rectied for operating more elaborate. means,` as, .for example, a pen recorder. v A

It can be appreciated thaty with .the above described arrangementfthe high-frequency wide band signals vorige inating from whatever source enteramplitier 22 and amplifier 30 with the 'following relative intensities With an arrangement such as this, regardless of the degree` of amplification by amplifier 22, the noise can be obliterated by amplifier 30, and therefore the amplification employed in amplifier 22 can be as high as desired, except for limitations engendered by noises generated by the tubes of the lamplifier itself, thus promoting :the sensitivity of my arrangement to corona in the cable.

The above described arrangement does not, of course, locate the defect or void in the reeled cable. For the purpose of locating the voids,the arrangement above described can be modified as illustrated in FIG. 2, wherein like` parts have been given like reference characteristics.

In the arrangement shown in FIG.Y 2 the cable is eX- tended, as distinguished `from being reeled, during testing, the cable, designated 34, passing from a reel 36 through grounded tank 3S, tube 40ct insulating materiaLand grounded tank '42 to take-up Yreel 44. The insulating tube 40 and the tanks 38 and 42 contain a semi-conducting fluid, such as water, and the tube 40 intermediate its ends is provided with a conduct-ing section or electrode 46. As in the construction illustrated in FIG.-v 1, one terminal of the secondary of the transformer 16 is grounded. The other terminal of the secondary of this transformer is connected to the electrode 46 of the tube 40, instead of being connected directly to the conductor of the cable, as in FIG. 1.

As set out in Francis H. Gooding Patent No. 2,794,- 168, dated May 28, 1957, corona-producing stresses are built up in the insulation of the cable as the cable passes from reel 36 to take-up reel 44 through the semi-conducting liquid in the tanks 38 and 42'and tube 40.

Antenna 28 is provided, as in FIG. l, -for the purpose of picking up interference, exactly `as described in connection with FIG. 1. The antenna 28 andthe lead to the electrode 46 are connected Vas shownv tothe apparatus described in connection with FIG. l, and the operation of this equipment will be obvious from the yforegoing description and lthat of thev Gooding Patent 2,794,168.v

As earlier stated, it is important that the -two -ampliiers 22 and 3 0 of FIGS. l and 2 be identical in characteristics. Also it is Vimportant that the phase inverterbalance the outputs of the two ainpliers.

From all of the foregoing it will appear that I have prvided for corona-testing of the insulation o f electric cables, one embodiment of the invention providing for testing a full yreel of cable, vthe other embodiment providing for the progressive testing of the cable, i .e.,V While the cable is extended asdistinguished from being reeled. In both embodimentscfthe inventionit will be seen that I have provided foreliminating outsideinterference from atmospheric disturbances as well as from motors, generators, switching mechanisms ,etc, K

It is to be understood thatchangesmay be made i-n the `detailsuot .construction 'and :arrangement of prtS hereinabove described withinthe purview of my invention.

What I claim is:

1. Apparatus for the corona-testing of electric insulation, said apparatus comprising, in combination, means for subjecting the insulation to a `sulnciently high electrical `stressfto cause corona kdischarge in defective portions of the insulation; a radio-frequency receiver, including an amplifier, r'responsive to high-frequency emanations resulting from said corona discharge; -a second radiofrequency receiver, including ian amplifier; an antenna, connected tosaid `second receiver, ktor receiving high- Vfrequency electrical ldisturbances from the' surrounding area, said antenna being positioned sufiiciently remote from the stressed insulation so that the high-frequency emanations resulting from said corona discharge will be attenuated before'reaching the antenna; a phase inverter to which the output of each ampliier is connected so as to transpose the highefrequency electrical disturbances picked up by the second receiver to one hundred'and eighty' degrees phase displacement relative to any high.-A frequency electrical disturbances picked` up by lthe first receiver, thereby to nullify the same; `and an indicating device connected to the output of the said phase inverter.

2. Apparatus for the corona-testing of electric insulation, said apparatus comprising, in combination, means for subjecting a reel of insulation to a sumciently high electrical stress to cause corona dischange in defective portions of the insulation; a radio-frequency receiver, including an amplifier, responsive' to high-frequency `emanations resulting from said corona dischargegfa second radio-frequency receiver, including an amplilierg' an antenna, connected to said second receiver, for receiving high-frequency electrical disturbances from the surrounding area, said antenna being positioned snliicientlyy remote from the stressed insulation vthat Vthe `high-frequency emanations from said corona discharge will be attenuated before reaching the antenna;`-a vphase inverter to which the output of each amplifier -is connected so as to trans` pose the high-frequency electrical disturbances picked up by the second receiver to one hundred vland eighty degrees phase displacement relative to any high-frequency electrical'disturbancespicked up by the first receiver', therebyy to nullify the same; andan indicating device connected to the output of the said phase inverter.

3. Apparatus for the corona-testing of electric insulation, said apparatus comprising, in combination, means for subjecting the insulation progressively to a suficiently high electrical stress to cause corona discharge in defective portions of the insulation; a radio-frequency receiver, including an amplifier, responsive to high-frequency emanations resulting from said corona discharge; a second radio-frequency receiver, including an amplifier; an antenna for said second receiver for receiving highfrequency electrical disturbances from the surrounding area, said antenna being positioned sufliciently remote from the stressed insulation that the high-frequency emanations from said corona discharge will be attenuated before reaching the antenna; a phase inverter to which the output of each amplifier is connected so as to transpose the high-frequency electrical disturbances picked up by the second receiver to one hundred and eighty degrees phase displacement relative to any high-frequency electrical disturbances picked up by the first receiver, thereby to nullify the same; and an indicating device connected to the output of the said phase inverter.

4. A device for detecting voids in electric insulation, said device comprising means for applying alternatingcurrent voltage to the insulation under test to induce corona discharge in the voids in the insulation; means detecting and responsive to said corona discharge and to similar high-frequency disturbances in the area surrounding the insulation under test; means detecting and responsive to said similar high-frequency disturbances and comprising an antenna positioned sufficiently remote from the stressed insulation that the high-frequency emanations resulting from said corona discharge will be attenuated before reaching the antenna so that said second detecting means is relatively insensitive to the said corona discharge in the insulation; and means superimposing the response of the second detecting means on the response of the first detecting means in ya manner to suppress the response of the first detecting means to high-frequency disturbances in the area surrounding the insulation.

5. A device for detecting voids in the insulation of an electric cable comprising a bath of semi-conducting liquid in which the cable is immersed; a source of high voltage between the liquid and the conductor of the cable, stressing the insulation to produce corona in the voids therein; a high-frequency detector closely coupled to the cable and responsive to high-frequency emanations resulting from said corona discharge and responsive to electrical disturbances in the surrounding area; a second high-frequency detector; an antenna connected to said second detector for receiving high-frequency electrical disturbances from the surrounding area, said antenna being positioned sufficiently remote from the stressed insulation that the high-frequency emanations resulting from said corona discharge will be attenuated before reaching the antenna; andmeans superimposing the response of the second detector on the response of the first detector in such a manner that the combined output of the two detectors is a result of substantially only corona discharge in the cable voids.

6. A device for testing the insulation of an electric cable comprising a tank containing a semi-conducting liquid; means for passing Ithe cable to be tested through the tank; means maintaining the cable conductor and the tank ends at the same potential; means, including an electrode intermediate the ends of the tank, generating an electrical stress in the cable insulation; a high-frequency detector and amplifier electrically coupled to the stressed cable insulation and responsive to high-frequency energy emanating therefrom generated by corona discharge in the insulation and responsive to electrical disturbances in the area; an antenna, high-frequency detector, and amplifer responsive to high-frequency energy emanations, said antenna being positioned so as to be responsive to electrical disturbances in the area and relatively insensitive to energy emanating from said corona discharge; and a phase inverter coupling the output of the two amplifiers with a resultant combined output relatively free of electrical disturbances.

7. Equipment for testing the insulation of a length of electric cable of the shielded type comprising, in combination, means for grounding the cable shield; a grounded high-tension source connected to the cable conductor; an antenna positioned for receiving electrical disturbances reaching the vicinity of the test leads and equipment and the cable conductor but relatively insensitive to corona discharge in voids in the insulation; detecting and amplifying means responsive to high-frequency energy emanating from corona discharge in the cable insulation; detecting and amplifying means responsive to energy emanations received by said antenna; and means for so superimposing the output of the second ampliier upon the output of the first amplifier that the combined output of the two amplifiers is substantially confined to the emanations resulting from corona discharge in the cable insulation.

References Cited in the file of this patent UNITED STATES PATENTS 1,524,645 Latour I an. 27, 1925 1,943,392 Paine et al. Jan. 16, 1934 2,150,223 Hollmann Mar. 14, 1939 2,170,813 Doble Aug. 29, 1939 2,194,303 Germeck Mar. 19, 1940 2,794,169 Gooding May 28, 1957 FOREIGN PATENTS 621,731 France Nov. 8, 1929