US1938732A - System for improving the shielding effect of ground wires and shield wires - Google Patents

Description

Dec. 12, 1933.

E. F. VAAGE 1,938,732

SYSTEM FOR IMPROVING THE SHIELDING EFFECT OF GROUND WIRES AND SHIELD WIRES Filed Aug. 20, 1931 l I. n u

Telephone Power Zine Jkieid FVire (a) f ([0) M :6

Ac) f (d) 1 /e yw (4Q) :12 yw (f) f INVENTOR E]? Wal a6 BY ATTORNEY Paten ted Dec. 12, 1933 UNITED STATES PATENT orrics SYSTEM FOR IMPROVING THE SHIELDING EFFECT OF GROUND WIRE S WIRES AND SHIELD Emil Fridstein' Vaage, Brooklyn, .N. .Y. assignor to American Telephone and Telegraph Company, a corporation of New York Application August 20,

' 10 Claims.

and alternately disposed earth connections in the 1 associated ground-wire. All these objects and other objects and advantages of my invention will becomeapparent on consideration of a limited number of specific embodiments of the in vention which I have chosen for disclosure by way of example in thefollowing' specification taken with the appended claims andthe'idrawing. It will be understood that the following disclosure relatesprincipally tothese particular.

illustrative examples of practice according to the invention, and that the scope of the invention will beindicated in the appended claims.

Referring to the drawing, p Figure 1,is a diagrammatic side elevation of a set of three-phase power wires and associated ground return wire adaptedfor the practice of my invention, Fig. 2 is a; corresponding crosssecticn; Fig. 3 is a diagrammatic side elevation of a power line and a telephone line protected by a ground wireaccording to my invention; Fig. 4 is a corresponding cross-section; and Fig. 5 is a-set of 1 vector diagramsmshowing the im- I provement that may be attained bythe. use of my,invention. V e

In Figs. 1 and 2, thethree conductors 1, 2 and 3 are the several conductors of a three-phase power transmission line; As is well known, in such a system the ideal condition is to: keep thevector values of the currents in the three conductors of the same absolute magnitude and 120 apart; vector sum of the three currents will be zero, and no additional return conductor need be provided, and the inductive, effect on a neighboring conductor-Will be smallif its distance: is considerable compared to the distance apart of the three conductors l, 2 and 3.

In practice, there frequently are earth con If this condition is realized the 1931. Serial No. 558,366

nections at the neutral points of three-phase transmission systems. Even so, .there would be no earth return current if the ideal conditions mentioned above were maintained. However,

these conditions cannot be realized in prac tice, and therefore there will-be an earth return current for the system which may be very large at times where one or two of the power conductors become accidentally grounded.

In the absence of ground-wires, the vector sum of the currents in the three conductors 1, 2 and 3 will be equal and opposite to this return current in the earth. This vector sum current is called the (residual current. Such currents are'particularlyv serious in-producing inductive interference in the neighboring ,telephone lines.

It is the common practice to. place one or more so-called ground-wires on the same pole line with the three- phase conductors 1, 2 and 3. Such ground-wire orwires give protection to the power line from lightning and'tend to reduce inductive interference with neighboring telephonelines. In Figs. 1 and 2, there is one such ground return wire, which is indicated by the numeral I and it is earthed at its ends and'at various'intermediate points,-one such point being indicated as A in Fig. 1. It carries part of the return current, the remaining .part thereof flowing inthe earth. Since this ground-wire 7 is placed comparatively near the three-phase conductors 1,.2 and-3,1it neutralizes the inductive effect of the residual current to the extentithat it carries a part of the earth return current. It has been a known practice, prior to my invention, to' make the ground wire con ductively continuous, with shunt connection's'to ground at intervals;

Ihave determined that by interposing series condensers, such as C, in the ground-wire, alternating with the intermediate ground connections when such are present, and givin ,the' proper values to the C's, a much greater proportion ofthe return currentcan be caused to flow in the ground-wire and a correspondingly less proportion inthe earth. .Thus, the inductive effect of theresidual current in the three- phase conductors 1, 2 and 3 will be neutralized in greater part, and the interference effect on neigh boring telephone lines will be correspondingly reduced.

a where Z17 and Z77 have the meanings correspondetor '7 in Fig. 1.

the sum of the three currents flowing toward the Consider the grounded point A of the conduc- According to Kirchhoffs law,

point A must be zero, taking the direction toward the point as positive for each such'current. That is, let IF represent thetotal residual current, as above defined; and let I represent the current that flows in the earth and Igw represent the current that flows in the ground-wire. Then,

wire with earth return and Z17 is the mean mutual impedance between such ground-wire and the several three-phase wires.

For a system having more than one groundwire, the formula will be different from the fore- I going. If there are two ground-wires, then I 2Z1! aw 77+ 7s F n and for a system having three ground-wires,

i 1010*. 77+ Z78g ing to those stated for Equation (2), and Z78 is the mean mutualimpedance between the ground wires.

- For the case of a single ground-wire i f 77 I? (5) Similarly to the derivation of Equation (5 for 'a single ground-wire, it maybe deduced from Equations 1) and (3) that the earth current for two ground-wires is I 1 71+ 7a j and for the case of three ground-wires Comparing from-Equation (5) to Equation (6) and then from Equation (6) toEquation '(7), it can readily be seen that increasing the number of g'round wires reduces the earth return current.

from Equation (5) to Equation (6), and from Equation (6) 'toEquation (7), We change the fraction after the minus sign within the parentheses by'adding nearly the same quantity to its numerator. and denominate thus causing'the' fraction to increase and approach' unity and thereby'making the entire expression Within the parentheses smaller and smaller. 1

Accordingly, the conclusion is that the desirable end of decreasing the earth current is at tained in a degree by increasing the number or" ground-wires. But these same-Formulas (5), (6) and (-7) also make-it apparent that'the earth;

current can be reducedby reducing the selfimpedance Z77'of each ground-wire and making" Z77 approach 317 in value. Indeed, in;Equation (5) the earth current would be zero-if Z17Z77.' Such equality of 217 and Z77 will not be possible,

in series in the ground-wire.

i The current in a single ground-wire with low 'earthingresistance is given by the formula:

1 a ifewspecific examples, 132- kv. three-phase line having a single ground This is because, for the system considered, the absolute value of Z17 isconsiderably less than that of Z77, whereas the absolute values of Z17 and Z78 are nearly equal. Hence, inpassing on account of the resistance component of Z77.

But as will be shown presently, the condition of equality can be approached to a substantial degree by the use of a condenser or condensers I IF n rent is expressed by:

where Z77 is the self-impedance of the ground- R 77 17 09 Differentiating (9). with respect to (X77Xc) it is found that, to make Ie a minimum, the folj lowing relation should exist 2' 12 V v c- V i V v. the required size of the condenser, in'farads will be: V r I and since where A is given by Equation (10). In a similarmanner the optimum capacityfto use with two or more ground Wires can be found. 7 The foregoing principles will be illustrated by Consider a certain wire. The numerical values of the impedances Z77 and Zi7 in. ohms per mile are given by the following equations: I

' I 'zn=m+ixi7 ote+3 5 jrisi Computing the ground wire and :the earth rcurrent from (2) and (5) respectively, the-result, when'no condenser is'used in series with the ground wire, is: I

capacity-to make Ie a'minimurn. This is found to be 6,0!)0 microfarads per mile, and" the selfimpedance of the gro'undwire reduces to: "z77=0.s0e+a'0,-97 1 v (18) Using this value for-the computation of the ground wire and earth current these become:

0.09+j0.55 0.808+j0,.97 F W Ffl G Theabove example shows "a reduction in the percentage earth current of 3.9 per cent.

Continuing to deal with the specific example mentioned'above, assume that the ground wire Now froinflo) and (13) solve for the required I is of stranded copper having across-section of 211,600 circular mils for which the self impedance- Z77 is 0.361+:i1-.41. Then from Equations (2) and (5) the ground-wire current and earth current without condenser are: I

in series with the ground wire the selfirnpedance per mile reduces to: Z'1-i0.361+:i0.647 (24) .And the ground wire current and earth current become:

which is a gain of 35 per cent in shielding on the basis of the same residual current.

If the ground wire were of a still larger size, say 500,000 circular mils, its self-impedance would be:

The required size of the condenser for this case would be:

C=3150 microfarad per mile ('28) The following table shows the results for the three above mentioned sizes of ground wires:

1., percent of Ir I. percent of In Size Wires Without With Without With condenconden condencondenser ser ser ser 136,000 cir. mils 34. 3 44. 0 69. O 66. 3 211,600 cir. mils 3S. 2 74. l 61. 9 39. 0 500,000 cir. mils 39. 1 92. 0 60.8 19. 1

It will be seen from the above table that for the highest conductivity ground wire assumed it is possible to reduce the percentage of earth current 69 per cent, while for the lowest conductivity ground wire, which has a resistance component comparable in magnitude with the reactance component of the mutual impedance, there is only a 4 per cent gain in shielding.

The vector diagrams of the quantities in the above table are shown in Fig. 5 of the drawing.

Diagrams (a) and (b) are fora ground wire of power line, a grounded shield wire may be inter-- posed as shown. Its beneficial effect may be ena harmed by interposed condensers according to the g principles disclosed i'n the foregoing and" more specifically demonstrated below. 7

a It" can be proved thatthe shielding: factor for in absence of shield wire.

Zst='mutua'l impedance between the shield and telephone line.

Z s:mutualimpedance between the power and shield wire.

Zss='self-impedance of shield wire. The expression'in (29) can also be written:

, Equation, (33) after inserting a condenser can be written:

Butthis is the same form of expression as Equation (8) for ground wires, and therefore, the same method of determining the optimum value of ca- 7 pacity. as was'used in the case of ground wires can be applied.

As indicated in the examples cited heretofore the size of the condensers will be of the order of 3000 to 6000 microfarads per mile. From a practical standpoint it would be somewhat diflicult and expensive to supply and install condensers of such values in the ground wires of power lines. However, electrolytic condensers, protected against lightning could be used with telephone shield wires.

I claim: 7

1. In combination, a power line, a grounded return conductor therefor, and a plurality of like condensers in series along said conductor, said condensers having the proper capacity value to effect a substantial reduction in the proportion of the return current flowing in the earth.

2. In the operation of a power line and a grounded return conductor, the method of increasing the proportion of the return current flowing in the said conductor which involves the development of a substantial capacity reactance current along said conductor between it and ground.

3. In combination, a polyphase power line, a grounded return conductor associated therewith, and condensers in series and alternate grounds along saidconductor to increase the proportion of the return current flowing therein.

4. In combination, a power line, a return conductor associated therewith and grounded at intervals'along its length, and series condensers in said conductor alternating with the ground connections, said condensers having the proper ca.- pacity values to substantially increase the proportion of the return current flowing in said conductor.

5. In combination, a polyphase power line where l=ratio of induced voltage in telephone line in presence of shield wire to induced voltage at points along said conductor, and shunting the subject to a-substantial degree ,ofrresidual'current, a grounded return conductor associated ,7 therewith, and alternating series condensers and shunt ground connections along said conductor to increase the proportion of return current flowinginsaid conductor.

6.'In combination, apolyphase power line subject to aresidualicurrent therein',-a plurality of return conductors associated therewith, said conductors being grounded at intervals along their length, and condensers in series in said conductors and in alternation with said grounds to wire to protect the telephone line from such inv duction, grounds at intervals along said shielding wire,and condensers interposed in seriesin alternation withsaid grounds.

' 9. In combination, a power line a neighboring telephone line subjectto induction therefrom, an

interposed shielding wire, said shielding wire being grounded at intervals, and condensers in series in said wire alternating with the grounds.

, '10. In the operation of a polyphase power line and a grounded return conductor, the ,methodof increasing the proportion ofthe return current flowing in the said conductor, which involves shunting the return current between the said conductor andground at-points distributed along the length of said conductor and at alternately ,disposed points along its length developing-a substantial capacity reactance in series-insaid con;

ductor.

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