occurs quietly within heavy grounded metal tanks. Modern high-voltage and low-voltage oil breakers are very compact, a 15-kv, 1,000,000-kva metal-clad structure running a little over 36 inch centers.

Air-blast breakers have also had their failures. This type of breaker has short arcing time, though any longer time extend-ing until the series disconnecting switch opens would be serious. The over-all open-ing time (the important feature) averages a little less than for commercial oil breakers but can be met by high-speed oil breakers.

The opening of an air-blast breaker is accompanied by considerable noise and flame, rather nerve-racking to an adjacent operator.

Because of limited air supply, an air breaker may be safely operated only two or three times successively. The operating high-pressure piping, valves, relays, and cylinders are rather an involved precision equipment and require high-grade main-tenance, particularly for outdoor service. Some of these moving parts are rather inaccessible, enclosed in high porcelains. A t Arvida the 154-kv breakers are mounted indoor.

W e note that, because of lack of suitable high-power laboratories, the authors can provide only "synthetic" test proof of their larger ratings, based on a few system tests. W e do not consider such data conclusive. During the development of our air-blast breakers, thousands of tests were made on all parts, and complete poles, to full rating, and overvoltage, in a laboratory of ample capacity.

In conclusion, the authors have given us two very interesting and instructive papers covering the development of this new equip-ment in Canada.

A. K . Leuthold: High-voltage air-blast circuit breakers can be equipped either with, or without, oil-immersed current transformers, but even in the former case, which is often preferred from the economical standpoint, the oil hazard, compared to an oil circuit breaker, is very small, because no arc interruption has to be carried out under oil.

The low-voltage air-blast breakers are also very compact in design. For example: seven-kilovolt breakers require a space from center to center of approximately 34 inches and 13-kv breakers of approximately 42 inches.

Neither with our outdoor nor our indoor type of air-blast circuit breakers can flames be noticed at the breaker exhaust openings. The noise from an air-blast breaker erected outdoors is not objectionably loud, while the noise from a breaker erected indoors is similar to that of a modern high-speed d-c breaker when interrupting a heavy short circuit.

Breakers equipped with automatic re-closing devices can open, reclose, and open a second time in less than 20 cycles without requiring refilling from the main air-storage tank. After two breaker opening opera-tions, automatic refilling takes place from the main storage tank in approximately four to six seconds, and further switching operations can then be carried out.

Experience has shown that the various pistons, applied for the fully automatic operation of the breaker, operate very satis-

factorily, even under very low temperature. On outdoor breakers, this piston assembly is carefully protected against snow and rain and, if desired, can be kept above freezing point by means of small heaters similar to those applied on the motor-control pedestals of outdoor oil circuit breakers.

The arcing chambers of high-voltage breakers are designed with inspection open-ings or other devices, which enable the movable arcing contacts to be inspected, or even replaced if necessary, in a few minutes. A t Arvida, the 150-kv air-blast circuit breakers are installed indoors, because they had to replace two oil circuit breakers pre-viously erected in the same building where two 75,000-kva transformer banks are located. This customer preferred to elimi-nate the oil hazard by applying two air-blast circuit breakers in place of the pre-vious oil circuit breakers.

The high-voltage air-blast circuit breakers equipped with potential-controlled multiple arc breaks, are tested in our large breaker test plant by breaking the partial inter-rupting capacity with one arc break only, which enables one to judge what the per-formance of this breaker will be under full-rated interrupting capacity. A definite conclusion can be drawn from these tests because the voltage distribution between the individual arc breaks is equal on ac-count of the potential control of the arc breaks, which is achieved by means of small capacities.

Through a series of developments, the manufacturer has achieved a very high per-formance from the high-voltage air-blast circuit breakers which, besides the short interrupting time, enables automatic single-or three-phase high-speed reclosure to be carried out.

This insight into the latest breaker de-velopments, including testing and operating experiences, should induce and encourage power companies to collect experience on their own networks by applying this new type of circuit breaker in order to gain valuable operating data for further im-provement in the protection and stability of their high-voltage transmission lines and power-distribution systems.

Design, Manufacture, and Installation of 120-Κ ν Oil-Filled Cables in Canada

Discussion a n d authors 1 c losure of oaper 4 2 -1 4 8 b y D . M . Farnham a n d O . W . Ti tus, p r e -sented at t h e A I E E Pacif ic Coast c o n v e n t i o n , V a n c o u v e r , B. C , C a n a d a , S e p t e m b e r 9 - 1 1 , 1 9 4 2 , a n d p u b l i s h e d in A I E E T R A N S A C -T I O N S , 1 9 4 2 , D e c e m b e r s e c t i o n , pages 8 8 1 -8 .

A. H . Frampton (The Hydro-Electric Power Commission of Ontario, Toronto, Ont., Canada) : Under the heading "System," section 2, the authors state that cable con-nections were used between the step-down transformer terminals and the station bus "as providing a substantial saving in steel structure, greater compactness, and better appearance, and much less exposed high-voltage circuit." In the apparently restricted

space available at the site of station B, the value of the second and fourth points may be granted. However, it is difficult to accept the inference that the saving of the cost of the additional structures required to accommodate open connections to the trans-formers had a controlling influence in the decision to use cable at this point. The ac-ceptance of this inference is made more diffi-cult by the assumptions that may be drawn from the statements under "System," sec-tion 3, that the line route had to be short-ened to approximately one third, and the acquisition of expensive right of way avoided in order to justify the main cable section as against the alternative overhead construc-tion.

I t would be interesting if the authors could expand on these points. Perhaps they would be prepared to suggest a rule-of-thumb figure as representing the approxi-mate value of right-of-way saving necessary to justify the adoption of cable constuction.

From Figure 2 of the paper it is presumed that this cable system operates in parallel with 60-kv overhead circuits. From the dis-cussion, it is inferred that varying amounts of power are required to be transmitted be-tween stations A and C, in order to maintain the most efficient loading on certain generat-ing sources. As these parallel paths would obviously have different impedance char-acteristics, it would appear that the main-tenance of a proper subdivision of load be-tween the parallel paths would occasion diffi-culties. I t would be of interest to know whether any special operating or other pro-cedures have been established in this con-nection.

Herman Halperin (Commonwealth Edison Company, Chicago, 111.) : The authors are to be complimented for the thoroughness of their past investigations of design and in-stallation details and of their present inves-tigations of operating matters. If, as in this case, engineers devote some time and efforts to the study of all details, problems relating to oil-filled cable systems become simple.

Records of duct temperatures, and so forth, as they are planned, should be of con-siderable value in obtaining the maximum allowable load rating for the cable in the ac-tual installation. As load ratings are most important, it would be of interest to have more detailed information given on the methods being used by the authors and on any special points relating to loading.

According to Table I of the paper, 70 de-grees centigrade is considered the maximum allowable copper temperature in accordance with the Association of Edison Illuminating Companies specifications. As indicated in my recent A I E E paper on "Load Ratings of Cable—II ," 1 such cable could be operated safely at higher temperatures. If this is at-tempted, one, of course, must watch out for limitations, such as reservoir capacity or in-duced sheath potentials.

The Canadian-developed reservoirs shown in Figure 6 of the paper are of interest. The arrangement may be less expensive than those used in the United States. What ma-terial is used for the cell walls?

The design of the conductor with flat strands reduces considerably the free oil space between conductor and paper as com-pared with conductor with ordinary round strands. Have any test data or calculations

1074 Discussions A I E E T R A N S A C T I O N S

eooi 500

400

»1300

200

100

6 AM

Figure 1 . Typical da i ly load curve for 1 2 0 - k v cable

Sheath currents no t flowing. 7̂ = 20 d e g r e e s c e n t i g r a d e

been made on the relation of the maximum stresses for these two types of conductors?

The average coefficients of friction of 0.25 to 0.30 for pulling cable into the ducts are less than usually obtained. We have found higher values in pulling cables into precast-concrete round ducts, using a heavy grease on the cable. While the grease is not a very good lubricant, as demonstrated by our tests, it is used mainly to retard or prevent corrosion of the sheath. It seems to me that the main considerations in determining maxi-mum allowable pulling strains are possible harmful effects on the conductor, on the rest of the material inside the sheath, and on the outer surface of the sheath.

In general, the methods and carefulness used on the whole project described in the paper were apparently of such a nature as to permit high allowable loading and still ob-tain a satisfactory operating record over a period of decades.

REFERENCE

1. L O A D R A T I N G S OF C A B L E -

perin. A I E E T R A N S A C T I O N S , pages 930 -42.

- I I , H e r m a n H a l -v o l u m e 61 , 1942,

6 P.M.

600,

500

400

u 3 0 0 |

i 200

\ / Ν

6 AM

This was in some measure due to the shape of the premises which made it desirable, even necessary/ to place the high-voltage outdoor structure along the south side of the prop-erty from east to west, using the entire width. I t was, therefore, necessary to place the four transformer banks in a row running from east to west and north of the high-volt-age structure. The steelwork required to feed these transformers by overhead con-struction, plus the cost of the overhead con-struction, was estimated to be higher than the cost of the oil-filled cables. Part of this advantage may have resulted from a reason-able cable-installation cost, since other oil-

9 12 3 6 9 12 PM

Figure 2 . T y p i c a l d a i l y l o a d curve for 1 2 0 - k v c a b l e

S h e a t h c u r r e n t s flowing. 7a = 10 d e g r e e s c e n t i g r a d e

As underground costs vary differently from those for overhead with difference in load and raw-material costs, we would not care to set up even an approximate rule-of-thumb figure such as Mr . Frampton sug-gests, particularly at times such as these.

Mr. Frampton's question as to how the present 60-kv ring system is made to carry its share of load when its impedance is so

T a b l e I. M a x i m u m A l l o w a b l e Current ( A m p e r e s )

A l l o w a b l e C o p p e r

C a b l e T e m p e r a -( T h o u s a n d s tu re C a b l e s S h e a t h Cur ren t s G r o u n d T e m p e r a t u r e

of ( D e g r e e s in ( D e g r e e s C e n t i g r a d e )

Ci rcular C e n t i - D u c t N o t

M i l s ) g r a d e ) B a n k F l o w i n g F l o w i n g 20 15 10 5

300 . . 7 0 . . . . 3 . . . . . . . . X . . . . . . . 3 8 2 . . . 403 422 . . . 4 4 2

300 . . 7 0 . . . . 3 . . . . . . . . X . . . 4 1 2 . . . 434 456 . . . 4 7 6

300 . 7 0 . . . . 6 . . . . . . . . X . . . . . . . 3 4 8 . . . 368 . . . . 387 . . . 4 0 5

3 0 0 . . 7 0 . . . . 6 . . . . X . . . 3 7 2 . . . 393 . . . . 4 1 3 . . . 4 3 2

300 . . . . 8 0 . . . . . . . 6 . . . . . . . . X . . . . . . . 3 8 0 . . . 398 415 . . . 4 3 1

650 . 7 0 . . . . 3 . . . . . . . . X . . . . . . . 5 2 8 . . . 558 585 . . . 6 1 2

650 . . . . 8 0 . . . . . . . 3 . . . . . . . . X . . . . . . . 5 7 6 . . . 602 . . . . 6 2 7 . . . 6 5 2

650 . . . . 7 0 . . . . . . . 3 . . . . X . . . 6 3 5 . . . 670 . . . . 7 0 3 . . . 7 3 6

6 5 0 . . . '.'...80. . . . . . .3 X . . . 6 9 3 . . . 724 754 . . . 7 8 3

D. M. Farnham and O. W . Titus: Mr. Frampton feels that the saving of steel alone would not be a controlling factor in the use of cable from the station bus to the trans-former. All projects, of course, contain a number of elements of cost. The sum total of all elements is the determinant. In this instance steel was one substantial element.

Figure 3

filled cables and equipment were being in-stalled at the time in the immediate vicinity. Appearance and "dead-front" characteris-tics might also be appraised as contributing factors if that were necessary.

Our opinion is that for short runs, as en-countered in this case, the problem is worth investigating thoroughly before a decision is reached to use either the overhead or under-ground method.

much greater than that of the cable is well taken.

This difficulty was very cheaply and ef-fectively overcome by splitting the high-voltage bus at station C and connecting the cable to one pair of overhead lines and the ring to the other pair. These pairs of lines were paralleled approximately 90 miles away.

Figure 4

5000 6000 7000 8 000 7500 8500 9500 10500 16-HOUR AMPERE TOTAL 18-HOUR AMPERE T O T A L

1942, V O L . 61 Discussions 1075

The combined impedances were such that the load divided correctly to take full advan-tage of both tie circuits.

The authors are grateful for Mr. Hal-perin's kind remarks. The Canadian reser-voirs employed plain steel diaphragms, prin-cipally because of wartime difficulty in se-curing stainless steel within the time permis-sible.

N o tests or calculations were made on the maximum dielectric stresses for the shaped conductors as compared with conventional stranding.

During the development of the pulling eyes, tension tests were made on conductors, paper-wrapped but without the lead sheath. These tests determined both the effective-ness of the pulling eye and the maximum tension on the conductor permissible with-out permanent deformation. In the field, pulling tensions were kept below these val-ues in all cases. Any added strength caused by the lead sheath was regarded only as added factor of safety.

In replying to the second paragraph of Mr. Halperin's discussion, the listed calcu-lations are samples of those used in estab-lishing the loading schedule of the cables :

Allowable peak load=7 at loss factor stated

Loss factor (by design) = 621/2 per cent =

(/avg) 2 2?

PR

Daily basis 100 per cent loss = Ρ X R X 24

62 1 / 2 per cent loss factor =

avg (P)XRX24

PXRX24 = 0.625

For any period of the day Let

i x 2 = avg( J 2 )

and

/ x 2 = avg (7 2 ) for the remainder of the 24 hours

Then

{Ix2XX h o u r s ) + ( / y 2 X Y hours) = 0.625/« X 24

Py-( 0 . 6 2 5 X / 2 X 2 4 ) - A T / x 2

a v g ( / 2 ) naor or = 0.625

or avg (7 2 ) =0.625/ 2

24 χ = 0.625 X / 2 X — — I x 2

y y

Take X as 18 hours and y as 6 hours then

Je= V ( 0 . 6 2 5 X / 2 X 4 - 3 / i 8 2 )

(a). Sheath currents flowing, TG = 20 degrees centigrade. Tc = 70 degrees centigrade—/ = 528.

(b). Sheath currents not flowing TG = 20 degrees centigrade. Tc = 70 degrees centi-g r a d e - / = 6 3 5 .

(For allowable maximum loading see Table I of this discussion.)

For (a) / 6 = V ( 0 . 6 2 5 X 5 2 8 2 X 4 - 3 / 1 8

2 ) = V3(232, 3 2 0 - / 1 8

2 ) . For (b) / e = V(0.625X 6352 X 4 - 3 / i 8 2 ) = V3(336, 021 - / 1 8

2 ) .

Curves in Figures 1, 2, 3, and 4 of this dis-cussion are made on the basis of these or similar calculations.

This allows cable loadings to a maximum for 18 hours each day, and from these total amperes a load rating can be had for the suc-

ceeding six hours, with a resultant loss factor of 62 V 2 per cent (see typical loading curves herewith).

This method ot scheduling the loading has been used since the cables went into service, with careful attention being paid to tempera-tures and duct-mouth movements. The idle duct (that is a duct adjacent to a loaded duct) temperature at various locations has been recorded and shows a variation of from 38V2 degrees centigrade to 4OV2 degrees centigrade, which would indicate a fairly constant copper temperature.

Duct-mouth movements have been re-corded both daily and weekly. The daily movements are in the nature of one eighth of an inch or less, while the weekly movement is in the order of three eighths of an inch. Outages or long periods of light loads in-crease these movements considerably.

The flow of oil to and from reservoirs has been recorded, the daily or even weekly quan-tity has been approximately one gallon per phase per section, this of course is divided on the two ends. The seasonal variation has so far been only a transfer from cables to reser-voirs and has amounted to about 13 gallons per phase per section. This quantity would have to be corrected for the temperature change of the reservoir immersion oil amounting to 0.085 gallon per degree centi-grade. The temperature change has been 45 degrees centigrade which is equivalent to 3.8 gallons; thus the actual oil expelled be-cause of seasonal change has been about 9V2 gallons per phase per section.

Up to the present sufficient power has been handled by using a maximum copper temperature of 70 degrees centigrade. How-ever, if the need arose, there would be little hesitation in increasing this limit to 80 de-grees centigrade or even higher. We are now confident that oil demands would not be excessive for the present reservoirs even at extreme temperatures.

1076 Discussions A I E E T R A N S A C T I O N S