US3827143A

US3827143A - Oil cable installation method

Info

Publication number: US3827143A
Authority: US
Inventors: J Johnsen; T Vralstad; G Wettre
Original assignee: International Standard Electric Corp
Current assignee: Alcatel Lucent NV
Priority date: 1973-04-12
Filing date: 1973-04-12
Publication date: 1974-08-06
Anticipated expiration: 1991-08-06

Abstract

A system and method are provided for working on oil filled cables where there are substantial height differences between levels of the cable ends. Oil is drained from the lower level to a predetermined low pressure while the upper portion is maintained under a vacuum. Oil at the lower level may be frozen to permit conventional terminating work to be done. Various valve and degasifying arrangements are employed. The cable is then refilled with degasified oil.

Description

United States Patent [191 Aug. 6, 1974 Johnsen et a1.

1 1 OIL CABLE INSTALLATION METHOD [75] Inventors: John Normann Johnsen, Oslo; Tor

Vralstad, Eiksmarka; Gunnar Wettre, Asker, all of Norway [73] Assignee: International Standard Electric Corporation, New York, NY.

22 Filed: Apr. 12,1973

21 Appl. No.: 350,677

[52] US. Cl 29/628, 29/424, 29/433, 29/428, 137/15, 174/15 C [51] Int. Cl. H01! 43/00, H05k [58] Field of Search 29/628, 624, 428, 424, 29/433; 137/15, 315; 61/721; 174/10, 15 C [56] References Cited UNlTED STATES PATENTS 1.965.540 7/1934 Atkinson l. 174/15 C 3/1973 Pugh 174/10 X 5/1973 Dima et a1. 29/628 X Primary Examiner-Charles W. Lanham Assistant Examiner-Victor A. DiPalma Attorney, Agent, or Firm.lohn T. OHalloran; Menotti .1. Lombardi, Jr.; Edward Goldberg 5 7] ABSTRACT A system and method are provided for working on oil filled cables where there are substantial height differences between levels of the cable ends. Oil is drained from the lower level to a predetermined low pressure while the upper portion is maintained under a vacuum. Oil at the lower level may be frozen to permit conventional terminating work to be done. Various valve and degasifying arrangements are employed. The cable is then refilled with degasified oil.

4 Claims, 2 Drawing Figures OIL CABLE INSTALLATION METHOD BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a method for installation of oil filled cables having a great difference of height between the upper and lower ends of the cable which causes high internal, static oil pressure to be developed at the lower end if precautions against this are not taken.

2. Description of the Prior Art The solid insulation in oil filled cables is normally impregnated with a mineral or synthetic oil having a very low viscosity. Commonly the viscosity at 20C is between 5 and 50 centistokes. In oil filled cables there are also longitudinal ducts through which the oil flows to and from expansion or pressure tanks, connected to the cable. When such a cable is laid out with a great height difference between the cable ends, a corresponding high internal pressure will be developed.

When any kind of mounting work has to be undertaken on such cables, such as mounting of a joint or a terminating socket, the cable always has to be opened at the end. The oil will then run freely out of the cable end and this results both in a great loss of oil and makes the mounting work difficult. The higher the pressure is, the more troublesome are these drawbacks.

It has previously been known to cool down or freeze the cable near its end to such a degree that the viscosity of the oil will increase until it practically forms an oil plug which prevents the flow of oil out of the cable. At moderate over pressures, up to 2-5 kp/cm corresponding to height differences of approximately -50 meters, dependent of the oil viscosity, the cable construction and the length of the cooled cable section, this cooling process may be undertaken by means of carbon dioxide ice at a temperature of 78C. At still higher pressures liquefied air or nitrogenwith a temperature of l94.5C and -1 96C respectively (boiling point atl Atm) is required. At these low temperatures some of the components in the cable may, however, be damaged. A sheath of plastic, such as commonly used as a corrosion protection, will easily be damaged at such temperatures. Such damage requires repair and adds greatly to the cost of the installation work. The containers with the liquefied air or nitrogen are also usually open, which causes rapid evaporation and emptying of the containers. When the joint is situated at a remote location having difficult transportation facilities, this causes further problems as a great deal of time is required to move new containers to the jointing area.

At oil pressures of 20 kp/cm or higher, the cables are also very rigid as a result of the very high internal pressure and therefore difficult to handle. Thus when the difference of height is about 200 meters or more, it is part of the cable is avoided and the cable may be jointed without a freezing process. However, another drawback is introduced. The degasified oil which remains in the emptied cable insulation will absorb gas until it is saturated. When the cable is refilled with oil after the mounting work, it is then evacuated. The quality of the insulation however, is reduced as it is nearly impossible to remove all gas absorbed. This is particularly disadvantageous in modern cable design where considerably higher voltage gradients are allowed than in older designs. This is obtained by rather sophisticated production methods, but in the field it is not possible to obtain a sufficiently good degasing of the remaining oil as required in modern cables. In the first case, the vacuum obtained is not high enough and it will also decrease with the cable length. In the second case, it should be very difficult to heat the cable to a desirable degasing temperature, even if the vacuum obtained should be high enough. Another known process is described in British Patent No. 886,643, issued Jan. 10, I962, wherein the upper end of the cable is sealed to create a vacuum while draining oil from the lower end.

SUMMARY OF THE INVENTION The primary object of this invention is therefore to provide an improved method for reducing the pressure at the lower end of an oil filled cable which is laid out in a cable path with a great height difference between the cable ends. This is obtained by a method in which the oil pressure at the lower end of the cable is maintained at such a low value that the mounting work at the cable end may be undertaken in a conventional manner. The method includes partly emptying the cable of oil through its lower end so that only the lower part of the cable is filled with oil to a predetermined level, which may be controlled by means of a manometer, while the remaining upper part of the cable from which the oil is removed, is kept under a high vacuum during the mounting and arranging of the cable in the lower end. This may be done by freezing the oil within the cable at a position situated at a proper distance from the lower end, whereupon the overpressure present in the part of the cable below the frozen section is reduced by setting the oil in the lower part in contact with the atmosphere via a valve which is closed when the pressure in the cable is reduced to a value that allows conventional mounting of the cable, and the cable is then refilled with oil when the mounting work is finished. The following explanation and the accompanying drawings will provide a clear and detailed description of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 schematically shows a cable pulled into a vertical shaft, and

FIG. 2 shows the cable pulled further along a horizontal shaft at the bottom of the vertical shaft including a vacuum plant and degasing apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The following examples relate to a case where the cable is laid down a vertical shaft between an outdoor switching plant and a transformer built into the ground. The switching plant is placed several hundred meters above the transformer. In this case the cable in the shaft is terminated in a stop joint'which after the mounting process can withstand the working pressure. The joint is connected to the terminal at or near the transformer by means of a cable. It may also be suitable to terminate the shaft cable directly in the terminal socket if this is sufficiently pressure resistant. The following operations will then be substantially unchanged.

In FIG. 1, the cable 1 is shown in a position in which it is partly paid off from the reel 2 and where the remote cable end 1 is lowered in the shaft to a proper height above the floor in the transformer hall 7. A suitable oil volume is removed from the cable into a container, or the like, not shown in the figure. A represents the upper level of the oil after this tapping process. The lower cable end 1 is provided with a valve 3 and a non-return valve 4 both mounted on the cable before it is paid off from the reel, preferably before the cable leaves the factory. The non-return valve is designed so that it is closed for internal overpressure. It should also preferably be prestressed so that it only will be opened forcibly when an adaptable equipment such as a manometer 5 is connected. This cable end is thus preferably provided with a pressure measuring equipment 5, such as manometer, which shows the static oil pressure in the cable and therefore the actual level A .of the oil. The manometer 5 is connected to the nonreturn valve 4 when the cable is laid out to the position shown in FIG. 1. During the transport and the laying out process the valves 3 and 4 are preferably protected against possible damage caused by outer strain. The near upper end 1" on the cable reel is also provided with a valve 8 (FIG. 2) which must be vacuum tight.-A pressure tank which is connected to the near end 1" of the cable during transportation, may be disconnected when end 1" is removed from the reel.

In FIG. 2, the cable is removed from the reel and its remote lower end 1 is arranged in a proper position for mounting. The freezing equipment 9 is also provided. During this time, the oil level is lowered from A to B such that a suitable pressure is obtained which will permit freezing with CO ice, or the pressure obtained is so low that mounting without freezing is possible. The upper end 1 with the valve 8 is disconnected from the reel and is connected to a vacuum pump 10. Apparatus for degasing of the oil is also connected. This comprises another vacuum pump 10', degasifier 11, oil pumps 12 and 13, an indicator 14 for measuring the degree of degasing, and an auxiliary equipment 15 comprising a vacuum tight preferably transparent container, referred to below as a control glass, to which the oil may be led either because it is not of a sufficiently high quality according to the degasifying indicator 14, or to indicate that the refilling process is completed after the mounting of the lower cable end 1. The oil pump 13 supplies the whole system with oil from the oil reservoir 29 via the supply pipe 28. The control glass 15 comprises a cylindrical glass with vacuum tight flanges at both ends and may also be used as a rough control of the degree of degasification of the oil. The control glass is provided with one upper and one lower valve 19 for filing and/or tapping. The various reference numbers l6, l7, l8, 19, 20, 21, 22, 23 and 24 are all valves. A flexible tube 26 is armored to withstand high internal oil pressures and may also be used to refill the cable with oil from the degasifying apparatus when the mounting of the lower end is completed. Therefore the oil tube 26 is provided with a non-return valve 27 at the lower end suitable for connection to the mounted socket at the lower end of the cable. By means of the arrangement shown in FIG. 2, the cable may be refilled with oil from the upper end 1" via valve 20 to raise the oil level at A if this is getting too low.

With the equipment as shown the following method may be used.

1. The cable I is partly paid off from the reel 2, with valve 3 closed until the remote end I is in the position shown in FIG. 1.

2. The flexible tube 26, which is prefilled with degasified oil is laid out.

3. The manometer 5 is connected to the non-return valve at the lower end of the cable.

4. The upper end 1", with valve 8 closed, is disconnected from the pressure tank in the reel and the cable is thus disconnected from the reel.

5. Oil is let out of the cable at its lower end into a suitable container, such as an empty oil barrel, which is not shown in the figure. The outflow of oil is continued until the oil level in the cable has reached the point A. This level is predetermined and is measured by manometer 5.

6. The cable is pulled further down until the position shown in FIG. 2 is reached. If the part of the cable situated in the horizontal shaft is long, it may be necessary to add oil to keep the oil level sufficiently high. The level of oil must be positioned in the vertical part of the shaft. This may conveniently be obtained by filling oil into the cable from its lower end via a connection to flexible tube 26, and possibly simultaneously evacuating the upper end.

7. The vacuum pump and the degasing equipment are connected as shown in FIG. 2. It is assumed that the degasing equipment is preconnected and arranged in a single unit and that the control glass 15 and part of the pipe system are already evacuated, and that the vacuum pumps always work when the valves 18, 21 and 23 are open. When a connection between the cable and the flexible tube 26 is established, the valves 20, 22, and 24 are also opened. When the remaining parts of the vacuum pipes are sufficiently evacuated, valve 8 is opened.

The freezer system 9 is put in place at the lower end of the cable.

8. The oil level B is adjusted. Normally this will be done by letting a small amount of oil out of the cable.

If the oil level has dropped below the predetermined level, it is possible to refill the oil from the degasifier ll via the pump 12 and the degasifying indicator 14. The

valves

23, 16, 17, 20 and 8 are open during the refilling process while the remaining valves are closed. The oil is led from the container 29 via pump 12 and the gas indicator 14 to the tube 26. When the pressure in the lower cable end is adjusted the degasifying ceases and the

valves

16, 17, 20 and 23 are closed while valve 22 is opened.

9. Freezing starts.

10. Mounting of the lower end is started and completed.

11. Just as the degasifying process starts again, the cable is filled with oil by connecting the non-return valve 27 on the tube 26 to a proper bushing on the mounted socket arrangement. This socket may also be equipped with a non-return valve connected to the valve 27 to avoid loss of oil when the valve 27 is connected or disconnected. The further filling is obtained by opening the

valves

17 and 25. In addition, valves 21 and 24 are opened as valve 22 is closed. Thus the oil will appear in the control glass when the cable is completely filled.

l2. When the cable is completely filled by degasified oil, the valve 8 may be closed and the upper end of the cable is disconnected from the degasifying apparatus and made ready for mounting. During this mounting process the pressure is maintained by means of the degasifying apparatus connected to the lower end of the cable via the tube 26.

In certain cases it may be convenient to place the degasifying apparatus at the lower end of the cable to fill the cable with oil. Then pipe 26 can be omitted. It would then be advantageous to place an additional control glass between the vacuum pump and the upper end of the cable. With this position of the degasifying pump, the oil pump has to be sufficiently effective to give the maximum oil pressure in the cable with a safe margin. This is necessary to withstand the pressure when the cable is filled with oil.

When the first cable is mounted, it may be used for filling a subsequently mounted cable by introducing a suitable connection between the first cable and the following cables. In this case the armored pipe is required only for the first cable laid out in the actual cable path.

If the cable end must be transported some distance in a horizontal direction after it has reached the bottom of the shaft and before it reaches the equipment to which it is to be connected, it may in some circumstances be convenient to reduce the pressure in the lower end of the cable in the following manner:

1. The cable is lowered into the shaft until its lower end reaches a required height above the floor of the shaft.

2. The oil is frozen into a plug at a location having a sufficient height above the floor, possibly after letting out part of the oil as earlier described.

3. The valve 3 at the lower end of the cable is opened until the pressure in that part of the cable which is below the oil plug is reduced to the extent that this end of the cable may be easily bent and handled. The pressure may be read out on the manometer 5. Then the valve 3 is closed.

4. The cable is pulled further in the horizontal direction towards its final position, until it is in the desired location or until the frozen oi] plug has reached the bottom of the shaft. If the cable has not yet reached its final position, the process must be repeated as a second oil plug is formed above the first one which then is melted. Then the pressure is again reduced through valve 3 and the cable is pulled further towards the final position.

The above description provides only a limited number of examples of how the method of the present invention may be carried out and many other variations and modifications may be made within the scope of the invention as set forth in the appended claims.

We claim:

1. A method for installation of an oil filled cable in a path having great differences of height between the upper and lower ends of the cable such that there would normally be a high static oil pressure at the lower end of the cable, and which provides a sufficiently low pressure to permit normal mounting and connections to be made at the lower cable end comprising the steps of partially emptying the cable of oil through its lower end so that only the lower part of the cable is filled with oil to a predetermined level and at a sufficiently low pressure to stop the flow of oil, applying a high vacuum at the upper part of the cable, maintaining the high vacuum while completing the mounting and connecting of the cable at the lower end, and refilling the cable with oil upon completion thereof.

2. The method of claim 1 including degasifying the oil while applying the high vacuum and refilling the cable with degasified oil.

3. The method of claim 1 including freezing the oil within the cable at a predetermined distance from the lower end to block the flow of oil, and permitting the lower cable end to be open to atmospheric pressure until the oil pressure is reduced to said low pressure.

4. The method of claim 3 and wherein after the oil is frozen it thereafter is pulled further in a horizontal direction until the frozen portion has reached a knee where the cable passes from vertical to a horizontal direction, whereupon a second portion is frozen in a second part of the cable, after which the first frozen oil portion is melted, whereupon the cable is pulled still further in a horizontal direction until the cable has reached its final position.