Contrastación de hipótesis Hipótesis general

Cable Construction

Pipe-type cables incorporate three cable phases installed in a common steel pipe (see Figure 3.2-1). Each cable phase consists of a stranded copper or aluminum con-ductor, with a layer of metallic (steel or copper) binder tapes intercalated with a carbonized black paper tape.

Larger conductors above 800 mm² (1500 kcmil) may be segmented to reduce ac resistance, and hence reduce ac losses. Over the conductor shield is a laminated Kraft paper or, for higher voltages, a laminated paper-polypropylene insulation. The insulation thickness is governed by voltage. Typical AEIC insulation thick-nesses are listed in Table 3.2-1.

The insulation wall thickness is important when evalu-ating reconductoring options or for considering the free area within the pipe for circulating dielectric liquid.

These concepts are discussed in Section 3.7.

Table 3.2-1 Typical Pipe Cable Insulation Thicknesses

Rated kV Phase-to-Phase Size of Conductors Insulation Thickness

(kcmil) (mm²)

Laminated Paper Polypropylene^a mils

(mm)

a. Large conductor sizes using laminated paper polypropylene insulation may require increased insulation wall thicknesses to control the minimum electrical stress to 1750 volts/mil (68.9 kV/mm) so as not to exceed the design limits of terminals and splices.

Paper mils (mm)

69 167.8-4000 85 – 2027 n.a. 270 (6.86)

300^b (7.62^b)

405-2,027 n.a. 435 (11.05)

405 (10.29)

507-2027 n.a. 575 (14.61)

515 (13.08)

230 1000-2000

2250-4000

507-1013

1140-2027 450 (11.43) 745 (18.92)

605 (15.37)

500 2000-4000 1013-2027 745 (18.92) 1100 (27.94)

765 2000-4000 1013-2027 1200 (30.48) n.a.

b. High-pressure gas-filled (HPGF) cable insulation thicknesses.

Over the insulation, it is common to see one or two met-alized Mylar tapes applied over a carbonized black paper tape. The Mylar tape acts as a moisture seal to limit insulation contamination and dielectric liquid drainage prior to installation. Metal shield tapes are then applied over the Mylar tape. Over the shield and moisture barrier tapes is one or two helical metal skid wires, typically constructed of stainless steel, zinc, brass, or bronze. The skid wires provide mechanical protection when the three cables are pulled into the installed cable pipe. On a few cable designs, a plastic “compression jacket” is applied over the insulation shield (more often on HPGF cables than HPFF cables) to limit the insula-tion impregnate from draining from the insulainsula-tion and mixing with the dielectric media within the cable pipe.

Cable Pipe

The pipe is generally ASTM A-523 Schedule 20 or 40 line pipe, 6.35 mm (¼-in.) wall with flared ends to facili-tate welding with chill rings. A cable trench is excavated for the installation of the cable pipe. Typically, the trench is usually backfilled with “thermal sand” or a Fluidized Thermal Backfill (FTB) that helps ensure good heat transfer away from the cable pipes (Figure 3.2-2).

Joints

Pipe cables may be 32 km (20 miles) long, but most installations are only a few kilometers (miles). Installa-tion secInstalla-tions are on the order of 350-1000 m (1200-3300 ft) and require manholes and joints to connect cable sections. Cable pipes enter both ends of the man-hole to facilitate joining the cables. Inside manman-holes, section casing lengths of 1-1.5 m (3-5 ft)— generally 1.5-3 times the cable pipe diameter—are used to connect pipe sections. Inside the casing, each cable phase is joined together using a compression connector and hand-applied paper or laminated-paper-polypropylene tapes. Joints may be one of three types:

• Normal Joint. The cable conductors are connected through the casing, and hydraulic continuity is per-mitted (see Figure 3.2-3).

• Semi-Stop Joint. The cable conductors are connected through the casing and hydraulic flow is stopped for differential pressures below 350 kPa (50 psi). Valves may allow complete hydraulic isolation from one side of the joint to the other.

Figure 3.2-1 Example of high-pressure fluid-filled (HPFF) pipe-type cable.

Figure 3.2-2 Pipe-type cable trench being backfilled with FTB.

Figure 3.2-3 Pipe-type cable manhole with joint casing.

• Full Stop Joint. The cable conductors are connected through the casing, but there is no hydraulic continu-ity as the full stop joint supports rated line pressure differential.

Terminations (Potheads)

The ends of a pipe-type circuit are terminated with a graded insulation that controls electrical stress from the paper-insulated cable to the air-insulated terminal. A

“cone” of insulation is applied within a porcelain termi-nation to provide hydraulic and electrical isolation for the cable end. Leading up to the terminations, the three cables within the common cable pipe are separated into individual stainless steel pipes through a trifurcating joint (see Figure 3.2-4). Nonmagnetic stainless steel pipes are used between the trifurcating joint and the ter-mination to avoid the high circulating currents and eddy current heating that would otherwise result if conven-tional carbon steel pipe were used. Stand-off insulators are used at the base of the potheads to isolate the pot-head from the support structure so that circulating cur-rents are not induced in the riser pipes between the trifurcator joint and pothead.

Fluid-Filled Cables

High-pressure fluid-filled (HPFF, also known as high-pressure oil-filled) cables are installed in cable pipes where the pipe is filled with very clean, very low

mois-ture dielectric fluid. Older HPFF cable systems (before 1970) typically used mineral oil for the pipe filling dielectric fluid. HPFF cable systems installed after 1970 have used alkyl benzene or polybutene dielectric fluid (polybiphenyl chlorine-based liquids were never used as an insulating liquid in pipe cables). The dielectric fluid is pressurized to 1400 kPa (200 psi) and is generally free to mix with the insulation impregnant, although this movement is limited.

Gas-Filled Cables

High-pressure gas-filled (HPGF) cables use pressurized dry nitrogen gas inside the cable pipe. HPGF cables still utilize dielectric-fluid impregnated into paper insulating tapes as insulation, but the dielectric fluid is generally of a much higher viscosity than fluid-filled cables to limit drainage. Also, the insulation thickness on HPGF cables is slightly greater than in HPFF cables as shown in Table 3.2-1. Nitrogen pressure is typically on the order of 1400 kPa (200 psig). Bottled nitrogen and a pressure regulator located near the terminal ends are used to maintain the pressure within the cable pipe.

Low-pressure alarms are utilized to ensure that the cable pipes are maintained at the required pressure to avoid damaging the pipe cable.

Other Equipment Pumping Plants

As was mentioned above, pipe-type cables are pressur-ized with either dry nitrogen or dielectric liquid. For the liquid-filled cables, a “pumping plant” or “pressuriza-tion plant” is needed to maintain and regulate the typi-cally 1400 kPa (200 psi) pressure within the cable pipe (Figure 3.2-5).

Cathodic Protection Equipment

The carbon steel pipe must be protected from corrosion to avoid leaks and deterioration of the pipe. The first level of protection is a corrosion protection layer that is applied over the outside of the pipe. Older cable systems used a hot applied tar coating or a somastic coating that is similar to concrete. More recent HPFF cable systems use pipe that is coated with a polymeric material such as high-density polyethylene. These corrosion protection coatings are effective in preventing corrosion if there are no holes (“holidays”) or cracks in the coatings. How-ever, some damage inevitably occurs to the pipe coating during installation or subsequent digging after the cable system has been placed in service. Consequently, it is necessary to further protect the cable pipe with impressed current cathodic protection systems or sacri-ficial anodes.

Some HPFF cable system pipes are corrosion protected with magnesium sacrificial anodes that are connected to Figure 3.2-4 Above ground trifurcator (spreader head)

and pipe-type cable potheads.

the pipe at manhole locations as well as at the substa-tions where the cable terminasubsta-tions are located.

Impressed current cathode protection systems must pro-vide enough current to maintain the cable pipe at a potential of –1.0 volt dc (or in some cases higher) with respect to the surrounding earth. The impressed cur-rent/pipe grounding system must also be designed to accommodate the maximum line-to-ground fault cur-rent while keeping the pipe potential close to ground potential.

Several types of impressed current systems have been used to cathodically protect and ground the cable pipe.

These are:

Resistor/Rectifier Cathodic Protection. In this type of cathodic protection system, the ends of the pipe are grounded through low resistance connections (several milliohms), and a relatively high-capacity dc current supply forces enough current through the resistor to maintain the dc pipe potential at approximately -0.85 to -1.0 V.

Polarization Cells with Rectifiers. In this type of cathodic protection system, a passive device called a polarization

cell is used to ground the cable pipe at the end point substations. A polarization cell, which is about the size of a car battery, is characterized by a relatively high resistance to dc voltages of several volts and a low resis-tance to ac currents. A relatively low-capacity dc recti-fier then supplies enough current to the pipe to maintain the pipe potential at -0.85 to -1.0 V.

Solid-State Pipe Grounding Devices with Rectifiers. The most recent development for pipe cathodic protection includes power electronic devices that are capable of conducting line-to-ground fault currents. These devices are direct replacements for the polarization cells described above.

3.2.2 Extruded Dielectric