Medium-Voltage Cable Constructions

Underground medium-voltage cables at plants are installed as one of three basic cable assembly configurations:

 Individual, insulated single conductors

 A twisted combination of the insulated single conductors, known as a triplexed assembly

 A jacketed three-conductor cable

In any of these assemblies, the insulated conductors share the same basic construction shown in Figure 4-1, with the exception of the jacketed three-conductor cable, which has an overall jacket over the insulated singles.

Cable Designs

Figure 4-1

Shielded, Single-Conductor, Medium-Voltage Cable Design

The conductor (A) is typically stranded copper or aluminum, with copper being more common in power plants. The insulation (C) used in conventional and plant medium-voltage cables is EPR or, to a lesser extent, XLPE. Medium-voltage cables in a few early plants had butyl rubber insulation. Shields (B and D), composed of semiconducting polymer in modern designs, help maintain a uniform voltage stress in the insulation. The metallic tape shield (E) provides a continuous drain for the insulation shield and a return path for fault currents. The jacket (F) adds mechanical protection as well as an additional barrier to moisture and external contaminants.

In early black EPR and butyl rubber cables, the insulation and conductor shields were formed from helically wrapped carbon black–loaded cotton tape. By the early 1970s, the insulation shield was made of a helically wrapped semiconducting polymer tape. The insulation shields were in tape form to enable installers to differentiate between the black shield and the black insulation. When strippable, extruded black semiconducting polymer shields became common, the industry converted to gray, pink, or brown EPR by eliminating some or all of the carbon black from the insulation. Extruded conductor shields became available for power plant use in the mid 1970s.

A modern construction, similar to that shown in Figure 4-1, is the water-impervious design shown in Figure 4-2. The sub-components are virtually the same, but the water-impervious cable is constructed to limit absorption of moisture and external contaminants through additional radial and axial design barriers. One design barrier is that the metallic tape shield (E) has been replaced with a continuous, corrugated tape shield that is copolymer coated and sealed at the overlap. A second design barrier is the use of water-swellable tapes or powders placed in the conductor strands to prevent water migration through the stranded core. These two design changes provide barriers to prevent moisture and external contaminants from penetrating into the layers below.

The corrugated metallic shield also should provide lower, more stable shield resistance compared to the helically wrapped copper tape design, which should aid in long-term testability. Other moisture-impervious designs exist, including those that use a fully sealed lead or aluminum sheath instead of a corrugated copper sheath with a glued overlap. For more information on cable design and selection, refer to the EPRI report Plant Support Engineering: Common Medium Voltage Cable Specification for Power Plants (1019159) [12].

Cable Designs

Figure 4-2

Medium-Voltage Shielded Cable, Water-Impervious Design

Two disadvantages of the water-impervious design are the higher cable cost and the increased difficulty in making field terminations. The cost differential might not be significant in

comparison to the overall cost of installation.

Using a lead sheath in place of the corrugated copper tape is also possible, but it has limited availability and requires additional care in pulling to preclude damage to the lead sheath.

Installation of lead-sheathed cable might not be practical when replacing cables that had tape shields or were nonshielded. The lead-sheathed cable will likely require larger ducts and larger-radius bends.

Figure 4-3 shows a nonshielded cable design. In this design, there is no insulation shield. This design has been applied in many instances for medium-voltage circuits of 4160 V and lower. The reason is that non-grounded systems, when designed properly, can tolerate a single phase-to-ground fault while remaining in service for a short period. However, the absence of a shield that confines the voltage stress to the insulation makes testing of the insulation system in a

meaningful way quite difficult. Testing is possible in a laboratory by submerging the cable in water and using the water for the ground plane. Submergence of an entire circuit is not practical in a plant and can provide confusing results because the jacket of the cable is in series with the insulation and can affect the test results.

Figure 4-3

Medium-Voltage Nonshielded Cable Design

Cable Designs

Figure 4-4 shows an example of the UniShield cable design. In this design, the outer polymer layer serves as both jacket and semiconducting insulation shield for the cable. The shield conductor system is composed of six longitudinal, corrugated, neutral wires embedded in the jacket. The design includes a compacted conductor. The result of this design is a smaller-diameter cable. The small-smaller-diameter design might be needed when existing ducts are relatively small in diameter because smaller-diameter cable had been previously used. Other designs exist that have small diameters, but they have not been commonly used in power plants to date.

Figure 4-4

UniShield Construction, Shield Wires Embedded in the Semiconducting Outer Jacket

4.2.1 Voltage Rating

Typical distribution service feeders are rated 5 kV through 35 kV. Plant auxiliary power

distribution system feeder cables are predominantly rated 5 kV; plants constructed after the late 1970s have 8 kV or 15 kV cable, as well. Medium-voltage systems in plants operate at 4.16 kV, 6.9 kV, 12.2 kV, or 13 kV. Plants with 12.2 kV or 13 kV systems generally also have 4.16 kV systems.

The combination of lower voltage rating (that is, 5–15 kV) and larger minimum conductor size (see Section 4.2.2, Conductors) means that cable insulations generally operate at the lower end of the range of electrical stresses at which water treeing occurs in XLPE and water-related degradation occurs in EPR. Therefore, such systems are less susceptible to moisture-related degradation, and water-related failures tend to occur later in cable life than has been recognized in distribution systems with smaller conductors and operating voltages of 33 kV and greater.

Insulation must be able to withstand the voltage stress experienced during normal operation, as well as voltage surges. Thicknesses of insulation material for a given voltage rating have been established for different materials by the Insulated Cable Engineers Association (ICEA) and AEIC for power plant applications. Commercial and industrial cable applications are governed by Underwriters Laboratories, Inc. and National Electrical Code standards. The voltage ratings are phase-to-phase ac that relates to the maximum phase-to-phase operating voltage being used in the system. The discrete ratings are 5 kV, 8 kV, 15 kV, 25 kV, and 35 kV [8].

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When a ground fault occurs on a power system, voltages higher than the cable’s voltage rating can occur. To address this, ICEA and AEIC specify thicker insulation if the overvoltage can last longer than 1 minute. The duration of the overvoltage depends on whether the neutral is solidly grounded or ungrounded. The insulation and resultant thicknesses are classified as 100%, 133%, or 173% insulation levels [8], as follows:

 100% Level. Relay protection normally clears ground faults within 1 minute.

 133% Level. The faulted cable de-energizes within 1 hour.

 173% Level. The time needed to de-energize the fault is indefinite. This level is recommended for resonant grounded systems.

Even if the 173% level of insulation is used, it does not mean that cables with a ground fault can be left in service. If a ground is suspected (for example, an alarm is received), action should be taken to remove the cable from service as soon as possible. The faulted phase can adversely affect the adjacent phases and lead to a phase-to-phase fault that will have extremely large currents, which could cause damage to the connected equipment. For plant use, some designers opted to use the 133% level as conservatism.

4.2.2 Conductors

The conductor (see A in Figure 4-1) is typically stranded copper or aluminum, the former being more common in power plants. The cross-sectional area size, the associated diameter, and number of strands in the conductor are standardized and unaffected by material choice. The conductor size is given in terms of American Wire Gauge (AWG) or kcmil in the United States and in terms of mm² elsewhere. The outside surface of the conductor can be smoothed to a near-perfect circle by compacting the strands to a compact-round standard, which can result in up to a 3% diameter reduction. The compression also aids in attaining a smoother interface between the strands and the conductor shield. Figure 4-5 shows a comparison of the conductor stranding compression options. Although it reduces flexibility somewhat, the compact-rounding helps to avoid potentially ionizing and discharging air gaps at the inside surface of the insulation, where the voltage gradient is greatest. The smallest diameter stranded conductor configuration results from compacting in which each strand is shaped to allow approximately 9% reduction in

diameter. This degree of diameter reduction is used in cables designed for smaller diameter ducts and conduits.

Figure 4-5

Conductor Stranding Configuration, Showing Compressed and Compacted Conductor Configurations

Cable Designs

Distribution and plant conductor design philosophies differ substantially. Distribution designers frequently use aluminum conductors and typically use solid conductors for aluminum wires up through 2/0 AWG (67.5 mm²) and copper up through 6 AWG (13.28 mm²). Stranded conductors are used for all larger sizes. Although the solid conductor is somewhat stiffer, it does effectively block the flow of water through the interior of the cable. In contrast, plants use stranded copper conductors to allow greater flexibility for installation. Water blocking of stranded conductors can be accomplished through the use of polymer fills, water-swellable powders, or a thixotropic gel filling for the strand interstices. However, water-blocking technologies were not available at the time of construction of most plants, but they are available for replacement cables and new plant use.

Historically, most cables were not compacted, with the exception of the UniShield construction, which is designed to be a small diameter for a given voltage rating.

4.2.3 Conductor Shield

Air gaps between the conductor and insulation result in high-voltage stresses that cause the gap to periodically discharge. Such discharges can damage the insulation and lead to failure. To eliminate the air gaps between the conductor and insulation, an effective conductor shield or a strand shield (see B in Figure 4-1) is required over the conductor, regardless of whether the insulation itself is shielded. Addition of a semiconducting layer between the insulation and the conductor prevents voltage from building in gaps between the insulation and the conductor by eliminating the gap and causing surface charge on the insulation to be drained to the conductor.

The semiconducting layer must be in intimate contact with the inner diameter of the insulation.

The conductor shield is typically a thin (~10–20 mil [0.254–0.508 mm]), extruded

semiconducting compound that is compatible with the primary insulation. Like the electrodes of a capacitor, the insulation shield on the opposite side of the cable insulation and the conductor shield help to confine the electric field and create symmetrical radial distribution of voltage stress within the dielectric.

Due to limits in extrusion technology, helically wrapped carbon black–loaded cotton tapes were used for the conductor shield in medium-voltage cable designs available in the late 1960s until shortly after 1970. With such tapes, stray fibers protruding from the conductor shield tape could become encapsulated in the insulation during extrusion. These protrusions became initiating sites for water-tree growth. The subsequent development of dual-pass and dual-tandem extrusion systems facilitated the use of polymeric conductor shields and the elimination of the inner tape.

The Kerite Company’s Permashield design uses an alternative stress reduction technique at the conductor to insulation interface. Instead of using a semiconducting conductor shield, a high-permittivity polymer layer is applied to the conductor and bonded to the insulation. The layer limits electrical stress at the conductor-to-insulation interface.

4.2.4 Insulation

The cable’s primary insulation (see C in Figure 4-1) is manufactured of materials that are designed with sufficient dielectric strength to withstand the voltage stress experienced during normal operation, as well as unusual voltage spikes and surges. The insulating material for most medium-voltage plant cables is either XLPE or EPR; however, some early plants have cables with butyl rubber insulation. Although XLPE has additives for fire retardance and processing, it

Cable Designs

is mostly XLPE polymer, but a copolymer (additional plastic type) is often used. Recent

experience within the transmission and distribution utility industry has led to the development of a tree-retardant enhancement of this insulating material. The additives to tree-retardant XLPE (TR-XLPE) do not totally eliminate water trees; rather, they greatly reduce their rate of generation and growth.

In contrast to the relatively limited number of variations in XLPE formulations, EPR insulations are comparatively complex compounds that vary substantially among polymer suppliers and cable manufacturers. Different improvements of the EPR materials have evolved and have been distinguished from one another by color, such as black, gray, brown, or pink. The colors alone are not directly related to the way these materials age, but they are indicative of the changes made to the formulations that improved the longevity of cables. Black EPR cables are early-generation EPR cables manufactured through the mid to late 1970s. Brown EPR appears to have been resistant to water-enhanced aging effects throughout the period of its use. Pink (or red) EPR cables are the more modern generation, available from the late 1970s through today. A few manufactures changed from black EPR to gray EPR in the late 1970s. The major shift in EPR formulation during that period was the transition from untreated clay to silane-treated clay. The silane treatment caused the EPR to bind more tightly to the clay, and it sealed the clay so that water uptake in the EPR was greatly reduced.

A typical dielectric constant for XLPE is ~2.3, whereas that for EPR is distinctly higher, at ~3.2.

Thus, EPR has higher dielectric losses per unit length of cable than XLPE, but it is generally more resistant to voltage stress and discharges. The dielectric loss through the EPR insulation drains any charge that could build in the insulation at imperfections and eliminates high localized stresses that result in water-enhanced aging. Because medium-voltage cables in power plants are usually much shorter than those in transmission and distribution applications, electrical system losses are not paramount in the design of station cable systems. Thus, a majority of underground medium-voltage cables at plants have EPR insulation, which has always had an expectation of greater operating life. Rubber insulation systems also have been chosen for in-plant applications due to their flexibility, which is important during installation in the tighter confines of power plant applications.

When PE insulation first became available for medium-voltage applications, it was hailed as the cure-all for many of the issues then facing the distribution industry. These PE materials had extremely low losses in a high-stress electrical field, were easier to compound than rubber systems, were lower in cost than either rubber or paper-insulated lead-covered cables, and were quite hydrophobic (tended not to absorb water or moisture), whereas conventional rubber systems were not. Thus, distribution utilities made widespread of various types of PE.

generating station designers gave relatively little weight to PE’s low loss characteristics because of the insignificant circuit lengths involved. Those few plant designers who did not choose rubber-insulated systems chose XLPE for its superior mechanical strength and thermal endurance.

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4.2.5 Insulation Shield

4.2.5.1 Semiconducting or High-Permittivity Shield Layer

Historically, 5-kV rated cables could be purchased with or without an insulation shield.

According to NEI 06-05 survey results [11], 30% of the respondents have some nonshielded cables installed. Typically, 8-kV and higher rated underground cables at plants are shielded.

When grounded, the shield confines the electric field within the insulation and produces a symmetrical radial distribution of voltage stress within the dielectric, minimizing the potential for surface discharges. In addition, the shielding limits radio-interference generation, allows for individual conductor insulation electrical testing, and if properly grounded, reduces a possible shock hazard to plant personnel. The insulation shield is composed of a semiconducting

polymeric insulation shield or screen (see D in Figure 4-2) and an overlying metallic component (see E in Figure 4-2). The Kerite Company offers an alternative shield design, called

Permashield, using a high-permittivity shield instead of a semiconducting layer. Kerite uses a Permashield layer as a conductor shield and offers either a semiconducting layer or a

Permashield layer for the insulation shield. When both shields are of the Permashield type, the cable is labeled “Double Permashield.”

The semiconducting or Permashield layer eliminates air gaps between the primary insulation and the ground plane of the metallic shield that could ionize, discharge, and, in time, degrade the insulation. Earlier cables used cotton tapes, which were ultimately problematic because cotton fibers could enter the insulation during manufacture, leading to high localized stresses in the insulation. Semiconducting tape insulation shields were introduced in the late 1960s and continued to be used into the early 1970s. The use of insulation shield tapes simplified production and ensured that the cable could be readily spliced or terminated. The tapes have printed statements indicating that they must be removed when splicing and are readily

discernable from the insulation. However, during the manufacture of this type of cable, the tapes are applied in a separate operation from the extrusion of the insulation. The exposure and

handling of the insulation before the tape was applied allowed the possibility of contamination of the interface. Contaminants occurring during manufacture were subsequently identified as the root cause of water trees. These designs were replaced with the higher-reliability, extruded semiconducting shields that are currently available.

The extruded semiconducting layers can be either thermoplastic or thermoset materials. As manufacturers switched to extruded semiconducting layers, so-called “dual-pass” extrusion systems were commonly used. Although the extruded insulation shield was a definite

improvement over the old tape method, the interface was still exposed to contamination before and during the second pass. When the significance of this contamination was recognized, manufacturers eliminated the exposure through the use of three extruders on a single production line. The so-called “1 + 2” extrusion, in which the conductor shield was applied just upstream of a tandem extruder that applied both the insulation and the insulation shield was introduced in the 1980s. Triple extrusion, in which all three layers—the conductor semiconducting shield, the insulation, and the insulation semiconducting shield—are applied at once, precluding contamination, did not become common until the 1990s.

Cable Designs

Care must be taken when splicing new cable to old black rubber cables with tape shield designs.

In modern cables, the insulation shield layer is easy to identify because it is black as opposed to the gray, brown, or pink insulation. Current splicing crews are familiar with the modern cable designs. However, they are unlikely to have seen woven fabric or polymer tape shields. They must be trained in how to make splices and terminations to these old style cables. If the

In modern cables, the insulation shield layer is easy to identify because it is black as opposed to the gray, brown, or pink insulation. Current splicing crews are familiar with the modern cable designs. However, they are unlikely to have seen woven fabric or polymer tape shields. They must be trained in how to make splices and terminations to these old style cables. If the