Focus on the fundamentals of shielded MV power cable

MV power cables are perceived as commodities, but are all makes alike?

Shielded, 90 [degrees] C (Type MV-90), medium-voltage(MV) power cane (5kV to 35kV) has long been considered a commodity product, with little differentiation seen between competing product offerings. Yes, the basic construction of this type of cable, as well as its physical and electrical performance properties, are very similar from one make to another. However, you have a choice of insulations, metallic shields, and jackets, all of which depend on the specific power system requirements, installation rigors, service conditions, and/or environmental factors.

More importantly, you should realize the shielded Type MV power cable is a more complex and sophisticated product than it's usually given credit for. It's manufactured to meet stringent engineering standards and requires special attention in terms of proper handling and installation.

Considering the widespread use of this type of cable in primary feeders in a wide range of large complexes and facilities as well as in utility applications, we think it's time that you take a fresh look at Type MV power cable fundamentals. We believe it will be both timely and extremely useful.

Key components

A Type MY-90 power cable consists of a current-carrying conductor; semiconducting strand shield, insulation, semiconducting insulation shield, metallic shield, and outer jacket.

Current-carrying conductor. As specified in NEC Art. 326, the conducting element in Type MV power cable must be either copper or aluminum. Copper-clad aluminum, while permitted, is rarely used. Copper is the preferred metal due to its higher current-carrying capacity. Also, copper has greater tensile strength than aluminum. In addition, aluminum requires a larger cross-sectional area than copper. As a result, the metal takes up more space in trenches or raceways.

Conductors can be solid or stranded, and either bare or with a tin or lead-tin alloy coating for additional corrosion protection. Type MV power cable can contain up to four conductors and can be manufactured in a variety of configurations to provide application flexibility.

Most Type MV power cables use annealed (soft drawn), stranded, Class B copper in a single conductor configuration. Strand arrangements and configurations are categorized by the American Society for Testing and Materials (ASTM) tables that use designation letters (Class B, for example) to identify individual classes for the size, number, and arrangement of conductor strands. Conductor sizes are governed by two scale systems: AWG (American Wire Gauge) and CMA (circular mill area).

Insulation. The performance of Type MV power cable is a function of its dielectric insulation, which is physically positioned directly over the strand shield. And, cable insulation material properties are directly related to overall cable performance. As such, the selection of insulation material is a direct function of the intended application. The higher the cable voltage, the thicker the insulation required.

Insulations are available in three general material categories: Thermoplastic, thermoset, and laminated. The most common insulation type is XLP (chemically cross-linked polyethylene), which is a hard thermoset compound having excellent properties and strong resistance to heat exposure within reasonable limits. XLP is chemically cross-linked, meaning that it's vulcanized during the manufacturing process, thus improving its mechanical properties.

A premium polymer compound, EPR (ethylene propylene rubber), is the next most common insulation choice. While EPR is a specially compounded thermoset material and does not quite have the dielectric properties of XLP, its nature does provide excellent thermal protection, allowing it to withstand higher temperatures with little effect on its physical properties.

XLP is harder than EPR and especially suitable for direct burial applications, yielding less dielectric loss per mile. However, EPR is more flexible and can be rated for higher ampacity when rated at 105 [degrees] C. (105 [degrees] C EPR, also known as MV-105, has recently gained NEC recognition.)

Another insulation material is tree-retardant XLP containing special additives that help resist the formation of "insulation trees." These are deterioration channels that can develop in the cable's insulation.

Laminated insulations are less commonly used today for 5kV through 35kV cables due to their poor resistance to moisture and difficulty in splicing and terminating.

Metallic shield. The ground-potential metallic shield is another important element in Type MV power cable construction because it serves to protect both the cable itself and the power system to which the cable is connected. It protects the cable itself by confining the cable's dielectric field, as shown in Fig. 1, and providing symmetrical radial distribution of voltage stress, as shown in Fig. 2. This limits the stress concentration at any one insulation point. It also helps dissipate heat away from the current-carrying conductor. The metallic shield can also protect the power system by conducting any fault current to the ground.

In addition to the above information, the metallic shield reduces interference with electronic equipment and also reduces the hazards of shock to anyone working with the cable.

A metallic shield can be configured as either concentric copper wire applied helically and closely spaced, spiral-wrapped copper metal tape with an overlap, or corrugated copper wires embodied in a semi-conducting jacket (available in one make of MV power cable). Each has its own advantages and disadvantages. A concentric wire shielded cable is more flexible and has a tighter minimum bending radius; however, the cable is more likely to suffer damage during installation if allowable pulling tensions and/or sidewall pressures are exceeded. A tape shield covers 100% of the insulation, resulting in greater physical protection for the cable. It also has a higher short-circuit capacity, which can be adjusted in the design stage to allow for even larger currents. The embedded corrugated wire metallic shield also has a high short-circuit capacity and is used to strip the jacket away from the insulation. As to shielding effectiveness, all work equally well. In fact, each possesses features that make it the best choice under certain conditions.

Outer jacket. As the exterior element, the jacket covers the cable's insulation and shield and protects them from the effects of moisture, chemicals, and mechanical abuse.

Jackets are available in a number of different compounds specifically formulated to withstand the physical abuse occurring during installation and from any environmental conditions encountered during the cable's lifetime. Jacket types include PVC (polyvinyl chloride), polyethylene, Neoprene[TM], Hypalon[TM], and 'thermoplastic CPE. (For certain applications, lead sheaths and interlocking or corrugated armors are available for multiconductor cables.)

Many jacket properties, such as sunlight and flame-resistance, can be improved by adding specific ingredients during the compounding process. As with insulation choices, jacket selection depends on the circumstances of where and how a cable will be used and on the prevailing exposure conditions, both during installation and while in service.