Various Land Cables Used in Practice

One obvious difference of a cable from an OHL is that the outer surface of the cable is insulated from the core conductor. This leads to the relatively large admittance of cables compared with OHLs.

The difference in the admittance can be observed not only between cables and OHLs but also between different types of cables. This section explains three types of land cables – XLPE, SCOF and HPOF cables – widely used in practice. These land cables use different insulating materials, and their different relative permittivities lead to different admittances, even with an equal length. The thickness of the insulation layer also affects the admittance of the cable. This section also discusses physical characteristics of land cables together with their electrical characteristics that affect cable system transients.

XLPE Cables

XLPE cables are the newest type of cables among the three major types. The practical application of XLPE cables into the distribution network started in the 1960s. Since then, the applied voltage has been gradually raised and reached the current maximum nominal voltage of 500 kV in 2000.

Until recently, it was more common to select SCOF cables or HPOF cables even for new cable lines. However, in recent years XLPE cables have become the most popular choice for the following reasons:

1. Satisfactory service experience

2. Environmental effect

3. No pressure system

4. No maintenance

For item 2, since XLPE cables use cross-linked PE as the insulating material, flammable oil or greenhouse gases will not leak into the soil or atmosphere even when the cable is damaged. For SCOF and HPOF cables, there is a risk of causing fire or ecological impact when the cable is damaged.

The difference in the insulating material also leads to items 3 and 4. For SCOF and HPOF cables, it is necessary to apply pressure to insulating oil in order to maintain insulation. In contrast, XLPE cables do not require the pressure system including large oil tanks. This can become a major advantage especially when the cable is laid in an urban center and the available space is limited.

Figure "Single-core XLPE cable. Courtesy of VISCAS Corporation" shows an example of the cross-section of a single-core XLPE cable. The cable is composed of the following layers:

1. Core conductor

2. Inner semiconducting layer

3. Insulation layer

4. Outer semiconducting layer

5. Metallic sheath

6. Outer cover

Single-core XLPE cable. Courtesy of VISCAS Corporation

This configuration of layers is basically identical for single-core XLPE cables with recent technology, regardless of the cross-section size and the adopted material for each layer. The following describes each layer from inside to outside.

Core Conductor

The core conductor carries load currents of the cable and is made of copper or aluminum. The copper or aluminum wire is wound to form a stranded conductor as shown in image below “Stranded conductor. Courtesy of VISCAS Corporation”. Even though it is stranded, the core conductor is considered to be identical to the solid conductor when building a cable model for EMT studies.

Stranded conductor. Courtesy of VISCAS Corporation

The a.c. resistance of a solid conductor is larger than its d.c. resistance due to the skin effect and the proximity effect. The conversion from d.c. resistance to a.c. resistance is made using;

The d.c. resistance in equation above can be calculated from the resistivity of copper and aluminum given in Table above "Resistivity of core conductors". The temperature correction is not normally performed since the temperature of the core conductor can be lower than 20 ∘C for the cable energization. However, it is sometimes necessary to consider the temperature correction when matching simulation results with field measurements, for example for a forensic analysis. It is becoming increasingly popular to choose an aluminum conductor due to recent copper price increases. Since aluminum has higher resistance than copper, a larger cross-section is required in order to provide the same transmission capacity.

Because of the skin effect, the density of an a.c. current is not uniform throughout the cross-section. In fact, the density of an a.c. current is higher at the outer surface of the solid conductor, which reduces the effective area of the cross-section. The formula for calculating the skin effect factor is given in IEC 60287-1-1. Most EMT-type programs can calculate the skin effect factor, and it is common practice to consider the skin effect when building a cable model for EMT studies.

The proximity effect occurs between conductors of the other phases. Because of the proximity effect, the density of an a.c. current becomes higher at the perimeter close to conductors of the other phases. This also reduces the effective area of the cross-section, but its effect is much smaller than that of the skin effect. The formula for calculating the proximity effect factor is given in IEC 60287-1-1.Many EMT-type programs cannot calculate the proximity effect factor, and it is common practice not to consider the proximity effect between conductors when building a cable model for EMT studies.

Both the skin effect and the proximity effect are more significant for a cable with a larger cross-section. In order to reduce these effects, a segmental conductor as in Figure "Segmental conductor. Courtesy of VISCAS Corporation" is normally adopted for a cable with a large cross-section. The segmental conductor is also called the Milliken conductor. In the segmental conductor, each segment is insulated from another by a semiconducting tape or sometimes by an insulating tape. Recently, an enameled conductor has also been considered in order to virtually eliminate the skin effect and the proximity effect.

Segmental conductor. Courtesy of VISCAS Corporation

When EMT-type programs calculate the skin effect factor, they cannot consider a segmental conductor and overestimate the skin effect factor. As a result, they produce higher a.c. resistance than the actual. The same is true for an enameled conductor, which is introduced for the same reason as the segmental conductor.When modeling these types of conductors, it becomes necessary to calculate the skin effect factor outside EMT-type programs depending on the type of study being performed.

Inner Semiconducting Layer (Conductor Screen)

The inner semiconducting layer is applied around the core conductor in order to equalize the electric field strength at the outer surface of the core conductor. This layer is also referred to as the conductor screen. Since the resistivity of the semiconducting layer is much larger than the core conductor, the current does not flow through the semiconducting layer. Therefore, it can be considered as a part of the insulation layer for calculating the series inductance of the cable.

In contrast, for calculating the capacitance of the cable, the inner semiconducting layer should be considered as a part of the core conductor. This is because the potential of the inner semiconducting layer becomes equal to the core conductor.

Insulation Layer

XLPE cables use cross-linked PE as the insulating material. In the planning stage, the relative permittivity of XLPE is normally set between 2.3 and 2.5 depending on the utility’s practice. In the implementation study stage it is given by a cable manufacturer and after the installation it can be calculated from field measurements.

Outer Semiconducting Layer (Insulation Screen)

The outer semiconducting layer is applied between the insulation layer and the metallic sheath in order to equalize the electric field strength at the inner surface of the metallic sheath. This layer is also referred to as the insulation screen. Similarly to the inner semiconducting layer, the outer semiconducting layer should be treated as a part of the insulation layer for the inductance calculation and should be treated as a part of the insulation layer for the capacitance calculation.

When building a cable model in EMT-type programs, it is common practice to set the radius of each layer for the accurate calculation of the cable inductance. That is, the inner and outer semiconducting layers are treated as a part of the insulation layer. This treatment of the semiconducting layers makes the cable capacitance smaller than the actual value. In order to compensate for the error in the cable capacitance, the relative permittivity of the main insulation needs to be converted as:

Metallic Sheath (Metallic Screen)

There are several types of metallic sheath. The most common types are the lead (alloy) sheath, aluminum tape sheath, and copper tape sheath. Figure "Single-core XLPE cable. Courtesy of VISCAS Corporation" above shows the corrugated aluminum sheath, but it is becoming more popular to choose the plain aluminum tape sheath. The lead sheath is manufactured by applying extruded lead alloy on the outer semiconducting layer. The aluminum tape sheath and the copper tape sheath are welded longitudinally to guarantee watertight construction in a radial direction. When water sealing in a longitudinal direction is necessary, the water swelling tape is placed inside these types of metallic sheath.

The lead sheath has a larger resistance than the aluminum tape sheath and the copper tape sheath. In addition, it is expensive, toxic, and heavy. In spite of these unfavorable characteristics, the lead sheath is still adopted due to its high resistance to corrosion especially when a cable is installed in a moist environment.

The copper wire sheath is often formed inside the lead sheath or the aluminum tape sheath in order to carry the required fault current and also to improve the conductivity of the metallic sheath. The copper wire sheath is wound to form a stranded conductor as shown in Figure "Copper wire sheath. Courtesy of VISCAS Corporation". When building a cable model for EMT studies, it is modeled as a solid conductor. The resistivity of copper is modified so that themodeled sheath has identical resistance to the actual copper wire sheath. The resistivity values of lead, copper, and aluminum are shown in Table "Resistivity of metallic sheaths" below.

Copper wire sheath. Courtesy of VISCAS Corporation

Outer Cover

The outer cover is normally made of PE or polyvinyl chloride (PVC). PVC has better performance than PE in terms of fire resistance. PVC is non-flammable whereas PE is flammable. However, since PVC releases toxic hydrogen chloride gas when burned, PE is the more preferred choice and is increasingly used by many utilities. Even though PVC has much larger permittivity than PE, it does not have a noticeable impact on cable system transients.

SCOF Cables

SCOF cables were preferred until recently over XLPE cables because of their reliability. They are less susceptible to a defect that can be introduced during a manufacturing process at a factory or a joint assembly process at a site. In addition, the defect can be found through routine maintenance and can be fixed before causing a cable failure and a subsequent blackout.

Due to the improvement in themanufacturing process ofXLPE cables and the joint assembly process, SCOF cables have lost their popularity to XLPE cables. Currently, it is not common to choose SCOF cables for the construction of a new cable line or even for the replacement of an old SCOF cable line. However, SCOF cables are still in themajority in terms of installed cables. Therefore, cable system transients with SCOF cables will be studied for the safe operation of existing cable lines and for forensic studies.

The physical and electrical characteristics of SCOF cables are very similar to those of XLPE cables. Figure "SCOF cable. Courtesy of VISCAS Corporation" shows the construction of a SCOF cable.

SCOF cable. Courtesy of VISCAS Corporation

There are two main differences between SCOF cables and XLPE cables:

1. Hollow conductor: The hollow conductor is used so that it can serve as an oil duct for insulation. In the setup of the cable model, the inner diameter of the conductor does not become zero for SCOF cables.

2. Insulating material: The impregnated paper is used as the insulating material. The paper wrapped outside the hollow conductor is impregnated with insulating oil. Kraft paper has been used as the insulating material, but in recent years, it is common to choose PPLP (polypropylene laminated paper) because of its better dielectric characteristic. The relative permittivity and tan𝛿 of the impregnated paper are shown in Table "Typical values of relative permittivity and tan𝛿 of the impregnated paper" below. Because of the larger relative permittivity, the charging capacity of SCOF cables is normally larger than that of XLPE cables.

HPOF Cables

HPOF cables have a significantly different structure compared with XLPE and SCOF cables. Normally, cables of three phases are enclosed in one steel pipe as shown in Figure "HPOF cable. Courtesy of VISCAS Corporation".

HPOF cable. Courtesy of VISCAS Corporation

The cable in the steel pipe has only the conductor and the main insulation. The material and the construction of the conductor and the main insulation are basically the same as those of SCOF cables. One difference is that it is not necessary to use a hollow conductor in HPOF cables as the steel pipe serves as an oil duct.

The steel pipe works as the metallic sheath offering a path for the current return in case of cable faults and shielding for the intrusion of humidity. It is shared by three-phase cables enclosed in it. Because of the larger resistivity and diameter of the steel pipe, compared with the aluminum or copper tape, HPOF cables have a higher zero-sequence impedance (not earth-return, but pipe-return propagation mode), both in resistance and reactance, compared with XLPE or SCOF cables. It is typical for HPOF cables that the zero-sequence impedance is larger than the positive-sequence impedance, which is not the case with XLPE and SCOF cables. The relative permeability of the steel pipe is normally specified as one.

In addition to the proximity effect between conductors of the other phases, the proximity effect occurs between inner conductors and the pipe in the case of HPOF cables. The formula for calculating the impedance and admittance, taking this proximity effect into account, is given in Reference [Ametani, A. (1980) General formulation of impedance and admittance of cables. IEEE Transactions on Apparatus and Systems, 99 (3), 902–10.].Many EMT-type programs can consider the proximity effect between inner conductors and the pipe of HPOF cables.