Cable Splicing and Terminating Theory

During the installation of medium-voltage cable circuits, connections might be necessary to create long lengths. These connections are referred to as joints or splices. Each circuit must have at least two ends known as terminations. Collectively, joints, splices, and terminations are often referred to as accessories in the literature.

As described in Section 4, Cable Designs, there are both shielded and nonshielded medium-voltage cables. This section concentrates on splicing and terminating cables with insulation shielding (commonly referred to only as shielding or shielded) because they are the dominant medium-voltage cables in plants and are closely related in most aspects. Nonshielded cables are connected the same way as low-voltage cables and the same way as shielded cables, with the exception of the two components of the insulation shielding system—the semiconducting tape and the metallic shield component.

The splicing of two pieces of cable can best be visualized as two terminations that are connected together. The most important deviation, from a theoretical view, between splices and

terminations is that splices are more nearly extensions of the cable. The splice simply replaces with field components all the various components that were made into a cable at the factory.

Instead of two lugs being attached at the center of the splice, a connector is used. At each end of the splice where the cable shielding component has been stopped, electrical stress relief is required, just as it is when terminating.

5.2 Gradients

5.2.1 Electric Fields

An electric field in a cable can be visualized with the use of equipotential and flux lines. The equipotential lines represent surfaces of constant potential difference between the two electrodes.

The flux lines define the boundaries of dielectric flux between two electrodes. For a shielded, medium-voltage cable, these lines are illustrated in Figure 5-1 [13].

Splicing and Terminating

Figure 5-1

Equipotential and Flux Lines in a Cable [13]

When the cable is cut so that the shield ends abruptly, the electrical stresses change from being in the semiconducting material to being in the air, as shown in Figure 5-2.

Figure 5-2

Electrical Stress Fields, Shield Removed

To reduce the electrical stress at the end of the cable, the insulation shield is removed for a sufficient distance to provide adequate leakage or creepage distance between the conductor and the shield. The distance depends on the voltage involved as well as the anticipated environmental conditions. The removal of the shield disrupts the coaxial electrode structure of the cable. In most cases, the resulting stresses are high enough that they cause dielectric degradation of the materials at the edge of the shield, unless steps are taken to reduce that stress. The concentration of electric stress is now located at the conductor and end of the insulation shield. The stress lines are the horizontal lines that curve upward at the end of the shield (identified by the arrow in Figure 5-2), and the flux lines are the curved lines that are at right angles to the stress lines. The stress lines are more closely spaced near the conductor, and the flux lines more closely spaced at the end of the shield. These forces are strong enough to actually decompose the factory

insulation at that interface and ultimately cause the cable insulation to fail.

The stress at the insulation shield remains great because the electrical stress lines converge at the end of the shield. The equipotential lines are closely spaced at the shield edge. If those stresses are not reduced, PD can occur. Electrical stress relief is required in most medium-voltage applications.

Splicing and Terminating

5.2.2 Stress Cones

To produce a termination of acceptable quality for long life, it is necessary to relieve voltage stresses at the edge of the cable insulation shield. The traditional method of doing this was to use a stress cone to control the capacitance in the area of high electrical stress (see Figure 5-3) [14].

Another method of stress control most often used for splicing and terminating in plants is the high-K material described in Section 5.2.3, Voltage Gradient Design.

Figure 5-3

Termination of an Insulation Shield with a Stress Cone [14]

A stress cone increases the spacing from the conductor to the end of the shield, as shown in Figure 5-3. This spreads out the electrical lines of stress as well as providing additional

insulation at this high stress area. The ground plane gradually moves away from the conductor and spreads the dielectric field, thus reducing the voltage stress per unit length. The stress relief cone is an extension of the cable insulation. Another way of saying this is that the electrostatic flux lines are not concentrated at the shield edge as they are in Figure 5-2; it follows that the equipotential lines are also spaced farther apart. Stress cones can be taped by hand or premolded.

Terminations that are taped achieve this increase in spacing by creating a lapped conical configuration of tape followed by a semiconducting layer that is connected electrically to the insulation shield, as shown in Figure 5-3. Premolded stress cones use the same concepts in the construction.

The classic approach to the design of a stress relief cone is to have the initial angle of the cone be a few degrees and take a logarithmic curve throughout its length. This provides the ideal

solution, but it was not usually needed for the generous dimensions used in medium-voltage cables. There is such a little difference between a straight slope and a logarithmic curve for medium-voltage cables that, for hand build-ups, a straight slope is acceptable. Premolded designs usually maintain that logarithmic shape.

In actual design, the departure angle is in the range of 3° to 7°. The diameter of the cone at its greatest dimension has generally been calculated by adding another insulation thickness to the diameter of the insulated cable at the edge of the shield; therefore, at the maximum diameter of the stress cone, the insulation thickness is twice that of the cable’s insulation. A major

disadvantage of such stress cones is that they require much more space between cables than the voltage gradient types that are described in Section 5.2.3, Voltage Gradient Design.

Splicing and Terminating

5.2.3 Voltage Gradient Design

Electrical stress relief can come in different forms. A high-permittivity material (high dielectric constant or high K) can be applied over the cable end, as shown in Figure 5-4 [15]. When materials with different permittivities are subjected to a voltage gradient across their combined thickness, the material with the lower permittivity is subjected to the highest stress. The high K material over the shield maintains the radial voltage gradient in the insulation. The equipotential lines emerge only gradually from the insulation, thus producing a stress gradient, as shown in Figure 5-4. This material can be represented as a long resistor connected electrically to the insulation shield of the cable. By having this long resistor in cylindrical form extending past the shield system of the cable, the electrical stress is distributed along the length of the tube. Stress relief is thus accomplished by using a material with a controlled resistance or capacitance. These are available in cold-shrink, heat-shrink, and hand-taped designs. Other techniques can be used, but the basic concept is to use a material with a high resistance, high dielectric constant, or nonlinear current and voltage characteristics to extend the lines of stress away from the edge of the cable shield.

Figure 5-4

Stress Relief with High Dielectric Constant or High Resistivity Materials

Capacitive-graded materials usually contain particles of silicon carbide or oxides of aluminum, zinc, or iron. Although they are not truly conductive, they become electronic semiconductors and have identical stress relief to that of a stress cone. They do not have a linear E = IR relationship, but rather produce a voltage gradient along their length.

One of their useful features is that the diameter is not increased to that of a stress cone. This makes them valuable for use in confined spaces. This voltage gradient does not depend on the IR drop but on an exchange of electrons from particle to particle.

Resistive-graded materials contain carbon black, but in proportions that are less than the semiconducting materials used for extruded shields for cable. They also provide a nonlinear voltage gradient along their length. With proper selection of materials and proper compounding, these products can produce almost identical stress relief to that of a stress cone.

A termination such as the one in Figure 5-4 obviously will fit in a smaller space than the stress cone design shown in Figure 5-3.

Splicing and Terminating