Laying Configurations of Cables Spread Throughout Areas

Burial Condition
Cables are, in most cases, buried in the following conditions:

• Directly buried
• Buried in a pipe or a duct
• Buried in a trough
• Buried in a tunnel

When cables are directly buried, they are laid in a flat formation or a trefoil formation as shown in Figure "Flat formation (a) and trefoil formation (b)". The flat formation is often preferred as the required depth of a trench can be shallower compared with the trefoil formation. However, the flat formation requires wider land space, which may prohibit its application when such wide space is not available.

Flat formation (a) and trefoil formation (b)

Typical phase spacing for EHV cables ranges from about 300 to 500 mm. Smaller phase spacing may be selected in order to reduce the cost for the digging work as long as the necessary transmission capacity is available.

In relation to cable system transients, this phase spacing affects the inductance of a cable line. In fact, the inductance of a cable line becomes larger for smaller phase spacing if other conditions are equal.This is apparent from the theoretical equation of the impedance of a cable line. Therefore, larger phase spacing is preferable in terms of both the transmission capacity and the inductance of a cable line.

When a cable is laid in a pipe or a duct, its phase spacing depends on the arrangement of the pipe or the duct. Their arrangement is affected by available land space and the cost for the digging work as in the directly buried cable. Obviously the cable arrangement affects the impedance of the cable line.When surplus pipes or ducts are available for the cable installation, it is recommended to calculate positive-sequence and zero-sequence impedances with different cable arrangements.

When a cable is laid in a trough, its phase spacing tends to be small compared with other laying conditions. Since three-phase cables are laid in a trough, phase spacing is limited by the size of the trough. When the trough is made of concrete, the cable is often modeled as a pipe-type cable, considering the trough as a pipe of the pipe-type cable.

When a cable is laid in a tunnel, some utilities decide to choose small phase spacing in order to save space in a tunnel and install more cables in it. Some cables are laid even next to each other with a cable of other phases like a triplex cable. This can be justified considering the high construction cost of a tunnel, especially around big cities where a tunnel needs to accommodate many cables.

Tunnel installed cables are normally modeled as a pipe-type cable considering the tunnel as a pipe of the pipe-type cable. Reinforced concrete is considered electrically conductive because of the steel and concrete.

Sheath Bonding

When considering the sheath bonding, two important factors are the sheath voltage and the sheath current. If only the former factor is considered, the most favorable sheath bonding is solid bonding. However, it causes higher sheath current, leading to a larger sheath circuit loss. Hence, it is only applied to submarine cables, which do not have joints in a cable line.

In contrast, if only the sheath current is considered, the most favorable sheath bonding is single-point bonding.The sheath current in the normal operating condition becomes zero when single-point bonding is adopted. However, it causes higher sheath voltage, which requires the installation of an earth continuity cable (ECC). In fact, single-point bonding is often adopted together with the cross bonding as discussed later in this section.

Cross bonding is applied considering both the sheath voltage and the sheath current. It can suppress the sheath voltage while limiting the sheath current. It is dominantly applied to a cable line with three or more cable sections.

Solid Bonding

An example of solid bonding is shown in Figure "Solid bonding" even though this configuration is not applied to land cables due to the large sheath current. As the sheath circuit is grounded at every joint (earthing joint, EJ) and termination, the sheath voltage is suppressed to virtually zero at these points. Only the sheath voltage caused by the grounding resistance remains at joints and terminations. The value of the grounding resistance is normally around 5–10 Ω at joints and 0.1–1 Ω at terminations. The grounding resistance at terminations is lower since the grounding circuit can be connected to the substation grounding grid, except for the termination at the transition point between the underground cable (UGC) and the OHL in the mixed UGC/OHL line.

Solid Bonding

The solid bonding is applied to submarine cables, which do not have joints in a cable line. The metallic sheath of a submarine cable is grounded every 2–4km in order to suppress the sheath voltage as shown in Figure "Solid bonding of a submarine cable". As a result, the sheath current of a submarine cable becomes larger than that of land cables. The metallic sheath of a submarine cable often has a large cross-section in order to reduce the sheath circuit loss.

Solid Bonding Of A Submarine Cable

Single-point Bonding

An example of single-point bonding is illustrated in Figure "Single-point bonding (two sections)". Single-point bonding is applied to a short cable line with one or two cable sections. The sheath circuit on the left side is insulated from the sheath circuit on the right side by sheath sectionalizing joints (SSJs). This creates open ends in the sheath circuit.

Single-point bonding (two sections)

The magnitude of the continuous sheath voltage, which is induced by positive-sequence power flow in a core conductor in normal operating conditions, is equivalent to that in the case of cross bonding. The short-term sheath voltage at the sheath open end becomes an issue in the case of single-point bonding. The short-term sheath voltage under the following conditions is studied, and the earth continuity cable (ECC) and sheath voltage limiters (SVLs) are installed as a countermeasure as shown in Figure "Single-point bonding (two sections) with ECC and SVLs":

• Single-line-to-ground (SLG) faults (external to the targeted major section)
• Three-phase faults (external to the targeted major section)
• Switching surges
• Lightning surges (only for mixed UGC/OHL)

The power-frequency component of the short-term sheath voltage under SLG faults and three-phase faults is calculated using the formulas. In addition, it is sometimes necessary to study the transient component of the short-term sheath voltage,especially in order to evaluate the performance of SVLs. The study is performed using EMT-type programs.

Single-point bonding (two sections) with ECC and SVLs

Cross Bonding

An example of cross bonding is illustrated in Figure "Cross bonding (three sections)". The cross bonding is applied to a cable line with three or more cable sections.

Cross bonding (three sections)

As in single-point bonding, the sheath circuit on the left side is insulated from the sheath circuit on the right side by SSJs. In the cross bonding, however, the sheath circuit on the left side is connected to the sheath circuit of a different phase cable on the right side as shown in Figure "Cross bonding (three sections)". For example, the sheath circuit expressed by the dotted line goes with the phase a cable in the first minor section, with the phase b cable in the second minor section, and with the phase c cable in the third minor section.

Thanks to this connection of the sheath circuit, the cross bonding can suppress the sheath voltage while limiting the sheath current. Assuming the following three conditions, the vector sum of the continuous sheath voltage of three minor sections becomes zero:

• Positive-sequence core current
• Trefoil formation (balanced circuit)
• Equal length of three minor sections

The continuous sheath voltage of three minor sections can be illustrated as in Figure "Continuous sheath voltage in a major section of a cross-bonded cable". This means that the sheath current becomes zero if these conditions are satisfied. However, in an actual installation, these conditions are not completely satisfied, which causes an imbalance in the continuous sheath voltage of the three minor sections. The sheath current flows due to this imbalance, but it is much smaller than that in the solid bonding.

Continuous sheath voltage in a major section of a cross-bonded cable

Even though the continuous sheath voltage is suppressed by the small sheath current and the balanced sheath voltage, the short-term sheath voltage cannot be suppressed by the cross bonding. As a countermeasure for the short-term sheath voltage, SVLs are installed at sheath sectionalizing joints as shown in Figure "Cross bonding (three sections) with ECC and SVLs".

Cross bonding (three sections) with ECC and SVLs

SVLs are normally arranged in a star formation, and the neutral point of SVLs is grounded as shown in Figure "Cross bonding (three sections) with ECC and SVLs" above. The short-term sheath voltage can exceed the ratings of SVLs when the grounding resistance at a SSJ is high. In this case, ECC can be installed connecting the neutral points of SVLs to grounding wires at terminations and EJs. The installation of ECC lowers the effective grounding resistance at the SSJ. Other countermeasures include changing the neutral point from solidly grounded to ungrounded and changing the SVL connection from the star formation to the delta formation.

When the number of minor sections is not a multiple of three, one or two minor sections cannot become a part of the cross-bonding configuration. For example, if the number of minor sections is five, two minor sections cannot become a part of the cross-bonding configuration. In such a situation, single-point bonding is applied to the remaining two minor sections as shown in Figure "Combination of cross bonding and single-point bonding". In the figure, SSJ/EJ and EJ/SSJ means that:

• The sheath circuit of the left side is insulated from the sheath circuit of the right side as in SSJ.
• The sheath circuit of the left side (EJ/SSJ) or the right side (SSJ/EJ) is grounded as in EJ.

Combination of cross bonding and single-point bonding

As the single-point bonding is applied to these minor sections, itis important to confirm that the sheath voltage of these minor sections does not exceed the ratings of SVLs, joints, and the sheath.