ELEC5205 Chap.10 Underground Cable Design
Underground Cable Design
Several limits compete for one design
Coaxial cable geometry produces high capacitance, low inductance and low surge impedance compared with an overhead line. Insulation thickness must satisfy working-field, power-frequency test, impulse and maximum radial-field checks, with the largest valid requirement governing.
Charging current increases with length, while dielectric loss and conductor heating impose additional thermal limits. Trench grouping saves civil cost but reduces cable rating through mutual heating.
Start with coaxial field geometry
The conductor and metallic sheath form nearby cylindrical equipotential surfaces.
Electric field is highest at the conductor screen and decreases radially, so average voltage divided by insulation thickness is not the maximum stress. Use phase voltage and the correct inner and outer radii in the logarithmic relation. Semiconductive screens smooth strand geometry and stabilise the boundary.
After calculating thickness, check that the selected standard construction still satisfies maximum conductor-surface field.
Let the largest applicable thickness govern
Working stress, power-frequency withstand, impulse withstand and radial-field limits produce alternative minimum thicknesses for one insulation layer. They are compared, not added.
Each calculation must preserve its own rms or peak voltage, duration, temperature factor and safety margin. The largest valid requirement governs, then manufacturing tolerance, joint design, bending and thermal performance are checked.
A smaller result from another case does not reduce the governing minimum.
Use charging current as a route constraint
Cable capacitance draws reactive current throughout the energised length. Per-phase charging current grows with voltage, angular frequency, capacitance and length, consuming part of the conductor and system reactive-current capability before real load is delivered.
A permitted fraction of current rating can therefore define a length ceiling.
Keep capacitance units per kilometre or per metre consistent, and state that this ceiling addresses charging only; dielectric loss, voltage control and thermal rating can be tighter.
Join electrical and thermal design
Dielectric loss grows with voltage squared, frequency, capacitance and loss angle, while conductor loss grows with current squared.
Soil thermal resistivity, burial depth, spacing and adjacent circuits decide how that heat escapes. Grouping cables in one trench reduces civil cost but introduces mutual heating and de-rating. Compare alternatives using total de-rated capacity and installed cost, then include emergency thermal duration, bonding, sheath losses, repair access and reliability before recommending a route.
What this chapter covers
- 01
Radial Electric Field
- 02
Insulation Thickness
- 03
Cable Capacitance
- 04
Charging Current
- 05
Dielectric Loss
- 06
Thermal Grouping
Worked example · free
Find the governing cable limit
- 2Compare the working, impulse and AC requirements.
- 3Select the maximum and verify radial field.
- 2State remaining thermal and installation checks.
Key terms
- Charging Current
- Charging Current is the reactive current required to cycle cable capacitance at the operating voltage and frequency.
- Loss Angle
- Loss Angle describes the dielectric phase departure that determines real insulation power loss through tan delta.
- Radial Field
- Radial Field is the nonuniform electric stress that peaks at the inner conductor of a coaxial cable.
Underground Cable Design FAQ
Why can charging current limit cable length?
Cable capacitance draws reactive current even when little real load is delivered. Because current grows with voltage, frequency, capacitance and length, a long AC cable can consume an unacceptable fraction of conductor rating.
How is insulation thickness selected?
Calculate each applicable requirement for working voltage, test voltage, impulse and radial peak field. Choose a practical thickness at or above the largest requirement, then check thermal, mechanical and manufacturing constraints.
Why does grouping cables reduce rating?
Adjacent cables heat the same soil or trench environment and reduce each other’s ability to reject heat. De-rating factors lower available ampacity, so shared civil cost must be balanced against lost transferable current.
Exam move
Practise one coaxial capacitance calculation, one charging-length ceiling and one maximum-of-checks insulation decision. Write phase voltage explicitly and include absolute permittivity. Compare cable and overhead-line L, C and surge impedance before addressing trench economics or overload duration. Maintain a design table that separates working, test, impulse and radial-field minima, then selects rather than sums them.
After rounding thickness upward, recalculate geometry-dependent capacitance and stress. For a route comparison, list charging compensation, dielectric heat, conductor temperature, grouping, emergency duration, bonding and repair access so the cheapest trench is not mistaken for the most capable cable system.
Test the preferred layout under one contingency case, since mutual heating, route access and reactive demand may change the result outside normal operation.
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