Medium Voltage Power Cables Catalogue - AEC Online

Annex D: Formulas
D.1 Resistance
The values of conductor DC resistance given in the previous
tables are based on 20 °C. In case the DC resistance is required
at any other temperature the following formula is used:
Where;
Conductor DC resistance at θ °C
Conductor DC resistance at 20 °C
Operating temperature
Resistance temperature coefficient
• 0.00393 for Copper
• 0.00403 for Aluminum
To get the AC resistance of the conductor at its operating
temperature the following formula is used:
Where
and are the proximity and skin effect factors, respectively,
which depend on the laying and operating frequency of the cable.
D.2 Inductance
Self and mutual inductance are formulated as follows:
Where;
Inductance
Constant depends on the
conductors’ number of wires
Conductor diameter
• 1 x S in case of trefoil formation
• 1.26 x S in case of flat formation
D.3 Capacitance
Where;
Capacitance
Relative permittivity of insulation
• XLPE = 2.5
Diameter over insulation,
excluding screen, if any
Conductor diameter, including
screen, if any
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D.4 Insulation Resistance
Where;
Insulation resistance
Constant depends on the insulation
material
Inner diameter of the insulation
Outer diameter of the insulation
D.5 Charging Current
The charging current is the capacitive current which flows
when an AC voltage is applied to the cables as a result of the
capacitance between the conductor and earth, and for a multi-core
cable in which cores are not screened, between conductors.
The value can be derived from following the equation:
Where
Charging current
Phase voltage
2 π f
Operating frequency
Capacitance to neutral
D.6 Dielectric Losses
The dielectric losses of an AC cable are proportional to the
capacitance, the frequency, the phase voltage and the power
factor. The value can be derived from the following equation:
Where;
Dielectric Losses
2 π f
Operating frequency
Capacitance to neutral
Phase voltage
Dielectric power factor
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