Formula Used
The electrostatic capacitance ($C$) of a single-core coaxial cable or a concentric cable system is determined using core geometry, insulation permittivity, and total cable length. The governing equation is:
- $C$ = Total capacitance (Farads)
- $\epsilon_0$ = Permittivity of free space ($8.854 \times 10^{-12} \text{ F/m}$)
- $\epsilon_r$ = Relative permittivity (dielectric constant) of the insulation material
- $L$ = Length of the cable
- $D$ = Inner diameter of the outer sheath or screen
- $d$ = Outer diameter of the inner conductor
Additional metrics like capacitive reactance ($X_c$), charging current ($I_c$), and generated reactive power ($Q_c$) are derived from standard AC power network formulas.
How to Use This Calculator
- Enter Conductor Dimensions: Input the inner conductor diameter ($d$) and outer insulation sheath diameter ($D$) using consistent units.
- Define Material Properties: Specify the relative permittivity ($\epsilon_r$) corresponding to your insulation type (e.g., XLPE, PVC, or EPR).
- Set Length and Network Metrics: Provide the total physical run length, pick the correct unit scale, input system frequency, and line voltage.
- Compute Results: Press the Calculate Capacitance button to view comprehensive electrical parameters instantly above the control form.
Understanding Electrical Cable Capacitance in Power Networks
Capacitance is an inherent electrical property present in all insulated cables. When two conductors are separated by an insulating dielectric medium, they form a capacitor capable of storing electrical energy. In alternating current (AC) power transmission and distribution networks, cable capacitance plays a significant role in determining overall system performance, voltage regulation, and protection coordination.
Unlike overhead transmission lines where conductors are separated by air, underground and submarine power cables place conductors in close proximity to metallic shields or earth paths with thick solid dielectrics like cross-linked polyethylene (XLPE) or paper insulation. Because of this compact configuration, the capacitance values in underground distribution systems are significantly higher. This high capacitive characteristic results in continuous charging currents even when the circuit operates under no-load conditions.
Significance of Charging Current and Reactance
As operating voltages increase, particularly in high-voltage (HV) and extra-high-voltage (EHV) underground cables, the charging current becomes a major operational factor. Power systems engineers must calculate capacitance accurately to size circuit breakers, configure protective relay settings, and design reactive compensation equipment such as shunt reactors. Neglecting cable capacitance can lead to severe voltage rise phenomena, especially during light-load periods, matching the classic Ferranti effect seen in long transmission lines.