How to Design an Underground Cable Network for Power Distribution

Underground cable networks are widely used for power distribution in urban and industrial areas due to their reliability, safety, and aesthetic advantages over overhead lines. The design of an underground power cable network involves several critical steps, including load assessment, cable selection, laying techniques, and protection considerations. This article provides a comprehensive guide on designing an efficient underground cable system with detailed calculations and examples.

Step 1: Load Assessment and Network Planning

Load Calculation

The first step in designing an underground power distribution network is to determine the total load demand. This involves:

• Identifying the connected load (in kW or MW)
• Determining the demand factor and diversity factor
• Calculating the maximum demand

Example:

Suppose a residential area requires a power supply with the following load distribution:

• 500 houses, each with a connected load of 5 kW
• Demand factor = 0.7
• Diversity factor = 1.5

Maximum demand = (Total connected load) × (Demand factor) / (Diversity factor)

$\text{Maximum demand} = \frac{(500 \times 5) \times 0.7}{1.5} = \frac{1750}{1.5} = 1167 \text{kW}$

The estimated maximum demand is 1167 kW or 1.167 MW.

Step 2: Selection of Cable Type and Size

Choosing the Right Cable

The choice of underground cable depends on factors such as voltage level, load capacity, soil thermal resistivity, and environmental conditions. Common types of underground cables include:

• XLPE (Cross-linked Polyethylene) insulated cables
• PILC (Paper Insulated Lead Covered) cables
• EPR (Ethylene Propylene Rubber) insulated cables

Cable Sizing Calculation

Cable sizing is crucial for ensuring minimal power losses and voltage drops while maintaining operational efficiency.

Parameters for Cable Sizing:

• Load current (I)
• Cable length (L)
• Voltage level (V)
• Permissible voltage drop (ΔV)
• Cable resistance (R) and reactance (X)

Using the formula: $I = \frac{P}{\sqrt{3} \times V \times \cos \theta}$

Where:

• $P$ = Load power (W)
• $V$ = System voltage (V)
• $\cos \theta$ = Power factor

Assuming a 415V, 3-phase system with a power factor of 0.9:

$I = \frac{1167 \times 10^3}{\sqrt{3} \times 415 \times 0.9} = 1794 \text{A}$

Using standard cable selection charts, an XLPE-insulated 400 mm² aluminum conductor cable can handle this load.

Step 3: Cable Installation and Laying Methods

Laying Methods

• Direct Buried Cables: Laid directly in trenches and covered with sand, protective tiles, and backfill.
• Ducted Cables: Placed inside PVC or HDPE ducts, suitable for congested urban areas.
• Tunneled Cables: Installed in cable tunnels for high-voltage networks.

Depth of Laying

• Low voltage (LV) cables: 0.5 - 0.75 m
• Medium voltage (MV) cables: 0.9 - 1.2 m
• High voltage (HV) cables: 1.2 - 1.5 m

Step 4: Protection and Safety Considerations

Cable Protection

• Mechanical Protection: Use of tiles, slabs, or warning tapes to prevent accidental damage.
• Electrical Protection: Overcurrent protection using circuit breakers and fuses.
• Earthing and Bonding: Proper grounding to avoid potential rise and faults.

Step 5: Voltage Drop and Short Circuit Calculations

Voltage Drop Calculation

Voltage drop (ΔV) is given by: $\Delta V = \sqrt{3} \times I \times (R \cos \theta + X \sin \theta) \times L$

Where:

• $I$ = Load current (A)
• $R$ = Resistance of cable (Ω/km)
• $X$ = Reactance of cable (Ω/km)
• $L$ = Cable length (km)

For a 2 km long cable with $R = 0.089 Ω/km$ and $X = 0.08 Ω/km$:

$\Delta V = \sqrt{3} \times 1794 \times (0.089 \times 0.9 + 0.08 \times 0.435) \times 2$ $\Delta V = 72.4 V$ 

This is within the permissible voltage drop of 5% (20.75V for a 415V system).

Short Circuit Calculation

Short-circuit current ($I_s$) is given by: $I_s = \frac{V}{Z}$ where $Z$ is the impedance of the cable.

FAQs

1. Why are underground cables more expensive than overhead lines?

Underground cables require specialized materials, installation techniques, and additional protective measures, increasing the cost.

2. How do you prevent overheating in underground cables?

Proper cable sizing, adequate spacing, and thermal backfill materials help in dissipating heat.

3. What are the advantages of using XLPE cables?

XLPE cables offer higher current carrying capacity, better thermal resistance, and improved mechanical properties.

Conclusion

Designing an underground cable network requires careful planning, precise calculations, and adherence to safety standards. By following the outlined steps and calculations, an efficient and reliable underground power distribution system can be implemented.