Electrical Distribution Networks: Aerial and Underground Systems
1. Definition and Types of Distribution Networks
A distribution network, whether public or private, distributes electric power. It’s defined by the voltage between a phase conductor and earth or between two phase conductors for directly grounded facilities, and by the voltage between two phase conductors for facilities not directly linked to ground.
2. BT Airline Installation Methods
- Installed on facades or walls.
- Installed on supports.
- Stretched over supports.
- Stretched on walls.
3. Common BT Airline Cable
Commonly used cables have 0.6/1 kV rated voltage insulation. Phase conductors are typically aluminum, while the neutral conductor is often an Al-Mg-Si alloy (almelec).
4. Arrow Definition
The straight-line distance between two conductor attachment points.
5. Span Definition
The distance between two supports or columns.
6. Main Brace Efforts
Braces primarily support the conductors and reduce longitudinal stress on the strand.
7. Support Classification by Function
- Alignment Supports: Support conductor weight with minimal tip effort. Placed in straight sections.
- Angle Supports: Support conductors at line angles.
- Anchor Supports: Provide strong points to limit and prevent line destruction.
- Line Supports: Withstand stresses from the line.
- Special Supports: Used where multiple derivations converge (e.g., cross over rail, river).
8. Anchor Support Positioning
Anchor supports can be either alignment or angle supports. They provide firm points on the line, limiting longitudinal stress spread. A line should have an anchor support approximately every 3 km.
9. Support Robustness Comparison
Angle supports that are also anchor supports are the most robust. They must withstand longitudinal stresses, forces from the line, and bending stress.
10. Line Support as Special Support
Yes, a line support can also be a special support, for example, when it carries a transformer.
11. Anchor Support Material
False, anchor supports can be made of any suitable material, not just metal.
12. Line Section Definition
The section of line between two anchor supports.
13. Braided Carrier Cable vs. Neutral Guarantor Cable
Neutral Conductor: Supports the cable assembly at various support points. Aluminum neutral conductors are often made of a special Al-Mg-Si alloy (ALMELEC) for increased strength.
Guarantor Cable: Attached to the network with clips. This steel cable provides mechanical support for the conductors and must have a tensile strength of at least 800 daN.
14. Minimum Cable Height and Mitigation
Minimum cable height is 2.5 meters. At lower heights, protection with a rigid tube or channel is required, along with measures to prevent water accumulation.
15. Distances from Windows and Balconies
Windows: 0.30 m from the top edge and 0.40 m from other sides.
20. Aerial Distribution Line Applications
Used in rural areas, areas with low to medium energy demand, strained networks, and urban installations.
22. REBT Instruction for Airlines
REBT-ITC-6
25. Low Voltage Underground Line Definition
Lines buried directly, in pipes, conduits, or galleries, located below ground.
26. Underground BT Line Installation Systems
- Directly buried
- In pipes
- In galleries
- Reviewable in culverts or channels
- In trays, brackets, or directly attached to walls
28. Underground Low Voltage Cable Characteristics
Cables typically have a black outer sheath. Single-core cables use class 2 aluminum conductors with XLPE insulation. The outer sheath can be PVC (RV-AL Cable) or polyolefin (RZ1-AL Cable). These cables are not fireproof and have a rated voltage of 0.6/1 kV. They are used in direct burial installations.
For industrial installations or specific requirements, multicore cables with copper or aluminum conductors can be used in trays or conduits. These cables have zero-halogen flame-retardant thermoplastic filling and a green thermoplastic polyolefin outer sheath. They are also non-fireproof but are flame retardant with low smoke and corrosive gas emission.
29. Protection Mechanisms for Directly Buried BT Lines
Concrete slabs, plastic shields, bricks, or equivalent mechanical protection.
30. Trenching and Installation Process for Buried BT Lines
- Excavate the trench.
- Place a sand bed, removing any damaging materials.
- Place pipes (160 mm for power cables, 40 mm for multiduct).
- Add another sand layer and warning tape (25 cm above pipes, 25 cm below ground).
- Lay the cables (one circuit per tube, marked with colored tape).
- Make necessary connections.
- Repair pavement.
31. Additional Training for Low Voltage Underground Lines
ITC-BT-07
32. Crossing, Near, and Parallel Conditions for Underground Networks
Must comply with ITC-BT-07 and other applicable regulations.
33. Underground Installation in Culverts
Cables are installed inside protected channels with ground-level caps and manipulated by hand. This system is restricted to industrial settings or enclosures containing electrical wiring with restricted access.
34. Overhead Line Installation
- Determine support locations and types.
- Dig foundations.
- Lift and install supports.
- Lay cables (with or without mechanical tension).
- Ground connection.
35. IT Distribution Scheme
The IT scheme has no point directly connected to ground. Receiving facility masses are grounded directly.
Protection Systems: Mass interconnection and grounding, first fault signaling, second fault current protection.
Operation: First insulation fault detection, fault location and removal, cutoff on two simultaneous faults.
Key Features: Ensures service continuity, preventative fault detection, suitable for specific MV/BT or BT/BT feeds, requires good network isolation.
36. TT Distribution Scheme
The TT scheme has a feed point (neutral or compensator) directly grounded. Receiving facility masses are connected to a separate ground.
Protection System: Mass grounding and residual current devices.
Operation: Cutoff on first insulation fault.
Advantages: Simple solution, requires only differential testing, differential protection limits leakage current and fire risk.
Disadvantages: Each fault causes a power failure.
37. TN Distribution Scheme
The TN scheme has a directly grounded feed point (neutral or compensator). Receiving facility masses are connected to this point via protective conductors.
Protection System: Mass, neutral, and ground interconnection. Cutoff on first fault with overcurrent protection.
Operation: Cutoff on first insulation fault (phase-to-neutral short circuit).
Key Features: Dedicated CT usage, requires grounding throughout the facility, protection sizing requires calculations.