6. Distribution Lines and LT Substation notes in english

 

6. Distribution Lines and LT Substation

Distribution lines are the essential components in the electrical power system that carry electricity from substations to homes, offices, industries, and other establishments. These distribution lines can be overhead or underground, and there are several aspects to consider when designing, estimating, and maintaining these lines. Let’s go into detail about each topic related to distribution lines and substations.

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6.1 Introduction to Overhead and Underground Distribution Lines

Overhead Distribution Lines:

• Definition: These are the most common type of distribution lines that are installed on poles or towers above the ground.
• Structure: They consist of conductors (wires), insulators (to hold the wires in place), crossarms (to support the insulators), and poles or towers (to hold everything up).
• Advantages:
  • Easier and cheaper to install.
  • Maintenance and repair are simpler since the lines are visible.
• Disadvantages:
  • Vulnerable to weather conditions like storms, high winds, and lightning.
  • Aesthetic concerns, as they can be seen above the ground.

Underground Distribution Lines:

• Definition: These are distribution lines that are buried beneath the ground to avoid exposure to weather and environmental factors.
• Structure: They consist of insulated cables that are buried in ducts or trenches underground.
• Advantages:
  • Less vulnerable to weather, offering greater reliability.
  • Better aesthetic appeal as no poles or wires are visible.
• Disadvantages:
  • Higher installation costs due to excavation and specialized equipment.
  • Difficult to maintain and repair because the lines are underground.

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6.2 Materials Used for Distribution Line HT and LV

The materials used in distribution lines (both High Tension (HT) and Low Tension (LT)) are crucial for ensuring the safe, reliable, and efficient transmission of electricity. The materials typically used include:

1. Conductors:

   • Aluminum Conductors: Most commonly used in both HT and LT lines due to their low cost and good conductivity. Aluminum is also lighter than copper, making it easier to handle.
   • Copper Conductors: Used less frequently, but they are more conductive and more durable than aluminum. Copper is more expensive and heavier, making it less ideal for long-distance transmission.

2. Insulators:

   • Porcelain Insulators: Commonly used in overhead lines. They are durable and provide excellent insulation.
   • Polymer Insulators: Made of synthetic materials and used in areas with high pollution or extreme weather conditions. They are lighter and more durable than porcelain insulators.

3. Poles:

   • Wooden Poles: Often used in rural areas due to their low cost.
   • Concrete Poles: More durable than wooden poles and commonly used for HT lines.
   • Steel Poles: Strong and durable, ideal for areas with high wind speeds or for long spans of wire.

4. Cables:

   • PVC or XLPE Insulated Cables: These are used for underground distribution lines. PVC cables are cheaper, while XLPE cables are more heat resistant.
   • Aerial Bundled Cables (ABC): These cables are used in overhead distribution and are insulated, reducing the risk of short circuits.

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6.3 Cables Used for Distribution Lines

Cables for Distribution Lines are the primary medium for carrying electricity, whether overhead or underground. They vary based on the type of line (HT or LT) and the specific requirements of the installation. Common types of cables used for distribution lines are:

1. Aerial Bundled Cables (ABC):

   • These are insulated cables used for overhead distribution lines. They consist of multiple conductors bundled together and are safer as they reduce the risk of accidents like short circuits and electrical shocks.

2. XLPE (Cross-Linked Polyethylene) Cables:

   • These cables are commonly used in underground distribution lines because they have better thermal resistance, greater mechanical strength, and enhanced insulation properties.

3. PVC Insulated Cables:

   • These cables are used in less critical installations. They are cheaper than XLPE cables but have lower temperature tolerance and are more prone to damage.

4. EPR Cables (Ethylene Propylene Rubber):

   • These are used for medium to high voltage applications and are ideal for distribution lines that may be exposed to chemicals, oil, or extreme temperatures.

5. Underground Cables:

   • These are specially designed cables that are buried underground for long-distance power transmission and distribution. They are often made of multiple layers of materials, including insulation, to ensure reliability and safety.

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6.4 Cable Laying and Cable Termination Methods According to IS (Indian Standards)

Cable Laying Methods:

• Trench Method:
  • The most common method for underground cables. It involves digging a trench and laying the cable at a specified depth (usually 0.6 meters or more, depending on the voltage level).
  • The cable is then covered with soil, and protective layers like sand and warning tapes may be added.
• Duct Method:
  • This method involves laying the cable inside ducts (pipes). Ducts are placed in trenches or under roads, and cables can easily be replaced or repaired by pulling them out through the ducts.
• Overhead Method:
  • For overhead cables, the conductors are mounted on poles using insulators. The cables should be placed with adequate spacing to avoid contact with trees or buildings.

Cable Termination Methods:

• Straight-through Joint: Used when two cables are to be connected directly in a line.
• End Termination: Used when a cable ends and needs to be connected to a device or equipment. The cable is stripped and attached to a termination kit to ensure proper insulation and safety.
• Ring Main Unit (RMU): A type of switchgear used for cable terminations in HT distribution lines. It ensures that electricity can still be supplied if there’s a fault by isolating faulty sections.

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6.5 Design, Estimation, and Costing of HT and LT Overhead and Underground Cabling

Design of HT and LT Cabling:

• HT Lines: High Tension lines generally carry voltages above 33kV. The design includes ensuring that the conductors can handle the high voltage, ensuring the correct spacing between poles, and selecting appropriate insulators to prevent breakdown due to electrical stress.
• LT Lines: Low Tension lines carry voltages up to 415V. These lines typically serve residential and commercial establishments. The design for LT lines ensures that cables are appropriately insulated, and poles are spaced to provide adequate support.

Estimation of HT and LT Cabling:

• Estimation involves calculating the total length of the cable, the number of poles, insulators, transformers, and other required materials.
• The cost is then estimated based on the materials' prices, labor costs for installation, and any additional accessories like cable joints or termination kits.

Costing:

• For HT Lines:

  • Higher cost due to the use of specialized materials like steel poles, thick conductors, insulators, and transformers.
  • Also, land and clearance costs for overhead lines need to be accounted for.

• For LT Lines:

  • Relatively lower cost since the voltage and materials used are less expensive. However, if underground cabling is involved, the cost can rise due to digging and trenching.

• Underground Cabling generally costs more due to the excavation, ducting, and the higher price of the cables (like XLPE).

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6.6 Types of 11kV Distribution Substations and Their Line Diagrams

An 11kV Distribution Substation is a vital part of the power distribution network. It receives high-voltage electrical power from transmission lines and steps down the voltage to 11kV for local distribution. Here’s an overview:

Types of 11kV Substations:

1. Outdoor Substation:

   • Typically used for areas with more space. Equipment like transformers, breakers, and switches are installed in the open air.
   • Line Diagram: It consists of a main power source (feeder), a transformer (step-down transformer from 33kV to 11kV), and distribution panels for different feeders.

2. Indoor Substation:

   • Commonly used in urban areas with limited space. All equipment is enclosed in a building for protection.
   • Line Diagram: Similar to outdoor substations but with additional protection measures, like fire suppression systems, since it’s indoors.

3. Compact Substation:

   • Used when space is a constraint. The entire substation is housed in a compact, weatherproof container or kiosk.
   • Line Diagram: It involves a compact transformer, circuit breakers, and other control equipment housed in a single unit.

11kV Distribution Line Diagram: A basic line diagram for an 11kV distribution substation will include:

• Feeder Line: The main line coming from the transmission network.
• Transformer: Steps down the voltage from 33kV or higher to 11kV.
• Bus Bar: Used for distributing electricity to different feeders.
• Circuit Breakers and Switchgear: For protection against faults.
• Outgoing Feeders: Lines that distribute power to the local areas.