Podcast
Questions and Answers
What is the maximum voltage rating of belted cables?
What is the maximum voltage rating of belted cables?
- 22KV
- 33KV
- 11KV (correct)
- 44KV
Which method involves digging a trench for cable installation?
Which method involves digging a trench for cable installation?
- Conduit laying
- Draw in System
- Direct laying (correct)
- Surface laying
What material is commonly used to protect the underground cable in the direct laying method?
What material is commonly used to protect the underground cable in the direct laying method?
- Concrete blocks
- Metal sheets
- Bricks and other materials (correct)
- Gravel
What is the primary function of providing spacing between multiple cables in a trench?
What is the primary function of providing spacing between multiple cables in a trench?
What voltage rating does a screened cable support?
What voltage rating does a screened cable support?
Which method is preferred for short length cable routes such as road crossings?
Which method is preferred for short length cable routes such as road crossings?
What depth is typically required when digging a trench for direct laying of cables?
What depth is typically required when digging a trench for direct laying of cables?
Which of the following is NOT a component typically found in the Draw in System?
Which of the following is NOT a component typically found in the Draw in System?
Which material is typically NOT used for insulation in underground cables?
Which material is typically NOT used for insulation in underground cables?
What is the primary purpose of a metallic sheath in an underground cable?
What is the primary purpose of a metallic sheath in an underground cable?
Which layer is applied to protect the metallic sheath from corrosion and mechanical damage?
Which layer is applied to protect the metallic sheath from corrosion and mechanical damage?
What classification of underground cable has a maximum voltage capacity of 11KV?
What classification of underground cable has a maximum voltage capacity of 11KV?
Which of the following cable types is associated with a voltage rating beyond 132KV?
Which of the following cable types is associated with a voltage rating beyond 132KV?
What material is used for armouring a cable?
What material is used for armouring a cable?
What is the main function of serving in underground cables?
What is the main function of serving in underground cables?
Which type of cable is classified as having a maximum capacity of 1000 V?
Which type of cable is classified as having a maximum capacity of 1000 V?
What is the primary advantage of using the solid system for cable laying?
What is the primary advantage of using the solid system for cable laying?
Why do underground cables generally require larger conductor sizes compared to overhead lines?
Why do underground cables generally require larger conductor sizes compared to overhead lines?
Which factor contributes to the higher construction complexity of underground cables?
Which factor contributes to the higher construction complexity of underground cables?
What is a significant disadvantage of underground cables in terms of fault management?
What is a significant disadvantage of underground cables in terms of fault management?
Which voltage range are underground cables primarily used for, due to their construction limitations?
Which voltage range are underground cables primarily used for, due to their construction limitations?
What is a notable difference regarding the cooling capability of overhead lines in comparison to underground cables?
What is a notable difference regarding the cooling capability of overhead lines in comparison to underground cables?
In terms of costs, how do underground cables compare to overhead lines?
In terms of costs, how do underground cables compare to overhead lines?
What is a common misconception about the mechanical strength of underground cables versus overhead cables?
What is a common misconception about the mechanical strength of underground cables versus overhead cables?
What does the permittivity ε of the insulation material represent?
What does the permittivity ε of the insulation material represent?
How is dielectric stress defined in the context of a single-core cable?
How is dielectric stress defined in the context of a single-core cable?
What is the relationship between electric intensity Ex and potential gradient g in a single-core cable?
What is the relationship between electric intensity Ex and potential gradient g in a single-core cable?
What does the inner sheath diameter D represent in a single-core cable?
What does the inner sheath diameter D represent in a single-core cable?
What happens to the dielectric stress as the distance x from the center of the cable increases?
What happens to the dielectric stress as the distance x from the center of the cable increases?
What is the role of the lead sheath in a single-core cable?
What is the role of the lead sheath in a single-core cable?
How is the surface area of a cylinder calculated in the context of a single-core cable?
How is the surface area of a cylinder calculated in the context of a single-core cable?
What does the charge per meter Q depend on in a single-core cable?
What does the charge per meter Q depend on in a single-core cable?
What is one of the main reasons underground cables are utilized instead of overhead lines?
What is one of the main reasons underground cables are utilized instead of overhead lines?
Which component of an underground cable is primarily responsible for protecting against moisture?
Which component of an underground cable is primarily responsible for protecting against moisture?
What type of material can the conductor in an underground cable be made from?
What type of material can the conductor in an underground cable be made from?
How many cores can an underground cable have at most?
How many cores can an underground cable have at most?
What effect does the wire screen in an underground cable have?
What effect does the wire screen in an underground cable have?
What determines the thickness of insulation on each core of an underground cable?
What determines the thickness of insulation on each core of an underground cable?
Which of the following is NOT a function of the cable's insulation in an underground cable system?
Which of the following is NOT a function of the cable's insulation in an underground cable system?
In what situation will underground cables typically be employed?
In what situation will underground cables typically be employed?
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Study Notes
Underground Power Cables
- Underground cables are used for reliable power transmission when overhead lines are impractical, such as near plants and substations.
- Installation costs for underground cables are higher than overhead lines, but they provide better protection from environmental factors.
Construction of Underground Cables
- Cores/Conductors: Can be made of aluminum or copper and can have multiple configurations (one to four cores).
- Cable Sheath: Protects against moisture.
- Wire Screen: Controls electric field and discharges fault currents.
- Insulating Layer: Insulates electric conductors; common materials include impregnated paper, varnished cambric, or rubber.
- Metallic Sheath: Made from lead or aluminum to protect from moisture and damaging liquids.
- Bedding: A fibrous layer protecting the metallic sheath from corrosion and mechanical damage.
- Armouring: Consists of galvanized steel wire or tape to prevent mechanical injury.
- Serving: A protective fibrous layer similar to bedding, guards armouring from atmospheric conditions.
Classification of Underground Cables
-
Voltage Capacity:
- LT Cables: Up to 1000 V (Low Tension)
- HT Cables: Up to 11 kV (High Tension)
- ST Cables: 22 kV to 33 kV (Super Tension)
- EHT Cables: 33 kV to 66 kV (Extra High Tension)
- Extra Super Voltage Cables: Greater than 132 kV
-
Construction Type:
- Belted Cables: Up to 11 kV
- Screened Cables: Up to 66 kV
- Pressure Cables: Over 66 kV
Methods of Laying Underground Cables
-
Direct Laying:
- Involves digging a trench (1.5 m deep, 45 cm wide) and using sand layers for protection.
- Requires inter-axial spacing of at least 30 cm for multiple cables to mitigate mutual heating effects.
-
Draw-in System:
- Utilizes ducts with manholes for cable pulling, ideal for short lengths like in workshops and road crossings.
-
Solid System:
- Involves laying cables in open pipes or troughs, providing good mechanical protection but at a higher cost than direct laying.
Comparison: Underground Cables vs. Overhead Lines
- Construction Complexity: Underground cables are more complex and costly to construct than simpler overhead lines.
- Conductor Size: Underground cables require larger conductor sizes due to limited heat dissipation compared to overhead lines.
- Voltage Capacity: Overhead lines support higher voltage capacities; underground cables are typically limited to 33 kV.
- Fault Detection: Overhead cables offer easier fault detection and repair than underground cables, which require more time and effort.
Dielectric Stress in Cables
- Dielectric stress refers to electrostatic forces acting on cable insulation under normal operating conditions.
- Electric intensity and potential gradient are equivalent at any point in the cable, affecting the insulation's performance.
- Variations in dielectric stress can impact the operational life and reliability of underground cables.
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