High Voltage Cable Layers

Question 1

List the 6 components of a typical dielectric cable

Answer 1

1. Conductor
2. Strand Shielding
3. Insulation
4. Insulation Shield
5. Metallic Shield
6. PVC Jacket

Question 2

What are 6 of the most common types of medium-voltage cables in general use today

Answer 2

1. Teck cable
2. Shielded power cable
3. Concentric neutral cable (URD)
4. Paper-insulated, lead-covered cable (PILC)
5. Submarine cable
6. Mining cable

Question 3

What does dielectric strength of the material refer to?

Answer 3

The ability of the dielectric to withstand electrical breakdown under the influence of voltage. Dielectric strength is stated in terms of voltage per unit thickness.

Question 4

What are two things that will affect the dielectric strength of a cable

Answer 4

Heat and Moisture.

Dielectric strength will decrease if a dielectric becomes heated. Temperature rise is due to heat generated by the conductor as well as the ambient temperature. Moisture content will dramatically lower dielectric strength.

Question 5

What are the two common kinds of plastic insulation that you will find in high voltage cables?

Answer 5

Thermoset and Thermoplastic

Question 6

What are 5 common types of insulation that you will find in high voltage cables?

Answer 6

1. XLPE is a thermoset plastic
2. PVC (polyvinyl chloride) is a thermoplastic material
3. PE (polyethylene) is a thermoplastic
4. Oil-impregnated paper is used for insulation in PILC (Paper-insulated, lead-covered) cables
5. Synthetic Rubber such as silicone rubber, butyl rubber, and ethylene-propylene rubbers

Question 7

What are the two principal types of shielding for HV cables?

Answer 7

1. Conductor shield (Strand Shield) a thin conducting or semi-conducting layer of a metallic or carbon-impregnated plastic
2. Insulation shield is commonly called the semi-con, meaning semi-conductor

Question 8

What must an insulation shield have if the cable carries a charge because of its proximity to the conductor?

Answer 8

Sheath grounding. If the cable is grounded at both ends, a complete circuit is formed, resulting in a sheath current. If the current flow in the shielding is large enough, it can cause sufficient rise in the cable insulation temperature for the conductor ampacity to require derating.

Question 9

Most HV cables have insulation levels of 100% or 133%. When would you use each?

Answer 9

100% insulation Permissible in solidly grounded systems, or in ungrounded systems where ground faults are eliminated in no more than one minute.

133% insulation For use in ungrounded systems, where ground faults are eliminated in no more than one hour.

Question 10

What purpose does cable armour serve?

Answer 10

Mechanical protection

Question 11

List four different forms cable armour can take

Answer 11

1. Interlocking Armour is used on Teck cables and is made of spirally interlocked steel or aluminum
2. Corrugated Metal Sheath is less flexible than interlocked armour and is a longitudinally corrugated metal tube of aluminum or steel
3. Wire Armour is a spirally-laid steel armour often used on submarine cables
4. Tape Armour is flat metal tape(usually steel) helically wound over a layer of jute that covers the insulation