High Voltage Flashcards
Main hazards associated with working in a high voltage facility
(5)
- inadvertent contact
- step and touch potential
- backfeed, mutual induction and cap charge
- arc flash/blast
- high voltage transients
Reduces the conductor skin effect, especially in higher voltages
Annular
Used to obtain a smaller cables diameter multi-conductor cable
Compact sector
Normally used with smaller conductors (2 AWG and less)
Solid
Used for single conductor cable where high current carrying capacity must be combined with minimum diameter and reduced skin effect
Segmental
Used to obtain a smaller diameter, but less flexible conductor with the same conducting area as the concentric round
Compact round
The first layer contains six strands of the same size, which are helically wrapped around a single centre strand.
Concentric round
Two types of thermoplastic insulation
- PVC (polyvinyl chloride)
- PE (polyethylene)
Name a type of thermoset insulation
XLPE (cross-linked polyethylene)
Name the two (2) types of insulation shielding
- non-metallic
- metallic
Nicking or disturbing the metallic shield in a high-voltage cable can cause high electrical _________ areas.
Stress
Terminations must provide four (4) functions
- Provide stress relief at end of terminated shield
- protect against tracking
- keep moisture out of cable in damp or humid locations
- protect against flashover
A class ___ termination is used in a clean dry location
3
A class ___ termination is track resistant, moisture proof and stress resistant.
1
Flashover and ___________ are always a concern in high-voltage cable splices and terminations.
Tracking
Distance along surface between the insulation shield and the bare conductor at termination is the
Creepage or flashover distance
When air voids are created in the insulation of high-voltage cable, the electrostatic forces cause ___________, which leads to _________ discharge.
Ionization; corona discharge
Most common method of reducing electrical stress points at terminations
Stress cone
Multi conductor cable having a layer of insulation over the assembled insulated conductors.
Belted type cable
Common to central point
Concentric
Results from ionization of air molecules into ozone
Corona
Thermoplastic material is converted to thermoset material
Cross-linking
Excellent insulator and has ability to sustain electrostatic field
Dielectric
Rate at which electric energy is lost to heat in a dielectric
Dielectric loss
The lower the value, better the dielectric material
Highest voltage a dielectric can withstand before breakdown occurs
Dielectric strength
Ability of a substance to return quickly to its original state
Elasticity
Can be stretched at room temp to at least 2x its length and returns to original shortly after
Elastomer
Arching caused by a high-voltage difference between the insulation shield and exposed conductor
Flashover
Semi or conducting tape or extrusion over insulation to create uniform radial voltage stress
Insulation shielding
Effect of high voltage on atmospheric molecules; major factor of insulation failure
Ionization
Rubber, metal or synthetic covering placed over a cable for mechanical protection
Jacket
Thermoset elastomer used as a jacketing material over insulated conductors.
Neoprene
Thermoplastic insulating material
PE (polyethylene)
Thermoset insulating material
XLPE (cross-linked polyethylene)
Electrostatic stress field perpendicular to the conductor or across the conductor
Radial stress
Reduces voltage on a cable to below which is necessary to cause corona
Shielding
Cable termination or spice constructed to reduce the concentration of high voltage stress points
Stress cone
Electrostatic stress field in parallel to the conductor
Tangential stress
Plastic that may discolour, blacken, turn clear, crack but does not soften above normal heat
Thermoset
Plastic that softens above normal heat
Thermoplastic
