High Voltage Flashcards
To ace the High Voltage topic.
Why Do We Use High Voltage?
To deliver bulk power economically. High voltage lines carry low currents.
Benefits of using HV
- reduced line current
- reduced power losses
- reduced weight
Factors affecting the voltage of transmission lines
- load requirement
- distance to load (1000V/mile is rule of thumb)
IEEE High Voltage Classifications
- Medium Voltage (2,400 - 69,000 V)
- High Voltage (115,000 - 230,000 V)
- Extra High Voltage (345,000 - 765,000 V)
Low Voltage characteristics
- For the same load, requires more amps and larger wires
- Length and size of conductors is determined by the voltage drop permissible.
- Electrocution requires direct contact.
High Voltage
- For the same load, requires less amps and smaller wires.
- Length of conductors is determined by the load and distance.
- Physical contact is not required for electrocution! HV jumps.
- Fault currents and fault energy are much greater as the magnetic fields create huge physical forces on equipment.
HV isolation
- Elevate
- Enclose
Fields created by HV
- magnetic field due to current flowing
- electrostatic field due to voltage alone
Voltage gradient
The flux concentrations decrease with distance, therefore the stress on the insulation decreases with distance.
Electric Stress
The stress created by the electrostatic field on the conductor insulation
Ionization
Air breaking down and becoming conductive.
Electric corona
Very high voltage between conductors in air produces electric stresses that cause the surrounding air (insulation) to break down. Corona represents a power loss in lines.
Strike distance
The minimum required separations between energized conductors.
Flashover
HV jumping to a grounded object when air breaks down.
How is corona reduced?
- increase cable diameter
- bundle cables with spacers
- use corona rings at weak points
Corona effect
- eats insulation
- interferes with TV and radio signals
Leakage (creepage) current
Current tracking across the insulation to the ground.
Creepage distance
The distance over the surface of an insulator
Creepage (tracking) causes
- contamination
- moisture
- high voltage
BIL
Basic Impulse Level is the level of lightning strike the equipment can withstand.
B.C. voltage generation
10 - 25 kV only
Source of voltage generation in B.C.
- hydroelectric
- natural gas
- coal
BC Hydro transmission voltages
Range: 60 kV - 500 kV
Transmission Line Galloping
It happens during storms or in the event of a short circuit.
Underground transmission disadvantages
- higher power losses are associated with insulated cables.
- insulated cables are expensive compared to steel.
- limited length of 25-30 miles, due to capacitive losses.
Distribution voltages
2 400 - 69 000 V (Medium Voltage). Most common in Vancouver is 13.2 kV.
DC Transmission Systems advantages
- No skin effect (a smaller volume of conductor material is required)
- No capacitive charging current
- No XL
- Power factor is unity, no reactor or capacitors are required
- smaller short circuit current on a faulted DC line.
DC Transmission Systems disadvantages
- Need converter stations on BOTH sides
- Harmonics are an issue. Need filters feeding converters.
- Switchgear for DC is larger.
- Limited to 320 kV on insulated cables.
HV Distribution Layouts
- Radial (uses only one HV line or feeder and is the cheapest system layout).
- Ring or loop (more expensive than radial and it requires TWO high voltage feeders.)
- Network or grid (a loop system with some radial. Has a maximum of THREE feeders.)
Radial system advantages
- simple, non-complicated layout
- not subject to “back-feeds”
Radial system disadvantages
- has only ONE feeder
- maintenance is difficult
Ring (loop) systems advantages
- more reliable than the radial system (TWO feeders)
- allows for maintenance
Ring (loop) systems disadvantage
Subject to backfeeds
Network (Grid) advantages
- very reliable (THREE feeders only)
- easy to perform maintenance
Network (Grid) disadvantage
Highest risk of back feeds
Insulation shield functions
- Makes the flux uniform in the cable insulation.
- Suppresses possible radio and TV interference by confining the flux.
- Acts to protect life in the event of mechanical damage to the cable.
The role of strand (conductor) shield
Prevents the flux lines to concentrate in the air pockets between the stranded conductors.
Insulation levels
- 100%
- 133%
- 173%
100% insulation use
Used in solidly grounded systems, or in ungrounded systems where ground faults are eliminated in no more than one minute.
133% insulation use
For use in ungrounded systems, where ground faults are eliminated in one hour or less.
173% insulation use
Used in ungrounded systems and required where there may be an indefinite time for ground fault clearance
Lightning arresters
- located on overhead lines, as close as possible to the equipment to be protected.
- always connected in parallel with equipment.
- diverts the over voltage to ground, then allows the regular voltage to go to the equipment.
Series reactor
When placed in series with HV equipment, they are used to limit the fault current and reduce mechanical/thermal stress on equipment.
Shunt reactors
Connected in parallel with HV lines, they are simply trying to lower the voltage on a long transmission line.
Dielectric strength
It’s the insulation ability to withstand electrical breakdown under the influence of voltage. It’s measured in kV/mm.
Factors that decrease dielectric strength
- high temperatures
- moisture
Insulation types
- thermoplastic (it burns)
- thermoset (it doesn’t burn)
Medium-voltage cables (2 400 V - 69 000 V)
- Teck cable
- Shielded power cable
- Concentric neutral cable
- Paper-insulated, lead-covered cable (PILC)
- Submarine cable
- Mining cable
Types of fuses
- current-limiting fuses
- solid-materials fuses
- liquid fuses
- distribution fuses cut-outs
Types of CB’s
- Air-magnetic
- Vacuum
- Oil
- Air-blast
- Sulphur hexafluoride (SF6)
Switchgear
The combination of electrical disconnect switches, fuses or circuit breakers used to control, protect and isolate electrical equipment.
Metal-Clad Switchgear
- all components are in separate metal compartments, providing excellent isolation in the event of a fault.
- the bus-bars are insulated and typically found at the back of the equipment.
Metal-Enclosed Switchgear
- looks similar to metal-clad switchgear
- circuit breakers are stationary
- bus-bars are not insulated.
- it has Plexiglas viewing windows.
Horn-Gap Switch
- strictly for outdoor pole-top applications.
- it has limited current interrupting ability of 15 amps.
- used mainly on overhead rural lines.
Switch ratings
- Maximum design voltage
- Rated voltage
Types of Switches
- Isolating switch
- Horn-gap switch
- Load-break switch
- Disconnect switch
Gradient mat size
1.2 m x 1.8 m
Key interlocking
A safety feature frequently used in HV installations for the protection of personnel and equipment (e.g. unit substation).
What is the stress cone for ?
To provide stress relief and/or anti-tracking and/or a seal to the environment.
What are the cable layers?
- Conductors
- Strand shield
- Dielectric or insulator
- Insulation shield
- Concentric shield (can serve as neutral) or tape ribbon shield (cannot be used as a neutral)
- Cable jacket
