High Voltage Flashcards
What are the main advantages to high voltage generation?
Current can be kept lower so the size of material is smaller and lighter, cost is lower, fewer power losses
What are the two main factors that affect line voltage?
Load requirement
Distance to load (1000V/m rule of thumb)
What does IEEE stand for?
Institute of Electrical & Electronics Engineers
What are the three classifications of high voltage regulated by the IEEE?
Medium: 2.4kV - 69kV
High: 115kV - 230kV
Extra high: 345kV - 765kV
What are the four steps in providing high voltage in order?
Generation > Transmission > Distribution > Utilization
Hydro alternators generate how much power?
Between 10-25kV
What is the purpose of hydro alternators being limited to 10-25kV?
Alternator windings would become too stressed under higher voltages. Step-up transformers are used to provide higher voltages after alternation.
What voltage are transmission lines generally?
60kV - 765kV.
In BC, lines are maximum 500kV
What is the pro and con to transmission towers?
Capital costs are very high, but power losses are relatively low.
How are transmission towers protected from lightning?
Steel ground wire is connected between each tower
How are transmission towers and lines protected against wind storms and hurricanes?
Dampers are installed on lines to counter the force of galloping.
What is Sub-Transmission?
Voltages that fall between transmission and distribution values. Usually about 60kV
What is the most common distribution voltage in BC?
12.47kV
What are the two types of distribution conductors?
ASCR - Aluminum steel-core
ACCC - Aluminum carbon-core.
Why is HV not typically ran underground?
Insulated cables cause high power losses, more expensive, and are limited to about 25-30 miles due to capacitive losses.
What are the three HV distribution systems?
Radial, Ring or Loop, Grid or Network
What are the pros and cons of Radial systems?
Pro: simple, cheap, least likely to backfeed
Con: Least reliable (only has one feeder)
What are the pros and cons of Ring or Loop systems?
Pro: More reliable with two feeders - power still available if one line goes down
Con: Subject to backfeeding
What are the pros and cons of a grid or network system?
Pro: very reliable with a minimum of three feeders, easiest maintenance
Con: Must follow switching sequence to prevent back feeding
What is corona?
Ionized air. Appears as a violet arc. Byproduct is ozone gas. Power loss.
Why is corona bad?
Power loss, eats insulation on cables and interferes with radio signals RFI on overhead lines
How do you avoid corona?
Increase conductor diameter, bundle cables with spacers, use corona rings at high stress points.
What is flashover?
When enough air breakdown occurs that current can flow to a grounded object through the air
What is the purpose of height clearances on HV equipment?
To prevent and avoid flashover in ionized air
What is tracking?
Conduction along the surface of insulation, creating carbon ‘tracks’, tracking can also lead to flashover
What is anti tracking?
Increasing the surface area of insulation with “skirts”
What is BIL?
Basic impulse level, the rating found in equipment for a lightning strike surge. Always much higher than system voltage rating.
What is a sub station?
Where voltage is transformed and cleaned up. Incoming transmission lines go through step down power transformer to lower the voltage for distribution
What is a switch yard?
Where voltage is rerouted for distribution. They do not contain transformers
What is a unit substation?
A factory made metal enclosure meant for indoors. Each compartment has its own isolating switch, load break switch, transformer etc.
What is a HV vault?
Used to house oil filled equipment indoors. Concrete construction, ventilation, floor drains steel doors.
How is a lightning arrestor connected?
In parallel with equipment
How does a lightning arrestor work?
Diverts the over voltage of a lightning strike to ground.
Normally open with a high ohmic value for normal operation, when over voltage occurs the lightning arrestor drops to a very low ohmic value acting as a closed switch for current to flow to ground.
What is the difference between lightning arrestor and lightning rod?
Lightning arrestor protects equipment, lightning rod protects building or structure.
What is a HV series reactor?
A coil of thick wire with an air core, used to limit fault current and reduce mechanical and thermal stress on equipment.
How does a Hv series reactor work?
A sudden increase in current appears as a high increase in Hz, creating a choking effect on the fault current. This allows bc hydro to use a cct breaker with lower interrupting rating.
What is the purpose of a shunt reactor?
Connected in parallel, they lower the voltage in a long transmission line. Long transmission lines make the Xc rise which increases voltage, the XL of the shunt reactor lowers the voltage back down.
What is key interlocking?
Safety measures used on switchgear
How do you determine if a piece of equipment is a TX or shunt reactor?
The only connections on a shunt reactor are to the HV bushing, there are no output connections.
The skin effect is directly proportional to what?
Frequency
What is the purpose of the conductor shield/strand shield?
Semi-con layer prevents ionization by bonding outer strands together.
What is the dielectric strength?
The ability of insulation to withstand electrical breakdown under voltage.
What is thermoplastic insulation?
High dielectric strength but poor thermal stability. Softens when heated, brittle when cold.
Ex) PVC
What is thermoset insulation?
High dielectric strength, most common type of doesn’t soften under heat. Resistant to moisture and chemicals.
Ex) XLPE
What are PILC cables?
Paper insulated lead covered. Oil impregnated paper. High dielectric strength, used in pothead terminations. Can operate at high temps.
What is the insulation shield?
Two layers (semi conductor and metal) electric flux lines are pulled evenly because the shortest path to ground is a straight line.
What percent insulation do ground or wye systems need?
100%, faults cleared in 1 minute or less
What percent insulation thickness is needed for ungrounded delta systems when faults are cleared in one hour or less?
133%
What percent insulation thickness is needed for ungrounded delta systems when faults are cleared in over an hour?
173%
What is the purpose of HV armour in cables?
Provides mechanical strength
What are the four types of armour on an hv cable?
Aluminum interlocked (bx), continuously corrugated, steel wire, and steel tape.
What is the Concentric neutral cable used for?
Underground residential distribution. The concentric wires on the outermost layer provide insulation shielding and return neutral
What is a stress cone?
A stress cone flares the insulation shielding outwards providing a thicker insulation on stress points where the jacketing would be striped back
What is a class 1 termination?
Requires stress relief, ant tracking, sealed to environment
What is class 2 termination?
Stress relief and anti tracking.
What is class 3 termination?
Requires only stress relief.
What is HVDC transmission used for?
Long distances (underwater to Vancouver island), tying together two AC systems that are not synchronized with the same frequency
What are the four fundamental duties of switchgear?
When closed, it should be an ideal conductor
When open, it should be an ideal insulator
When closed, it must interrupt assigned current without causing dangerous over voltage
When open, it must close promptly under short circuit conditions
What are the two categories that HV switches fall under?
Load break, non load break
Define loadbreak switch
Break currents up to their rated normal current
Define disconnects
Used for no load closing and opening. Isolate downstream and/or upstream devices
Define grounding switches
Used to ground lines for safety purposes
Define isolating switches
Used to isolate a circuit or equipment from its source, not intended to interrupt the flow of current
Define a theee position disconnect
A disconnect and grounding switch in one
Define surge arrestor
Used to discharge loads caused by lightning strikes or switching operations
How are cct breakers and switches made to safety interrupt large fault currents?
Use of arcing contacts, arc shutes, blowout coils, resistors, multiple breaks, and high speed contact separation.
What is the main component of a load break switch?
Flicker blades and arc shutes.
What is the current interrupting rating of a horn gap switch?
15 Amps
What two things determine the length of an arc?
Magnitude of voltage, current being interrupted
What two things are essential for arc interruptions?
High speed contact separation, arc cooling
What is the current rating of an air magnetic circuit breaker?
Up to 3000A
What is an air blast circuit breaker?
Compressed air at 450-800psi is driven across the separating contacts. Used up to 800kV
What is inverse time characteristic in fuses?
The larger the overcurrent the faster the interruption must be
What are the three ratings found on fuses
Voltage, continuous current, interrupting current
Define the continuous current of a fuse?
The amount of current the fuse can carry without blowing
Define interrupting current of a fuse
The amount of current the fuse can interrupt without sustaining physical damage
What is the purpose of a current limiting fuse?
Load break switch, prevents single phasing, proper orientation of the pin is important
Is boric acid fuse expulsion or non expulsion?
Expulsion, water Vapor from heat and everything else is pushed out the bottom
Is the boric acid fuse a current limiting fuse?
No, it can take many cycles for it to melt and work properly
What is a distribution cut out?
Drop out type fuse, used primarily in utilities, uses a replaceable fuse link, when the fuse melts inside the tension is released and the fuse tube drops 180°
What type of fuse is a load break switch?
Current limiting fuse
Is a disconnect considered a LB or NLB switch?
No load break switch
Where is a series reactor connected in the circuit?
Between source and load
