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Electrician Apprentice - Level 4 - Canadian > High Voltage > Flashcards

High Voltage Flashcards

To ace the High Voltage topic.

1
Q

Why Do We Use High Voltage?

A

To deliver bulk power economically. High voltage lines carry low currents.

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2
Q

Benefits of using HV

A
  • reduced line current
  • reduced power losses
  • reduced weight
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3
Q

Factors affecting the voltage of transmission lines

A
  • load requirement

- distance to load (1000V/mile is rule of thumb)

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4
Q

IEEE High Voltage Classifications

A
  • Medium Voltage (2,400 - 69,000 V)
  • High Voltage (115,000 - 230,000 V)
  • Extra High Voltage (345,000 - 765,000 V)
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5
Q

Low Voltage characteristics

A
  • 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.
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6
Q

High Voltage

A
  • 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.
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7
Q

HV isolation

A
  • Elevate

- Enclose

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8
Q

Fields created by HV

A
  • magnetic field due to current flowing

- electrostatic field due to voltage alone

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9
Q

Voltage gradient

A

The flux concentrations decrease with distance, therefore the stress on the insulation decreases with distance.

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10
Q

Electric Stress

A

The stress created by the electrostatic field on the conductor insulation

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11
Q

Ionization

A

Air breaking down and becoming conductive.

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12
Q

Electric corona

A

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.

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13
Q

Strike distance

A

The minimum required separations between energized conductors.

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14
Q

Flashover

A

HV jumping to a grounded object when air breaks down.

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15
Q

How is corona reduced?

A
  • increase cable diameter
  • bundle cables with spacers
  • use corona rings at weak points
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16
Q

Corona effect

A
  • eats insulation

- interferes with TV and radio signals

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17
Q

Leakage (creepage) current

A

Current tracking across the insulation to the ground.

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18
Q

Creepage distance

A

The distance over the surface of an insulator

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19
Q

Creepage (tracking) causes

A
  • contamination
  • moisture
  • high voltage
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20
Q

BIL

A

Basic Impulse Level is the level of lightning strike the equipment can withstand.

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21
Q

B.C. voltage generation

A

10 - 25 kV only

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22
Q

Source of voltage generation in B.C.

A
  • hydroelectric
  • natural gas
  • coal
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23
Q

BC Hydro transmission voltages

A

Range: 60 kV - 500 kV

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24
Q

Transmission Line Galloping

A

It happens during storms or in the event of a short circuit.

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25
Q

Underground transmission disadvantages

A
  • 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.
26
Q

Distribution voltages

A

2 400 - 69 000 V (Medium Voltage). Most common in Vancouver is 13.2 kV.

27
Q

DC Transmission Systems advantages

A
  • 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.
28
Q

DC Transmission Systems disadvantages

A
  • 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.
29
Q

HV Distribution Layouts

A
  • 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.)
30
Q

Radial system advantages

A
  • simple, non-complicated layout

- not subject to “back-feeds”

31
Q

Radial system disadvantages

A
  • has only ONE feeder

- maintenance is difficult

32
Q

Ring (loop) systems advantages

A
  • more reliable than the radial system (TWO feeders)

- allows for maintenance

33
Q

Ring (loop) systems disadvantage

A

Subject to backfeeds

34
Q

Network (Grid) advantages

A
  • very reliable (THREE feeders only)

- easy to perform maintenance

35
Q

Network (Grid) disadvantage

A

Highest risk of back feeds

36
Q

Insulation shield functions

A
  • 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.
37
Q

The role of strand (conductor) shield

A

Prevents the flux lines to concentrate in the air pockets between the stranded conductors.

38
Q

Insulation levels

A
  • 100%
  • 133%
  • 173%
39
Q

100% insulation use

A

Used in solidly grounded systems, or in ungrounded systems where ground faults are eliminated in no more than one minute.

40
Q

133% insulation use

A

For use in ungrounded systems, where ground faults are eliminated in one hour or less.

41
Q

173% insulation use

A

Used in ungrounded systems and required where there may be an indefinite time for ground fault clearance

42
Q

Lightning arresters

A
  • 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.
43
Q

Series reactor

A

When placed in series with HV equipment, they are used to limit the fault current and reduce mechanical/thermal stress on equipment.

44
Q

Shunt reactors

A

Connected in parallel with HV lines, they are simply trying to lower the voltage on a long transmission line.

45
Q

Dielectric strength

A

It’s the insulation ability to withstand electrical breakdown under the influence of voltage. It’s measured in kV/mm.

46
Q

Factors that decrease dielectric strength

A
  • high temperatures

- moisture

47
Q

Insulation types

A
  • thermoplastic (it burns)

- thermoset (it doesn’t burn)

48
Q

Medium-voltage cables (2 400 V - 69 000 V)

A
  • Teck cable
  • Shielded power cable
  • Concentric neutral cable
  • Paper-insulated, lead-covered cable (PILC)
  • Submarine cable
  • Mining cable
49
Q

Types of fuses

A
  • current-limiting fuses
  • solid-materials fuses
  • liquid fuses
  • distribution fuses cut-outs
50
Q

Types of CB’s

A
  • Air-magnetic
  • Vacuum
  • Oil
  • Air-blast
  • Sulphur hexafluoride (SF6)
51
Q

Switchgear

A

The combination of electrical disconnect switches, fuses or circuit breakers used to control, protect and isolate electrical equipment.

52
Q

Metal-Clad Switchgear

A
  • 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.
53
Q

Metal-Enclosed Switchgear

A
  • looks similar to metal-clad switchgear
  • circuit breakers are stationary
  • bus-bars are not insulated.
  • it has Plexiglas viewing windows.
54
Q

Horn-Gap Switch

A
  • strictly for outdoor pole-top applications.
  • it has limited current interrupting ability of 15 amps.
  • used mainly on overhead rural lines.
55
Q

Switch ratings

A
  • Maximum design voltage

- Rated voltage

56
Q

Types of Switches

A
  • Isolating switch
  • Horn-gap switch
  • Load-break switch
  • Disconnect switch
57
Q

Gradient mat size

A

1.2 m x 1.8 m

58
Q

Key interlocking

A

A safety feature frequently used in HV installations for the protection of personnel and equipment (e.g. unit substation).

59
Q

What is the stress cone for ?

A

To provide stress relief and/or anti-tracking and/or a seal to the environment.

60
Q

What are the cable layers?

A
  • 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

Electrician Apprentice - Level 4 - Canadian (13 decks)

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