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Electrical - Level 4 > High Voltage > Flashcards

High Voltage Flashcards

1
Q

What classifies extra low voltage in the CEC?

A 
AC = less than or equal to 30V
DC = less than or equal to 42.4V
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2
Q

What classifies low voltage in the CEC?

A 
AC = more than 30V, less than or equal to 1000V
DC = more than 42.4, less than or equal to 1060V
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3
Q

What classifies High Voltage in the CEC?

A 
AC = more than 1000V
DC = more than 1060V
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4
Q

IEEE/ANSI medium voltage?

A

100 - 69,000V

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5
Q

IEEE/ANSI High voltage?

A

69,000 - 230,000V

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6
Q

IEEE/ANSI Extra high voltage?

A

230,000 - 800,000V

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7
Q

IEEE/ANSI Ultra high voltage?

A

more than 800,000V

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8
Q

What does IEEE stand for?

A

Institute of Electrical and Electronic Engineers

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9
Q

What does ANSI stand for?

A

American National Standards Institute

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10
Q

Characteristics of High Voltage?

A
  • Delivers bulk power
  • has strong electrostatic fields
  • creates high temperature & strong magnetic fields during fault conditions
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11
Q

Why is generated voltage lower than transmission voltage?

A

Due to insulation constraint in the alternators

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12
Q

What is transmission line voltage determined by?

A
  • Load
  • Distance
  • Existing infrastructure
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13
Q

What is the general rule for voltage per mile?

A

1000 Volts per mile

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14
Q

What are the 3 classifications for High voltage networks?

A
  • Radial
  • Ring or Loop
  • Network or Grid
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15
Q

Advantages and Disadvantages of Radial Network?

A 
Advantages:
-Least expensive
-simplest
-least likely to backfeed
 Disadvantages:
-Only one supply
-least reliable
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16
Q

Advantages and Disadvantages of Ring/Loop Network?

A 
Advantages:
-More reliable
-Gives two feeds incase one fails
-Allows maintenance
Disadvantages:
-Costs more
-Subject to back feed
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17
Q

Advantages and Disadvantages or Network/Grid

A 
Advantages:
-Greatest reliability
-most efficient
-power delivered by several routes
Disadvantages:
-Most expensive
-requires complex switching
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18
Q

list 3 methods of isolating AC transmission systems?

A
  1. Elevation
  2. Substations
  3. Vaults
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19
Q

List 4 types of overhead conductors?

A
  1. Stranded bare copper (heavy & not common)
  2. Solid Copper (sometimes for trolly lines)
  3. Aluminum conductor steel reinforced (ACSR)
  4. All Aluminum Conductor (AAC)
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20
Q

What prohibits broad use of insulated cables?

A

capacitive losses

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21
Q

List 3 insulator materials?

A
  1. Porcelain
  2. Glass
  3. Polymer Resins (silicon)
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22
Q

Insulators have ______ to increase ________?

A
  1. Skirts or petticoats

2. Creepage distance

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23
Q

What are the 4 types of insulators?

A
  1. Pin
  2. Post
  3. Suspension
  4. Strain
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24
Q

Pin insulator properties?

A
  • less than 35 KiloVolts

- conductor fastened with binding

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25
Q

Post insulator properties?

A
  • Like a Pin but stronger
  • Higher voltages
  • Conductor fastened with clamp
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26
Q

Suspension insulator properties?

A
  • Conductor hangs on the bottom

- Modular (individual skirts are stacked for higher voltage)

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27
Q

Strain Insulator properties?

A
  • Used where line dead-ends or turns
  • Strong tensile strength
  • Individual skirts stacked for higher voltages
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28
Q

Underground & Underwater Cables properties?

A
  • Expensive (high initial cost, ongoing power losses, continuous capacitor charging current)
  • Max distance about 40-50 km
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29
Q

High voltage DC cable properties?

A
  • No SHRED (DC has no hysterisis or eddy-current losses, no skin effect)
  • Smaller conductors
  • No capacitive losses
  • Expensive cable
  • requires rectifier at source and inverter at load
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30
Q

What does AC and DC both produce?

A

an electrostatic field (ESF)

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31
Q

What does the size of the ESF depend on?

A

Voltage

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32
Q

What can be used to spread or disperse ESF?

A
  • Larger Conductor than required

- Corona Rings

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33
Q

What is voltage gradient?

A
  • ESF strength with respect to distance

- Voltage density

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34
Q

What is Dielectric strength?

A

The voltage an insulator can withstand before breaking down (flashover)

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35
Q

What is Dielectric strength measured in?

A
  • V/mm or V/in
  • Air = 3MV/m or 3000V/mm
  • Vacuum = 10 to the 12 V/m
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36
Q

What is Dielectric Constant?

A

-the measure of the ability of a material to store a charge
Vacuum = 1
Air = 1.0006
Mica = 3

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37
Q

What is Corona?

A

An electrical discharge caused by ionization (breakdown) of air due to ESF

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38
Q

What are the symptoms of Corona?

A
  • Radio Frequency Interference
  • Hissing/crackling
  • Violet/Blue light
  • Ozone (poisonous ionized air)
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39
Q

What happens to overhead lines due to corona?

A
  • causes power loss

- sharp bends may cause concentrated ESF and Corona

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40
Q

What do Corona Rings do?

A
  • Corona rings or parallel conductors reduce corona

- is self-limited and WILL NOT flashover

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41
Q

What happens to insulated Conductors due to corona and what disperses it?

A
  • Will cause insulation breakdown and lead to flashover

- Strand Shielding disperses ESF

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42
Q

What does BIL stand for?

A

Basic Impulse Insulation Level

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43
Q

Explain BIL?

A

-Insulation must be capable of system voltage and transient or impulse voltages

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44
Q

What can BIL ratings be and what equipment must be BIL rated?

A
  • Ratings can be 5-30 times the system voltage

- Equipment over 25kV must be BIL rated

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45
Q

What is a vault?

A

Fire resistant construction for housing transformers, etc…

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46
Q

Where can vaults be located?

A

Above or Below ground

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47
Q

How are lightning arrestors connected?

A

Connect in parallel with the equipment or line to ground with one per phase

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48
Q

How does a lightning arrestor work?

A
  • Air gap provides insulation

- Lightning jumps the air gap to ground

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49
Q

What must lightning arrestors have?

A

a lower breakover voltage than the BIL rating of the equipment it protects

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50
Q

What is another name for a Low Voltage Surge Suppressor?

A

Transient voltage surge suppressor

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51
Q

Low voltage surge suppressors are usually _______?

A

Metal oxide varistors (MOVs)

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52
Q

Where are low voltage surge suppressors used?

A
  • Power bars
  • Surge protected receptacles
  • hardwired in panel
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53
Q

How are low voltage surge suppressors made and how do they work?

A
  • 2 insulated plates with peaks and valleys
  • Transient voltages jump from peak to peak
  • have a limited life span before all peaks are gone
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54
Q

What do lightning rods do?

A

protect structures

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55
Q

Are lightning rods connected to equipment?

A

NO

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56
Q

How are lighting rods wired?

A

Electrode on top of structure wired as short and straight as possibly to an isolated ground electrode

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57
Q

What is a series air core reactor used for and what does it allow?

A
  • to limit fault current

- allows for less expensive CB’s having a lower interrupting current

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58
Q

What does a series reactor do if a fault occurs?

A

it “chokes” the short circuit current

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59
Q

What can achieve similar goals of a series reactor?

A

Impedance Grounding

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60
Q

What are instrument transformers?

A
  • Voltage Transformers (VT’s or PT’s)

- Current Transformers (CT’s)

61
Q

Characteristics of a Voltage Transformer?

A
  • Low VA rating
  • 120V secondary usually
  • Primary 3A typically
  • Ground secondary
62
Q

Characteristics of a Current Transformer?

A
  • in series with primary
  • 5A secondary standard
  • 1A secondary emerging
  • ground secondary
63
Q

What does a Class 1 Termination provide?

A
  • Electric stress control (geometric or capacitive)
  • Tracking Protection
  • A seal to external environment
64
Q

What does a Class 2 Termination provide? Is it Common?

A
  • Electric stress control
  • Tracking protection
  • Not common
65
Q

What does a Class 3 termination provide?

A

-Electric Stress Control

66
Q

What is Tracking Protection?

A

External leakage insulation between the H.V. conductor and ground

67
Q

What causes Tracking?

A

Conductive carbon paths from surface leaked currents

68
Q

Name and the dielectric constant of capacitive stress control?

A

-Hi-K, 25

69
Q

Name the types of electric stress control?

A

Geometric (stress-cone)

Capacitive (Hi-K)

70
Q

What 3 conditions must exist to have tracking?

A
  1. Contamination
  2. Moisture
  3. Voltage
71
Q

Name some contaminations that cause tracking?

A
  • Dust
  • Chemicals
  • Salt
  • Other airbourne particals
72
Q

Name some types of moisture that can cause tracking?

A
  • Humidity
  • Fog
  • Condensation
  • Mist
  • Snow
  • Rain
73
Q

What is the equation for tracking?

A

contamination x moisture x voltage

if any is 0 then tracking equals zero

74
Q

How much voltage do most alternators generate?

A

between 10kV and 20kV

some over 30kV

75
Q

What voltages are normally used in transmission systems? What is the highest used by BC Hydro?

A

60 kV to 765 kV

500kV

76
Q

What is leakage or creepage current?

A

current that tries to track across the insulation to ground

77
Q

How do you keep creepage current low?

A

ensuring that the distance over the surface of an insulator is sufficiently long

78
Q

WCB specifies that only _____ and ______ persons may be allowed to work on HV

A

trained and qualified

79
Q

What effect does electric stress have on insulation?

A

it weakens insulation

80
Q

A rural overhead distribution line is most likely to be a ______ distribution system

A

Radial

81
Q

What is the purpose of viewing windows in a substation?

A

for visible indication of a switch being opened or closed

82
Q

what are the main parts of a surge arrestor?

A

air gap & thyrite material

83
Q

what are two functions of an instrument transformer in a HV system?

A
  • monitor voltage and current levels

- isolate monitoring devices from HV system

84
Q

is step voltage or touch voltage more likely to be higher?

A

touch

85
Q

is a ground mat made of insulating or conducting material?

A

conducting

86
Q

what is the minimum safe distance to be maintained when work is to be done near a 25 kV line?

A

3 meters

87
Q

what are the general steps to be taken in a safe switching sequence?

A
  1. disconnect
  2. lock out
  3. test
  4. ground and screen off nearby live parts
88
Q

what is single strand used for and what are the normal sizes?

A
  • terminations where flexibility is not required

- #2 or smaller, usually #10 or smaller

89
Q

What are the properties of Concentric Stranding?

A
  • typical strandings are 7, 19, 37, etc…
  • large undesirable air voids
  • difficult to remove insulation
90
Q

Compressed stranding properties?

A
  • 97% of the concentric stranding cross section

- smaller air voids

91
Q

Compact stranding properties?

A
  • 90% the size of concentric

- Preferred for HV

92
Q

What do air voids lead to in insulated cables?

A
  • corona
  • ionization
  • RFI
  • insulation damage
93
Q

All Aluminum Conductor Properties?

A
  • OK for bends
  • light
  • not strong enough for long spans
94
Q

Aluminum Conductor Steel Reinforced properties?

A
  • Center strand steel for strength
  • steel center called the messenger
  • outer strands aluminum
95
Q

Aluminum Conductor Composite Reinforced properties?

A
  • Strong and light
  • carbon fiber or fiberglass messenger
  • alumina fibers
96
Q

Why are bare overhead conductors preferred over insulated cables?

A

Due to the lack of continuous capacitive charging and discharging losses from insulation

97
Q

at what voltage is shielding required?

A

over 5kV

98
Q

List 5 types of high voltage cables?

A
  • Armoured (teck, acwu)
  • Wire armoured (submarine cable)
  • Tape armoured
  • Paper insulated lead covered (underground by utilities)
  • Concentric neutral
99
Q

Where are concentric neutral cables used? What is special about it?

A
  • Underground residential distribution (URD)

- has a shielding which also serves as a neutral

100
Q

How does a concentric neutral work in a 3-phase 4 wire system?

A

each neutral uses 33%, connect all 3 neutrals = 100%

101
Q

What is the purpose of cable jackets?

A
  • provide mechanical protection
  • seal out contaminations
  • reduce friction
  • identify cable specifications
102
Q

List 4 cable dielectrics?

A
  1. Rubber
  2. Thermoplastic
  3. Thermoset
  4. Paper insulated lead covered
103
Q

Rubber dielectric properties?

A

600V - 15 kV

  • moisture resistant
  • replaced by PVC plastics in 1950s
104
Q

Thermoplastic dielectric properties? (ratings?)

A

5 kV - 15 kV

  • softens at higher temps and brittle at low temps
  • Polyvinyl Chloride (PVC)
  • Polyethylene (PE)
105
Q

Thermoset dielectric properties?

A

5 kV - 69 kV

  • does not tend to soften with increased temp.
  • Cross-linked polyethylene (XLPE)
  • Ethylene propylene rubber (EPR)
  • Silicon
106
Q

What happens to stranded conductors with air voids?

A

will have concentrated ESF between the strands

107
Q

What does a strand shield?

A

a semi-conductor material that evens out the ESF

108
Q

What is the strand shield layer called?

A

“Semi-Con”

109
Q

What is an Insulation Shield?

A

a semi-con placed around the insulation to evenly disperse the ESF

110
Q

What will bonding the insulation at both ends do?

A

may result in excessive sheath currents which may increase conductor temp.

111
Q

what will bonding the insulation sheath at one end do?

A

can result in extremely dangerous voltages at the open end

112
Q

what surrounds the insulation shield and why?

A

a conductive layer surrounds it so that its entire length is bonded well

113
Q

What must insulation be rated for?

A

Line voltage

114
Q

What are the insulation ratings?

A

100% - grounded or ungrounded systems where faults are eliminated in less than a minute
133% - ungrounded systems where faults are eliminated in less than an hour
173% - rarely used, where may be an indefinite time for ground fault clearance

115
Q

Cables used in high-voltage and extra-high voltage are likely to be __________?

A

liquid or gas filled

116
Q

what is the most common solid-dielectric used for cables?

A

Cross-linked polyethylene (XLPE) or ethylene-propylene rubber (EPR)

117
Q

which of the cable shields must be grounded?

A

Insulation shield

118
Q

what is the primary purpose of lead sheathing on a cable?

A

Prevent the ingress of moisture into the cable

119
Q

what is the secondary purpose of a lead sheath on a cable?

A

mechanical protection

120
Q

Which medium voltage cable is the most widely used for heavy industrial applications?

A

Teck Cable

121
Q

What are the two most common voltage ratings of medium voltage cables used for power applications?

A

15 kV and 25 kV

122
Q

two main factors which determine the voltage of an overhead transmission line?

A
  • size of the load

- distance between the source and load

123
Q

How is HV AC usually transmitted over long distances?

A

Bare conductors suspended and insulated from steel towers

124
Q

List two advantages that DC has over AC for HV transmission?

A
  1. smaller conductor for same energy

2. no continuous capacitive charging & discharging

125
Q

list 3 structural methods employed to safely isolate HV equipment?

A
  1. elevation
  2. substations
  3. vaults
126
Q

Switchgear includes: (6)

A
  • CB’s
  • Fuses
  • Load break switches
  • disconnect/isolation switches
  • “protective relay” circuitry
  • Instrument transformers
127
Q

Switchgear may be located where?

A
  • outdoor

- indoor (metal clad, metal enclosed)

128
Q

Metal clad properties?

A
  • Gear “racks out and in”
  • Insulated bus bars
  • shutters close
  • DONT RACK UNDER LOAD
129
Q

Metal Enclosed properties?

A
  • Bare bus bar
  • fixed switches and CB’s
  • Viewing windows
  • louvers for ventilation
130
Q

Arc Interruption methods : (6)

A
  • Fast separation of contacts using compressed springs
  • Auxiliary flicker contacts and arc chutes
  • magnetic force
  • temperature rise
  • compressed air
  • contacts immersed in Oil, SF6, or a vacuum
131
Q

Horn Gap Switch properties?

A
  • can interrupt small currents, 15A max
  • Main and Aux contacts
  • Air break
  • Rural Overhead Distribution
132
Q

Load break switch properties:

A
  • can make and interrupt current
  • compressed spring opens contact fast
  • main contact opens
  • AUX contact opens in Arc chute
133
Q

Load break switch/fuse combo properties?

A
  • cheaper than CB’s
  • current limited fuse interrupts faults
  • “striker pin” of fuse trips load break switch
  • all 3 lines open
134
Q

What do load break switch/fuse combos prevent?

A

Single phasing

135
Q

isolation switch properties?

A
  • carry rated loads
  • cannot make or break any current flow
  • isolates after circuit is de-energized
  • provides visual verification of contacts
136
Q

disconnect switches properties?

A
  • carry rated current
  • can switch minimal current (less than .5A)
  • used to isolate equipment
137
Q

list 5 different circuit breakers?

A
  1. Air/Air magnetic
  2. Oil
  3. Inert Gas
  4. Vacuum
  5. Air blast
138
Q

Air/Air magnetic CB properties? Max ratings?

A
  • 15kV and 3000A max
  • Arc chutes or baffle plates
  • magnetic blow out coils
  • may incorporate a puffer
139
Q

Bulk oil CB properties?

A
  • All poles within a chamber
  • indoor or outdoor
  • older style
140
Q

Minimum Oil CB properties?

A
  • One oil vessel per pole
  • sight gauges for checking oil
  • containment curbs
141
Q

Inert Gas CB properties?

A
  • Sulphur Hexaflouride (SF6)
  • maintained at low positive pressure
  • very high dielectric strength
  • all voltage ranges
142
Q

Vacuum CB properties?

A
  • Typically up to 35kV
  • uses motor charged spring
  • real estate friendly
143
Q

Air blast CB properties?

A
  • compressed air extinguishes arc
  • up to 800 PSI
  • All voltage ranges especially extra high voltage
  • silencers may be required
144
Q

What are CB reclosers used for and how do they work?

A
  • exclusively for overhead lines
  • burns branches off bare conductors
  • may attempt reclosure 3 or 4 times
  • 10-20 second delay after each trip
145
Q

Common ratings of fuses for HV?

A

2.4kV to 69kV

.5A to 400A

146
Q

Distribution fuse properties?

A
  • Pole top “distribution cut-out”

- inner fuse blows releasing cartridge

147
Q

Power fuse properties?

A
  • Current limited like HRC fuses
  • one time use
  • very fast, 1/4 of a cycle
148
Q

Solid material fuse properties?

A
  • not current limiting, may take several cycles to open
  • explusion type
  • during fault, boric acid forms steam & water
149
Q

What is a liquid fuse filled with?

A

oil

Electrical - Level 4 (8 decks)

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