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
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
3
Q
What classifies High Voltage in the CEC?
A
AC = more than 1000V DC = more than 1060V
4
Q
IEEE/ANSI medium voltage?
A
100 - 69,000V
5
Q
IEEE/ANSI High voltage?
A
69,000 - 230,000V
6
Q
IEEE/ANSI Extra high voltage?
A
230,000 - 800,000V
7
Q
IEEE/ANSI Ultra high voltage?
A
more than 800,000V
8
Q
What does IEEE stand for?
A
Institute of Electrical and Electronic Engineers
9
Q
What does ANSI stand for?
A
American National Standards Institute
10
Q
Characteristics of High Voltage?
A
- Delivers bulk power
- has strong electrostatic fields
- creates high temperature & strong magnetic fields during fault conditions
11
Q
Why is generated voltage lower than transmission voltage?
A
Due to insulation constraint in the alternators
12
Q
What is transmission line voltage determined by?
A
- Load
- Distance
- Existing infrastructure
13
Q
What is the general rule for voltage per mile?
A
1000 Volts per mile
14
Q
What are the 3 classifications for High voltage networks?
A
- Radial
- Ring or Loop
- Network or Grid
15
Q
Advantages and Disadvantages of Radial Network?
A
Advantages: -Least expensive -simplest -least likely to backfeed Disadvantages: -Only one supply -least reliable
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
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
18
Q
list 3 methods of isolating AC transmission systems?
A
- Elevation
- Substations
- Vaults
19
Q
List 4 types of overhead conductors?
A
- Stranded bare copper (heavy & not common)
- Solid Copper (sometimes for trolly lines)
- Aluminum conductor steel reinforced (ACSR)
- All Aluminum Conductor (AAC)
20
Q
What prohibits broad use of insulated cables?
A
capacitive losses
21
Q
List 3 insulator materials?
A
- Porcelain
- Glass
- Polymer Resins (silicon)
22
Q
Insulators have ______ to increase ________?
A
- Skirts or petticoats
2. Creepage distance
23
Q
What are the 4 types of insulators?
A
- Pin
- Post
- Suspension
- Strain
24
Q
Pin insulator properties?
A
- less than 35 KiloVolts
- conductor fastened with binding
25
Post insulator properties?
- Like a Pin but stronger
- Higher voltages
- Conductor fastened with clamp
26
Suspension insulator properties?
- Conductor hangs on the bottom
| - Modular (individual skirts are stacked for higher voltage)
27
Strain Insulator properties?
- Used where line dead-ends or turns
- Strong tensile strength
- Individual skirts stacked for higher voltages
28
Underground & Underwater Cables properties?
- Expensive (high initial cost, ongoing power losses, continuous capacitor charging current)
- Max distance about 40-50 km
29
High voltage DC cable properties?
- 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
30
What does AC and DC both produce?
an electrostatic field (ESF)
31
What does the size of the ESF depend on?
Voltage
32
What can be used to spread or disperse ESF?
- Larger Conductor than required
| - Corona Rings
33
What is voltage gradient?
- ESF strength with respect to distance
| - Voltage density
34
What is Dielectric strength?
The voltage an insulator can withstand before breaking down (flashover)
35
What is Dielectric strength measured in?
- V/mm or V/in
- Air = 3MV/m or 3000V/mm
- Vacuum = 10 to the 12 V/m
36
What is Dielectric Constant?
-the measure of the ability of a material to store a charge
Vacuum = 1
Air = 1.0006
Mica = 3
37
What is Corona?
An electrical discharge caused by ionization (breakdown) of air due to ESF
38
What are the symptoms of Corona?
- Radio Frequency Interference
- Hissing/crackling
- Violet/Blue light
- Ozone (poisonous ionized air)
39
What happens to overhead lines due to corona?
- causes power loss
| - sharp bends may cause concentrated ESF and Corona
40
What do Corona Rings do?
- Corona rings or parallel conductors reduce corona
| - is self-limited and WILL NOT flashover
41
What happens to insulated Conductors due to corona and what disperses it?
- Will cause insulation breakdown and lead to flashover
| - Strand Shielding disperses ESF
42
What does BIL stand for?
Basic Impulse Insulation Level
43
Explain BIL?
-Insulation must be capable of system voltage and transient or impulse voltages
44
What can BIL ratings be and what equipment must be BIL rated?
- Ratings can be 5-30 times the system voltage
| - Equipment over 25kV must be BIL rated
45
What is a vault?
Fire resistant construction for housing transformers, etc...
46
Where can vaults be located?
Above or Below ground
47
How are lightning arrestors connected?
Connect in parallel with the equipment or line to ground with one per phase
48
How does a lightning arrestor work?
- Air gap provides insulation
| - Lightning jumps the air gap to ground
49
What must lightning arrestors have?
a lower breakover voltage than the BIL rating of the equipment it protects
50
What is another name for a Low Voltage Surge Suppressor?
Transient voltage surge suppressor
51
Low voltage surge suppressors are usually _______?
Metal oxide varistors (MOVs)
52
Where are low voltage surge suppressors used?
- Power bars
- Surge protected receptacles
- hardwired in panel
53
How are low voltage surge suppressors made and how do they work?
- 2 insulated plates with peaks and valleys
- Transient voltages jump from peak to peak
- have a limited life span before all peaks are gone
54
What do lightning rods do?
protect structures
55
Are lightning rods connected to equipment?
NO
56
How are lighting rods wired?
Electrode on top of structure wired as short and straight as possibly to an isolated ground electrode
57
What is a series air core reactor used for and what does it allow?
- to limit fault current
| - allows for less expensive CB's having a lower interrupting current
58
What does a series reactor do if a fault occurs?
it "chokes" the short circuit current
59
What can achieve similar goals of a series reactor?
Impedance Grounding
60
What are instrument transformers?
- Voltage Transformers (VT's or PT's)
| - Current Transformers (CT's)
61
Characteristics of a Voltage Transformer?
- Low VA rating
- 120V secondary usually
- Primary 3A typically
- Ground secondary
62
Characteristics of a Current Transformer?
- in series with primary
- 5A secondary standard
- 1A secondary emerging
- ground secondary
63
What does a Class 1 Termination provide?
- Electric stress control (geometric or capacitive)
- Tracking Protection
- A seal to external environment
64
What does a Class 2 Termination provide? Is it Common?
- Electric stress control
- Tracking protection
- Not common
65
What does a Class 3 termination provide?
-Electric Stress Control
66
What is Tracking Protection?
External leakage insulation between the H.V. conductor and ground
67
What causes Tracking?
Conductive carbon paths from surface leaked currents
68
Name and the dielectric constant of capacitive stress control?
-Hi-K, 25
69
Name the types of electric stress control?
Geometric (stress-cone)
| Capacitive (Hi-K)
70
What 3 conditions must exist to have tracking?
1. Contamination
2. Moisture
3. Voltage
71
Name some contaminations that cause tracking?
- Dust
- Chemicals
- Salt
- Other airbourne particals
72
Name some types of moisture that can cause tracking?
- Humidity
- Fog
- Condensation
- Mist
- Snow
- Rain
73
What is the equation for tracking?
contamination x moisture x voltage
| if any is 0 then tracking equals zero
74
How much voltage do most alternators generate?
between 10kV and 20kV
| some over 30kV
75
What voltages are normally used in transmission systems? What is the highest used by BC Hydro?
60 kV to 765 kV
| 500kV
76
What is leakage or creepage current?
current that tries to track across the insulation to ground
77
How do you keep creepage current low?
ensuring that the distance over the surface of an insulator is sufficiently long
78
WCB specifies that only _____ and ______ persons may be allowed to work on HV
trained and qualified
79
What effect does electric stress have on insulation?
it weakens insulation
80
A rural overhead distribution line is most likely to be a ______ distribution system
Radial
81
What is the purpose of viewing windows in a substation?
for visible indication of a switch being opened or closed
82
what are the main parts of a surge arrestor?
air gap & thyrite material
83
what are two functions of an instrument transformer in a HV system?
- monitor voltage and current levels
| - isolate monitoring devices from HV system
84
is step voltage or touch voltage more likely to be higher?
touch
85
is a ground mat made of insulating or conducting material?
conducting
86
what is the minimum safe distance to be maintained when work is to be done near a 25 kV line?
3 meters
87
what are the general steps to be taken in a safe switching sequence?
1. disconnect
2. lock out
3. test
4. ground and screen off nearby live parts
88
what is single strand used for and what are the normal sizes?
- terminations where flexibility is not required
| - #2 or smaller, usually #10 or smaller
89
What are the properties of Concentric Stranding?
- typical strandings are 7, 19, 37, etc...
- large undesirable air voids
- difficult to remove insulation
90
Compressed stranding properties?
- 97% of the concentric stranding cross section
| - smaller air voids
91
Compact stranding properties?
- 90% the size of concentric
| - Preferred for HV
92
What do air voids lead to in insulated cables?
- corona
- ionization
- RFI
- insulation damage
93
All Aluminum Conductor Properties?
- OK for bends
- light
- not strong enough for long spans
94
Aluminum Conductor Steel Reinforced properties?
- Center strand steel for strength
- steel center called the messenger
- outer strands aluminum
95
Aluminum Conductor Composite Reinforced properties?
- Strong and light
- carbon fiber or fiberglass messenger
- alumina fibers
96
Why are bare overhead conductors preferred over insulated cables?
Due to the lack of continuous capacitive charging and discharging losses from insulation
97
at what voltage is shielding required?
over 5kV
98
List 5 types of high voltage cables?
- Armoured (teck, acwu)
- Wire armoured (submarine cable)
- Tape armoured
- Paper insulated lead covered (underground by utilities)
- Concentric neutral
99
Where are concentric neutral cables used? What is special about it?
- Underground residential distribution (URD)
| - has a shielding which also serves as a neutral
100
How does a concentric neutral work in a 3-phase 4 wire system?
each neutral uses 33%, connect all 3 neutrals = 100%
101
What is the purpose of cable jackets?
- provide mechanical protection
- seal out contaminations
- reduce friction
- identify cable specifications
102
List 4 cable dielectrics?
1. Rubber
2. Thermoplastic
3. Thermoset
4. Paper insulated lead covered
103
Rubber dielectric properties?
600V - 15 kV
- moisture resistant
- replaced by PVC plastics in 1950s
104
Thermoplastic dielectric properties? (ratings?)
5 kV - 15 kV
- softens at higher temps and brittle at low temps
- Polyvinyl Chloride (PVC)
- Polyethylene (PE)
105
Thermoset dielectric properties?
5 kV - 69 kV
- does not tend to soften with increased temp.
- Cross-linked polyethylene (XLPE)
- Ethylene propylene rubber (EPR)
- Silicon
106
What happens to stranded conductors with air voids?
will have concentrated ESF between the strands
107
What does a strand shield?
a semi-conductor material that evens out the ESF
108
What is the strand shield layer called?
"Semi-Con"
109
What is an Insulation Shield?
a semi-con placed around the insulation to evenly disperse the ESF
110
What will bonding the insulation at both ends do?
may result in excessive sheath currents which may increase conductor temp.
111
what will bonding the insulation sheath at one end do?
can result in extremely dangerous voltages at the open end
112
what surrounds the insulation shield and why?
a conductive layer surrounds it so that its entire length is bonded well
113
What must insulation be rated for?
Line voltage
114
What are the insulation ratings?
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
Cables used in high-voltage and extra-high voltage are likely to be __________?
liquid or gas filled
116
what is the most common solid-dielectric used for cables?
Cross-linked polyethylene (XLPE) or ethylene-propylene rubber (EPR)
117
which of the cable shields must be grounded?
Insulation shield
118
what is the primary purpose of lead sheathing on a cable?
Prevent the ingress of moisture into the cable
119
what is the secondary purpose of a lead sheath on a cable?
mechanical protection
120
Which medium voltage cable is the most widely used for heavy industrial applications?
Teck Cable
121
What are the two most common voltage ratings of medium voltage cables used for power applications?
15 kV and 25 kV
122
two main factors which determine the voltage of an overhead transmission line?
- size of the load
| - distance between the source and load
123
How is HV AC usually transmitted over long distances?
Bare conductors suspended and insulated from steel towers
124
List two advantages that DC has over AC for HV transmission?
1. smaller conductor for same energy
| 2. no continuous capacitive charging & discharging
125
list 3 structural methods employed to safely isolate HV equipment?
1. elevation
2. substations
3. vaults
126
Switchgear includes: (6)
- CB's
- Fuses
- Load break switches
- disconnect/isolation switches
- "protective relay" circuitry
- Instrument transformers
127
Switchgear may be located where?
- outdoor
| - indoor (metal clad, metal enclosed)
128
Metal clad properties?
- Gear "racks out and in"
- Insulated bus bars
- shutters close
- DONT RACK UNDER LOAD
129
Metal Enclosed properties?
- Bare bus bar
- fixed switches and CB's
- Viewing windows
- louvers for ventilation
130
Arc Interruption methods : (6)
- 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
Horn Gap Switch properties?
- can interrupt small currents, 15A max
- Main and Aux contacts
- Air break
- Rural Overhead Distribution
132
Load break switch properties:
- can make and interrupt current
- compressed spring opens contact fast
- main contact opens
- AUX contact opens in Arc chute
133
Load break switch/fuse combo properties?
- cheaper than CB's
- current limited fuse interrupts faults
- "striker pin" of fuse trips load break switch
- all 3 lines open
134
What do load break switch/fuse combos prevent?
Single phasing
135
isolation switch properties?
- carry rated loads
- cannot make or break any current flow
- isolates after circuit is de-energized
- provides visual verification of contacts
136
disconnect switches properties?
- carry rated current
- can switch minimal current (less than .5A)
- used to isolate equipment
137
list 5 different circuit breakers?
1. Air/Air magnetic
2. Oil
3. Inert Gas
4. Vacuum
5. Air blast
138
Air/Air magnetic CB properties? Max ratings?
- 15kV and 3000A max
- Arc chutes or baffle plates
- magnetic blow out coils
- may incorporate a puffer
139
Bulk oil CB properties?
- All poles within a chamber
- indoor or outdoor
- older style
140
Minimum Oil CB properties?
- One oil vessel per pole
- sight gauges for checking oil
- containment curbs
141
Inert Gas CB properties?
- Sulphur Hexaflouride (SF6)
- maintained at low positive pressure
- very high dielectric strength
- all voltage ranges
142
Vacuum CB properties?
- Typically up to 35kV
- uses motor charged spring
- real estate friendly
143
Air blast CB properties?
- compressed air extinguishes arc
- up to 800 PSI
- All voltage ranges especially extra high voltage
- silencers may be required
144
What are CB reclosers used for and how do they work?
- exclusively for overhead lines
- burns branches off bare conductors
- may attempt reclosure 3 or 4 times
- 10-20 second delay after each trip
145
Common ratings of fuses for HV?
2.4kV to 69kV
| .5A to 400A
146
Distribution fuse properties?
- Pole top "distribution cut-out"
| - inner fuse blows releasing cartridge
147
Power fuse properties?
- Current limited like HRC fuses
- one time use
- very fast, 1/4 of a cycle
148
Solid material fuse properties?
- not current limiting, may take several cycles to open
- explusion type
- during fault, boric acid forms steam & water
149
What is a liquid fuse filled with?
oil
