High Voltage Flashcards
CEC Extra Low Voltage
≤ 30V AC
≤ 42.4 V DC
CEC Low Voltage
> 30V AC ≤ 1000V AC
> 42.4V DC ≤1060V DC
CEC High Voltage
> 1000V AC
> 1060V DC
IEEE/ANSI Medium Voltage
100V to 69,000V
IEEE/ANSI High Voltage
69KV to 230KV
IEEE/ANSI Extra High Voltage
230KV to 800KV
IEEE/ANSI Ultra high voltage
> 800KV
IEEE stands for?
Institute of Electrical and Electronic Engineers
ANSI stands for ?
American National Standards Institute
The 2 main factors in determining transmission line voltage is?
Load and Distance
also existing infrastructure
Generated voltage is less than transmission voltage due to?
Insulation contraint in the alternators
Gen Station typical voltage
10KV to 30KV
Transmission line voltage?
60kv to 800kv
Distribution Line voltage
`2.4kv to 69kv
Utilization line voltage
600V or less
For transmission voltages a general rule is ?
1000V per mile
Three classifications for HV networks?
Radial
Ring or Loop
Network or Grid
AC Transmission system types of overhead conductors
Stranded bare copper
Solid copper
Aluminum conductor steel reinforced (ACSR)
All aluminum conductor (AAC)
AC Transmission insulator materials
Porcelain
Glass
Polymer Resins (ie. silicon)
Insulators have _______ or ________ to increase ______ _______.
Skirts
Petticoats
Creepage distance
Types of insulators
Pin
Post
Suspension
Strain
Pin insulator characteristics
< 35 KV
Conductor fastened with a binding
Post insulator charac.
Like pin but stronger
Higher voltages
Conductor fastened with clamp
Suspension insulator charac.
Conductor hangs on the bottom
Modular (individual skirts are stacked for higher voltages)
Strain insulator charac.
used where line dead-ends or turns
strong tensile strength
individual skirts are stacked for higher voltage
Max distance of an underwater/underground cable?
40-50 km typically
Underground and underwater cable charc.
Expensive (high initial cost)
On going power losses with AC
High voltage DC cable charac.
No shred (DC has no hysteresis or eddy current losses)(no skin effect)
No capacitive losses
Expensive
Requires rectifier (at source) and inverter (at load)
What does ESF stand for?
Electrostatic Field
Electrostatic field charac.
- a voltage AC or DC produces an electrostatic field
- the size of the field depends only on voltage, not current nor AWG
- ESF is constantly changing in AC, steady in DC
- ESF forces may be concentrated or dispersed by conductor shapes
Voltage Gradient
ESF strength with respect to distance
Voltage density
Dielectric Strength
the voltage an insulator can withstand before breaking down
-in high voltage this is called “flashover”
- V/mm or V/in
Air = 3MV/m or 3000V/mm
Vacuum = 10 to the 12 V/M
Dielectric Constant
- how good an insulating material is as a capacitive medium
dielectric strength relative to a vacuum
vacuum is assigned “1”
air = 1.0006
mica = 3
Striking distance v.s. Creepage Distance
“flashover” vs “tracking”
Corona
- ionization (breakdown) of air caused by ESF
- Symptoms - RFI, hissing/crackling sound, violet/blue light, ozone.
What does BIL stand for?
Basic Impulse Insulation Level
BIL ratings can be __ to __ times the system voltage
5 to 30
Equipment over ___kv must be BIL rated
2.5
What is a Vault?
Fire resistant construction for housing HV elec equipment ie. tranformers etc.
Lightning Rods
- protect structures
- not connected to equipment
- electrode on top of a structure wired as short and straight as possible to an isolated ground electrode
- CEC 10-108, App. B and G
Series Reactors
- used to limit fault currents
- very little effect under normal conditions
- if a fault occurs, short circuit current is ‘choked’
- can allow for less expensive CBs having a lower interrupting rating
- also impedance grounding can achieve similar goals
2 types of instrument transformers used in HV
Voltage transformers (VTs and PTs) Current transformers (CTs)
Voltage transformers
- low VA rating
- 120 V secondary rating usually
- primary 3 A typical
- ground secondary
Current transformers
- in series with the primary
- 5 A secondary
- 1A secondary emerging
- ground secondary
Radial HV system charac.
Least expensive Simplest Only one supply Least reliable Least likely to backfeed
Ring/Loop HV system charc.
Requires 2 feeds Costs more More reliable May use auto transfer switch Subject to back feeds Allows maintenance via isolation
Network/Grid HV system charc.
Greatest reliability Most expensive Requires complex switching Used for hospitals, airports, penetentiaries Most danger of back feed
Methods of isolating AC transmission systems
Elevation (poles, towers)
Substations (fences)
Vaults
The WCB specifies that only persons who are _____ and _____ are allowed to work on HV.
Trained and qualified
Rural distribution systems primarily use?
Overhead lines
What effect does electric stress have on insulation?
Weakens insulation
Why is creepage distance kept relatively long?
to keep creepage current/leakage to a very low value
What is the function of a surge arrestor?
to divert a high voltage surge to ground by providing an easy conducting path (overvoltage)
What are the main parts of a surge arrestor?
Air gap
Thyrite material
What are the 5 general steps to be taken in a safe switching sequence?
- Disconnect
- Lock out
- Test for absent voltage
- Ground and short circuit
- Screen off neighbouring live parts
Single strand HV cable
- used for termination where flexibility is not required
- #2 or smaller, usually #10 or smaller
Concentric Stranding HV cable
- typical strandings are 7, 19, 37 etc.
- large undesirable air voids
- flexible
- difficult to remove insulation
Compressed stranding HV cable
- 97% of the concentric stranding cross section
- smaller air voids
- improves on concentric
Compact stranding HV cable
- 90% of the size of concentric
- preferred for HV
Bare overhead aluminum HV conductors
- OK for bends
- light
- not strong enough for long spans
Bare overhead aluminum conductor steel reinforced (ACSR)
- center strand is steel for strength
- steel center is called the “messenger”
- outer strands are aluminum
Bare overhead aluminum conductor composite reinforced (ACCR)
- strong and light
- carbon fibre or fibreglass messenger
- alumina fibers
Are bare conductors or insulated conductors preferred for HV?
Bare, due to lack of continuous capacitive charging and discharging losses associated with insulation
Types of HV cable
Armoured cable - (TECK)(ACWU), MI
Wire Armoured - Submarine cable
Tape Armoured - steel tape armoured (STA)
Paper Insulated Lead Covered (PILC) - underground
Concentric Neutral - underground
Another name for a concentric neutral cable?
Underground Residential Distribution (URD)
Concentric neutral cable (URD) characteristics
- cable has shielding which also serves as a neutral
- neutral has 100% of ampacity of center conductor
- 3 Ø 4W uses 33% concentric neutral
- connect all 3 neutrals = 100% ampacity
Purpose of cable jackets?
- provide mechanical protection
- seal out contaminants
- reduce friction
- identify cable specs
Cable dielectrics =
Insulation
Types of cable insulation (dielectrics)
- Rubber
- Thermoplastic
- Thermoset
- Paper insulated lead covered (PILC)
Rubber insulation charc.
- 600V to 15KV
- moisture resistant
Thermoplastic insulation charac.
- 5KV to 15KV
- tends to soften at higher temps and get brittle at lower temps
- Polyvinyl chloride (PVC)
- Polyethylene (PE)
Thermoset insulation charac.
- 5KV to 69KV
- does not tend to soften with increase in temp
- cross linked polyethylene (XLPE)
- ethylene propylene rubber (EPR)
- silicon
Paper insulated lead covered insulation charc.
- Paper insulated
- Lead covered
Concentrated ESF can lead too?
Corona which will compromise the insulation and lead to flashover
What will even out the ESF surrounding the entire conductor?
“strand shield” of semiconductor material
The stranded shield layer is often referred to as?
Semi-con layer
What causes the ESF to be evenly distributed throughout the insulation?
“Insulation shield” which is at ground potential
The insulation shield is usually bonded?
At both ends but not always
B/C the insulation shield is not an ideal conductor, a _________ surround it so that it’s entire length is bonded as well.
Conductive layer (copper or aluminum tape)
or
Conductive neutral
Cables are rated ____ ____?
Line voltage ( 15KV, 28KV, 35KV )
HV cable insulation are rated by?
Percentage (100%, 133% or 173%)
100% insulation
cables in grounded wye system or ungrounded system where faults cleared in 1 minute or less
133% insulation
- cables in ungrounded systems where faults are cleared in 1 hour or less
- most common
173% insulation
- cables in ungrounded systems where faults are cleared in more than 1 hour
- not common
Percentages represent _______ ?
Thickness
The thicker the insulation, the more _____ ______ it can withstand before electrical breakdown.
Voltage stress
Dielectric strength is
- the ability of the dielectric to withstand electrical breakdown under the influence of voltage
- voltage per unit thickness
What is the most commonly used insulation for medium voltage cables?
XLPE (cross-linked polyethylene)
Cables rated over ______ V require a strand shield?
1000V
Sheath voltage depends on?
Current magnitude
Cable length
Insulation thickness
What is sheath voltage?
an induced voltage in the shield caused by AC current flowing in the conductor
Which cable shield must be grounded?
Insulation shield
A concentric neutral cable is?
- One whose metallic insulation shield has as much ampacity as the current-carrying conductor
- A group of bare or tin-coated copper wires wound helically over the insulation shield
What factor determines the minimum available AWG size of a HV cable?
Voltage
The ampacity of a non-shielded cables is based on an ambient temp. of?
30 degrees
The ampacity of a shielded cable is based on an ambient temp. of?
40 degrees
What are the 2 most common voltage ratings of medium-voltage cables used for power applications?
15KV and 25KV
Where will there be a high concentration of stress lines when removing the insulation shield?
at the point where the insulation shield is discontinued
The 2 most common types of stress relief are?
Geometric (Stress cone)
Capacitive (Hi-K)
Geometric stress relief
A factory made stress cone is used to build up the thickness of the insulation in the region of high electric stress.
Termination Classifications
Class 1 - requires stress relief, anti-tracking and a seal to the environment.
Class 2 - requires stress relief and anti-tracking
Class 3 - only requires stress relief
All cables rated over ____KV require an insulation shield?
5KV
Switchgear includes
Circuit breakers Fuses Load break switches Disconnect/Isolation switches Protective relay circuitry Instrument Transformers
Metal Clad switchgear
Gear “racks out”
Insulated bus bars
Shutters close
Dont rack under load
Metal enclosed switchgear
Bare bus bars Fixed switches and CBs Viewing window to see contacts Louvers for ventilation Sections may or may not be divided
Arc interruption methods
Fast separation of contacts using compress springs
Auxiliary “flicker” contacts and “arc chutes”
Magnetic force
Temperature rise
Compressed air
Oil immersed contacts
Horn gap switch
Can interrupt small currents, 15A max
Air break
Load break switch
Can make and interrupt current Compressed spring opens contacts fast Main contact opens Aux contact opens in arc chute Arc is extinguished
LBS/Fuse Combo
Cheaper than CBs Current limiting fuses interrupt faults Striker pin of fuse trips load break All 3 lines are opened Prevents single phasing
2 purposes of the striker pin?
Trips load break switch
Visual indicator of blown fuse
Isolation switches
Carry rated load
Cannot make or break any current flow
Isolated after circuit is de-energized
Provides visual verification of contacts
Disconnect switches
Carry rated current
Can switch minimal current <0.5A
IE. magnetizing current
Used to isolate equipment
Circuit Breakers types
Air/air magnetic Oil (bulk oil/dead tank or minimum oil/oil poor) Inert gas Vacuum Air blast
Air/air magnetic CB
Maximum ratings - 15KV, 3000A
Arc chutes
blow out coils
puffer
Bulk Oil CB
Oil is a good dielectric
Older style
Indoor or outdoor
All poles within a common oil chamber
Minimum Oil CB
One oil vessel per pole
Sight gauges for checking oil
Containment curbs
Inert Gas CB
Sulphur Hexaflouride
Maintained at a low positive pressure
Very high dielectric strength
Used in all voltage ranges
Vacuum CB
Typically up to 35kv Uses a motor charged spring Vacuum is an excellent dielectric The fast action may cause voltage spikes Real estate friendly
Air blast CB
Compressed air extinguishes the arc Compressed air also may operate switches May be up to 800 PSI All voltage ranges especially extra high voltage (800KV) Silencers may be required
CB Reclosers
Used exclusively for overhead lines Burns branches off bare conductors May attempt reclosure 3 or 4 times 10 - 20 second delay after each trip Lock out if unsuccessful Part of a coordinated system
Distribution Fuses
Pole top “distribution cut out”
Tensioned inner fuse link blows releasing cartridge
Cartridge swings down
Can be used indoors with a muffler/condenser
Power fuses
Current limiting similiar to HRC fuses One time use Very fast May cause voltage transients Non expulsion type Striker pin operates LBS
Solid material fuse
Not current limiting - may take several cycles to open
Expulsion type - boric crystals surround fuse link, during fault boric acid forms steam and water, pressure helps extinguish the arc
Some may have replaceable cartridge
Liquid Fuse
Mostly obsolete
Oil filled
Non renewable
Why must the shielding be removed for an adequate distance when terminating a cable?
to minimize leakage currents
Why are outdoor terminations of the skirting type?
to increase creepage distance
Cable Insulation specs
Prevents ground faults and short circuits line to line
Leakage currents exist in all insulators except a perfect vacuum
Resistance - In M ohms range, tested with high DC voltage, negative temp coefficient.
Dielectric Absorption
Dangerous effect of cable capacitance after DC (Megger) is removed
In AC circuits the capacitive effect is power loss
Megger Testing specs
- Generally non-destructive
- H.V. DC applied for some time
- DC values = 500V - 15KV typically
- Applied time depends on the capacitance of cable or equipment
- Readings are taken after they stabilize
- Isolate equipment/cable from ALL sensitive loads
- Isolate the cable ends from ground and “corona proof”
How should you connect the megger for HV testing?
- positive lead to ground (return/path/tape shield)
- negative lead to test conductor/equipment
- guard lead to dielectric
Reverse polarity megger test is preferred due to?
Electroendosmosis
Guard lead specs
Also a return path but it bypasses the meter
May connect to the dielectric to shunt surface current around the meter
Capacitive charging current
Initial “inrush”
Varies depending on cable/equipment
May last for several seconds
Absorption Current
Charge is building in the cable/equipment
Bare bus bars = short duration
Long cables = long duration
Proof Test
Test the cable after it is pulled in to prove it is good
Before terminating so time and materials may be saved
Short time test
60 second duration
Polarization Index Test
Two readings - 1 minute duration and 10 minute duration
1 min reading
Ratio below 1.5 indicates a problem
Step voltage test
- Voltage is stepped up in 60 second intervals
- Current should increase in linear pattern and resistance should remain constant
- If M ohms decreases with increasing voltage, insulation may be bad.
Dielectric Absorption Tests (Megger test)
Readings are taken over a 10 minute period
The shape of the curve is interpreted
Hi-Pot Testing
- DC or AC
- Possibly destructive
- More specialized than megger
- May use voltages of 30KV, 60KV or higher
- Highly trained personnel
Hi-Pot Thumping
- Hi-Pots may be used to find underground cable faults
- A spark gap is connected in parallel to the cable
- Hi-Pot is connected and the gap sparks
- Increasing the gap space eventually cause the sparking to stop
- The cable fault begins sparking/thumping
- A ground microphone and headset can usually locate the fault location
Voltage classifications for rubber gloves Beige
Class 00
Proof - 2500V AC/10,000V DC
Max Usage - 500V AC/ 750V DC
Voltage classifications for rubber gloves Red
Class 0
Proof - 5000/20000V
Max use - 1000/1500V
Voltage classifications for rubber gloves White
Class 1
Proof - 10,000/40,000V
Max use - 7500/11250V
Voltage classifications for rubber gloves Yellow
Class 2
Proof - 20KV/50KV
Max use - 17kv/25.5kv
Voltage classifications for rubber gloves Green
Class 3
Proof - 30kv/60kv
Max use - 26.5kv/39.75kv
Voltage classifications for rubber gloves Orange
Class 4
Proof - 40kv/70kv
Max use - 36kv/54kv
Runway lighting circuits can be loaded to a maximum of ___KW for an operating voltage of _____ V AC.
20kw
3030V AC
6.6A
What HV measuring tool is used for non contact voltage testing?
Modiewark
What voltages do the primary and secondary wires typically carry for runway lighting?
Primary 5000V +
Secondary less than 50V
What does SCADA stand for?
Supervisory Control And Data Aquisition
What does ASLC stand for?
Airport series lighting cable
Most common failure for runway lighting?
Isolation transformer
What is the purpose of the strand shield on HV cables?
prevents the corona from developing in the air pockets that might be between the conductor and insulation by bonding the outer strands together
What is the main difference between concentric neutral cables and shielded cables?
Concentric neutral has the ability to use its insulation shield as a return path or neutral, tape does not.
A DC Hi-Pot tester measures 3 types of current?
Leakage
Capacitive
Absorption
Name the three most common HV switches used today?
Load break
Disconnect/Isolation
Horn gap
Name the fastest acting HV fuse and a unique feature it has?
Current limiting
Striker pin to open the load break to prevent single phasing
What two functions does a distribution cut out offer when it drops out?
Large re striking distance
Visual indictation
What three things are required in order from tracking to occur?
Moisture
Contamination
HV
