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72 Cards in this Set
- Front
- Back
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CCVT [ capacitive Voltage transformer] |
Communication |
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On-Load tap changers |
Voltage regulation |
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What can be used to overcome touch and step voltage? |
By using Gradient Control mat. |
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Purpose of CTs and PTs in HV system |
-To monitor voltage and current levels -isolate themselves from hv systems |
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Function of Surge arrestors |
Protect against over voltage (i.e. Lightning) |
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How does a gradient control mat work and what is it's purpose? |
A metal grid connected to the ground structure, a person standing on it will have the same voltage potential |
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Potheads |
Terminations |
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Why is HV more dangerous than LV? |
-HV can jump across air gaps to grounded objects -short cct between phases + ground = high 'fault current'
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Dielectric Is another term for |
Insulation |
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The most widely used insulation for medium-Voltage cables? |
XLPE [ thermoset plastic type] |
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What are the primary factors that determine the voltage magnitude used on a HV overhead line or underground cable? |
Its load and distance |
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Corona is |
High voltage between conductors breakdowns causing the air to become ionized - represents a power loss in lines and can cause radio and TV interference |
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All cables over ___ require an insulation shield |
5kv |
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Why must the shielding be removed for an adequate distance when terminating a cable? |
To cut down leakage current |
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Is it permissible to run HV and LV cables in the same cable tray? |
Yes but they have to be separated by sheet of metal |
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What is dielectric strength and what unit is it measured in? |
Ability of the insulator to withstand ( EBUV) electrical breakdown under voltage. V/mm |
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What are the Two principal types of shielding for HV cables |
-Strand [conductor] shield -insualtion shield |
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With a Strand shield installed the flux lines emanate from there material instead of the conductor strands. Why is this significant? |
The flux line no longer pass through air voids and ionization is prevented. |
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Why is the insulation shield grounded? |
In order for the magnetic flux lines to be in uniform |
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Medium voltage is classified as_______ High voltage is classified as ______
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2400v - 69000 v 115000V-230k 345k extra hv |
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Name each part |
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What factors determine the minimum available AWG size of a HV cable? |
Higher the voltage larger minimum conductor size |
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Which switch gear offers better isolation in the event of a fault? |
Metal-clad switchgear |
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Which switch gear uses "rackable" or "drawn out" circuit breaker and what is its purpose? |
Metal-clad switchgear, permits physical separation of contacts |
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Metal-Enclosed switch gear uses a _____ style, its circuit breakers are_______ and the bus bars are ________ |
Cubicle / stationary / not insulated |
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Most load break switches are |
Air break type |
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LBS require ________ unlike cct breakers. |
Fuses |
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What occurs During an Arc chute with LBS |
main contacts open and Aux contacts close up then open to extinguish the arc |
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HV switches are rated for which three primary electrical quantities |
Voltage, frequency, current |
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4 types of hv switches are |
Isolating Horn gap Load break Disconnect |
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Isolation switches |
-operation under no load - no aux contacts, strictly for isolation -may be operated using a hot stick |
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Horn gap switches |
-Interrupts small load current [ 15A] - aux and main contacts |
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Disconnect switch |
-Visual proof of HV Isolation -up to 60kV loads - in series with CB |
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What 3 things are required to interrupt a arc? |
- High speed contact separation - Arc Cooling [air, oil] - zero current |
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What factors determine how long an arc lasts? |
higher the voltage, higher the current, longer the arc |
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Five types of circuit breakers found on HV installations are |
- Air Magnetic -vacuum -oil [ bulk / minimum] -Air Blast -sf6 |
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The Basic impulse level [BIL] is |
- the longer an insulation is exposed to HV stress the less it can effectively withstand stress - usually much higher than the nominal voltage rating |
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Explain Air Magnetic CB |
- For medium Voltage, up to 15kV nominal / 3000 A -interrupts arc in air, aided by baffle plates and blow out coil [ series connected coil of low impedance] |
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Explain Vacuum CB |
-Compact unit compared to air magnetic -very quiet operation - requires little maintenance - shortest contact travel distance [6 -25mm] -high dielectric strength [ 5MV/in] |
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Oil CB |
- higher dielectric strength than air magnetic - comes in two types, Bulk [older type] and minimum [Modern design] |
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What are negatives of Bulk oil CB? |
- oil is flammable - very large - leakage is environment hazard |
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Minimum Oil CB |
-used for 34.5kV applications-only a few litres of oil are used - oil forced under high pressure across contacts to cool and interrupt current arc |
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Air blast CB |
-outdoor use, up to 800kV - loud noise operation - compressed air [400-800 psi] is driven across serpating contacts - filters keep air clean and dry |
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SF6 [ sulphur hexafluroride] |
- expensive but reliable, low maintenance, compact size -motor-charge spring, hydraulic or pneumatic method for contact movement - consists of fixed and moving contacts -cools aswell as extinguishes current arc |
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Label each |
A. Generating Station B. Transmission Line C. Distribution line D. Utilization E. Substation |
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Voltage's used in transmission systems are generally |
60 KV to 765 KV [BC Hydro's highest is 500 KV] |
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Identify the three classes of HV Distribution Systems |
- Radial - least expensive and simplest - Ring [Loop] - supplied from two directions - Network - reliable but most expensive |
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Contact movement is nearly always done with |
Charged Springs [hydraulic and pneumatic] |
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What type of medium voltage requires a pothead? |
Pilc |
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Circuit breakers can be identified by |
The medium in which the current arc is interrupted |
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What is the primary function of key interlocking? |
To prevent the switch from being open or closed under load conditions |
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Function of a recloser CB and its location |
Able to open and close for temporary faults, located on OH power lines |
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5 steps of safe switching |
1. Disconnect 2. Lockout 3. Test for no voltage 4. Ground and short cct 5. Screen off neighbouring live parts |
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Allows the breaker to utilize momentary absences of current flow? |
Current zero |
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Minimizes the time it takes an Arc to form while the contacts separate |
High speed switching |
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Which of the 5 common CBs is not used in medium Voltage installations? |
Air blast [higher voltage applications] |
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Which of the 5 common CBs had traditionally been used on all HV levels? |
Oil |
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Which of the 5 common CBs does not have arching [aux] contacts? |
Vacuum [uses fixed and moving contact] |
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Which of the 5 common CBs uses a "puffer" and for what purpose? |
Air magnetic, To aid in arc extinction |
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The 3 benefits of using HV are |
- Reduced line current - Reduced power losses - Reduced weight |
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Stress on the insulation decreases with distance is called |
Voltage gradient |
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4 types of hv fuses |
- Distribution cutout Power Fuses: - current limiting - non expulsive [one time use] - solid material ( boric acid fuse) - expulsions - liquid type - non expulsion [one time use] |
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ALL fuses have a Inverse time characteristic which means... |
larger the overcurrent then faster the interruption |
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Fuses are rated for |
> Voltage - higher the voltage rating the longer the fuse must be > Current rating - magnitude of current it can CARRY continuously without blowing > Interruption current rating - magnitude of current it can INTERRUPT without damaging the fuse |
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The current limiting fuse is a ________ and must be ________ |
One-time fuse / replaced after it blows |
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Current limiting fuse only requires a quarter of a cycle to |
Limit a short circuit current, open and extinguish the arc |
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On a current limiting fuse what activates the trip mechanism on the LBS and what does it prevent? |
The striker pin, it prevents single phasing |
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Why is solid material fuse [boric acid] not referred to as a current limiting fuse? |
It simply limits the duration not the magnitude. [Many cycles to catch the fault] |
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What is required to use a solid material fuse indoors? |
A condenser, Muffler or Snuffer in order to collect the expelled materials |
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A liquid fuse is |
Non expulsion and one time use only. no longer used |
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Distribution cutout uses what kind of fuse? |
A Drop-out type [ flexible pigtail] |
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What two things are accomplished by using a Dropout action? |
Visible identification and low chance of a restrike |
