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Year 4 Electrical > L HV > Flashcards

L HV Flashcards

1
Q

IEEE defines High Voltage as

A

2 400 to 69 000 Medium Voltage
115 000 to 230 000 High Voltage
345 000 to 765 000 Extra High Voltage

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2
Q

What voltage does Distribution system use?

A

Medium voltage

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3
Q

Generated voltages are between

A

10 kV to 20 kV 13.8 kV is common

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4
Q

Transmission voltages are between

A

60 kV to 765 kV

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5
Q

Electrostatic field

A

an invisible field of force surrounding energized conductors

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6
Q

Electric Stress

A

the electrostatic field surrounding a conductor creates an electric stress on the insulation that surrounds a conductor

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7
Q

Describe stress gradient or voltage gradient

A

as the flux lines leave the surface and diverge outward the stress on the insulation decreases with distance

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8
Q

Ionization and corona

A

when air breaks down and becomes a conductor it is said to be ionized
Electric corona occurs when the voltage in the air layers closest to the conductor become so high that the insulation breaks down.

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9
Q

Define leakage or creepage current

A

the current trying to track across the insulation to ground

Skirting on HV insulators and cable terminations increase creepage distance

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10
Q

BIL

A

basic impulse insulation level
the ability of insulation to withstand stresses
System voltage stresses and transient voltages

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11
Q

an electrostatic field is due to

A

voltage

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12
Q

3 general HV distribution classifications

A

Radial
Ring
Network

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13
Q

Lightning arrestors

A

diverts a high-voltage surge to ground by using thyrite and an air gap

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14
Q

Series Reactors

A

used to limit fault currents in the system

permit the installation of CBs having lower interrupting ampacity than might otherwise be required.

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15
Q

Viewing windows in Unit substations

A

visible indication of a switch being open or closed

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16
Q

What is key interlocking

A

safety feature used in HV installations that will unload a non-load break switch

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17
Q

list the steps for safe switching

A 
Disconnect
Lock out
Test for absence of voltage
Ground and short-cct
Screen off neighbouring live parts
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18
Q

Step and touch voltage

A

works on voltage gradient

touch voltage is the most lethal for of step voltage

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19
Q

Ground mat effectively overcomes the problem of

A

step and touch voltages

20
Q

Some medium voltage cable

A 
Teck
Shielded power cable
Concentric neutral cable
PILC
Submarine cable
Mining cable
21
Q

Types of dielectrics

A

XLPE (thermoset) softer and more flexible
PVC (thermoplastic)
PE (thermoplastic)
Synthetic Rubber

22
Q

Purpose of the strand shield

A

an insulating semi-conductor layer applied directly over a stranded conductor that bonds the conductor stands making the electrostatic line emanate from it instead of the strands thus preventing ionization

23
Q

what does the ionization of air in the air gaps of a stranded conductor of a HV cable cause

A

breakdown of the insulation because the ionized air will burn into the insulation
plus ionized air gives of ozone which further break down the insulation.

24
Q

List the function of the grounded shield (semi-con)

A

makes the flux uniform in the cable insulation
Suppresses radio and TV interference
Acts to protect life in the event of mechanical damage

25
Q

Sheath currents

A

Most HV cables have their shields grounded at both ends to prevent the CT effect

26
Q

100% insulation

A

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

27
Q

133% insulation

A

for use in ungrounded systems, where ground faults are eliminated in no more than one hour

28
Q

Describe Concentric neutral cable

A

a cable whose metallic insulation shield has as much ampacity as the current-carrying conductor (URD)

29
Q

Can a shielded cable’s insulation-shield metallic tape(s) to be used as a return neutral

A

NO

30
Q

Two common types of stress relief

A

Geometric

Capacitive

31
Q

Three types of HV termination Classifications

A

Class 3 Stress relief only
Class 2 Stress relief and anti-tracking
Class 1 Stress relief, anti-tracking and seal to environment

32
Q

Metal-clad switchgear

A

components and CB can be readily removed from the cubicle

33
Q

Metal-enclosed switchgear

A

more difficult to remove parts and CB

34
Q

HV switches are primarily rated for

A

Voltage
Hz
Current

35
Q

HV switches may be broadly classified as

A

Load-break

Non-load-break

36
Q

list types of HV switches

A

Isolating made for operation under no load
Horn Gap
Load-break can make, carry, and interrupt normal-load current
Disconnect provides a visible isolation distance and can open or close a cct if there is no load and at same potential

37
Q

HV fuses are classified as

A

expulsion or non-expulsion

38
Q

four types of HV fuses

A

Current-limiting non-expulsion type
Solid-materials or boric-acid
Liquid (dated)
Distribution cut-outs (most often used)

39
Q

five types of HV CB

A 
Air-magnetic
Air-blast
Vacuum
Oil
SF6
40
Q

What provides the force that moves the contacts in most CBs

A

springs

41
Q

The ideal insulator will have

A

infinite resistance (no conductivity)
all absorption phenomena and dielectric losses are absent
a perfect vacuum is the only known perfect insulator

42
Q

the purpose of insulation

A

to prevent current flow between an energized line and ground

43
Q

Insulation resistance decreases with rise in temp because

A

insulators have a negative temperature coefficient of resistance

44
Q

reasons for insulation deterioration

A 
high temp
electric stresss
vibration
chemical changes
moisture
45
Q

Dielectric loss (power loss)

A

when insulation is subjected to AC voltage the electrons in the atoms are strained producing heat

46
Q

Dielectric Absorption

A

when the positive and negative charges separate when a DC voltage is applied.
People that test insulation must be alert to the this potentially hazardous dielectric absorption phenomenon

Year 4 Electrical (6 decks)

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