L HV

Question 1

IEEE defines High Voltage as

Answer 1

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

Question 2

What voltage does Distribution system use?

Answer 2

Medium voltage

Question 3

Generated voltages are between

Answer 3

10 kV to 20 kV 13.8 kV is common

Question 4

Transmission voltages are between

Answer 4

60 kV to 765 kV

Question 5

Electrostatic field

Answer 5

an invisible field of force surrounding energized conductors

Question 6

Electric Stress

Answer 6

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

Question 7

Describe stress gradient or voltage gradient

Answer 7

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

Question 8

Ionization and corona

Answer 8

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.

Question 9

Define leakage or creepage current

Answer 9

the current trying to track across the insulation to ground

Skirting on HV insulators and cable terminations increase creepage distance

Question 10

BIL

Answer 10

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

Question 11

an electrostatic field is due to

Answer 11

voltage

Question 12

3 general HV distribution classifications

Answer 12

Radial
Ring
Network

Question 13

Lightning arrestors

Answer 13

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

Question 14

Series Reactors

Answer 14

used to limit fault currents in the system

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

Question 15

Viewing windows in Unit substations

Answer 15

visible indication of a switch being open or closed

Question 16

What is key interlocking

Answer 16

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

Question 17

list the steps for safe switching

Answer 17

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

Question 18

Step and touch voltage

Answer 18

works on voltage gradient

touch voltage is the most lethal for of step voltage

Question 19

Ground mat effectively overcomes the problem of

Answer 19

step and touch voltages

Question 20

Some medium voltage cable

Answer 20

Teck
Shielded power cable
Concentric neutral cable
PILC
Submarine cable
Mining cable

Question 21

Types of dielectrics

Answer 21

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

Question 22

Purpose of the strand shield

Answer 22

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

Question 23

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

Answer 23

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.

Question 24

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

Answer 24

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

Question 25

Sheath currents

Answer 25

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

Question 26

100% insulation

Answer 26

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

Question 27

133% insulation

Answer 27

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

Question 28

Describe Concentric neutral cable

Answer 28

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

Question 29

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

Answer 29

NO

Question 30

Two common types of stress relief

Answer 30

Geometric | Capacitive

Question 31

Three types of HV termination Classifications

Answer 31

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

Question 32

Metal-clad switchgear

Answer 32

components and CB can be readily removed from the cubicle

Question 33

Metal-enclosed switchgear

Answer 33

more difficult to remove parts and CB

Question 34

HV switches are primarily rated for

Answer 34

Voltage Hz Current

Question 35

HV switches may be broadly classified as

Answer 35

Load-break | Non-load-break

Question 36

list types of HV switches

Answer 36

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

Question 37

HV fuses are classified as

Answer 37

expulsion or non-expulsion

Question 38

four types of HV fuses

Answer 38

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

Question 39

five types of HV CB

Answer 39

``` Air-magnetic Air-blast Vacuum Oil SF6 ```

Question 40

What provides the force that moves the contacts in most CBs

Answer 40

springs

Question 41

The ideal insulator will have

Answer 41

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

Question 42

the purpose of insulation

Answer 42

to prevent current flow between an energized line and ground

Question 43

Insulation resistance decreases with rise in temp because

Answer 43

insulators have a negative temperature coefficient of resistance

Question 44

reasons for insulation deterioration

Answer 44

``` high temp electric stresss vibration chemical changes moisture ```

Question 45

Dielectric loss (power loss)

Answer 45

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

Question 46

Dielectric Absorption

Answer 46

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