L HV Flashcards
IEEE defines High Voltage as
2 400 to 69 000 Medium Voltage
115 000 to 230 000 High Voltage
345 000 to 765 000 Extra High Voltage
What voltage does Distribution system use?
Medium voltage
Generated voltages are between
10 kV to 20 kV 13.8 kV is common
Transmission voltages are between
60 kV to 765 kV
Electrostatic field
an invisible field of force surrounding energized conductors
Electric Stress
the electrostatic field surrounding a conductor creates an electric stress on the insulation that surrounds a conductor
Describe stress gradient or voltage gradient
as the flux lines leave the surface and diverge outward the stress on the insulation decreases with distance
Ionization and corona
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.
Define leakage or creepage current
the current trying to track across the insulation to ground
Skirting on HV insulators and cable terminations increase creepage distance
BIL
basic impulse insulation level
the ability of insulation to withstand stresses
System voltage stresses and transient voltages
an electrostatic field is due to
voltage
3 general HV distribution classifications
Radial
Ring
Network
Lightning arrestors
diverts a high-voltage surge to ground by using thyrite and an air gap
Series Reactors
used to limit fault currents in the system
permit the installation of CBs having lower interrupting ampacity than might otherwise be required.
Viewing windows in Unit substations
visible indication of a switch being open or closed
What is key interlocking
safety feature used in HV installations that will unload a non-load break switch
list the steps for safe switching
Disconnect Lock out Test for absence of voltage Ground and short-cct Screen off neighbouring live parts
Step and touch voltage
works on voltage gradient
touch voltage is the most lethal for of step voltage
Ground mat effectively overcomes the problem of
step and touch voltages
Some medium voltage cable
Teck Shielded power cable Concentric neutral cable PILC Submarine cable Mining cable
Types of dielectrics
XLPE (thermoset) softer and more flexible
PVC (thermoplastic)
PE (thermoplastic)
Synthetic Rubber
Purpose of the strand shield
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
what does the ionization of air in the air gaps of a stranded conductor of a HV cable cause
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.
List the function of the grounded shield (semi-con)
makes the flux uniform in the cable insulation
Suppresses radio and TV interference
Acts to protect life in the event of mechanical damage
Sheath currents
Most HV cables have their shields grounded at both ends to prevent the CT effect
100% insulation
permissible in solidly grounded systems, or in ungrounded systems where ground faults are eliminated in no more than one minute
133% insulation
for use in ungrounded systems, where ground faults are eliminated in no more than one hour
Describe Concentric neutral cable
a cable whose metallic insulation shield has as much ampacity as the current-carrying conductor (URD)
Can a shielded cable’s insulation-shield metallic tape(s) to be used as a return neutral
NO
Two common types of stress relief
Geometric
Capacitive
Three types of HV termination Classifications
Class 3 Stress relief only
Class 2 Stress relief and anti-tracking
Class 1 Stress relief, anti-tracking and seal to environment
Metal-clad switchgear
components and CB can be readily removed from the cubicle
Metal-enclosed switchgear
more difficult to remove parts and CB
HV switches are primarily rated for
Voltage
Hz
Current
HV switches may be broadly classified as
Load-break
Non-load-break
list types of HV switches
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
HV fuses are classified as
expulsion or non-expulsion
four types of HV fuses
Current-limiting non-expulsion type
Solid-materials or boric-acid
Liquid (dated)
Distribution cut-outs (most often used)
five types of HV CB
Air-magnetic Air-blast Vacuum Oil SF6
What provides the force that moves the contacts in most CBs
springs
The ideal insulator will have
infinite resistance (no conductivity)
all absorption phenomena and dielectric losses are absent
a perfect vacuum is the only known perfect insulator
the purpose of insulation
to prevent current flow between an energized line and ground
Insulation resistance decreases with rise in temp because
insulators have a negative temperature coefficient of resistance
reasons for insulation deterioration
high temp electric stresss vibration chemical changes moisture
Dielectric loss (power loss)
when insulation is subjected to AC voltage the electrons in the atoms are strained producing heat
Dielectric Absorption
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
