High Voltage Flashcards
The alternator at a large hydro dam will probably generate power at the 138kV level. (T/F)
False
10-20kV
Most common 13.8kV
Name the invisible field of force surrounding energized conductors
Electrostatic field
What does BIL stand for?
Basic impulse insulation level
What is the significance of the BIL with regards to the insulation on HV equipment?
Must be rated higher than nominal system voltage
What are the three general classifications on HV distribution systems?
Radial - cheap, least reliable
Loop
Network - most reliable
Which of the three general classifications of HV distribution is the simplest and cheapest?
Radial
Which of the three general classifications of HV distribution would be typically used to supply a major airport or hospital?
Network/grid
What is the primary purpose of a series reactor in a HV system?
Limit fault currents
Give two advantages of utilizing high voltage/low current for transmission purposes
Smaller conductors
Less line drop/loss
Which is more likely to be higher:
a. Step voltage
b. Touch voltage
Touch voltage
Give the four steps, in order, for a high voltage fuse disconnect
- Open disconnect
- Remove key
- Open fuse compartment
- Change fuse
What type of grounding system can be installed in order to overcome the dangers of step voltage/touch voltage?
Ground matt or grid
What is this?
A lightning arrestor
What is the minimum safe distance (limit of approach) for voltages up to 75kV?
3m/10ft
What are two major physical differences between LV cables and HV cables?
HV cables have shielding
HV cables have thicker insulation
What material do most ‘medium voltage’ cables use for their insulation?
Thermoset
XLPE (cross linked polyethlene)
Name three (of the six) most common types of medium voltage cables
Teck
Mining
Concentric neutral
Submarine
Shielded power cables
PILC
Define dielectric strength
The ability of a material to withstand break down when subjected to a voltage
What are two types of shielding for HV cables?
Strand/conductor shield
Insulation shield
Which of the two shielding methods is used in order to overcome the problem of ionized air between stranded conductors?
Strand/conductor shield
Would a HV cable have its shield grounded at one end or both ends?
Both ends, in order to avoid having a potentially dangerous voltage appear on the shield
A dielectric is more commonly known as an……
Insulator
What percent voltage does a cable connected to a grounded system use?
100%
What percent voltage does a cable connected to an ungrounded system use?
133%
Not a question. Study this bitch
Yaga
Why are the ampacities of tables 1,3 higher than 2,4?
Better heat dissipation when run in free air
When referring to HV cable installation, the term ‘stress relief’ refers to a device that relieves a physical strain due to gravitational stresses. (T/F)
False
What are two types of stress relief?
Geometric
Capacitive
Why is a conductor shield required?
To avoid ionisation of free air
Why is an insulation shield required?
To avoid concentration of electrostatic flux
A stress relief device is required if the cable is not of the shielded type. (T/F)
False
A circuit breaker can be readily removed from….
Metal-clad switchgear
What is the name used for the long fibreglass tube used for opening and closing distribution cutouts?
Hot stick
What is a fuses current rating?
The continuous current rating (max normal current)
What is a fuses interrupting rating?
The max fault current it can handle
What is the main function of the horns on a horn-gap switch?
Spare the main contacts from arcing
List the three main ratings for a HV fuse
Voltage
Continuous current
Interrupting current
A 25kV URD cable will have strand and insulation shielding in its construction. (T/F)
True
Extra high voltage cables will likely use a thermoplastic insulation. (T/F)
False
Dielectric strength of a material is the ability of the dielectric to withstand electrical breakdown under the influence of frequency. (T/F)
False
Dielectric strength will increase if a dielectric becomes heated. (T/F)
False
XLPE is a thermoset plastic. (T/F)
True
Most HV alternators generate electrical power at voltages between 20kV and 40kV to reduce line loss. (T/F)
False
10kV-20kV
An AC test of insulation integrity is achieved by comparing the watts of the cable to the VA of the cable. The insulation under test is considered good when the power factor of the cable is closest to zero. (T/F)
True
The conductor shield overcomes the problem of ionized air between the conductor strands. (T/F)
True
The insulation shield creates an equidistant radial flux in the insulation. (T/F)
True
If the insulation shield was constructed of lead, it would also prevent the ingress of moisture into the cable. (T/F)
True
A pothead is a termination method that has been traditionally used on shielded power cables. (T/F)
False
How are lightning arrestors connected to the load?
In parallel
How are reactors connected to the load?
In series
