High Voltage Terms and Concepts

Question 1

In a typical one-line diagram of a High Voltage distribution system, what are the 4 areas typically shown?

Answer 1

1. Generation
2. Transmission
3. Distribution
4. Utilization

Question 2

What is a typical voltage you would see at the generating station along a High Voltage distribution system?

Answer 2

13.8 kV

Question 3

What is a typical voltage you would see along the transmission line of a High Voltage distribution system?

Answer 3

500 kV

Question 4

What is a typical voltage you would see along the distribution line of a High Voltage distribution system?

Answer 4

34.5 kV

Question 5

What is a typical voltage you would see along the utilization line of a High Voltage distribution system?

Answer 5

600 v

Question 6

Canadian Electrical Code defines high voltage as voltages of 750 V and above, what ranges do the IEEE define as medium voltage, high voltage and extra high voltage?

Answer 6

Medium-voltage 2 400—69 000 V
High-voltage 115 000—230 000 V
Extra-high-voltage 345 000—765 000 V

Question 7

What is an electrostatic field?

Answer 7

Voltage produces an invisible field of force surrounding energized conductors. The direction and strength of the field depends on the instantaneous polarity and magnitude of the voltage. The conductor may or may not be carrying a current.

Question 8

Cable insulation can adversely be affected from High Voltage electric fields that can cause (5)

Answer 8

1. electric stress
2. voltage gradient
3. ionization and corona
4. strike distance and flashovers
5. creepage distance and tracking.

Question 9

When air breaks down and becomes a conductor, it is said to be?

Answer 9

Ionized

Question 10

What is electric corona?

Answer 10

It is ionized air which occurs when the voltage between conductors in air becomes so high that the electric stresses cause the surrounding air (insulation) to break down. The voltage gradient is highest on the air layers closest to the conductor.

Question 11

Corona is sometimes manifested on High Voltage overhead lines by a violet-coloured arc or streamer around the lines. What are some of the negative effects of corona?

Answer 11

1. There is a hissing sound
2. Ozone which is released by chemical reactions and smells
3. It represents a power loss in lines
4. Can cause radio and TV interference

Question 12

What is strike distance?

Answer 12

The purpose of insulation is to separate energized conductors from ground points and other phases. The higher the voltage the greater the separation required where the strike distance is this minimum distance to prevent flashover.

Question 13

In High Voltage systems, leakage or creepage current will always try to track across the insulation to ground. What do we do to prevent this current from damaging insulation?

Answer 13

We apply skirting on HV insulators and cable terminations to increase the creepage distance

Question 14

What do the initials BIL stand for and why is it important?

Answer 14

BIL is basic impulse insulation level which refers to the ability to withstand stresses due to voltage and is normally several multiples of the nominal voltage rating