Lecture 10 - Short circuits and protection systems

Question 1

Name some causes of faults

Answer 1

• Faults in apparatus
• Branches and trees on OH-lines
• Damage cables
• Vandalism
• Lightning

Question 2

What is the most common fault? How many % of all faults is this? And what is the reason this fault occur?

Answer 2

1-phase to ground (Earth faults)
80-90% of all faults
Due to faults in components, branches on OH-lines, lightning, damage on cables.
In this fault the fault current directly flows to the ground.

Question 3

Describe what a fault is

Answer 3

Unwanted connection between phases or between phase and ground

Question 4

Name some types of grounding systems

Answer 4

• Direct
• Isolated
• Resistans grounded
• Resistans and rectance grounded

Question 5

Describe what is meant by bolted short circuit. Why is this interesting?

Answer 5

It’s when there is no fault impedance and therefore gives maximum fault current. It is used to calculate fault level of an electric power system, and it gives the worst case scenario (=maximum fault current). All components (switchgear and conductor for example) are designed to withstand this high current.

Question 6

Name some short circuit contributions

Answer 6

• The grid
• Individual turbines (small contribution)
• Loads with rotating mass (motorers without frequency converters) (small contribution)
• Loads with rotating mass (motorers with frequency converters) (very small contribution)

Question 7

What is subtransient current and when is it the highest?

Answer 7

(Har inte fattat än hur jag ska förklara vad det är men ska fylla på här)

It is highest just after the fault (<10 ms)

Question 8

What are the mechanical impacts of fault current for 1) busbars, 2) disconnector and 3) circuit breakers?

Answer 8

1. Bending
2. “Welded” shut
3. May not be able to be operated

Question 9

What is considered when dimensioning for thermal impact of fault current?

Answer 9

At dimensioning due to thermal withstand is the current I_1 during 1 s considered. I_s too short time to make an impact.
I_1 = standard for all types of cables, dependent on cross section area and t_sc (=disconnection time / clearing time)
I_s = subtransient current

Question 10

What is short circuit power?

Answer 10

Made up, generally used and a measure of the strength of the grid.

Short-circuit power is the theoretical current that will run in the event of a short circuit if the protection has not yet intervened (https://www.eleq.com/en/short-circuit-power-and-short-circuit-resistance/#)

Question 11

What is short circuit capacity?

Answer 11

Made up, generally used and a measure on how much the component/system will stop the short circuit current

Question 12

What is a circuit breaker?

Answer 12

• Break up to fault current
• No visible breaking
• Remote operating possible
• Can break any current (See Question 3 on Quiz 2 from 2021-04-08)

Question 13

What is a disconnector?

Answer 13

• Can NOT break current
• Visible breaking/lockable
• Remote operating often not possible
• Used to ensure that an electrical circuit is completely de-energized for service or maintenance

Question 14

What is a switch?

Answer 14

• Can break up to load current

- Often visible breaking

Question 15

What is a fuse?

Answer 15

• Can break fault current
• Cheap <3
• Can’t be “controlled”
• Up to 72 kV
• Its essential component is a metal wire or strip that melts when too much current flows through it, thereby stopping or interrupting the current
• Needs to be changed when has been used