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Short circuits and protection systems - Coggle Diagram
Short circuits and protection systems
Causes of faults
Faults in apparatus
Branches and tree on OH-lines
Damage cables
Ice loaded OH-lines (can cause cables to swing more during wind -> the cables cross each other, temporary problem)
Vandalism
Lighting
Types of faults
An unwanted connection between phases or between phases and ground
3-phases (at symmetry, the same...
3-phases and ground ... maximum current)
2-phases
2-phases and ground
1-phase to ground (earth faults) (lowest current, most common)
Earth faults:
Single phase earth fault is the most common, 80-90% of all faults
Due to; faults in components, branches on OH-lines, lighting, damage on cable
Different types of grounding systems
Direct
Isolated
Resistance grounded
Reactance grounded
Resistance and reactance grounded
Used to fix voltage towards ground and control the fault current
Faults impedance
Impedance between conductors and/or ground. ''Bad connect'' can arise due to several resons eg. branch against a OH-line
Fault impedance reduce fault current which can make it hard to detect
If there is no fault impedance it is called bottled short circuit. This gives maximum fault current
Short circuits
Short circuit contribution?
The grid
Individual turbines (small contribution)
Loads with rotating mass (motors without frequency converters) (small contribution)
Loads with rotating mass (motors with frequency converters) (very small contribution)
Short circuit current
Current due to a short circuit
Not only in the fault location
''Keep the voltage up''
Significantly higher than the load current
Skriv slides: 12-14
Short circuit power
made up
generally used
a measure on the strength of the grid
Ssc=sqrt(3) Isc Un
Short circuit capacity
made up
generally used
a measure on how much the component/system will stop the short circuit current
Sssc=U^2n/xc
Power system model for short circuit calculations
Do not consider the load current
The transformers are just an impedance
Reduction of short circuit power
Reactance in series
Sectioning substations
Subtransient current
In the beginning of the faults (<10ms) there might be a very large current but it will not last
Important for mechanical dimensioning
Size of subtransient current
Rule of thumb: > 600V At generator: Is= 3 Isc otherwise: Is ≈ 2.5 Isc
< 600V Is ≈ 2 Isc
Läs vidare mechanical impact
Mechanical impact of fault current
Busbars: bending
Disconnector: ''welded shut''
Circuit breakers: may not be able to be operated
Thermal impact of fault current
When dimensioning due to thermal withstand, the current I1 (during 1s) is considered. The time is to short for the current Is to make an impact (Energy: RIsc^2t)
se slide 19
Switching/operating/protection devices
Circuit breaker
Break up to fault current
No visible breaking
Remote operating possible
Disconnector
Can not break current
Visible breaking/lockable
Remote operating often not possible
Switch
Can break up to load current
Often visible breaking
Fuse
Can break fault current
Cheap
Can't be controlled
Up to 72kv
Protection philosophy
It's a philosophy
The fault needs to be disconnected as fast as possible
Always more than one protection, backup needed
Selectivity, as little part of the system should be disconnected as possible