Philosophy Flashcards
Why do protection systems operate in zones?
a) in the event of a fault, only the protection for that zone will trip; b) nothing is left unprotected
How is a ‘zone’ defined?
CTs define the boundaries of a zone (i.e. current can be checked on both sides of plant).
Why might HV primary system faults vary in their impedence?
Might have a low-impedance, short-circuit fault, or a high-impedance fault (e.g. lightning strike stretching from phase to ground).
Define a ‘fault’
a ‘disruptive event resulting in a departure of the power system from the normal operation condition’
What is Powerlink’s basic protection philosophy?
To allow for a fault anywhere on the system and for any one thing to go wrong (hence the X and Y protection)
What is ‘unit protection’? Give an example.
A scheme that protects a specific area (e.g. transformer). Based off Kirchoffs Current Law. Examples: bus zone and transformer differential protection
What is ‘non-unit protection’?
Doesn’t have exact zone boundaries. Based off ‘time grading’ of relays; i.e. closest relay repsonds first.
What are some required operating times for 250kV+ protection?
CB fail: ~260ms
faulted end: 100ms
Why is ‘stability’ important in a unit protection system?
If a fault occurs on a feeder close to a substation, there will be abnormally high bus currents (all the other feeders will try and feed more current in to this fault). The unit bus must stay ‘steady’ (i.e. stable; not operate) and let the feeder protection do its thing.
In other words, it ensures protection won’t operate outside its zone.
