Short Circuit Study Flashcards
What are the types of fault?
1) Series fault (open circuit fault)
* single conductor open
* Double conductor open
2) Shunt fault(short circuit fault)
* Symmetrical faults
* Unsymmetrical faults
Obejectives of short circuit study?
1) To find short time(circuit rating of all equipment’s)
2) Breaking current( I rms)
3) making current(I p)
4) DC breaking current (I dc)
5) to find V raise during faults
Standard for transformer
IEEE 60076
In short circuit analysis,current and power is high?what will be z and z tolerance
Z is low
Z tolerance is negative
Except short circuit study ,all other studies etap considers positive z tolerance
For what peak and initial sum rms used for ?
Design of circuit breaker
What asymmetrical current depends on?
1) which voltage angle at fault occurs
2) ………
Duration of DC current depends upon on what?
Depends upon network reactance and resistance(X/R)
X/R is high or infinity- DC current takes huge time to decay or never comes to zero
X/R is low- DC current takes less time to decay
X/R=0 - no DC current in the system
Time varying reactance
Xd “>Xd’>Xd
19%<28%<155%
I f”> I f’> I f
Why I DC takes huge time to decay in Generators?
Generator is purely inductive in nature so R is negligible ,X is high, so X/R is high .so DC component takes huge time to decay
I dc component plays major role in designing circuit breaker
Reactive power input of the motor depends upon the what?
Power factor of motor
What is LRC?
Locked rotor current
A current taken during starting
LRA formula?
LRA= FLA * LRC
What is short circuit contribution of induction motor formula?
(LRC* C.Factor)/1000
Does short circuit contribution of synchronous motor is higher than induction motor ?why?
Yes because synchronous motor has filed excitation so short circuit contribution is high and induction motor has no field excitation so short circuit contribution is less
I SC formula for generator
FLA/X d”
Fault current I f
Formula
I f = FLA sc /%z
Types of founding
1) Solid grounding
2) Resistance grounding
3) Reactance grounding
4) Resonance grounding
If rotating machines are present what grounding is preferred?
Resistive grounding to prevent core damage
What is disadvantage of ngr or resistive grounding?
Resistance grounding causes over voltage so insulation levels required is high which is expensive as wells difficult to manufacture
If we cannot measure fault current at lv side what is the remedy?
We use earthing transformer (zigzag transformer)to detect wart fault currrent
Where star solidly grounded/delta transformer is used
In industries(steel industries)
What is the disadvantage of ungrounded system?
1) you need to use over voltage protection
2) you cannot detect fault
Standards for circuit breaker
ANSI/ieee c37.04 (asymmetrical)
Symmetrical
IEC 622271 -100 -HV
IEC 60947-2
IEC 60898-1 (over current protection)
What is breaking current
RMS symmetrical current value
Peak =sqr root of 2 * rms