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
What is making current?
Asymmetrical peak value
Ability of circuit breaker to close
What is making capacity formula?
= multiplication factor * symmetrical breaking capacity
Relationship between making current and breaking current
n= SC making capacity/ SC breaking capacity
TRV?
Transient recovery voltage
FPC?
First pole close factor
- 3 for solidly grounded
- 5 for resistance grounded or ungrounded systems
In islanders system relay coordination is difficult or not?why and remedy?
Relay coordination is difficult . Because if fault current is less than full load current relay cannot detect fault due to time varying reactance of Generator
Remedy:
we use compound excitation which tries to sustain fault current so the relay can detect fault current
Methods for limiting fault current?
Low voltage:
1) Increase in cable length
2) Lighting transformer
Medium voltage; 1)Current limiting reactor(series reactor) 2)Unit ratio transformer Above two methods are costly 3)IS limiter(fuse)
High voltage:
1) current limiting reactor
2) network splitter
What happens to fault current if we add generator?
If we add generator fault current increase
If MVA increase what happens to Short circuit current?
Increases
In high voltage why go for star in hv side instead of delta?
If delta in hv side means ,over voltage at each phase so you must uniformly insulate cable through out which is not possible so we use star connection by using graded insulation at winding
Bus voltage and angle of current is influenced by what?
Power factor
Transformer should be sized for max demand or connected load?
Maximum demand
For above 10mva transformer which cooling is preferred?
ON/AN and ON/AF
If transformer rating is 45 MVA ,it is output or input
Output- ANSI
Input- IEC
Key points to remember for manual calculations?
1) c factor
2) impedance tolerance of transformer and generator
3) please don’t add z directly(use R+jX)
4) correction factors for generator,transformer etc
5) fault contribution from various sources
6) impact of grid fault MVA
Major cases of short circuit studies
With maximum short circuit
1) to select short time rating of the breaker,switch gear etc
2) optimise the relay setting of relay
With minimum short circuit
1) motor starting analysis
2) harmonic analysis
3) transient stability
Advantages of solidly grounded system?
1) Accurate detection and location of ground fault
2) Transient over voltage is minimum and neutral shift during ground fault is less compared to ungrounded system
3) lightning arrest or rated for 80% can be used
Disadvantages of solidly grounded system?
1) it permits 3 rd harmonics and multiples of 3 rd harmonic current (zero sequence current)
2) Ground fault current magnitude is high,the core damage at a point of fault in rotating machines like generator and motor will be high
What is other name for reactance grounding system?
Resonance grounding and Peterson coil
In reactance grounded system,how the value of reactor is selected?
It is selected such that,for ground fault,current through reactor is equal to total capacitive current
Advantages of reactance grounded system?
1) current through the reactor almost nullify capacitive current for ground fault
2) Fault current is practically zero,hence the possibility of restrict is less and arc is self extinguishing
Need for grounding?
1) enables ground fault current to flow so that selective isolation of faulted section is feasible
2) under fault condition,it minimise the neutral shift and limits over voltage systems
3) ensures human safety
Why sometimes grounded system is prefered over ungrounded system?
In ungrounded system,due to absence of return path , the ground fault current is low.so service is not interrupted and fault can be located and isolated at leisure time.soon problems like transient over voltages ,arching ground and insulation failures happened.to over come this problem ,grounding the neutral was considers as a possibile solution.so the over voltage problem in ungrounded system is replaced by over current problem in grounded system
Disadvantage of ungrounded system
The possibility of restroom in a circuit breaker is high when opening capacitive current of 10A max 15A .this leads to insulation failure
During ground faults there is over voltage on healthy phases and neutral shift is high.the result is following design considerations
1) neutral has to be fully insulated
Bus voltage and angle of current is influenced by what?
Power factor
Transformer should be sized for max demand or connected load?
Maximum demand
For above 10mva transformer which cooling is preferred?
ON/AN and ON/AF
If transformer rating is 45 MVA ,it is output or input
Output- ANSI
Input- IEC
Bus voltage and angle of current is influenced by what?
Power factor
Transformer should be sized for max demand or connected load?
Maximum demand
For above 10mva transformer which cooling is preferred?
ON/AN and ON/AF
If transformer rating is 45 MVA ,it is output or input
Output- ANSI
Input- IEC
Where series fault occurs?
Nuclear power plant
In EHV line ,why fault clearing time should be minimum as possible?
If fault clearing time is not minimum,generator will go out of step and generator looses synchronisation
what are the assumption for short circuits?
1) for duration of shortcircuit, there is no change in the type of short circuit involved and there is no change in the network
2) impedance of trafo is refered to the tap at nominal position .so impedance correction factor is introduced
3) arc resistance is not taken in account
4) all capacitance ,shunt admittance,non rotating loads,except those zero sequence system is neglected
effects of short cuircuit?
1) electro dynamic and thermal stress
2) undesirable current flow
3) excessive magnitude of current may lead to equipment damage
4) excessive over voltage
5) hazardous to working personal
6) gen out of step
standards
IEC 60909 2016 ANSI IEEE 551 2006 IEC 61363 ANSI IEEE 3002.3 AS 3851 VDE 0102
what is arc resistance?
insulation provides some resistance when short circuit occurs
fault location should be considered as what source?
voltage source
why c factor is necessary for iec standard?
1) voltage variation depend on time and place
2) changing of transformer taps
3) for neglecting loads and capacitance by calculation
4) for th subtransient behaviour of gen and motors
for 50 hz and 60hz system
what is x/R ?
1) for 50HZ,X/R is 14
2) for 60HZ,X/R is 17
if X/R ratio and short circuit ratio(current) of 3phase system and single phase system is same in grid then system is
solidly grounded system
if X/R ratio and short circuit ratio(current) of 3phase system and single phase system is different in grid then system is(3phase is greater)
resistance grounded system
X/R increases what happens to rms current and peak current?
1)peak current increases
2)rms current is unchanged
verify
why short circuit currents are assymetrical?
in a reactive circuit,if SC occurs at peak then the current will start from zero and this wave will be symmetrical.but for the same case ,if the fault occurs at voltage zero then the current will start at zero but will beassymetrical.this assymetry will happen only if the current is displaced from zero axis
assymetrical nature of fault current depends upon?
1) point at which the sc occurs on voltage waveform
2) x/R ratio of the system
during fault faraway from generator,what happens to ac component and dc compenent?
Ac component remain constant
dc component decays
during fault near to generator,what happens to ac component and dc compenent?
Ac component decays and
dc component decays
short circuit contribution calculation?
refer in note book
zero sequence switchin diagram for trafo?
refer in note book
bus and branch fault current calculation refer in note book?
.
for lv system what grounding is used?
solidly grounded system coz of sensitive fault detection
and fast fault clearance
for mv system what grounding is used?
resistance grounding system
where reactance grounding system is used?
used for generators
where resonance grounding system is used?
used by utilities to improve transient performance of hv system
for hv system,above what voltage we go for solidly grounded system?
132KV
to detect a fault in ungrounded system what we should do?
go for earthing(zigzag)trafo
with max sc current what can we do?
1) can select short time rating of breakers and switch gears
2) optimize relay setting
3) ARC FLASH STUDIES
with min sc current what can we do?
1) motor starting analysis
2) harmonic analysis
3) transient stability
4) Transformer energisation
in distribution system what is status of R,X/R and dc current?
R is high,so X/R is less,so dc current is less
in MV system what is status of R,X/R and dc current?
X/R is high,so dc current is present.so dc current carrying capability of breaker should be 36% of rms current
in 11/0.4 KV system what should be the max rating of trafo?
should not exceed 4MVA
