Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

Power System Studies > Short Circuit Study > Flashcards

Short Circuit Study Flashcards

1
Q

What are the types of fault?

A

1) Series fault (open circuit fault)
* single conductor open
* Double conductor open
2) Shunt fault(short circuit fault)
* Symmetrical faults
* Unsymmetrical faults

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

Obejectives of short circuit study?

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

Standard for transformer

A

IEEE 60076

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

In short circuit analysis,current and power is high?what will be z and z tolerance

A

Z is low
Z tolerance is negative

Except short circuit study ,all other studies etap considers positive z tolerance

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

For what peak and initial sum rms used for ?

A

Design of circuit breaker

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

What asymmetrical current depends on?

A

1) which voltage angle at fault occurs

2) ………

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

Duration of DC current depends upon on what?

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

Time varying reactance

A

Xd “>Xd’>Xd
19%<28%<155%
I f”> I f’> I f

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

Why I DC takes huge time to decay in Generators?

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

Reactive power input of the motor depends upon the what?

A

Power factor of motor

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

What is LRC?

A

Locked rotor current

A current taken during starting

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

LRA formula?

A

LRA= FLA * LRC

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

What is short circuit contribution of induction motor formula?

A

(LRC* C.Factor)/1000

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

Does short circuit contribution of synchronous motor is higher than induction motor ?why?

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

I SC formula for generator

A

FLA/X d”

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

Fault current I f

Formula

A

I f = FLA sc /%z

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

Types of founding

A

1) Solid grounding
2) Resistance grounding
3) Reactance grounding
4) Resonance grounding

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

If rotating machines are present what grounding is preferred?

A

Resistive grounding to prevent core damage

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

What is disadvantage of ngr or resistive grounding?

A

Resistance grounding causes over voltage so insulation levels required is high which is expensive as wells difficult to manufacture

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

If we cannot measure fault current at lv side what is the remedy?

A

We use earthing transformer (zigzag transformer)to detect wart fault currrent

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

Where star solidly grounded/delta transformer is used

A

In industries(steel industries)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

What is the disadvantage of ungrounded system?

A

1) you need to use over voltage protection

2) you cannot detect fault

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
23
Q

Standards for circuit breaker

A

ANSI/ieee c37.04 (asymmetrical)

Symmetrical
IEC 622271 -100 -HV
IEC 60947-2
IEC 60898-1 (over current protection)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
24
Q

What is breaking current

A

RMS symmetrical current value

Peak =sqr root of 2 * rms

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
25
Q

What is making current?

A

Asymmetrical peak value

Ability of circuit breaker to close

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
26
Q

What is making capacity formula?

A

= multiplication factor * symmetrical breaking capacity

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
27
Q

Relationship between making current and breaking current

A

n= SC making capacity/ SC breaking capacity

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
28
Q

TRV?

A

Transient recovery voltage

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
29
Q

FPC?

A

First pole close factor

  1. 3 for solidly grounded
  2. 5 for resistance grounded or ungrounded systems
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
30
Q

In islanders system relay coordination is difficult or not?why and remedy?

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
31
Q

Methods for limiting fault current?

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
32
Q

What happens to fault current if we add generator?

A

If we add generator fault current increase

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
33
Q

If MVA increase what happens to Short circuit current?

A

Increases

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
34
Q

In high voltage why go for star in hv side instead of delta?

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
35
Q

Bus voltage and angle of current is influenced by what?

A

Power factor

36
Q

Transformer should be sized for max demand or connected load?

A

Maximum demand

37
Q

For above 10mva transformer which cooling is preferred?

A

ON/AN and ON/AF

38
Q

If transformer rating is 45 MVA ,it is output or input

A

Output- ANSI

Input- IEC

39
Q

Key points to remember for manual calculations?

A

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

40
Q

Major cases of short circuit studies

A

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

41
Q

Advantages of solidly grounded system?

A

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

42
Q

Disadvantages of solidly grounded system?

A

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

43
Q

What is other name for reactance grounding system?

A

Resonance grounding and Peterson coil

44
Q

In reactance grounded system,how the value of reactor is selected?

A

It is selected such that,for ground fault,current through reactor is equal to total capacitive current

45
Q

Advantages of reactance grounded system?

A

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

46
Q

Need for grounding?

A

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

47
Q

Why sometimes grounded system is prefered over ungrounded system?

A

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

48
Q

Disadvantage of ungrounded system

A

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

49
Q

Bus voltage and angle of current is influenced by what?

A

Power factor

50
Q

Transformer should be sized for max demand or connected load?

A

Maximum demand

51
Q

For above 10mva transformer which cooling is preferred?

A

ON/AN and ON/AF

52
Q

If transformer rating is 45 MVA ,it is output or input

A

Output- ANSI

Input- IEC

53
Q

Bus voltage and angle of current is influenced by what?

A

Power factor

54
Q

Transformer should be sized for max demand or connected load?

A

Maximum demand

55
Q

For above 10mva transformer which cooling is preferred?

A

ON/AN and ON/AF

56
Q

If transformer rating is 45 MVA ,it is output or input

A

Output- ANSI

Input- IEC

57
Q

Where series fault occurs?

A

Nuclear power plant

58
Q

In EHV line ,why fault clearing time should be minimum as possible?

A

If fault clearing time is not minimum,generator will go out of step and generator looses synchronisation

59
Q

what are the assumption for short circuits?

A

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

60
Q

effects of short cuircuit?

A

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

61
Q

standards

A 
IEC 60909 2016
ANSI IEEE 551 2006
IEC 61363
ANSI IEEE 3002.3
AS 3851
VDE 0102
62
Q

what is arc resistance?

A

insulation provides some resistance when short circuit occurs

63
Q

fault location should be considered as what source?

A

voltage source

64
Q

why c factor is necessary for iec standard?

A

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

65
Q

for 50 hz and 60hz system

what is x/R ?

A

1) for 50HZ,X/R is 14

2) for 60HZ,X/R is 17

66
Q

if X/R ratio and short circuit ratio(current) of 3phase system and single phase system is same in grid then system is

A

solidly grounded system

67
Q

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)

A

resistance grounded system

68
Q

X/R increases what happens to rms current and peak current?

A

1)peak current increases
2)rms current is unchanged
verify

69
Q

why short circuit currents are assymetrical?

A

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

70
Q

assymetrical nature of fault current depends upon?

A

1) point at which the sc occurs on voltage waveform

2) x/R ratio of the system

71
Q

during fault faraway from generator,what happens to ac component and dc compenent?

A

Ac component remain constant

dc component decays

72
Q

during fault near to generator,what happens to ac component and dc compenent?

A

Ac component decays and

dc component decays

73
Q

short circuit contribution calculation?

A

refer in note book

74
Q

zero sequence switchin diagram for trafo?

A

refer in note book

75
Q

bus and branch fault current calculation refer in note book?

A

.

76
Q

for lv system what grounding is used?

A

solidly grounded system coz of sensitive fault detection

and fast fault clearance

77
Q

for mv system what grounding is used?

A

resistance grounding system

78
Q

where reactance grounding system is used?

A

used for generators

79
Q

where resonance grounding system is used?

A

used by utilities to improve transient performance of hv system

80
Q

for hv system,above what voltage we go for solidly grounded system?

A

132KV

81
Q

to detect a fault in ungrounded system what we should do?

A

go for earthing(zigzag)trafo

82
Q

with max sc current what can we do?

A

1) can select short time rating of breakers and switch gears
2) optimize relay setting
3) ARC FLASH STUDIES

83
Q

with min sc current what can we do?

A

1) motor starting analysis
2) harmonic analysis
3) transient stability
4) Transformer energisation

84
Q

in distribution system what is status of R,X/R and dc current?

A

R is high,so X/R is less,so dc current is less

85
Q

in MV system what is status of R,X/R and dc current?

A

X/R is high,so dc current is present.so dc current carrying capability of breaker should be 36% of rms current

86
Q

in 11/0.4 KV system what should be the max rating of trafo?

A

should not exceed 4MVA

Power System Studies (10 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions