1. BEE 3243
BEE 3243 –
– CHAPTER 7
CHAPTER 7
BEE 3243
BEE 3243 –
– CHAPTER 7
CHAPTER 7
BEE 3243
BEE 3243 –
– CHAPTER 7
CHAPTER 7
Fault in Electric Power System
Fault in Electric Power System
BEE 3243
BEE 3243 –
– CHAPTER 7
CHAPTER 7
Fault in Electric Power System
Fault in Electric Power System
2. Module Outline
1. Introduction
2. Symmetrical/ Balanced Faults
3 U t i l/ U b l d F lt
3. Unsymmetrical/ Unbalanced Faults
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3. Introduction
• Analysis types:
power flow - evaluate normal operating conditions
fault analysis - evaluate abnormal operating conditions
• Fault analysis is also known as short circuit study.
• In normal condition, a power system is operating at
balanced 3-phase AC system.
• Whenever a fault occurred, the bus voltages and
fl f t i th t k l t t ff t d
flow of current in the network elements get affected.
• Faults can cause over current at certain point of
t
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power system.
4. Introduction
• Faults occur in power system due to:
insulation failure in the equipments
flashover of lines initiated by lightning
mechanical damage to conductors and towers
mechanical damage to conductors and towers
accidental faulty operation
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5. Introduction
• Fault types:
Symmetrical/ balanced faults (3-phase)
Unsymmetrical/ unbalanced faults
i l li t d d d bl li t d
single-line to ground and double-line to ground
line-to-line faults
• The relative frequency of occurrence of various
The relative frequency of occurrence of various
faults in the order of severity are as follows:
balanced 3-phase fault 5%
double line to ground fault 10%
line to line fault 15%
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single line to ground fault 70%
6. Introduction
• When a fault occurs in a power system, bus
voltages reduces and large current flows in the
lines.
• This may cause damage to the equipments.
• The magnitude of the fault currents depends on:
the impedance of the network
the internal impedances of the generators
th i t f th f lt ( i t )
the resistance of the fault (arc resistance)
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7. Introduction
• Faulty section should be isolated from the rest of
the network immediately.
• This can be achieved by providing relays and circuit
breakers.
• The protective relays sense the occurrence of the
f l d d i l i i b k h
fault and send signals to circuit breakers to open the
circuit under faulty condition.
P l tti d l di ti
• Proper relay setting and relay coordination are
required for effective protection.
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8. Introduction
• The main purposes of fault analysis:
specifying ratings for circuit breakers and fuses
protective relay settings
specifying the impedance of transformers and generators
• Network impedances are governed by
generator impedances
transformer connections and impedances
transmission line impedances
transmission line impedances
Load impedances
grounding connections and resistances
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grounding connections and resistances
9. Subtransient and transient
• Generator behavior is divided into three periods
sub-transient period lasting for the first few cycles during
sub-transient period, lasting for the first few cycles during
which current decrement is very rapid
transient period, covering a relatively longer time during
which current decrement is more moderate
steady state period
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y p
10. Subtransient and transient
T i t St d t t
X’d
X’’d
Xd / Xs
Sub transient Transient Steady state
DC component
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12. Subtransient and transient
• Sub-transient reactances, XG = Xd”
determine the interrupting capacity of HV circuit breakers
determine the interrupting capacity of HV circuit breakers
determine the operation timing of the protective relay
system for high-voltage networks
• Transient reactances, XG = Xd’
determine the interrupting capacity of MV circuit breakers
determine the operation timing of the protective relay
system for medium-voltage networks
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13. Subtransient and transient
2
2
E
i 2
E
i
dc
ac
rms i
i
i 2
2
d
dc
X
i
'
'
max 2
d
ac
X
i
'
'
max
2
2 E = phase voltage
2
'
'
2
'
'
max 2
d
X
E
d
X
E
irms
E = phase voltage
rms
X
E
i
'
'
max 3
Momentary short circuit current
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d
X
14. Subtransient and transient
• Short circuit current
In theory it should be multiplied by multiplying factor
In theory, it should be multiplied by multiplying factor
of 3
d
rms
X
E
i
'
'
max 3
But in practice, it is recommended to use multiplying
factor of 1 6
d
X
factor of 1.6
Multiplying factor depends on the speed of CB.
Slower breaker (i.e. 8 cycle breaker) = 1.0
( y )
5 cycle breaker = 1.1
2 cycle breaker = 1.4
1 cycle breaker = ?
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1 cycle breaker = ?
15. Percentage Resistance/Reactance
• Percentage resistance, Rp
• Defined as resistance of that value which has a
Defined as resistance of that value which has a
resistance drop of Rp percent of normal voltage value
when carrying full load current.
100
V
IR
Rp
Where R = resistance in ohm, I = full load current,
V = rated voltage
• Percentage reactance ?
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16. • Percentage reactance, Xp
• Defined as reactance of that value which has a
Defined as reactance of that value which has a
reactive drop of Xp percent of the normal voltage
value when carrying full load current.
100
V
IX
p
X
where X = reactance in ohm, I = full load current,
V = rated voltage
V = rated voltage
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17. Rearrange the equation will give
V
p
X
Multiply & divided by V will give
100
I
p
X
100
2
IV
V
p
X
X
100
2
VA
in
output
voltage
p
X
If expressed in KV and KVA
100
2
kVA
kV
p
X
X
kVA
kV
p
X
X
10
2
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18. Symmetrical Three-phase Fault
• The balanced fault is a phenomenon where the
three phases are short circuited simultaneously.
three phases are short circuited simultaneously.
• Since the network is balanced, it is solve on per
phase basis.
p
• A fault represents a structural network change
equivalent to the addition of an impedance at the place of
q p p
the fault
if the fault impedance is zero, the fault is referred to as a
b lt d f lt lid f lt
bolted fault or solid fault
• For small networks, it can be solved by the
Thévenin’s method and for large networks it is
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Thévenin s method, and for large networks, it is
solved by the Bus Impedance Matrix method.
19. Symmetrical Three-phase Fault
Three Phase Fault on No Load Generator:
• The current and reactance are defined by the following
equation, provided the altenator was operating at no load
b f th f 3 h f lt
before the occurance of 3-phase fault:
2
]
[
Xd
Eg
Oa
I
•Eg = No load voltage
of the generator
Steady state current
'
2
]
'
[
2
Xd
Eg
Ob
i
Xd
•Xd = direct axis
synchronous
reactance
Xd’ di t i
Transient current
"
2
]
"
[
'
2
Xd
Eg
Oc
i
Xd
•Xd’=direct axis
transient reactance
•Xd” = direct axis
subtransient reactance
Subtransient current
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"
2 Xd subtransient reactance
20. Symmetrical Three-phase Fault
Three Phase Fault on Loaded Generator:
• Illustration of generators fault:
• The current following the fault occurs is IL, the voltage at the fault is
Vf and the terminal voltage of the generator is Vt
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g g
21. Symmetrical Three-phase Fault
• When a three-phase fault occurs at P, switch S is closed
p ,
and the value of Eg” can be obtained using the following
equation:
• For transient and steady state internal voltage is given
"
" jILXd
Vt
Eg
• For transient and steady state internal voltage is given
as follow:
'
' jILXd
Vt
Eg
jILXd
Vt
Eg
jILXd
Vt
Eg
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