College of Electrical and Mechanical Engineering
Department of Electrical Engineering
EE-863 : Power System Analysis
Lecture#13
Fault Analysis
Instructor: Dr Sarmad Majeed Malik
1
Fault Analysis
•
When a fault occurs, the voltage and current at any part of the
power system is different than normal operating conditions.
•
Fault Analysis: Finding the voltage and current distribution of the
system when the fault occurs.
•
Fault
•
Series fault/Open circuit fault
•
Shunt fault/Short circuit fault
•
L-L-L: All 3 lines are short circuited
•
L-L-L-G: All 3 lines short circuited with ground
•
L-G: Single line to ground fault
•
L-L: Line to line fault
•
L-L-G: Line to line to ground fault
Symmetrical and Unsymmetrical
faults
•
Symmetrical faults (L-L-L, L-L-L-G): Although the fault currents
will be abnormally high, currents will be balanced.
•
Magnitude of currents in 3 lines will be equal.
•
Phase difference of 120 degrees.
•
Unsymmetrical faults (L-G,L-L, L-L-G)
•
Symmetrical faults are easier to analyze (in terms of calculations)
but these faults are most severe in power systems.
•
High burden on circuit breakers.
Symmetrical Faults
•
Assumptions involved while calculating symmetrical faults
•
At the time of fault, system was operating at no-load (load
current is zero)
•
All the emfs of the generators in the system are operating at pu.
•
All transformers in the system will be assumed as having only
leakage reactances.
•
All transmission lines are represented as inductances.
•
All resistances in the system are neglected and only reactive
inductances are being considered
Symmetrical Faults
•
Steps involved while calculating symmetrical faults
•
Draw single line diagram of the system
•
For each element of power system, evaluate per unit reactances
X pu
•
Draw single line reactance diagram on per phase basis (with
respect to neutral.
•
Evaluate equivalent reactance as seen from the fault point.
Symmetrical Faults
•
So fault current is
__
If 
•
Ef
jX eq

10
pu
jX eq
We also know
I f ( actual)  I f ( pu)  I base
•
How to find I base?
Symmetrical Faults
•
Finding base current
S
3VL I L  3V p I p
I base 
•
S
S

3VL 3V p
Once actual value of fault current is obtained, we can calculate
short circuit MVA (fault current MVA/fault level).
Short Circuit MVA  V f ( prefault )( pu )  I f ( pu )
where V f ( prefault)( pu) is pre-fault voltage at point where fault has
occurred.
•
Since we have assumed that no voltage drop occurs and load
current is zero, we have
V f ( pu)  I f ( pu)  E f ( pu)  I f ( pu)
Symmetrical Faults
Short Circuit MVA( actual)  Short Circuit MVA( pu)  S
where S is the total capacity of 3-phase system
•
Note: If fault impedance Zf is given, perform all calculations in a
similar fashion except
__
If 
Ef
__
jX eq  Z f
Problem
For the given power system topology, calculate:
(i) Fault current.
(ii) Short Circuit MVA.
(iii) Current supplied by G1 and G2.
Solution
Solution
Solution
Solution
Problem
Five alternators each rated 5MVA, 13.2 kV with 25% of reactance
on its own base are connected in parallel to a bus bar. Find the
short circuit level in MVA at the bus bar.
Solution
Solution
Problem
Solution
Solution
Solution
Solution
Solution
Quiz 2
•
For the system shown in the figure, the three lines have a
series impedance of 0.02+0.08j pu and a total shunt
admittance 0.02j pu. Find the values of V2 ,  2 , Q3and  3 at
the end of the first iteration using Newton Raphson method
of load flow.
Sequence Networks of Power System
Sequence Networks of Power System
Sequence Networks of Power System
Sequence Networks of Power System
Sequence Networks of Power System
Sequence Networks of Power System
Sequence Networks of Power System
Sequence Networks of Power System
Sequence Networks of Power System
Sequence Networks of Power System
Sequence Networks of Power System
Sequence Networks of Power System
Sequence Networks of Power System
Sequence Networks of Power System
Sequence Networks of Power System
Sequence Networks of Power System
Problem
Solution
50