UNIT- III FAULT ANALYSIS - BALANCED FAULT

EE 1352 POWER SYSTEM ANALYSIS

Questions and Answers

UNIT- III FAULT ANALYSIS - BALANCED FAULT

What is meant by a fault?
A fault in a circuit is any failure which interferes with the normal flow of current. The faults are associated with abnormal change in current, voltage and frequency of the power system.
Why faults occur in a power system?
The faults occur in a power system due to
Insulation failure of equipment
Flashover of lines initiated by a lighting stroke
Due to permanent damage to conductors and towers or due to accidental faulty operations.
List the various types of faults.
Series fault or open circuit fault
One open conductor fault
Two open conductor fault
Shunt fault or short circuit fault.
Symmetrical fault or balanced fault
Three phase fault
Unsymmetrical fault or unbalanced fault
Line to ground (L-G) fault
Line to Line (L-L) fault
Double line to ground (L-L-G) fault
Write the relative frequency of occurrence of various types of faults.
Types of fault Relative frequency of occurrence of faults
Three phase fault 5%
Double line to ground fault 10%
Line to Line fault 15%
Line to ground fault 70%
.
State and explain symmetrical fault or balanced three phase fault.
This type of fault is defined as the simultaneous short circuit across all the three phases. It occurs infrequently, but it is the most severe type of fault encountered. Because the network is balanced, it is solved by per phase basis using Thevenins theorem or bus impedance matrix or KVL, KCL laws.
What is the need for short circuit studies or fault analysis?
Short circuit studies are essential in order to design or develop the protective schemes for various parts of the system .To estimate the magnitude of fault current for the proper choice of circuit breaker and protective relays.


What is bolted fault or solid fault?
A Fault represents a structural network change equivalent with that caused by the addition of impedance at the place of a fault. If the fault impedance is zero, the fault is referred as bolted fault or solid fault.
What is the reason for transients during short circuits?
The faults or short circuits are associated with sudden change in currents. Most of the components of the power system have inductive property which opposes any sudden change in currents, so the faults are associated with transients.
What is meant by doubling effect?
If a symmetrical fault occurs when the voltage wave is going through zero then the maximum momentary short circuit current will be double the value of maximum symmetrical short circuit current. This effect is called doubling effect.
Define DC off set current.
The unidirectional transient component of short circuit current is called DC off set current.
What is synchronous reactance or steady state condition reactance?
The synchronous reactance is the ratio of induced emf and the steady state rms current. It is the sum of leakage reactance (Xl) and the armature reactance (Xa).
X_d=X_a+X_l







What is sub transient reactance?
The synchronous reactance is the ratio of induced emf on no load and the sub transient symmetrical rms current.











X_d^''=X_l+1/(1/X_a +1/X_f +1/X_dw )
What is transient reactance?
The synchronous reactance is the ratio of induced emf on no load and the transient symmetrical rms current.











X_d^'=X_l+1/(1/X_a +1/X_f )
Define short circuit capacity of power system or fault level.
Short circuit capacity (SCC) or Short circuit MVA or fault level at a bus is defined as the product of the magnitude of the prefault bus voltage and the post fault current.
SCC or Short circuit MVA = |V_prefault |× |I_f |
Or
SCC = 1/X_th p.u MVA
Find the fault current in fig., if the prefault voltage at the fault point is 0.97 p.u.?







j0.2 and j 0.15 are in series. j0.2+ j 0.15 = j 0.35
J0.35 is in parallel with j 0.15
Z_th=(j0.35×j0.15)/(j0.35+j0.15)=j0.105 p.u
Fault current I_f=V_th/Z_th = 0.97/j0.105= -j 9.238 p.u
What is bus impedance matrix?
Bus impedance matrix is the inverse of the bus admittance matrix.Z_bus=[Y_bus ]^(-1)
The matrix consisting of driving point impedance and transfer impedances of the network is called as bus impedance matrix. Bus impedance matrix is symmetrical.
Give the methods available for forming bus impedance matrix.
Form bus admittance matrix and take the inverse to get bus impedance matrix.
Using bus building algorithm.
Using L-U factorization of Y-bus matrix.