UNIT- IV FAULT ANALYSIS – SYMMETRICAL COMPONENTS AND
UNBALANCED FAULT
TWO MARKS
1. What are the symmetrical components of a 3 phase system?
In a 3 phase system, the unbalanced vectors (either currents or voltage) can be
resolved into three balanced system of vectors.
They are Positive sequence components
Negative sequence components
Zero sequence components
Unsymmetrical fault analysis can be done by using symmetrical components.
2. What are the positive sequence components?
It consists of three components of equal magnitude, displaced each other by 120˚ in phase and
having the phase sequence abc .
Ic1
120
Ia1
120
120
Ib1
3. What are the negative sequence components?
It consists of three components of equal magnitude, displaced each other by 120˚ in
phase and having the phase sequence acb .
Ib2
120
Ia2
120
120
Ic2
4. What are the zero sequence components?
It consists of three phasors equal in magnitude and with zero phase displacement
from each other.
Ia0
Ib0
Ic0
Ia0 = Ib0 = Ic0
5. What is sequence operator?
In unbalanced problem, to find the relationship between phase voltages and phase
currents, we use sequence operator ‘a’.
a = 1∠120˚ =
= - 0.5+j0.866
6. Write down the equations to convert symmetrical components into unbalanced
phase currents. (Or) Determination of unbalanced currents from symmetrical
currents.
Let, Ia, Ib, Ic be the unbalanced phase currents
Let, Ia0, Ia1, Ia2 be the symmetrical components of phase a
7.
Write down the equations to convert unbalanced phase currents into symmetrical
components. (Or) Determination of symmetrical currents from unbalanced
currents.
Let, Ia, Ib, Ic be the unbalanced phase currents
Let, Ia0, Ia1, Ia2 be the symmetrical components of phase a
8. What are sequence impedance and sequence network?
The sequence impedances are the impedances offered by the power system
components or elements to +ve, -ve and zero sequence current.
The single phase equivalent circuit of power system consisting of impedances to
current of any one sequence only is called sequence network.
9. Draw the positive, negative and zero sequence network of all power system
components.
-----To refer Sequence impedance and network Table 4.1----10. Write the equation to determine fault current for L-G, L-L and L-L-G fault with
impedance.
-----To refer Unsymmetrical fault current Table 4.2----11. Draw the equivalent sequence network diagram for L-G, L-L and L-L-G fault .
-----To refer Unsymmetrical fault sequence network Table 4.3-----