Electrical Faults can be classified into two categories: Shunt Faults and Series Faults. Shunt faults include power conductor or conductors to ground or short circuit between the conductors.
Shunt faults are characterized by increase in current and decrease in voltage and frequency whereas Series faults are characterized by increase in voltage and frequency and decrease in current in the faulted phase.
Shunt faults are classified as:
1) Line-to-Ground Fault
2) Line-to-Line Fault
3) Double Line-to-Ground Fault
4) Three phase fault
Of the above faults, first three faults are unsymmetrical fault as the symmetry is disturbed in one / two of the phases. The method of Symmetrical Components shall be applied for the analysis of such unbalance and fault.
Three phase fault is balanced fault which can also be analyzed using concept of symmetrical components.
Series faults are classified as:
1) One Open Conductor
2) Two Open Conductors
These faults also disturb the symmetry and therefore these faults are unbalanced faults and hence shall be analyzed using concept of symmetrical components.
Neutral Voltage during Fault:
The potential of neutral when it is grounded through some impedance or is isolated will not be at ground potential under unbalance condition as in unsymmetrical fault rather it will have some finite value with respect to ground.
The potential of neutral is given as Vn = -InZn where Zn is neutral grounding impedance and In is neutral current. Notice the negative sign before the expression of neutral voltage Vn, it indicates the flow of current from ground to the neutral point and therefore the potential of neutral point will be less than the ground potential.
For a three phase system we know that,
Ia + Ib+ Ic = 3Ia0
You may also like to read Calculation of Symmetrical Components
Vn = -3Ia0Zn
Notice that only zero sequence current flows through the neutral and therefore voltage drop across neutral will be only due to zero sequence currents.