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Possible faults that may occur on a transmission line are
2 ┬╖ L-L-G fault
3 ┬╖ L-L fault
4 ┬╖ L-G fault
The decreasing order of severity of the fault from the stability point of view is
4
2
4
2
4
The severity of a fault on a transmission line in terms of stability is defined by its impact on the power system's ability to maintain synchronism ┬╖ The 3-phase fault is the most severe, followed by the L-L-G (double line-to-ground) fault, the L-L (line-to-line) fault, and finally the L-G (line-to-ground) fault, which is the least severe.
The severity of a fault on a transmission line in terms of stability is defined by its impact on the power system's ability to maintain synchronism ┬╖ The 3-phase fault is the most severe, followed by the L-L-G (double line-to-ground) fault, the L-L (line-to-line) fault, and finally the L-G (line-to-ground) fault, which is the least severe.
PmaxтАЛ=XeqтАЛV1тАЛV2тАЛтАЛsin(╬┤) тАФ Power transfer equation showing dependency on reactance XeqтАЛ
IfaultтАЛ=ZthтАЛVthтАЛтАЛ тАФ Standard fault current calculation where ZthтАЛ includes positive, negative, and zero sequence components
Stability is determined by the amount of power that can be transferred during the fault ┬╖ AтВГ-phase fault causes the greatest reduction in the transfer capability and the highest surge in current, often forcing the voltage to zero across all phases, which leads to the most significant power swing.
Symmetrical faults (3-phase) are balanced and affect all three phases equally.
Unsymmetrical faults (L-G, L-L, L-L-G) introduce sequence networks that must be connected in specific series or parallel configurations.
L-G faults are the most common in power systems, accounting for approximately 70-80% of all faults.
Stability analysis often considers the worst-case scenario, which is the symmetrical 3-phase fault.
Knowing the fault severity helps in designing optimal relay settings.
Categorization allows for prioritized protection schemes.
3-phase faults, while most severe, are rare.
Sequence network analysis for unsymmetrical faults is mathematically intensive.
Transient stability analysis studies.
Design of circuit breakers and protective relays.
Determination of short-circuit MVA ratings.
The severity order is strictly 3-phase (1) > L-L-G (2) > L-L (3) > L-G (4).
Option A lists the decreasing order as 1 > 2 > 3 > 4, which matches the physical reality of transient stability impact.
A is correct тАФ The decreasing order of fault severity is 3-phase > L-L-G > L-L > L-G.
Always remember that while L-G faults are the most frequent, 3-phase faults are the most severe for transient stability calculations!