Fault Severity in Transmission Line

When a fault occurs on a transmission line, it can lead to a disruption in the power flow and stability of the system. The severity of the fault depends on various factors such as fault type, fault impedance, fault location, and the characteristics of the power system.

In this question, we are given four possible faults that can occur on a transmission line: 3-phase fault, L-L-G fault, L-L fault, and L-G fault. We need to determine the decreasing order of severity of these faults from a stability point of view.

Explanation:

To determine the order of fault severity, we need to consider the impact of each fault on the stability of the power system. The stability of a power system refers to its ability to maintain synchronous operation under normal and abnormal conditions.

1. 3-phase fault: A 3-phase fault is the most severe type of fault as it involves a complete short circuit between all three phases. This fault can cause a rapid increase in fault current and can lead to severe damage to the equipment. It can also result in a large transient disturbance in the system and can cause the system to become unstable. Therefore, 3-phase fault has the highest severity.

2. L-L-G fault: A L-L-G fault is a fault between two phases with a ground connection. This fault can also lead to a significant fault current, but it is less severe than a 3-phase fault. The presence of the ground connection can limit the fault current and reduce the impact on the stability of the system. Therefore, L-L-G fault has a lower severity compared to a 3-phase fault.

3. L-L fault: A L-L fault refers to a fault between two phases without any ground connection. This fault can result in a high fault current, but it is less severe than a L-L-G fault. The absence of a ground connection can limit the fault current to some extent and reduce the impact on the stability of the system. Therefore, L-L fault has a lower severity compared to a L-L-G fault.

4. L-G fault: A L-G fault is the least severe type of fault among the given options. It involves a fault between one phase and ground. The fault current in this case is limited by the ground impedance, which is typically higher than the line impedance. As a result, the impact on the stability of the system is relatively lower compared to the other fault types. Therefore, L-G fault has the lowest severity among the given options.

Conclusion:

Based on the above analysis, the decreasing order of severity of the faults from a stability point of view is:
a) 3-phase fault
b) L-L-G fault
c) L-L fault
d) L-G fault

Therefore, the correct answer is option 'A'.