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In which type of fault, positive sequence and negative sequence voltages are equal
Line to line fault
Double line to ground fault
Line to ground fault
None of the above
Double line to ground fault
In a Double Line to Ground (DLG) fault, the two faulted phases are connected to the ground through a common impedance. Under the condition of zero fault impedance, the sequence network equations show that the positive and negative sequence voltages at the fault point become equal.
In a Double Line to Ground (DLG) fault, the two faulted phases are connected to the ground through a common impedance. Under the condition of zero fault impedance, the sequence network equations show that the positive and negative sequence voltages at the fault point become equal.
V1тАЛ=V2тАЛ=V0тАЛ тАФ Relationship in DLG fault with zero fault impedance
IaтАЛ=0 тАФ Zero sequence current condition for unfaulted phase in DLG
In a DLG fault, the fault currents for two phases (say b and c) are equal and directed towards the ground. According to the symmetrical component transformation, the boundary conditions at the fault location lead to the sequence networks being connected in parallel. Specifically, the relationship V1тАЛ=V2тАЛ arises because the positive and negative sequence networks are effectively connected in parallel across the sequence voltage source.
In a DLG fault, the positive, negative, and zero sequence networks are connected in parallel.
For solid DLG faults (ZfтАЛ=0), the sequence voltages are identical: V1тАЛ=V2тАЛ=V0тАЛ.
Positive sequence voltage is always present in a power system, while negative and zero sequences represent the degree of unbalance.
Simplifies analysis of complex unsymmetrical fault conditions.
Allows use of sequence network modeling for power system stability studies.
Requires complex conversion of phase quantities into symmetrical components.
Assumptions like zero fault impedance are idealized and not always present in real-world scenarios.
Short circuit current calculation in transmission lines.
Relay coordination and setting for unsymmetrical fault protection.
Line-to-line faults involve only positive and negative sequence networks connected in anti-parallel (V1тАЛ=тИТV2тАЛ).
Line-to-ground faults involve all three sequence networks (positive, negative, and zero) connected in series.
B is correct тАФ In a double line to ground fault with zero fault impedance, the positive and negative sequence networks are connected in parallel, resulting in equal sequence voltages.
Remember: LG faults connect sequence networks in series, LL faults in anti-parallel, and DLG faults in parallel. This is the golden rule for sequence network fault analysis!