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If all the sequence voltages at the fault point in a power system are equal, then the fault is a
Three-phase fault
Line to ground fault
Line to line fault
Double line to ground fault
Double line to ground fault
In a double line-to-ground (DLG) fault, the sequence networks (positive, negative, and zero sequence) are connected in parallel. At the fault point, the boundary condition VaтАЛ=V1тАЛ=V2тАЛ=V0тАЛ must hold, where V1тАЛ,V2тАЛ,V0тАЛ are the sequence voltages and VaтАЛ is the phase voltage of the faulted phases.
In a double line-to-ground (DLG) fault, the sequence networks (positive, negative, and zero sequence) are connected in parallel. At the fault point, the boundary condition VaтАЛ=V1тАЛ=V2тАЛ=V0тАЛ must hold, where V1тАЛ,V2тАЛ,V0тАЛ are the sequence voltages and VaтАЛ is the phase voltage of the faulted phases.
V0тАЛ=V1тАЛ=V2тАЛ=3VaтАЛтАЛ тАФ Sequence voltage equality at fault
IfтАЛ=I1тАЛ+I2тАЛ+I0тАЛ тАФ Total fault current expression
During a DLG fault occurring on phases B and C, the voltages of the faulted phases become zero (VbтАЛ=0,VcтАЛ=0). By applying the transformation V0тАЛ=31тАЛ(VaтАЛ+VbтАЛ+VcтАЛ), V1тАЛ=31тАЛ(VaтАЛ+aVbтАЛ+a2VcтАЛ), and V2тАЛ=31тАЛ(VaтАЛ+a2VbтАЛ+aVcтАЛ), substituting VbтАЛ=VcтАЛ=0 yields V0тАЛ=V1тАЛ=V2тАЛ=3VaтАЛтАЛ. Thus, all sequence voltages become identical at the fault point.
DLG faults involve the parallel connection of positive, negative, and zero sequence networks.
The fault is unsymmetrical, involving two phases and the reference ground.
The condition V0тАЛ=V1тАЛ=V2тАЛ is a unique signature for the double line-to-ground fault.
Allows for simplified sequence network analysis using Thevenin equivalents.
Facilitates the calculation of unsymmetrical currents in power system protection studies.
Highly complex fault analysis compared to symmetrical three-phase faults.
Requires knowledge of both positive and zero sequence impedances of the transformers and lines.
Relay coordination and setting calculations.
Power system stability and transient analysis studies.
For a Line-to-Ground fault, only the sequence networks are connected in series.
For a Line-to-Line fault, the positive and negative sequence networks are connected in parallel, but the zero sequence network is not involved (V0тАЛ=0).
D is correct тАФ In a double line-to-ground fault, the positive, negative, and zero sequence networks are connected in parallel, leading to the equality of sequence voltages at the fault location.
Always remember that in DLG faults, sequence networks are in parallel; for LG, they are in series; and for LL, positive and negative are in parallel with no zero sequence participation.