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Zero sequence current flows in a transmission line when there is
Double line to ground fault
An over voltage in a line caused by charged load
Line to line fault
Fault across all the lines
Double line to ground fault
Zero sequence current (I0тАЛ) flows in a power system only when the fault involves the ground, as this provides a path for the return current through the earth. In a double line-to-ground (DLG) fault, the symmetry of the system is disrupted in a way that generates zero sequence components for each phase.
Zero sequence current (I0тАЛ) flows in a power system only when the fault involves the ground, as this provides a path for the return current through the earth. In a double line-to-ground (DLG) fault, the symmetry of the system is disrupted in a way that generates zero sequence components for each phase.
I0тАЛ=31тАЛ(IaтАЛ+IbтАЛ+IcтАЛ) тАФ Definition of Zero Sequence Current
IaтАЛ=I0тАЛ+I1тАЛ+I2тАЛ тАФ Transformation of phase currents using symmetrical components
The zero sequence current is defined as I0тАЛ=31тАЛ(IaтАЛ+IbтАЛ+IcтАЛ). In balanced conditions, IaтАЛ+IbтАЛ+IcтАЛ=0, resulting in I0тАЛ=0. When a ground fault occurs (such as LG or LLG), the phase currents no longer sum to zero, creating a net current that flows through the neutral or ground path, thus resulting in a non-zero I0тАЛ.
Zero sequence current requires a return path through ground.
Balanced systems have zero sequence current equal to zero.
LLG (Double Line to Ground) faults result in the connection of two phases and ground, creating non-zero I0тАЛ.
Line-to-Line (LL) faults do not involve ground, hence I0тАЛ=0 for LL faults.
Used in protective relaying for sensitive earth-fault detection.
Essential for analyzing unbalanced system performance.
Increases complexity in fault analysis calculation.
Requires zero-sequence impedance modeling of transformers and lines.
Ground fault protection relay coordination.
Modeling of transmission lines for unbalanced power flow.
A line-to-line fault (Option C) does not involve ground, so the sum of currents is still zero (IaтАЛ+IbтАЛ+IcтАЛ=0), meaning no zero sequence current.
A three-phase fault (Option D) is a balanced fault; hence, no zero sequence current exists.
A is correct тАФ Zero sequence current flows only when a fault path involves the ground, which is present in a double line-to-ground fault.
Remember: I0тАЛ exists if and only if there is a path to ground. If the fault does not involve ground (like L-L or 3-phase faults), the zero sequence component is always zero.