Definition: An unbalanced current flows during an earth fault in an electrical circuit. This current is known as the zero sequence current or the DC component of the fault current. When the magnitude of the three phases has zero phase displacement, it is called the zero phase sequence. The zero sequence current is represented by three vector lines, which can be determined by adding the vector of three phases current. The following formula expresses the zero-phase sequence current:
Zero Sequence Current in Delta-Connected Winding
The figure below illustrates the delta-connected winding. The zero sequence current in phases a, b, and c has the same magnitude and phase. As displayed in the figure, this current circulates in the phase windings of the delta connection. The existence of zero sequence voltage produces these zero sequence currents.
The current at node a is,
The current at no node b is;
At nod c, the current is,
The equation clearly demonstrates that the delta connection eliminates any presence of zero sequence current because there are no return paths for such current.
In a delta-connected circuit with zero sequence impedance Z0, the impedance of the circuit becomes infinite due to the lack of a return path for the zero sequence current in the line. This infinite impedance is represented with zero sequence impedance Z0 by the open circuit at point P in the single-phase equivalent zero sequence network.
Within the delta circuit, there is a closed path for the zero sequence current. We connect the zero sequence impedance Z0 to the zero sequence current to represent this.
Zero Sequence Current in Star-Connected Winding with Neutral Isolated from Ground
Consider a star-connected winding without a neutral return, as shown in the figure below.
In this case,
The above equation shows a three-wire system without neutral return exhibits zero sequence current.
Zero Sequence Current in Star-Connected Winding Without Neutral Return
The figure displayed below shows a star-connected winding that is neutrally grounded.
Here,
The above equation shows that the zero sequence current in a three-phase grounded system flows from the phase winding and the lines.
Solved Problem
Calculate the zero-sequence current during a line-to-ground fault in the faulted phase, which is 600 amperes.
Solution:
Fault Current (IF)= 600 A
