Unbalanced Fault Analysis: Double Line to Ground Fault

Double line to ground fault when both phases on the three phase line are accidentally connected to the ground. In this case, fault current will flow from the line to the ground within the involved phases, say, Phase B and Phase C. 

Double Line to Ground Fault - Phase B and Phase C

From this scenario, the system parameters can be considered as follows: 

• Fault current at phase A = 0 (since no fault current is flowing in phase A)
• Fault current at phase B = If-B
• Fault current at phase C = If-C. 
• Here we can also see that voltages at phase B and phase C will be equal to zero (neglecting ground impedance). 

See article: How to Develop Sequence Network in an Unbalanced System?

Using the symmetrical components equation matrix formula plus the values we get from the above conditions, we can plot the current equation as follows: 

Symmetrical Components Matrix Equation of Current for DLG Fault

From this matrix equation, we can get the following values: 

• Ia0 + Ia1 + Ia2 = 0 (the sum of all sequence currents is equal to zero)
• IA = 0 (since there is no fault current flowing in Phase A during the fault. 

In the same case, the symmetrical components for Voltages is: 

Symmetrical Components Matrix Equation of Voltage for DLG Fault

From the above matrix equation, we can get the following voltage values:

• Va0 = Va1 = Va2 = Va/ 3

From the obtained voltage and current values, we can demonstrate it using the following sequence network,

Equivalent Sequence Network of DLG Fault

This figure satisfies the values obtained based on the given conditions.

For sample calculation, see: Example: Double Line to Ground Calculation