Causes of Higher L-G Fault Current than 3-Phase Fault Current

1.

Low Zero Sequence Impedance

Near a transformer or distribution substation, the zero sequence impedance (Z0) is typically lower than the positive and negative sequence impedances (Z1 and Z2). This is because of the presence of a transformer, which provides a low impedance path for zero sequence currents.2.

High Positive and Negative Sequence Impedances

As you move further away from the transformer, the distribution lines or cables become longer, increasing their resistance and inductance. This results in higher Z1 and Z2 impedances, which reduce the 3-phase balanced fault current (I3ph).

Explanation:

During an L-G fault, the zero sequence current path is closed through the faulted phase, the transformer windings, and the grounding system. The low Z0 allows a higher magnitude of zero sequence current (I0) to flow.In contrast, during a 3-phase balanced fault, the fault current divides equally between the three phases. Due to the higher Z1 and Z2 impedances, the phase currents (Iph) are lower than the zero sequence current (I0) that flows during an L-G fault.Therefore, in certain locations where the Z0 is low and Z1 and Z2 are high, the L-G fault current can exceed the 3-phase balanced fault current. Flag for review