Arcing Ground (explained)

Arcing Ground Phenomenon

Already explained in previous blog arcing ground is a phenomenon which is observed in ungrounded 3 phase system. The voltage across this capacitance is phase voltage. During fault this voltage reduced to zero in faulted line where as in other phase increased by a factor of √3.
The arc between the faulty conductor and ground gets extinguished and restarts many times, this repeated initiation and extinction across the fault causes severe voltage oscillations of order nearly 3 to 4 times that of nominal voltage. Which damages the equipment insulation breakdown.

Above Diagram shows circuit diagram (a) and vector diagram (b) of a ungrounded system under normal condition:

Under normal condition the capacitance between lines are negligible compared to line to ground capacitance.

Here line to ground capacitance C_R=C_Y=C_B=C(say)* and voltages across capacitors are phase voltage i.e V_RN,V_YN and V_BN.

So, capacitive current I_R=I_Y=I_B=V_ph/X_c.The capacitive currents I_R , I_Y and I_B lead their respective phase voltages V_RN, V_YN and V_BN by 90° as shown in the phasor diagram (b). The three capacitive currents are equal in magnitude and are displaced 120° from each other. Therefore, their phasor sum is zero, so no current flows to ground and the potential of neutral is the same as the ground potential. Therefore, ungrounded neutral system poses no problems under normal conditions.

Following diagram (c) and (d) shows the circuit and vector diagram of a ungrounded system with a fault in single line( line B) to ground fault at point F.

 

 

The voltage driving I_R and I_Y through line R and Y are V_BR and V_BY. Here V_BR and V_BY are line voltages and I_R and I_Y are essentially capacitive.

So, I_R=I_Y=√3*V_ph/X_c
The fault current I_C in line B is vector sum of I_R and I_Y,hence I_C=I_R+I_Y=3*V_ph/X_c.

Conclusion:

• The capacitive current in the two healthy phases increase to √3 times normal per phase capacitive current.
• The capacitive fault current I_C becomes 3 times the normal per phase capacitive current.
• The potential of the faulty phase becomes equal to ground potential. However, the voltages of remaining healthy phases increased to √3 times of phase voltage i.e line voltage which is sufficient to start arcing ground and insulation breakdown of the equipment.

*assuming transposed line