Ground Fault Current in the Ungrounded System    

Non-grounded systems are still used in many chemical industries. The distribution transformer may be 3.3KV or 6.6KV on the output side, and the medium-voltage motor outputs hundreds to thousands of horsepower. Since the transformer uses a non-grounded system, the winding on the output side of the transformer has no neutral. The point can be grounded, or there is a neutral point but it is deliberately not grounded. The ground current generated by a grounding event in a non-grounded system is discussed below. It is assumed that the secondary side of a three-phase distribution transformer (Distribution Transformer) is delta-connected and not grounded. Although There is no intentionally grounded conductor, but there will still be a shielding layer outside the insulation layer (such as XLPE) of the medium voltage cable. To protect the safety of workers, the shielding layer will implement protective earthing (PE). Therefore, the ungrounded system is actually Through a capacitive high-impedance grounded system (as shown in the figure below), under normal circumstances, each phase has only a small leakage current (IA, IB, IC), and the ground current is 0A (Io=IA+IB+IC=0 ).


 


Therefore, when the power supply system is normal, there is a small amount of leakage current in each phase, and the ground current should be 0A.
     Assuming that phase C of this power supply system is grounded, the capacitive impedance of phase C is bypassed, so no current will pass through the capacitance of phase C to ground, the voltage of phase C to ground is 0V, and the voltage of phase A and phase B to ground rises. 1.732 times higher, and the phase angle also shifts by 30o. The ground current will increase from 0A to 3 times the normal leakage current. Even so, the ground current is limited by the ground resistance and ground capacitance and is still very low. According to the grounding The capacitive reactance of the capacitor is different, which may only range from a few hundred milliamperes (mA) to several amperes. The following is an explanation of their relationship :