For systems with an ungrounded neutral point or grounded through an arc suppression coil, when a single-phase ground fault occurs, since a short-circuit loop cannot be formed, the ground current is often much smaller than the load current, so this type of system is called a small current grounding system (neutral point non-effectively grounded system). Generally, systems of 66kV and below often adopt this type of system.
When a single-phase ground fault occurs in a system with an ungrounded neutral point, since no loop is formed, the current flowing through the fault point is a capacitive current formed by the capacitance to ground, with a very small value. Meanwhile, the neutral point voltage of the entire system shifts relative to ground (the degree of shift depends on the degree of the ground short circuit; in the case of a complete metallic short circuit, the neutral point voltage to ground rises to the phase voltage), and the voltage to ground of the ungrounded phases also rises (in a metallic short circuit, it rises to the line voltage). However, the voltage of each phase to the neutral point and the line-to-line voltages remain unchanged, so the entire system can continue to operate. But since the rise in voltage to ground tests the insulation of the entire system to ground, the fault must be eliminated promptly before the insulation is damaged, and it cannot operate in this state for a long time, generally not exceeding 1-2 hours.
Small current grounding systems include: systems with an ungrounded neutral point, systems with a neutral point grounded through an arc suppression coil, and systems with a neutral point grounded through a large resistor. (Neutral point non-effectively grounded systems) When a fault occurs, the ground current is relatively small.
There is no need to elaborate much on systems with a directly grounded neutral point. In a single-phase ground fault, a large short-circuit current flows between the fault point and the neutral point, causing severe voltage asymmetry in the entire system, making normal operation impossible. Therefore, immediate tripping is required. In such systems, zero-sequence protection should be used as the main protection.
For systems with a directly grounded neutral point (including grounding through a small impedance), when a single-phase ground fault occurs, the ground current is generally relatively large, so it is called a large current grounding system. Generally, systems of 110kV and above adopt large current grounding systems.
A large current grounding system refers to a system with a directly grounded neutral point, such as our three-phase four-wire system, because the ground current is relatively large when a fault occurs.
