transformer neutral grounding resistor is one of the most vital type of tool that is a kind of new resistance product that is applied in power system. In addition to this it is connected between the neutral point of the transformer of the power system and the earth. In case of ground fault occurs in the system, the resistor system will limit the fault current and remove the faulty line or issues a fault alarm signal through the transformed current signal. It is done as the system is connected to the resistor; while the insulator of the line and equipment can be avoided and the economic loss can be reduced.
The essence of transformer neutral grounding is to establish a reference potential point to regulate system voltage and ensure equipment safety. Its principle depends on the grounding method and grid requirements, and its core functions can be classified into three major aspects:
1. Stabilizing system voltage and suppressing overvoltage
Direct neutral grounding (large-current grounded system): In high-voltage networks of 110 kV and above, the transformer neutral point is directly connected to the earth. When a single-phase-to-ground fault occurs, the fault current flows directly into the ground through the grounding device, causing the fault phase voltage to drop rapidly close to zero. Meanwhile, the non-faulted phase voltages remain at their rated phase-to-ground value (line voltage/√3), thus preventing excessive voltage rise of the healthy phases. For example, in a 220 kV system, during a single-phase-to-ground fault, the non-faulted phase voltage will not exceed 250 kV, effectively preventing insulation breakdown due to overvoltage.
Resistance grounding of transformer neutral: This method is an intermediate form between direct grounding and ungrounded systems, achieved by inserting a grounding resistor. Its main role is to limit single-phase-to-ground fault current to 100–1000 A, thereby avoiding the high fault current impact of direct grounding, resolving protection sensitivity issues in ungrounded systems, and dissipating fault energy to suppress overvoltage. It is widely applied in urban distribution networks (10 kV/20 kV), plant auxiliary power systems, as well as mining and chemical facilities, where it helps reduce outage scope, protect equipment, and lower safety risks.
Arc-suppression coil grounding (small-current grounded system): In medium-voltage networks such as 35 kV and 10 kV, the transformer neutral point is grounded via an arc-suppression coil. When a single-phase-to-ground fault occurs, the coil generates a compensating current in the opposite direction of the fault current, offsetting the capacitive ground current. This quickly extinguishes the fault arc (avoiding repeated restriking overvoltages) and limits the ground fault current to less than 5 A. The system can continue operating with the fault for 1–2 hours, allowing time for maintenance.
2. Facilitating fault detection and protection operation
With neutral grounding, the system can use zero-sequence current protection and zero-sequence voltage protection to rapidly identify ground faults. For example, in a solidly grounded system, a single-phase-to-ground fault generates a large current (up to several kiloamperes), which can be immediately detected by zero-sequence current relays, triggering circuit breaker tripping to isolate the faulty line. In contrast, in an ungrounded system, the fault current is very small (only the capacitive current, typically several tens of amperes), making it difficult for protection devices to detect, which may lead to fault escalation.
3. Reducing step voltage and touch voltage risks
When a transformer or transmission line experiences a ground fault, the ground current produces a potential gradient on the earth surface. Neutral grounding, together with a grounding grid, disperses the fault current into the earth, reducing ground potential differences:
Step voltage: The potential difference between a person’s two feet when walking near a ground fault point. With effective grounding, the step voltage can be controlled below 50 V (safe limit), preventing electric shock.
Touch voltage: The potential difference between equipment enclosure and the ground when a person touches the equipment during a fault. Neutral grounding combined with equipment grounding (such as enclosure earthing) ensures touch voltage remains within a safe range, protecting personnel from electric shock.
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