Transformer Neutral Grounding Methods and Applications for High-, Medium-, and Low-Voltage Systems

Writer： admin Time：2025-09-19 13:51:54 Browse：174℃

Neutral Point Grounding of Typical Three-Phase Equipment: Examples and Scenario Analysis

Transformers and generators have different functions, voltage levels, and operational scenarios, so their neutral point grounding methods (direct grounding, resistor grounding, arc-suppression coil grounding, etc.) vary. However, the core goal is always safety, reliability, and economy. The following explains with specific equipment examples:

(A) Transformer Neutral Point Grounding: Adapting Grounding Methods to Voltage Levels

Transformers are the energy conversion hub of the power grid. The neutral point grounding method should be determined according to the grid’s voltage level and fault-handling requirements. Common scenarios are as follows:

1. High-voltage side (110 kV and above): Direct grounding

• Application scenarios: Main transformers in high-voltage transmission networks of 110 kV, 220 kV, 500 kV, etc., such as regional substation main transformers in urban grids.

• Principle and purpose: High-voltage grids transmit large power. In the event of a single-phase-to-ground fault, the fault must be quickly isolated to prevent widespread power outages. With direct neutral grounding, the single-phase fault current is extremely high (up to tens of kiloamperes), allowing protection devices (e.g., zero-sequence current protection) to operate within 0.1 seconds, limiting the fault to a single line or transformer. Direct grounding also strictly fixes the neutral potential, preventing overvoltage stress on high-voltage equipment (e.g., transformer bushings, transmission line insulators).

• Example: At a 220 kV substation, the main transformer (capacity 120 MVA) has its high-voltage side (220 kV) star-connected, with the neutral directly connected to the substation grounding grid. When a single-phase fault occurs on the high-voltage line, the fault current flows through the line, fault point, grounding grid, and transformer neutral back to the winding. The zero-sequence current transformer detects a sudden change, triggering the breaker to trip in less than 0.05 seconds. The low-voltage side (10 kV) supplying residents and industries is unaffected.

2. Medium-voltage side (35 kV, 10 kV): Grounding via arc-suppression coil or small resistor

• Application scenarios: 35 kV distribution networks (e.g., industrial parks), 10 kV distribution networks (e.g., urban or rural grids) and their distribution transformers.

• Principle and purpose: In medium-voltage distribution networks, users are dense. Direct grounding would produce excessive single-phase fault current (several kiloamperes), risking line damage and extended outages; ungrounded systems could generate arc-overvoltages (repeated extinguishing and reignition of fault arcs, producing 2–3 times overvoltage).

• Therefore, medium-voltage transformer neutrals are often grounded via an arc-suppression coil (compensating the capacitive fault current so the arc self-extinguishes, allowing 1–2 hours of operation under fault for maintenance) or a small resistor (limiting fault current to several hundred amperes, ensuring protection operation without equipment damage).

• Example: In a rural 10 kV network, the distribution transformer neutral is grounded via an arc-suppression coil. When a single-phase fault occurs due to a tree contacting the line, the inductive current of the coil cancels the grid’s capacitive current, instantly extinguishing the fault arc. The automated distribution system detects the fault via zero-sequence voltage, isolates only the faulted line branch, while other users (irrigation pumps, households) remain powered. Maintenance clears the tree obstruction within an hour, restoring full supply and improving reliability.

3. Low-voltage side (0.4 kV, i.e., 380V/220V): Direct grounding (TT/TN systems)

• Application scenarios: Distribution transformers for residential areas, office buildings, and factory workshops, providing 220V single-phase and 380V three-phase power.

• Principle and purpose: Direct grounding of the low-voltage neutral (called functional grounding) stabilizes three-phase voltages, preventing overvoltage on any phase due to unbalanced load (e.g., 220V rising to 300V, damaging appliances). It also establishes a protective grounding path: if an appliance (e.g., washing machine) has a leakage, current flows through the casing, grounding wire, and transformer neutral back to the source, tripping the RCD within 0.03 seconds to prevent electric shock.

• Example: In a residential complex, the low-voltage side neutral of a 1000 kVA transformer is directly connected to the site’s grounding grid (ground resistance ≤ 4 Ω). When an electric water heater develops a casing leakage (≈5 A), the current flows through the protective path, and the RCD trips instantly, protecting residents without affecting other users. Only the faulty household loses power temporarily.