Earthing Transformer: The "Safety Foundation" And "Invisible Guardian" of Power Systems

In the complex electrical network, the vast majority of faults begin with an abnormal connection between a line and the earth—a single-phase-to-earth fault. However, in many distribution systems (such as 10kV or 35kV networks with delta-connected windings), the system inherently lacks a defined "neutral point" for direct earthing. This is where a seemingly low-key but vital piece of equipment comes into play: the Earthing Transformer. Its core mission is not power transmission, but to artificially create a safe, controllable "virtual neutral point" for the system, serving as the foundation for ensuring grid safety and reliable operation.

I. Core Mission: Why Do We Need to "Create" a Neutral Point?

To understand the value of an earthing transformer, one must first understand the three core problems it solves:

1. Providing a Defined Earth Path

• Problem: In unearthed or isolated neutral distribution networks, a single-phase earth fault results in a very small fault current (only capacitive current). While the system can operate briefly, the arc at the fault point is not easily extinguished and can trigger dangerous overvoltages.

• Solution: The earthing transformer provides a neutral point, which is then earthed through a resistor or an arc suppression coil (Petersen coil). This creates a low-impedance path for the fault current, helping to quickly extinguish the arc, limit overvoltage, and provide a clear signal for protective relays to operate.

2. Establishing a Zero-Sequence Current Path

• Problem: Protective relay systems rely on zero-sequence current to sensitively detect earth faults. Without a neutral point, zero-sequence current has no path to flow, rendering the protection "blind."

• Solution: The earthing transformer provides a circulation path for zero-sequence current, enabling protective devices to accurately identify and quickly isolate the faulty line.

3. Providing a Reliable Neutral for Station Auxiliary Power

• Problem: Substations or power plants require a 380/220V three-phase four-wire low-voltage supply for auxiliary equipment (lighting, controls, etc.), which must be derived from the main power system.

• Solution: An earthing transformer (often dual-purpose as a station service transformer) can not only create an earthing point but also directly provide a stable low-voltage auxiliary power supply, serving two functions in one unit.

II. Core Principles and Main Types

The design goal of an earthing transformer is to provide a special magnetic circuit with low zero-sequence impedance and high positive-sequence impedance. Its main types and characteristics are as follows:

Note: Earthing transformers typically work in conjunction with an earthing resistor cabinet or an arc suppression coil (Petersen coil). The former limits the fault current to a safe value; the latter compensates for the capacitive current to assist in arc extinction.

III. Key Performance and Technical Focus

Unlike power transformers which prioritize power transfer efficiency, earthing transformer design focuses on safety and reliability:

• High Short-Circuit Withstand Capability: Must withstand the sustained short-time (typically 10 or 30 seconds) fault current during a system single-phase-to-earth fault without damage.

• Low Zero-Sequence Impedance: This is its core performance indicator, ensuring an effective neutral point and sufficient fault current for protection operation.

• Flexible Compatibility with Earthing Methods: The design must match the specific parameters of the downstream earthing equipment (high-resistance, medium-resistance, low-resistance resistor, or arc suppression coil).

• Low No-Load Losses: As it is permanently connected to the system, no-load loss is a significant economic factor.

IV. Typical Application Scenarios

1. Urban & Industrial Distribution Networks: Provides a neutral point for 10kV, 35kV delta-connected cable or mixed line networks, a standard configuration for modern distribution grid automation.

2. Renewable Energy Sector:

• Wind/Solar Step-Up Stations: Collection lines are often cables with high capacitive current. Earthing transformers paired with resistor/arc suppression coils are critical for safe grid integration.

• Distributed PV Point of Interconnection: Used at the user-side interconnection point to establish a safe local earthing system.

3. Power Plants & Large Substations: Serves as a reliable station service power source and earthing point for the auxiliary system.

4. Special Environments (Mines, Oil & Gas, Marine): Acts as a critical safety device in isolated networks with extremely high power supply safety requirements.

V. Selection and Application Considerations

Selecting an earthing transformer requires systematic consideration of:

• System Voltage & Rated Capacity: The short-time thermal capacity (kVA) is determined by the earth fault current and its duration.

• Earthing Method: Clarify the resistance value, current rating, time rating of the earthing resistor, or the compensation capacity of the arc suppression coil.

• Installation Environment: Indoor or outdoor, standard or special environments (high altitude, high humidity).

• Functional Integration: Whether it also needs to function as a station service transformer (choose Yn-d type).

Conclusion: The Silent Guardian

The earthing transformer never participates in daily power transmission. It sits silently connected to the system bus, always ready to step in at the moment a fault occurs. It does not create value but guards the very foundation upon which the entire power grid creates value: safety and stability. With the rapid development of smart grids and renewable energy, the demands for grid earthing safety and precise fault management are increasing. High-performance, reliable earthing transformers have thus become indispensable "Invisible Guardians" of modern power systems.