An earthing transformer, also known as a grounding transformer, is a type of auxiliary transformer used in three – phase electric power systems. It is typically employed in power systems without a natural neutral point to provide an artificial neutral connection for earthing either directly or via an impedance like an arc suppression reactor, resistor, or current – limiting reactor. During line – to – ground faults, it offers a low – impedance path for zero – sequence fault currents (while presenting high impedance to positive and negative sequence currents), limiting fault currents and transient overvoltages to ensure reliable operation of the grounding protection system; moreover, it normally carries short – circuit ground current until the circuit breaker clears the fault, thus having short – time ratings. The kVA rating of an earthing transformer depends on the normal line – to – neutral voltage and the fault current value within a specified time, such as seconds to minutes. Additionally, it may adopt a secondary (low – voltage) winding to continuously supply power to substation stations, and enables delta – connected three – phase systems to accommodate phase – to – neutral loads by providing a return path for current to the neutral; during single – phase faults, it limits the fault current in the neutral to improve power line restoration.
1. Yₙ,d-connected grounding transformer
It is a three-phase transformer with a wye-connected (Yₙ, with a neutral lead) primary winding and a delta-connected (d) secondary winding.
The delta-connected secondary winding can carry circulating current to balance the current in the primary winding.
The delta secondary winding can also be connected as an open delta; by inserting resistors or reactors at the open end, the zero-sequence impedance of the grounding transformer can be adjusted.
Moreover, the terminals of the secondary winding can be led out to serve as an auxiliary power source for the substation.
2. Zₙ-connected (zig-zag-connected) grounding transformer
It is a three-phase transformer with zig-zag-connected windings.
Due to the inherent connection mode of zig-zag windings, fault currents can be mutually balanced between two series-connected windings.
A low-voltage winding can be added to this transformer to act as an auxiliary power source for the substation.
Additional Notes on Operation and Structure
- Structure: Grounding transformers are structurally similar to ordinary three-phase core-type power transformers.
- Normal operation: Only excitation current flows through the primary side of the grounding transformer; the secondary side (if present) has no current.
- Single-phase ground fault: Both the delta-connected windings of the substation’s main transformer and the three-phase windings of the grounding transformer carry short-circuit current. By properly selecting the current-limiting impedance Z, the per-phase short-circuit current can be controlled to not exceed the rated phase current of the main transformer windings. The standard duration of such short-circuit current is 10 seconds.
The working principle of earthing transformer
In normal operation, the primary winding of the earthing transformer connects to the phase conductors of the electrical system, while its secondary winding is grounded. At this time, the transformer operates like a conventional transformer, stepping voltage up or down according to requirements.
For limiting fault currents, the impedance of the earthing transformer, along with any additional grounding resistors or reactors, restricts the magnitude of fault currents flowing through the system. By controlling these fault currents, the earthing transformer helps maintain system stability and protects sensitive equipment from damage.
When a fault (such as a line-to-ground fault) arises in the system, fault currents flow through the secondary winding of the earthing transformer to the ground. This creates a low-impedance path for fault currents to dissipate safely, preventing equipment damage and reducing the risk of electrical hazards.
In terms of safety and protection, the earthing transformer ensures the safety of personnel and equipment in the electrical system by providing a reliable path to ground. It helps prevent electrical shocks, fires, and other hazards associated with fault conditions, thus contributing to a safer working environment and improved system reliability.
The Function of Earthing Transformer
Earthing transformer is a specialized electrical equipment developed to address the lack of neutral points in specific power grid configurations and ensure the safe operation of the system when ground faults occur. Its core functions and working characteristics are mainly reflected in the following aspects:
1. Provide an Artificial Neutral Point for Key Equipment
In small-current grounding systems, the arc suppression coil is crucial for compensating the grounding capacitive current when the power grid has a single-phase ground fault. However, the delta-connected side of the main transformer (a common configuration for the distribution voltage side of main transformers in 6kV, 11kV, and 33kV power grids) has no natural neutral point, making it impossible to install the arc suppression coil directly.
The earthing transformer solves this problem by creating an artificial neutral point. This neutral point not only enables the effective connection of the arc suppression coil but also provides a connection point for the grounding resistor. When the power grid adopts an ungrounded neutral operation mode (a common mode in the early stage of power grid construction due to its simplicity and low investment), the artificial neutral point laid by the earthing transformer becomes a key prerequisite for subsequent fault protection.
2. Mitigate Risks of Ungrounded Neutral Systems and Ensure Reliable Protection Action
In ungrounded neutral systems, although the line voltage remains symmetrical when a single-phase ground fault occurs (having little impact on users’ continuous power consumption), this advantage only holds when the grounding capacitive current is small (less than 10A; transient faults can even extinguish automatically). With the expansion of the power industry and the increase in urban cable circuits, the grounding capacitive current often exceeds 10A, leading to three major risks:
Intermittent extinction and reignition of the grounding arc, generating arc grounding overvoltage (up to 4U, where U is the peak value of the normal phase voltage) that damages equipment insulation;
Continuous arcs causing air dissociation, which easily leads to phase-to-phase short circuits;
Ferromagnetic resonance overvoltage, which may burn out voltage transformers or cause arrester explosions.
By connecting a grounding resistor to the artificial neutral point, the earthing transformer provides sufficient zero-sequence current and zero-sequence voltage for the system. This allows the highly sensitive zero-sequence protection device to quickly identify single-phase ground faults and cut off the faulty line in a short time, fundamentally preventing the above risks from expanding and safeguarding the insulation of grid equipment and the overall safe operation of the power grid.
3. Exhibit Special Electromagnetic Characteristics to Adapt to Fault Conditions
The earthing transformer has unique impedance characteristics for different types of currents, which is the key to its stable operation:
High impedance to positive and negative sequence currents: Under normal operating conditions, only a small excitation current flows through the earthing transformer’s windings. At this time, the transformer is in an unloaded state (many earthing transformers even have no secondary windings, further simplifying their structure for this unloaded scenario).
Low impedance to zero-sequence currents: The earthing transformer usually adopts Z-type (zigzag) wiring, where each phase coil is wound on two iron core poles respectively. When zero-sequence current is generated due to a ground fault, the two windings on the same iron core pole are connected in reverse polarity in series. Their induced electromotive forces are equal in magnitude and opposite in direction, canceling each other out-resulting in extremely low zero-sequence impedance (about 10Ω, far smaller than that of ordinary transformers). This low impedance ensures that zero-sequence current can flow smoothly through the neutral point grounding resistor and the earthing transformer, creating conditions for fault protection.
This impedance characteristic also determines the earthing transformer’s operating mode: long-term unloaded operation and short-term overload operation. It only functions during the period from the occurrence of a ground fault to the moment the zero-sequence protection cuts off the faulty line, and the fault current passes through it only briefly.
4. Improve Matching Efficiency and Reduce Investment Costs
Compared with ordinary transformers, the earthing transformer has obvious advantages in matching with arc suppression coils: regulations stipulate that when ordinary transformers are used with arc suppression coils, the capacity of the arc suppression coil cannot exceed 20% of the transformer’s capacity; while Z-type earthing transformers can match arc suppression coils with 90%~100% of their own capacity, significantly improving the efficiency of capacitive current compensation.
In addition, some earthing transformers can be connected to secondary loads while realizing grounding protection functions. This means they can replace ordinary distribution transformers in specific scenarios, integrating two functions into one device and effectively reducing the overall investment cost of power grid construction.
In summary, the earthing transformer is not only a “neutral point builder” for power grids lacking natural neutral points but also a “fault protector” that optimizes current impedance characteristics and ensures reliable protection action. Its special structure and operating mode make it an indispensable key equipment in modern power grids, especially in urban power grids with large capacitive currents.
Application of earthing transformer
The core function of an Earthing Transformer is to provide a neutral grounding point for ungrounded or low-current grounded power systems. It is mainly used in scenarios where grounding is required to achieve fault protection and voltage stability, covering distribution networks, industrial fields, new energy systems, etc.
- Medium and Low-Voltage Distribution Networks
This is the most primary application field of Earthing Transformers, especially suitable for medium-voltage distribution systems such as 10kV and 20kV.
- Most medium-voltage distribution networks adopt the “ungrounded neutral” or “neutral grounded via arc suppression coil” mode, and inherently lack a natural neutral grounding point.
- Earthing Transformers provide a neutral terminal via star (Y) connection, which is then connected to the ground with a grounding resistor or arc suppression coil to achieve single-phase ground fault handling.
- Function: When a single-phase ground fault occurs in the line, it can limit fault current, prevent equipment damage from overvoltage, and help relay protection devices quickly locate the fault point.
- Industrial High-Voltage Equipment Systems
High-voltage motors, transformers, and other equipment in large factories and industrial parks often require Earthing Transformers to ensure operational safety.
- In industrial systems, high-voltage motors (6kV, 10kV), rectifier equipment, etc., if designed with ungrounded neutral, are prone to phase-to-phase short circuits due to insulation breakdown.
- Earthing Transformers provide a neutral grounding point for the power supply system of such equipment, and cooperate with grounding protection devices to realize fault current detection and rapid tripping.
- Typical Scenarios: High-voltage power supply systems in petrochemical, metallurgical, and mining industries, which need to ensure continuous production and prevent fault expansion.
- New Energy Power Generation Systems
Earthing Transformers are key supporting equipment in the booster stations and collection lines of photovoltaic power plants and wind farms.
- Inverters and box-type transformers in new energy systems usually adopt the “ungrounded neutral” design to reduce the impact of ground faults on power generation efficiency.
- Earthing Transformers provide neutral grounding points for the 110kV and 35kV systems in booster stations, and cooperate with grounding resistors to limit fault current, protecting precision equipment such as inverters and transformers.
- Function: Prevent the shutdown of the entire power generation unit caused by single-phase ground faults, and improve the power supply reliability of new energy systems.
- Special-Scenario Power Supply Systems
Some special scenarios with high safety requirements also require Earthing Transformers to achieve precise grounding protection.
- Railway Traction Power Supply: In the traction substations of high-speed railways and subways, the 27.5kV traction network adopts single-phase power supply. Earthing Transformers are needed to balance voltage and suppress zero-sequence current.
- Offshore Wind Power/Oil Platforms: Equipment insulation in marine environments is prone to corrosion. Earthing Transformers, together with corrosion-resistant grounding devices, ensure the safe discharge of current in case of faults, preventing equipment damage or personal electric shock.
Key Factors for Selecting an Earthing Transformer
- System Voltage and Grounding Mode
Match the transformer’s rated voltage to the grid (6kV/11kV/33kV) for insulation compatibility. Select based on grounding type: arc suppression coil systems need models supporting high-capacity coil matching; small-resistance grounding requires low zero-sequence impedance to ensure protection activation.
- Winding Design and Zero-Sequence Impedance
Prioritize Z-type (zigzag) windings, which provide ultra-low zero-sequence impedance (~10Ω) and enable 90%–100% utilization of arc suppression coil capacity. Ensure impedance aligns with the system’s fault current requirements to facilitate effective zero-sequence current transmission.
- Grounding Capacitive Current and Capacity Sizing
Calculate the grid’s total grounding capacitive current (critical for systems >10A). Size the transformer to handle either the arc suppression coil’s compensation current or the short-term fault current from grounding resistors, preventing overload during faults.
- Operational Traits and Withstand Capacity
Adapt to its “long-term no-load, short-term overload” operation: check short-time withstand current (to tolerate fault currents for seconds) and prioritize low no-load loss to reduce energy waste during normal operation.
- Environmental and Installation Requirements
For harsh environments (dust, humidity, high temperatures), choose models with appropriate protection levels (e.g., IP54) and corrosion/heat resistance. In space-constrained areas (urban stations, indoor switchgear), opt for compact designs.
- Standards Compliance and Certifications
Ensure adherence to international (IEC 60076) or national (e.g., GB/T 6451) standards. Verify valid certifications (CE, CCC) to guarantee safety, compatibility, and reliability in grid operation.
Drawbacks of Transformer Neutral Point Ungrounded Operation
Transformer neutral point ungrounded operation has the following five disadvantages:
- High insulation level requirements and cost: When a single-phase grounding fault happens, the voltage of the non-fault phase increases by √3 times. As a result, electrical equipment in the power system needs to have a higher insulation grade, which significantly raises both the manufacturing cost and subsequent maintenance cost of the equipment.
- Danger of arc grounding overvoltage: If the single-phase grounding current is small, the arc will extinguish when the current passes through zero, and the fault will disappear. However, when the current exceeds 30 amperes, a stable arc will be generated, forming continuous arc grounding. This not only damages equipment but may also cause two-phase or even three-phase short circuits.
- Difficulty in selecting grounding relay protection: It is hard to realize sensitive and selective protection. Especially for power grids with arc suppression coils, the configuration and accurate operation of such protection become more difficult, which easily affects the timely detection and isolation of faults.
- Disconnection may cause resonance overvoltage: Actions such as wire breakage, switching operations of switches at different times, and fuse fusing at different periods can all lead to ferroresonance overvoltage. This overvoltage may cause lightning arrester explosion, reverse phase sequence of load transformers, and insulation flashover of electrical equipment.
- Resonance overvoltage of electromagnetic voltage transformer: Due to the asymmetry of power grid parameters, neutral point displacement often causes ferroresonance overvoltage, which frequently blows the high-voltage fuse of the electromagnetic voltage transformer. In severe cases, it can even burn out the transformer itself.
Advantages of transformer neutral point ungrounded operation
- High power supply reliability: Little change in three-phase voltages/currents during single-phase grounding faults; no immediate tripping, with faults cleared within ~2 hours, ensuring continuous power.
- Low interference to communication/signal systems: Weak electromagnetic interference under symmetrical three-phase operation; small grounding current causes minimal impact; arcs self-extinguish in small systems (e.g., rural grids).
- Facilitates fault detection & location: Distinctive small grounding current helps protection devices identify and locate faults.
- Reduces demand for current-limiting devices: Small grounding current eliminates the need for large-capacity current-limiting equipment, cutting costs and simplifying design.
- Better overvoltage control in specific scenarios: Easier to control voltage fluctuations during normal/transient processes, lowering overvoltage damage risks.
- Enhances transient system stability: Easier to maintain three-phase voltage balance during transients, reducing impacts on key equipment and avoiding cascading issues.
