What is the working grounding of a transformer?

     The working grounding of a transformer refers to the measure of reliably connecting the neutral 

point of the transformer (or a certain point of the winding) to the earth through a conductor to

 ensure the normal operation of the transformer and the power system. The following are its core

 functions, principles, implementation methods, and precautions:

 I. Core Functions of Working Grounding

1. **Stabilize the System Voltage**

   - After the neutral point is grounded, it can balance the three-phase voltage and prevent the neutral 

point from shifting (for example, when the three-phase load is unbalanced, the voltage at the neutral 

point approaches zero).

   - Example: In a three-phase four-wire system, if the neutral point is not grounded, the operation of

 a single-phase load may cause the voltage at the neutral point to rise, damaging the equipment.

2. **Provide a Path for Fault Current**

   - When a single-phase grounding fault occurs, the grounding current forms a loop through the neutral 

point, enabling the protective device (such as a circuit breaker) to act quickly and cut off the fault.

3. **Reduce Insulation Requirements**

   - After the neutral point is grounded, the voltage between the winding and the ground is the phase

voltage (such as 220V), rather than the line voltage (such as 380V), which can reduce the insulation cost 

of the transformer and the lines.

 II. Wiring Methods of Working Grounding

1. **Direct Grounding of the Neutral Point**

   - **Applicable Scenarios**: High-voltage systems of 110kV and above, and low-voltage distribution 

systems of 380/220V (such as the TN system).

   - **Wiring Method**: Connect the neutral point of the transformer (such as the neutral point of the 

Y-shaped winding) to the grounding device (ground electrode) with a copper busbar or cable.

2. **Ungrounded Neutral Point/Grounded Through a Resistor**

   - **Applicable Scenarios**: 3-35kV systems (to improve power supply reliability and allow short-term

 operation in case of a single-phase grounding fault).

   - **Principle**: Limit the grounding fault current through a high resistance to avoid instantaneous 

tripping.

 III. Technical Requirements for Working Grounding

1. **Grounding Resistance**

   - For low-voltage systems: ≤4Ω (as specified in GB 50169-2016 "Code for Construction and Acceptance

 of Grounding Devices").

   - For high-voltage systems: ≤0.5Ω (specifically in accordance with the design specifications).

2. **Selection of Ground Electrodes**

   - Vertical ground electrodes: Galvanized angle steel (50×50×5mm) or steel pipe (φ50mm), with a

 length of ≥2.5m.

   - Horizontal ground electrodes: Galvanized flat steel (40×4mm), with a burial depth of ≥0.7m.

3. **Differences from Protective Grounding**

IV. Working Grounding in Common Systems

1. **TN System (such as TN-C, TN-S)**

   - The neutral point of the transformer is directly grounded, and the neutral wire (N) and the protective

 wire (PE) are either shared or separated.

   - **Example**: In household electricity, the 220V live wire is connected to the load, and the neutral wire 

(N) is grounded through the neutral point of the transformer.

2. **TT System**

   - The neutral point of the transformer is grounded, and the equipment casing is grounded separately,

 with no electrical connection between the two.

   - **Applicable Scenarios**: Decentralized power supply in rural areas, which needs to be used in conjunction

 with a residual current device (RCD).

 V. Precautions

1. **Prohibit Mixed Wiring**

   - The working grounding and lightning protection grounding should maintain a distance of ≥3m to 

avoid damage to the equipment caused by the backflash voltage during a lightning strike.

2. **Regular Detection**

   - Measure the grounding resistance with a grounding resistance tester every year to ensure compliance

 with the standards.

3. **Fault Handling**

   - When the neutral point is poorly grounded, it may lead to:

     - Unbalanced three-phase voltage, causing abnormal heating of electrical appliances;

     - Failure of the protective device to operate, posing a risk of electric shock.

Conclusion

The working grounding of a transformer is the "reference point" of the power system. By grounding 

the neutral point, it achieves voltage stabilization, fault protection, and insulation optimization. In actual 

operation, the type of grounding should be selected according to the system voltage level and power 

distribution method, and the grounding resistance and construction specifications should be strictly f

ollowed.