Why are grading rings necessary?

1.Why are grading rings necessary?

A grading ring, also known as a voltage equalizing shielding ring, is a large metal ring installed at both ends (or one end) of the insulator string in high-voltage Transmission lines (typically 220kV and above). Its main function is to improve the electric field distribution around the insulator string, prevent local corona discharge and arcs, thereby protecting the insulators and fittings and ensuring the safe and stable operation of the line.

2.Why are grading rings necessary? (Function and necessity)

Under ultra-high and extra-high voltage conditions, the electric field intensity is extremely high and the distribution is highly uneven, which brings several serious problems:

Uneven electric field distribution: In the absence of grading rings, the capacitance of each component of the insulator string (such as Steel caps and steel feet) to the conductor is different, resulting in extremely uneven voltage distribution along the insulator string. The insulator pieces closer to the conductor bear higher voltages, while those farther away bear much lower voltages. This can cause some insulator pieces to be overburdened, making them prone to premature aging or breakdown.

Corona discharge:

At locations where the electric field is highly concentrated (such as the steel caps of insulators and the ends of line clamps), the electric field intensity can exceed the breakdown field strength of air, causing ionization of the air and resulting in corona discharge.

Hazards: Corona discharge can generate audible noise (buzzing), radio interference (affecting broadcasting and communication), and power loss (though small, it can be considerable over a long period), and it can also corrode metal parts and degrade the silicone rubber material of composite insulators.

Arc formation: In extreme cases, an uneven electric field may cause arc flashover on the surface of insulators or in air gaps, leading to line tripping and power outages.

The grading ring, through its large curvature radius and ring-shaped structure, can:

The hardware at both ends of the insulator is shielded, making it "invisible" under the protection of the ring to prevent tip discharge.

Compensate for the influence of ground capacitance to make the voltage distribute more evenly throughout the insulator string.

Controlling and optimizing the distribution of the surrounding electric field, the high electric field intensity area is transferred to the area around the ring itself. Due to the large curvature radius of the ring, the electric field intensity on its surface will be significantly reduced, thereby effectively suppressing the generation of corona and arc.

3.Types and Installation Positions of Voltage Equalizing Rings

Voltage equalizing rings are usually used in pairs, but this varies depending on the voltage level and insulator type:

Disk-shaped suspension insulators (porcelain or glass insulator strings):

High-voltage end (conductor side): Install a grading ring. This is the most crucial one, mainly used to protect the line clamp and the first insulator, and to evenly distribute the voltage.

Grounding end (tower side): Sometimes a smaller ring, known as a shielding ring, is installed, whose main function is to protect the steel feet of the insulator and prevent corona.

Composite insulator (silicone rubber insulator):

Equalizing rings must be installed. Due to the relatively poor arc resistance of the core rods of composite insulators (typically fiberglass reinforced plastic), without the protection of equalizing rings, the high temperature and ozone generated by corona discharge at the end fittings will severely erode the silicone rubber sheath and core rods, leading to fatal faults such as "brittle fracture".

A large grading ring is usually installed at the high-voltage end, and a smaller shielding ring is installed at the grounding end.

Voltage level reference:

For 220kV: Usually, only one grading ring is installed on the conductor side.

For 500kV: Equalizing rings must be installed at both ends of the insulator string (larger on the conductor side and smaller on the ground side).

For 750kV/1000kV (ultra-high voltage), the design of grading rings is more complex and larger in scale. Sometimes, multiple rings or rings of special shapes are adopted to precisely control the electric field.

4.Design Considerations

The design of grading rings is not arbitrary and requires consideration of multiple factors:

Voltage level: The higher the voltage, the larger the diameter of the ring is usually.

The installation position: The ring on the conductor side is larger than that on the ground side.

The diameter of the pipe: The diameter of the metal pipe used for the ring needs to be determined based on the thermal stability and mechanical strength of the current passing through (such as short-circuit current).

Ring diameter (R): This is the most crucial design parameter, which needs to be determined through electric field simulation calculations to ensure the best voltage equalization and shielding effects.

Material: Usually, aluminum alloy with good electrical conductivity is adopted, which is light in weight and resistant to corrosion.

5.Summary

Characteristics Descriptions

Chinese Name Grading Ring, Corona Ring

English Name Grading Ring, Corona Ring

Main Function Improve the electric field distribution of insulator strings, suppress corona discharge and arcs, and protect insulators and fittings

Working Principle Utilize a large-radius metal ring to shield the electric field concentration points, compensate for the capacitance to ground, and equalize the voltage

Installation Location Primarily installed on the high-voltage end of the conductor side. For ultra-high voltage and above grades, it is also installed on the grounding end.

Key Design Optimized through electric field simulation for the diameter (ring diameter), pipe diameter, and material of the ring

Application Scenarios 220kV and above voltage level transmission lines, especially for composite insulators

In conclusion, the grading ring is a seemingly simple yet crucial "guardian" in ultra-high voltage transmission systems. It silently ensures the reliability and efficiency of the power grid by smoothing the electric field.