Defects And Faults of Ceramic Insulators And Corresponding Measures

Porcelain and glass insulators account for two-thirds of the insulators used in overhead lines. Common defects and faults during operation include zero value of porcelain insulators, broken strings, high self explosion rate of glass insulators, rust, etc. Once these problems occur, they will seriously affect the safety of the line. Today, we will discuss the causes and countermeasures of these defects and faults.

The principle of ceramic insulator string breakage

For porcelain insulators, when there are minor cracks and other defects in the porcelain components at the head of the insulator, the insulation resistance at the head of the insulator decreases (forming a zero value insulator) with the invasion of moisture during operation. Once the insulator string is struck by lightning or other reasons that cause flashover, due to the extremely small head resistance and long discharge distance along the umbrella disk, the resistance is relatively high. Therefore, most of the energy is discharged from the discharge channel at the head of the porcelain insulator. The heat generated when the arc passes through the head of the insulator causes the insulator to explode, resulting in a broken string.

Figure 1Head defect porcelain insulator breakdown path

Cause of defect

1.Poor quality of insulators:

Poor bonding interface technology and poor material of porcelain insulators can lead to the generation of zero value insulators.

There are materials with different thermal expansion coefficients, such as porcelain, cement, and steel, at the interface of ceramic insulator adhesive installation. Generally, asphalt and other materials need to be coated as buffer layers. When the buffer layer is too thin or the compatibility with materials such as porcelain and cement is insufficient, the insulator can withstand external temperature changes and cause interface cracking.

There are sand holes in ceramic insulators or ceramic components with excessively large grain sizes, which can cause micro cracks inside the ceramic components during long-term operation. The cracks can expand to a certain extent, leading to the generation of low zero value insulators.

2.Multiple lightning strikes:

When the tower is struck by lightning, the insulator may withstand high voltage surges with both steepness and amplitude. At this point, the porcelain component can withstand the highest voltage (at the head of the component) and partial discharge damage may occur at areas with relatively weak insulation. After experiencing multiple steep wave impacts, the damage to the head of the insulator ceramic component will gradually expand, leading to the formation of low value insulators to a certain extent.

3.Excessive mechanical load:

Severe icing and strong winds exceeding the design wind speed can cause insulators to bear excessive mechanical stress, which may lead to cracking of the insulator steel feet and the generation of low zero value insulators.

Countermeasures

1.Strengthen zero value detection

Spot check and laboratory testing: Conduct spot checks on porcelain insulators in the multi lightning area, and replace some insulators in the laboratory for detailed sampling testing. Especially when the air humidity is high, insulation resistance testing is carried out because some low value insulators are difficult to measure when dry. At the same time, conduct steep wave impact testing to check if there is any damage to the head of the porcelain component.

Increase the detection voltage: To improve the efficiency of zero value detection, the insulation resistance test voltage can be appropriately increased.

2.Adopting guiding lightning protection measures

Parallel clearance and lightning arrester: Parallel clearance and lightning arrester are used on the insulator string to prevent lightning current from directly passing through the insulator and reduce the probability of insulator damage.

3.Improve the lightning resistance level of the line

Renovation of grounding and strengthening of insulation: By renovating grounding and strengthening insulation measures, the amplitude of impulse voltage borne by insulators after being struck by lightning is minimized as much as possible. For example, shortening the measurement cycle of tower grounding resistance, timely grounding renovation of towers with unqualified grounding resistance, and controlling the grounding resistance of towers below 10 Ω.

Comprehensive lightning protection measures: Strengthen insulation, install controllable lightning rods, and prevent winding lightning rods in the vulnerable pole section to improve the lightning resistance level of the line.