Insulation Resistance and Withstand Voltage Testing of Insulators: An Overview

Insulators are essential components used extensively in power grids. Currently, porcelain insulators are the most widely used, followed by glass insulators. In recent years, organic or composite insulators have also seen increasing application across domestic networks.

1. Classification of Insulators

Insulators come in various shapes and sizes, categorized by their application and structure:

• By Application: Line insulators and station insulators; indoor and outdoor types.
• By Shape/Structure: Suspension, pin-type, post, long-rod, bushing, and stay (guy) insulators.
• Special Types: Anti-pollution (dust-proof) insulators and insulating cross-arms.

The porcelain or glass components provide both electrical insulation and high mechanical strength. To ensure durability, these parts are often glazed to increase chemical stability, mechanical toughness, and resistance to moisture and pollution.

2. Deterioration: Low-Value and Zero-Value Insulators

During transport, installation, or operation, insulators may suffer damage from collisions, lightning strikes, or the long-term combined effects of mechanical load and high voltage. This leads to insulation deterioration:

• Low-Value Insulators: When the internal breakdown voltage drops below the dry surface flashover voltage.
• Zero-Value Insulators: When the internal breakdown voltage drops to zero.

The presence of low or zero-value insulators in a string significantly increases the risk of pollution flashovers or even explosions under normal operating voltages, posing a severe threat to grid reliability. Timely detection and replacement are critical to eliminating these hidden hazards.

3. Mandatory Test Projects

According to industry regulations, the testing of post and suspension insulators primarily includes the following four items:

1. Zero-Value Detection: Mandatory for systems of 66kV and above.
2. Insulation Resistance Measurement: To evaluate the basic dielectric condition.
3. AC Withstand Voltage Test: To verify the ultimate dielectric strength.
4. Equivalent Salt Deposit Density (ESDD) Measurement: To assess the level of surface contamination.

Note on Glass Insulators: For tempered glass suspension insulators, the three electrical tests (zero-value, resistance, and withstand voltage) are generally not required. This is because glass insulators possess a “self-shattering” characteristic—they will spontaneously shatter upon failure, allowing for easy visual identification and replacement.

Conclusion

Regular testing is the cornerstone of power system stability. By implementing a rigorous regimen of zero-value checks, resistance measurements, and withstand tests, utilities can significantly reduce trip rates and ensure the safe transmission of electricity.