Measurement of Insulator Insulation Resistance
Insulation resistance measurement is a fundamental method for evaluating the condition of porcelain, tempered glass, and composite insulators. The primary objective is to detect insulation deterioration, dielectric breakdown, and other defects that could compromise grid safety.
1. Equipment and Standards
For overhead transmission lines ranging from 35kV to 220kV, a 5000V Megohmmeter is recommended for accurate measurement.
- New Insulators: Insulation resistance should be ≥ 500 MΩ.
- In-Service Insulators: Insulation resistance should be ≥ 300 MΩ.
2. Criteria for Insulation Degradation
The state of an insulator is determined based on the following resistance thresholds:
- Low-value Insulator: Resistance < 300 MΩ but > 240 MΩ.
- Zero-value Insulator: Resistance < 240 MΩ.
Note: This resistance test method is generally not applied to composite insulators.
3. Factors Leading to Deterioration in Disc Insulators
Several factors can cause the insulation of disc-type suspension insulators to degrade over time:
- Temperature: Resistance decreases as temperature rises due to intensified dielectric polarization.
- Humidity: Moisture absorption forms a water film on the surface, significantly increasing leakage current.
- Mechanical Overload: Structural damage caused by excessive tension.
- Porcelain Material Issues: Moisture absorption or internal/external stress overlap in porcelain parts.
- Cement & Fitting Issues: Saturated expansion or freezing expansion of the cement used for casting steel caps; galvanic corrosion of steel caps and feet.
- Electrical Stress: Deterioration caused by overvoltage or inherent internal defects.
4. Measurement Methods
A. De-energized Measurement
Conducted during power outages using a 2500V or 5000V Megohmmeter. While both provide similar accuracy within the 10,000 MΩ range, the 5000V unit is superior for high-resistance insulators, capable of reading up to 200,000 MΩ, whereas a 2500V unit would simply indicate infinity (∞).
B. Live-line (Energized) Measurement
This method utilizes a high-resistance rod connected to the energized insulator, keeping the Megohmmeter at ground potential. A grounding capacitor (0.01–0.05 μF, rated for 3000V+ DC) is used to ensure safety and measurement accuracy. The insulator’s resistance is calculated by subtracting the rod’s resistance from the total measured value.
5. Final Judgment Rules
- Standard Units: Each element of a pin-type post insulator or each disc of a suspension string should not be lower than 300 MΩ.
- Ultra-High Voltage (500kV): Insulators used on 500kV lines should maintain a resistance of no less than 500 MΩ.
- Precaution: When measuring multi-element post insulators, copper wire should be wrapped around the bonded layers to ensure consistent contact and avoid misjudgment due to varying measurement positions.
Safety Tip: Insulation resistance measurements should be suspended during periods of high humidity to avoid inaccurate readings caused by surface moisture.
