Power Frequency AC Withstand Voltage Test for Insulators

The Power Frequency AC Withstand Voltage Test is the most direct, effective, and authoritative method for assessing the dielectric strength of insulators. It is a mandatory requirement during commissioning (handover) tests.

In preventive maintenance, this test can replace zero-value detection and insulation resistance measurements, or serve as the final judgment for insulators flagged by other methods. For single-unit post insulators, it remains the most efficient and straightforward testing approach.

1. Voltage Standards and Requirements

• 35kV Pin-type Post Insulators:
  • For two-part bonded units: 50kV per unit.
  • For three-part bonded units: 34kV per unit.
• Disc Suspension Insulators: For units with a mechanical failing load of 60–300kN, the AC withstand test voltage is standardized at 60kV.
• Equipment Recommendation: High-voltage test equipment in the 100kV class is recommended for these procedures.

2. Test Scope and Objectives

The scope of this test covers porcelain, tempered glass, and composite insulators. The primary objectives are:

• To identify insulators with compromised insulation that may have been missed by voltage distribution or insulation resistance methods.
• To effectively detect internal defects by applying steady voltage over a specified duration.
• To provide the ultimate verification of an insulator’s quality and reliability.

3. Judgment Criteria for Success

An insulator is considered compliant if it meets the following conditions:

• 1-Minute Duration: The insulator must withstand the standard test voltage for 1 minute without any flashover or breakdown.
• Voltage Ramp-up: During the increase of voltage (at a rate of 3–5kV/s) up to the standard test level, there should be no abnormal discharge sounds, no flashover, and no significant fluctuations in the voltmeter pointer.

4. Key Precautions and Procedures

To ensure safety and data accuracy, the following precautions must be observed:

1. Emergency Shutdown: Power must be disconnected immediately if any of the following occur during the test: overheating of the specimen, breakdown, flashover, abnormal discharge sounds, or violent oscillations of the voltmeter pointer.
2. Test Setup: Insulators should be placed in a sand tray at ground potential. The steel foot of the insulator must be securely connected to the high-voltage terminal of the test transformer.
3. Comprehensive Documentation: All test items must be numbered according to their installation sequence. Records must include:
   • Tower number and phase orientation.
   • Individual unit ID.
   • Environmental conditions: Temperature, humidity, and atmospheric pressure.
   • Final withstand test results.