Very Low Frequency (VLF) testing is an AC testing method used for medium and high-voltage power cables (typically 10kV and above) to perform withstand voltage tests and diagnose insulation condition.
Its core characteristics and principles are as follows:
The test voltage frequency is typically around 0.1 Hz (Hertz) (common range is 0.01 Hz to 0.1 Hz). This is significantly lower than power frequency (50Hz or 60Hz).
2.AC Withstand Voltage:
Although the frequency is very low, it is essentially an AC voltage test. This differs from traditional DC withstand voltage testing.
VLF testing applies an AC voltage substantially higher than the cable's rated operating voltage (usually 2 or 3 times the rated phase voltage) for a specific duration (typically 15 to 60 minutes).
3.Purpose:
Withstand Test: The primary goal is to verify that the cable and its accessories (joints, terminations) possess sufficient insulation strength to withstand stresses higher than normal operating voltage without breaking down. This is a critical acceptance test after cable installation or repair.
Diagnostic Testing: By monitoring parameters such as leakage current (or its harmonic components), loss tangent (tan delta), or partial discharge (PD) during the test, the overall aging condition of the cable insulation or localized defects (like water trees, electrical trees, installation damage) can be assessed. Even if the cable passes the withstand test (no breakdown), these diagnostic parameters provide valuable information about insulation health.
4.Working Principle:
The VLF AC voltage generates an alternating electric field within the cable's dielectric insulation.
Defects in the insulation (e.g., voids, impurities, water trees, interface issues) exhibit changes in their electrical characteristics (like capacitance, resistance) under this alternating field, different from sound insulation.
These changes are reflected in the magnitude, waveform, and harmonic content of the leakage current flowing through the insulation, or in the dielectric loss tangent (tan delta) value.
By precisely measuring and analyzing how these parameters change over time or with voltage, the presence and severity of insulation defects can be determined.
If severe defects exist, breakdown may occur under the high voltage, exposing the problem during testing rather than in service.