Very Low Frequency (VLF) testing is fundamentally an Alternating Current (AC) test, despite its extremely low frequency (typically 0.1 Hz). It differs fundamentally from Direct Current (DC) in its physical characteristics and effects on cable insulation.

Here are the key explanations:

1.Periodic Voltage Polarity Reversal:

The core characteristic of Alternating Current (AC) is that its voltage magnitude and direction change periodically over time.

Although the VLF test voltage changes very slowly (0.1 Hz means a complete positive-negative-positive cycle takes 10 seconds), its voltage value gradually shifts from a positive maximum to a negative maximum and back again, repeating in a cycle. This is a standard sine wave or cosine wave (or other AC waveform).

Direct Current (DC) voltage maintains a constant direction (only its magnitude might change, as in pulsating DC).

2.Periodic Electric Field Reversal:

Due to the periodic reversal of voltage polarity, the direction of the electric field applied to the cable's insulation material also reverses periodically.

This is the most fundamental difference between AC testing (whether power frequency or VLF) and DC testing. DC testing produces an electric field with a constant direction.

3.Avoids Space Charge Accumulation (Key Advantage):

For modern extruded insulation cables like Cross-Linked Polyethylene (XLPE), DC withstand testing has a significant drawback: a constant-direction electric field can inject and accumulate space charges within the insulation material. These charges concentrate near insulation defects (e.g., impurities, micro-voids, water tree tips), creating extremely high local electric fields. This can induce new water trees or electrical trees, or even directly cause insulation damage or accelerated aging.

VLF, as an AC test, features a continuously reversing electric field (albeit slowly). This effectively neutralizes or disperses these space charges, avoiding the potential insulation damage risks associated with DC testing. This makes VLF a safer and more appropriate testing method for XLPE and similar cables.

4.Closer to Actual Operating Conditions:

Power cables in service experience power frequency AC voltage (50Hz or 60Hz). VLF testing also applies an AC voltage, albeit at an extremely low frequency. While the frequency response of the physical processes it induces in the insulation (dielectric polarization, loss mechanisms, etc.) differs, its nature is still closer to actual operating conditions than DC testing. It is better at exciting and detecting defects that manifest specifically under AC service conditions (e.g., interface problems).