FAQ 1: Why is DC Hipot testing considered a "hidden danger" for XLPE cables?
Answer: The primary danger is Space Charge accumulation. Under a high DC voltage, electrical charges become "trapped" within the molecular structure of the Cross-Linked Polyethylene (XLPE). Unlike traditional oil-paper insulation, XLPE does not dissipate these charges quickly. When the cable is returned to AC service, these trapped charges can oppose or double the local electric field, causing premature insulation failure-often within days of a "successful" DC test.
FAQ 2: Can VLF testing detect "Water Trees" better than DC testing?
Answer: Yes, significantly better. DC stress is distributed according to the resistivity of the insulation, which often overlooks moisture-related defects like "Water Trees." VLF testing (at 0.1Hz) creates a capacitive stress distribution that mimics real-world 50/60Hz operation. This "AC-like" stress forces moisture-laden trees and voids to trigger a controlled breakdown during the test, preventing a catastrophic failure while the cable is under load.
FAQ 3: How does the 0.1Hz frequency make the equipment so much more portable?
Answer: The key is the physics of Charging Current (IcIc). The current required to energize a capacitive cable is directly proportional to frequency (Ic=2πfCVIc=2πfCV). By reducing the frequency from 50Hz to 0.1Hz (a 500-fold reduction), the power required to drive the test also drops by 500 times. This allows a lightweight, 25kg VLF generator to perform the same task that would otherwise require a multi-ton power-frequency transformer.
FAQ 4: Does VLF testing damage a healthy cable?
Answer: No. When performed according to standards like IEEE 400.2, VLF testing is a non-destructive diagnostic. It provides enough stress to eliminate "latent defects" (those already on the verge of failing) while leaving healthy insulation completely untouched. Unlike DC, which causes chemical and molecular "aging" via polarization, VLF is a pure AC stress test that preserves the life of the asset.
FAQ 5: What should I do if a cable "passes" VLF but I still suspect aging?
Answer: A withstand test is only a "Pass/Fail" check. For a true health assessment, you should upgrade to VLF Tan Delta (TD) or Partial Discharge (PD) diagnostics.
Tan Delta measures the "aging" of the bulk insulation.
Partial Discharge locates specific "tumors" or defects in joints and terminations. Only an AC source like VLF allows for these advanced diagnostic overlays, providing you with a "Repair List" rather than just a "Pass Sheet."