Equipment Composition and Functions A typical VLF high voltage generator includes: Boosting Unit: Core inverter circuit, converting DC or power frequency AC to 0.1Hz high voltage AC. Control Unit: Microcomputer control, automatically completing voltage boosting, reducing, timing, and pressure holding. Protection System: Equipped with overcurrent, overvoltage, and flashover (breakdown) protection. Once the cable breaks down, it can cut off the voltage within milliseconds without damaging the cable fault point (this is better than DC withstand voltage, which often causes permanent damage after breakdown). Coupling Capacitor/Voltage Divider: Used to measure the actual voltage value applied to the cable. Optional Module - Dielectric Loss Measurement (Tanδ): High-end models integrate dielectric loss measurement function. During the boosting process, not only is it checked for breakdown, but the dielectric loss factor (Tanδ) of the cable insulation and its curve with voltage changes are also monitored in real time. Significance: Tanδ can sensitively reflect overall insulation aging, moisture, or water tree defects, achieving "diagnostic testing" rather than just a "pass/fail" assessment. 5. Comparison with Other Test Methods Table Test Method Frequency Equipment Weight Impact on Cable Applicability Current Status DC Withstand Voltage (DC) 0 Hz Light Extremely Large (Easy to accumulate space charge, causing "memory effect", prone to explosion after commissioning) Oil-paper cables (old) Prohibited for XLPE cables (obsolete) Power Frequency AC (AC) 50/60 Hz Extremely Heavy (Requires resonant device) Minimal (Most realistic) Short cables Long cables require series resonance, bulky equipment Very Low Frequency (VLF) 0.1 Hz Lightweight Small (Equivalent to power frequency) All lengths of XLPE cables Mainstream recommendation (best cost-effectiveness) Oscillating Wave (OWTS) Damped oscillation Medium Small + Locatable Distribution network cables Emerging technology, can accurately locate fault points 6. Operational Precautions Discharge Time: Due to the large capacitance of cables, full discharge must be performed after the test (usually the equipment comes with an automatic discharge rod) to prevent residual charge from harming people. Wiring Check: Ensure the high-voltage line is firmly connected and the grounding is reliable. Environmental Humidity: High humidity may affect surface flashover, and attention should be paid to the treatment of terminal heads. Diagnostic Combination: If conditions allow, it is recommended to combine with Tanδ dielectric loss testing to detect potential hazards earlier, rather than waiting until breakdown occurs. 7. Summary In your previous equipment list: Microcomputer relay protection tester -> Tests secondary protection logic. Transformer test transformer -> Tests transformer main insulation. Winding deformation tester -> Tests transformer mechanical structure. Induced voltage withstand device -> Tests transformer longitudinal insulation. Very low frequency high voltage generator -> Specifically tests long cable insulation. It is a "must-have tool" for power cable maintenance personnel, solving the largest capacitive load testing problem with the smallest equipment size. Do you need to understand the test voltage standards and time requirements for specific voltage levels (such as 10kV or 35kV), or do you want to know how to interpret dielectric loss (Tanδ) test data?
