is a test method specifically designed for insulation performance testing of high-voltage cables (especially long-distance cross-linked polyethylene XLPE cables)
Very Low Frequency (VLF) High Voltage Withstand Test is a test method specifically designed for insulation performance testing of high-voltage cables (especially long-distance cross-linked polyethylene XLPE cables).
It uses a sinusoidal or cosine square wave AC voltage with a frequency of 0.1Hz (or even lower, such as 0.01Hz-0.5Hz) to replace the traditional power frequency (50Hz/60Hz) AC withstand test or DC withstand test.
The following is a detailed analysis of this technology:
1. Why is VLF test needed?
In the field of power cable testing, traditional methods have obvious defects, and VLF technology has emerged as the best solution:
Disadvantages of DC withstand test:
For XLPE (cross-linked polyethylene) cables, DC withstand voltage will cause space charge accumulation in the insulation layer. After the test, these charges are difficult to dissipate. When the cable is put back into operation, they are superimposed on the operating voltage, which can easily lead to insulation breakdown.
The DC electric field distribution depends on resistivity, while the AC electric field distribution depends on permittivity. The two are different, so the DC test cannot truly simulate the stress state of the cable during operation.
Conclusion: International standards (such as IEEE 400.2, IEC 60502) have explicitly prohibited DC withstand tests on new or in-service XLPE cables.
Power frequency (50Hz):
The cable is equivalent to a large capacitor. For long-distance cables, the power frequency withstand test requires huge reactive power (
I=UωC, when the frequency is reduced from 50Hz to 0.1Hz, the required test current and capacity are reduced by 500 times.
This makes the test equipment very lightweight (usually only a few tens of kilograms), suitable for field and narrow space operations, while maintaining the ability of AC voltage to detect insulation defects (such as tree aging, water trees).
2. Working principle and waveform
Frequency range: usually 0.1Hz (period 10 seconds), some high-end equipment supports 0.05Hz or 0.02Hz.
Waveform types:
Sinusoidal wave: closest to actual operating conditions, but equipment control is more complex.
Cosine rectangular wave: currently the most mainstream waveform. It transitions smoothly near the zero crossing, effectively exciting insulation defects while reducing harmonic interference, and the equipment implementation is relatively easy.
Equivalence: Studies have shown that the insulation breakdown characteristics at 0.1Hz are highly equivalent to those at 50Hz, and can truly reflect the insulation strength of the cable under AC operating voltage.
3. Main application objects
High-voltage power cables: especially 6kV ~ 35kV cross-linked polyethylene (XLPE) cables, and some 110kV and above cables (need to be combined with series resonance).
Rotating machines: insulation testing of stator windings of large generators and motors.
Other capacitive loads: such as long busbars, capacitor banks, etc.
4. Test standards and specifications
In China and internationally, the following standards are mainly followed:
Chinese standards:
DL/T 849.4-2019 "General Technical Conditions for Power Special Test Equipment Part 4: Very Low Frequency High Voltage Generator"
GB/T 3048.8 "Test Methods for Electrical Properties of Electric Cables and Wires Part 8: AC Voltage Test" (involving related principles)
Q/GDW 11316-2014 "Test Procedures for Power Cable Lines" (State Grid enterprise standard, which clearly specifies the voltage value and time of VLF test)
International standards:
IEEE 400.2 "Guide for Field Testing of Shielded Power Cable Systems Using Very Low Frequency (VLF)"
IEC 60502-2 / IEC 60840
5. Typical test parameters (reference)
Take common 10kV and 35kV XLPE cables as examples (specific parameters should refer to the latest regulations):
10kV cable:
Test voltage: usually 2.0~3.0 times U0 (U0 is phase voltage, for 10kV cable U0=8.7kV). For example: 17.4kV ~ 26kV.
Test time: 15 minutes ~ 60 minutes (depending on whether combined with partial discharge detection or only withstand test).
35kV cable:
Test voltage: usually 2.0 times U0 (for 35kV cable U0=21kV), i.e., about 42kV.
Test time: 60 minutes.
Note: For commissioning tests (before new cables are put into operation), the requirements are usually stricter; for preventive tests (in operation), the voltage level may be appropriately reduced or a diagnostic test mode may be adopted.
6. Equipment features (mainstream level in 2026)
Modern VLF withstand testers usually have the following functions:
Integrated design: high voltage source, controller, and protection unit integrated in one chassis, lightweight (<30kg).
Intelligent diagnosis: not only withstand test, but also combined with Tanδ (dielectric loss factor) measurement and partial discharge (PD) detection, achieving "withstand + diagnosis" integration.
Safety protection: with overvoltage, overcurrent, flashover protection, and can quickly cut off and record fault location (combined with traveling wave method or bridge method) at the moment of breakdown.
Data management: built-in storage, supports Bluetooth/WiFi report upload, automatically generates test reports in compliance with State Grid/Southern Power Grid formats.
7. Summary: comparison of advantages and disadvantages
Table
Characteristic DC Withstand (DC) Power Frequency AC Withstand (50Hz) VLF Withstand (0.1Hz)
Applicable cables Oil-paper insulation (old) Short cables XLPE cables (mainstream)
Equipment weight Light Very heavy (requires resonance) Lightweight
Insulation damage risk High (space charge) Low Low (no space charge)
Test authenticity Poor (different electric field distribution) Best Good (highly equivalent)
Field operability Easy Difficult Easy
Core recommendation:
For current 10kV and 35kV cross-linked polyethylene (XLPE) cables, VLF withstand test is the preferred solution. DC withstand test is strictly prohibited to avoid damaging cable insulation; if the cable is extremely long or the voltage level is extremely high (such as 220kV+), a combination of "VLF + series resonance" or pure series resonance power frequency test may be required.
