Very Low Frequency Withstand Test
The Very Low Frequency Withstand Test (VLF) is a test method that uses extremely low frequency (typically 0.1 Hz) AC high voltage to assess the insulation strength of power equipment. By greatly reducing the test frequency to decrease capacitive current, it significantly lowers the capacity requirements for test equipment, and is currently one of the mainstream solutions for withstand tests of XLPE insulated cables rated 35 kV and below.
Working Principle
Cables are typical large capacitive loads. Under power frequency (50 Hz) conditions, they generate very large capacitive charging currents, resulting in bulky test equipment and extremely high power requirements. The core idea of VLF technology is "frequency reduction to reduce capacity"—lowering the frequency from 50 Hz to 0.1 Hz increases the loop capacitive reactance to 500 times that at power frequency, thereby theoretically reducing the required test power supply capacity to one five-hundredth of that for power-frequency withstand testing while the specimen capacitance remains unchanged.
The specific implementation process is: convert the 50 Hz power-frequency AC voltage into DC through rectification and filtering, then invert it into a 1 kHz AC voltage through an inverter, and finally output a 0.1 Hz high-voltage sine wave (or cosine square wave) through amplitude modulation by a 0.1 Hz sine wave oscillator.
Why Very Low Frequency Is Needed
Comparison Item Power-Frequency Withstand Test (50 Hz) DC Withstand Test Very Low Frequency Withstand Test (0.1 Hz)
Equipment Capacity Extremely large, bulky equipment Relatively small Extremely small, lightweight equipment
Equivalence of Insulation Assessment Most realistic Poor, prone to "false pass" Close to power frequency, better than DC
Applicability to XLPE Cables Good Poor (space charge accumulation) Good
On-Site Portability Poor Average Good
Detection of Water Tree Aging Defects Good Poor Good
Compared with DC withstand testing, VLF withstand testing is closer to the actual operating conditions of cables. AC voltage can periodically change the direction of the electric field, making it easier to expose defects such as mechanical damage and water tree aging inside the insulation, and avoiding the "false pass" phenomenon that may occur in DC withstand testing.
Scope of Application
Mainly applicable to: handover tests and preventive tests of XLPE insulated power cables rated 35 kV and below
Can also be used for: withstand tests of large generator stator windings, large motors, capacitors, and other capacitive equipment
Not applicable to: cables rated 110 kV and above (variable-frequency series resonance method should be used)
Test Standards and Parameters
Handover Test (according to GB 50150-2016)
Test frequency: 0.1 Hz
Test voltage: 3U₀ (RMS)
Duration: 60 min
Example: For a U₀/U=8.7/10 kV cable, the test voltage is 26.1 kV; for a U₀/U=26/35 kV cable, the test voltage is 78 kV
Preventive Test
DL/T 596-2021 2.5U₀ 15–60 min
North China Power Grid Regulations 2.1U₀ 5 min
Jiangsu Electric Power Company Regulations 3U₀ 20 min
Since you are in Jiangsu, the preventive test may refer to the "Jiangsu Electric Power Company Preventive Test Regulations for Power Equipment," with a test voltage of 3U₀ and a voltage application time of 20 minutes.
Very Low Frequency Withstand Test for Generators
The peak test voltage is 1.2 times the peak value of the power-frequency test voltage
It is more effective than power-frequency withstand testing in detecting insulation defects at generator ends (under very low frequency, the voltage drop across the semiconductor corona protection layer decreases, and the end insulation bears a higher voltage)
Operating Procedure
Pre-Test Preparation
Disconnect the cable from external equipment and fully discharge and ground the cable
Use a 5000 V megohmmeter to measure and record the insulation resistance
Set up safety fences and warning signs; unauthorized personnel are strictly prohibited from entering the test area
Check that the equipment is reliably grounded and confirm that overvoltage and overcurrent protection functions are normal
Wiring and Testing
Connect the high-voltage end to the conductor of the phase under test, and reliably ground the non-tested phases and metallic shield/armor
Testing may be performed phase by phase, or with the three-phase conductors connected in parallel simultaneously (provided that the capacitance value is within the equipment load capacity range)
Set the test voltage and test time, and start the equipment for automatic voltage rise
During the voltage rise process, monitor the high-voltage circuit for abnormal sounds; start timing after reaching the target voltage
Continuously monitor changes in leakage current during the withstand test
End of Test
After the equipment automatically reduces the voltage to zero, cut off the power supply
Use a discharge rod to fully discharge the specimen (at least 2 minutes)
Re-measure the insulation resistance and compare and analyze it with the pre-test data
Confirm that there is no residual charge before disconnecting the wires
Acceptance Criteria
Pass: No destructive discharge throughout the entire withstand test process (no breakdown, no flashover); no upper limit for leakage current is specified
Fail: Breakdown, flashover, sudden increase in leakage current, sharp voltage drop, or discharge sounds inside the equipment
After the test, the insulation resistance shows no obvious decrease compared with that before the test
Output Frequency 0.1 Hz (optional 0.05 Hz, 0.02 Hz)
Output Waveform Sine wave / cosine square wave selectable
Output Voltage Up to 80 kV–130 kV (depending on model)
Load Capacitance 0.1 μF–1.0 μF (depending on model)
Voltage Accuracy ±3%
Protection Functions Overvoltage, overcurrent, breakdown flashover, zero-position protection
Overall Weight Approximately 25 kg–35 kg (portable type)
Operating Temperature -20℃–+50℃
Comparison Item Very Low Frequency (0.1 Hz) Variable-Frequency Series Resonance
Applicable Voltage Level 35 kV and below All voltage levels (especially 110 kV and above)
Equipment Size/Weight Small/light (can be carried by one person) Relatively large/heavy
On-Site Portability Excellent Average
Detection Sensitivity Sine wave suitable for withstand testing, cosine square wave suitable for partial discharge detection Good waveform, distortion rate ≤0.3%
Standard Basis DL/T 849.4 DL/T 849.1
Typical Scenarios Handover/preventive testing of distribution network cables Handover testing of high-voltage/extra-high-voltage cables
For cables rated 35 kV and below, VLF equipment is more portable and more cost-effective; for cables rated 110 kV and above, series resonance is the only choice; for 35 kV cables, either can be selected.
Implementing Standards
GB 50150-2016: Standard for Handover Test of Electrical Equipment in Electrical Installation Engineering
DL/T 596-2021: Preventive Test Code for Electric Power Equipment
DL/T 849.4-2004: General Technical Specifications for Very Low Frequency High Voltage Generators
DL/T 474.4-2018: Guide for Implementation of On-Site Insulation Tests—AC Withstand Voltage Test
GB/T 3048.8: Test Methods for Electrical Properties of Electric Wires and Cables—AC Voltage Test
