Very Low Frequency Withstand Voltage Test
The Very Low Frequency (VLF) Withstand Voltage Test is an insulation strength testing method that uses a 0.1Hz (or 0.05Hz, 0.02Hz) ultra-low frequency AC voltage to replace the traditional power frequency (50Hz) AC withstand voltage test. It is mainly used for insulation withstand voltage testing of cross-linked polyethylene (XLPE) insulated power cables rated at 35kV and below, as well as large and medium-sized generators and motors, serving as an ideal alternative to power frequency withstand voltage tests.
Working Principle
The VLF high-voltage generator internally uses digital frequency conversion technology to convert 50Hz power frequency AC voltage into DC through rectification and filtering, then inverts it to 1kHz AC voltage. After modulation by a 0.1Hz sine wave oscillator, it outputs a 0.1Hz high-voltage sine wave. Its core principles are:
Equivalence: The stress effect of 0.1Hz AC voltage on insulation is equivalent to that of power frequency AC voltage, effectively simulating the real electric field stress during cable operation.
Lightweight: Since the frequency is reduced to 1/500 of the power frequency, the capacitive current of the test object is greatly reduced, requiring only about 1/500 of the power capacity of power frequency equipment, significantly reducing equipment size and weight.
Non-destructive: Compared to DC withstand voltage tests, VLF AC withstand voltage does not cause cumulative space charge damage to XLPE cable insulation, making it a "non-destructive" withstand voltage testing method.
Scope of Application
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Equipment Type Applicable Description
XLPE power cables rated 35kV and below The primary application object, suitable for both acceptance tests and preventive tests.
Large and medium generators More effective in detecting end-winding insulation defects than power frequency withstand voltage.
High-voltage motors Acceptance, overhaul, partial winding replacement, and routine tests.
Cables rated 110kV and above Not applicable; variable frequency series resonance AC withstand voltage tests should be used.
Test Parameter Standards
Cable Test Parameters (Acceptance Test)
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Rated Voltage U₀/U (kV) Test Voltage Test Duration
1.8/3 3U₀ (5kV) 60min
3.6/6 3U₀ (11kV) 60min
6/10 3U₀ (18kV) 60min
8.7/10 3U₀ (26kV) 60min
12/20 3U₀ (36kV) 60min
21/35 3U₀ (63kV) 60min
26/35 3U₀ (78kV) 60min
Preventive Test Parameters
Different regulations have varying parameters for preventive tests:
DL/T 596-2021: Test voltage 2.5U₀, duration 15-60 minutes.
North China Power Grid Regulation: Test voltage 2.1U₀, applied for 5 minutes.
Jiangsu Provincial Electric Power Company Regulation: Test voltage 3U₀, applied for 20 minutes (60 minutes for acceptance and moisture checks).
Since you are in the Yangzhou area, it is recommended to prioritize the requirements of the "Jiangsu Provincial Electric Power Company Preventive Test Regulations for Electric Power Equipment" for preventive tests.
Generator Test Parameters
Peak test voltage calculation formula: Umax = √2 × β × K × U₀
β: Equivalence coefficient between 0.1Hz and 50Hz voltage, taken as 1.2.
K: Usually taken as 1.3 to 1.5.
U₀: Rated voltage of the generator stator winding.
Test Equipment
Main technical parameters of the VLF high-voltage generator (also known as VLF withstand voltage tester):
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Item Parameter
Output Voltage 0-90kV (single-stage direct output)
Output Frequency Adjustable at 0.1Hz, 0.05Hz, 0.02Hz
Waveform Sine wave or cosine square wave
Measurement Accuracy ±(3% of full scale + 0.5kV)
Load Capacity Up to 1.1μF at 0.1Hz, 2.2μF at 0.05Hz
Protection Functions Overvoltage protection (millisecond response), dual overcurrent protection for high and low voltage
Equipment Size Only 1/500 of traditional power frequency equipment
Some high-end models also integrate dielectric loss testing functions, allowing measurement of cable dielectric loss (tanδ), capacitance, and insulation resistance during the withstand voltage test, achieving "multi-function in one machine".
Test Procedure
Disconnect connections: Disconnect all electrical equipment connected to the test object.
Insulation resistance measurement: Use a 5000V or 10000V megohmmeter to measure insulation resistance of each phase separately; proceed with the withstand voltage test only after passing.
Reliable grounding: Ground the test equipment reliably using bare copper wire.
Wiring: Use dedicated flexible connection cables to connect the equipment to the test object. Connect the high-voltage terminal to the conductor of the phase under test, and reliably ground the non-tested phases and metal shields/armor.
Parameter setting: Set the test frequency (0.1Hz), test voltage, test duration, and overcurrent/overvoltage protection values.
Voltage application: Begin raising the voltage, closely monitor the high-voltage circuit, and listen for any abnormal sounds from the test object.
Withstand timing: Start timing once the test voltage is reached, and monitor voltage and current stability.
Voltage reduction and completion: After the time elapses, the instrument automatically stops. First reduce the voltage to zero, then cut off the power.
Full discharge: Use a discharge rod to fully ground and discharge the test object before disconnecting wires.
Re-measure insulation: After the test, re-measure insulation resistance and compare with pre-test data for analysis.
Acceptance Criteria
Core criterion: No destructive discharge (no breakdown) throughout the entire withstand voltage process; no upper limit for leakage current is specified.
Additional criterion: After voltage application, there should be no obvious heating at intermediate joints or terminations.
Abnormal handling: If waveform distortion, abnormal current increase, voltage instability, unusual odor/smoke/abnormal sounds or flashover from the test object occur during voltage rise, stop the test immediately and investigate the cause.
Interruption handling: If the test is interrupted due to non-test-object defects, after identifying the cause, the full-duration withstand voltage test must be restarted; simply "making up the time" is not allowed.
Comparison with Power Frequency and DC Withstand Voltage Tests
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Comparison Item Power Frequency Withstand (50Hz) VLF Withstand (0.1Hz) DC Withstand
Equipment Size Large and heavy Small and portable (about 1/500 of power frequency) Medium
Damage to XLPE Cables No cumulative damage No cumulative damage Causes space charge accumulation damage
Equivalence Closest to real operating conditions Good equivalence with power frequency Electric field distribution differs from operating conditions
Defect Detection Capability Good Good Insensitive to certain defects
Field Applicability Long cables require high-capacity equipment Suitable for field testing of long cables Equipment is relatively lightweight
Standard Recommendation First choice Alternative when power frequency conditions are not met Gradually being replaced
Applicable Standards
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Standard Number Name
GB 50150-2016 Standard for Acceptance Test of Electric Equipment Installation Engineering
DL/T 596-2021 Regulations for Preventive Tests of Electric Power Equipment
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 Performance of Wires and Cables - AC Voltage Test
DL/T 849.4-2004 General Technical Conditions for Special Testing Instruments for Electric Power Equipment
Operational Precautions
Safety first: Strictly follow the procedures of power outage, voltage verification, and grounding wire installation; set up safety barriers and warning signs in the test area.
Megohmmeter first: Insulation resistance must be measured and pass before conducting the withstand voltage test.
Full discharge: Use a discharge rod to fully discharge the test object before and after the test to prevent residual voltage hazards.
No open circuit: The grounding terminal and the test object must not be left open-circuited.
Environmental requirements: If abnormalities occur due to air humidity or surface contamination, clean and dry the test object before retesting.
Equipment calibration: Voltage meter accuracy should not be lower than 1.5 class, and equipment should be calibrated annually.
Major Brands
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Category Brand Features
Domestic Wuhan Moen Intelligent MEVLF series programmable VLF high-voltage generator, touch screen operation, supports dielectric loss testing.
Domestic Huayi Electric Lightweight design, single-stage direct output 0-90kV.
Domestic Zhicheng Test & Control ZC-524 model, supports testing for both cables and generators.
Domestic Wei'an Electric Multiple specifications and models, covering different voltage level requirements.
Relationship with Instruments You Previously Learned About
Among the several power test instruments you have already learned about, the VLF withstand voltage test serves as the "ultimate test" of insulation strength - the previous instruments are responsible for "finding problems", while the withstand voltage test verifies whether the equipment can withstand operating voltage as the "final checkpoint":
Circuit resistance tester: Checks whether the conductive circuit contacts are good.
Transformer turns ratio tester: Checks whether the winding turns ratio is correct.
Dielectric loss tester: Evaluates the overall aging and moisture level of insulation materials.
Zinc oxide arrester tester: Checks the condition of arrester valve blocks.
Partial discharge detector: Captures discharge signals from tiny internal defects in insulation.
VLF withstand voltage test: Uses high voltage to "test" whether the insulation can withstand it, serving as the final determination of insulation qualification.
