It solves the problem that traditional power frequency (50Hz/60Hz) withstand voltage test equipment is too bulky to test long cables on site, while avoiding the risk of cumulative damage to cable insulation caused by DC withstand voltage tests.
The following are the core knowledge points about VLF withstand voltage tests:
1. Core Principle
Frequency definition: Typically uses a 0.1 Hz sine wave or cosine square wave voltage (standard range is usually between 0.01 Hz and 0.1 Hz).
Why 0.1 Hz?
Significant reduction in capacity requirements: The power supply capacity required for AC withstand voltage tests
Reducing the frequency from 50 Hz to 0.1 Hz theoretically reduces the required test equipment capacity to 1/500 of that of power frequency equipment. This makes the equipment very lightweight and suitable for field and on-site operations.
Equivalence: At 0.1 Hz, the distribution characteristics of the electric field in the insulating medium are very close to those at power frequency (50 Hz), effectively detecting insulation defects without causing space charge accumulation effects like DC withstand voltage (space charge can accelerate aging or mask defects).
2. Main Application Objects
High-voltage power cables: Especially 6kV ~ 35kV (some up to 66kV or higher) cross-linked polyethylene (XLPE) cables.
Capacitors, generator stator windings: Also applicable under certain specific standards.
Non-applicable scenarios: Generally not used for main insulation testing of transformer windings or GIS (Gas Insulated Switchgear) (these typically use series resonance power frequency withstand voltage).
3. Comparison with DC Withstand Voltage and Power Frequency Withstand Voltage
Table
Characteristic DC Withstand Voltage (DC) Power Frequency Withstand Voltage (50Hz AC) Very Low Frequency Withstand Voltage (0.1Hz VLF)
Equipment Size Small Very large (long cables require giant resonant devices) Small (portable, suitable for field)
Impact on Insulation Harmful (easily generates space charge, accelerates aging) No damage (most realistic simulation of operating conditions) Basically no damage (close to power frequency effect)
Defect Detection Type Mainly detects through-type defects, easy to miss water trees Can detect all types of defects Effectively detects water trees, air gaps, and other defects
Applicable Standards Gradually phased out (no longer recommended for XLPE cables) Ideal but limited by field conditions Currently mainstream recommended solution
Test Duration Short Relatively long Relatively short (usually tens of minutes)
Important Note: According to Chinese power industry standards DL/T 474.4-2018 and GB 50150-2016, for cross-linked polyethylene (XLPE) cables, DC withstand voltage tests are strictly prohibited as acceptance or preventive tests, because the DC electric field can cause space charge accumulation inside the insulation, which may lead to breakdown when the cable is put into operation after the test. VLF withstand voltage is the best alternative to DC withstand voltage.
4. Test Standards and Parameters (Reference)
According to DL/T 474.4-2018 "Guide for On-site Insulation Tests Part 4: AC Withstand Voltage Tests":
Waveform: Usually cosine rectangular wave or sine wave.
Test Voltage Value:
New cables (acceptance test): Typically (where U0 is the rated phase voltage of the cable), duration 60 minutes (or according to specific standards, such as 30 minutes).
Old cables (preventive test): Typically
, duration depends on the situation.
Note: Specific multiples must strictly refer to the latest countermeasures of the local power grid company or national standards.
Frequency: 0.1 Hz ± 20%.
5. Brief Operation Procedure
Disconnect and discharge: Disconnect the cable under test from other equipment, fully discharge and ground.
Wiring: Connect the VLF high voltage generator, voltage divider, and cable terminals.
Parameter setting: Set the target voltage (e.g.,
frequency (0.1Hz) and time on the controller.
Voltage ramp-up: The equipment automatically ramps up the voltage at the set rate.
Withstand voltage: Maintain the specified voltage and time. If the cable insulation has defects, breakdown may occur under high electric field, and the equipment will automatically trip and record the breakdown voltage.
Discharge: After the test, the cable under test must be fully discharged using a dedicated discharge rod (because the capacitive energy storage at 0.1Hz is still significant).
6. Summary of Advantages and Disadvantages
Advantages:
Lightweight equipment, can be carried by one person, no need for large power supply vehicles.
No damage to cable insulation, safe and reliable.
Effectively detects early defects such as water tree aging.
Disadvantages:
Low frequency; if there are defects that only appear at high frequencies (very rare), it may be less sensitive than power frequency (but in cable applications, 0.1Hz has been proven sufficiently effective).
Test duration is slightly longer than DC withstand voltage.
If you need specific wiring diagrams, operation videos of certain brand equipment (such as Wuhan Moen, Huatian Electric Power, etc.), or need to query the test voltage standard table for specific voltage level cables, please let me know!
