VLF Withstand Voltage Test is currently the mainstream method for insulation strength testing of high-voltage cables and large generators in the power industry.
Simply put, it uses a 0.1Hz (or even lower) ultra-low frequency AC voltage to replace the traditional 50Hz power frequency AC voltage for withstand voltage testing.
Combined with the latest industry standards in 2026 (such as DL/T 849.4-2024) and technical practices, I will provide you with a detailed interpretation of the core logic, operation process, and key parameters of this test:
1. Core Principle: Why Use "Ultra-Low Frequency"?
The core of this technology lies in solving the testing difficulties of "large capacitance" equipment.
Pain Points of Power Frequency Testing: Long cables or large generators are equivalent to huge capacitors. At 50Hz power frequency, the capacitive reactance is very small, and testing requires a huge current (I=2πfCU), which means you need a giant test transformer weighing hundreds of kilograms or even several tons, making on-site handling extremely difficult.
Advantages of Ultra-Low Frequency:
Significant Capacity Reduction: When the frequency drops from 50Hz to 0.1Hz, the frequency is reduced by 500 times, and the required test current and power supply capacity theoretically also reduce to 1/500 of the power frequency.
Lightweight Equipment: Equipment that originally required truck transportation can now be made into a suitcase type or small cart type, which can be carried by a single person to cable wells or field operations.
Non-Destructive Testing: Compared to DC withstand voltage (which easily accumulates space charges in XLPE cables, causing insulation damage), ultra-low frequency is AC, which does not produce charge accumulation and is more friendly to insulation.
2. Scope of Application and Standards
Main Objects:
Power Cables: Especially cross-linked polyethylene (XLPE) cables of 35kV and below (this is the most common application scenario).
Rotating Machines: Stator windings of large generators and motors.
Others: Capacitive equipment such as power capacitors and high-voltage bushings.
Implementation Standards:
DL/T 849.4-2024 "General Technical Specifications for Special Testing Instruments for Power Equipment Part 4: Ultra-Low Frequency High Voltage Generators" (latest standard implemented in June 2025).
DL/T 596 "Preventive Test Code for Electric Power Equipment".
GB 50150 "Standard for Acceptance Test of Electrical Equipment Installation Engineering".
3. Key Test Parameters (How to Test?)
According to the regulations, different equipment and test purposes have different voltage and time requirements. The following are common reference standards:
A. Power Cables (Taking XLPE Cables as an Example)
Table
Test Type Test Voltage (Peak) Duration Description
Acceptance Test (New Cables) 3U₀ 60 minutes U₀ is the rated phase voltage of the cable. For example, for a 10kV cable, U₀≈5.8kV, test voltage≈17.4kV.
Preventive Test (Old Cables) 2.1U₀ 5 minutes Some regions or regulations may require 3U₀ for 20 minutes, depending on local standards.
B. Generators/Motors
Test Voltage: Usually calculated according to the formula or refer to the manufacturer's manual. Generally, it is 2-3 times the rated voltage.
Advantage: Ultra-low frequency voltage can more effectively detect defects in the end insulation of generators (under power frequency, due to large capacitive current, the voltage drop at the end is large, making it difficult to detect end defects).
4. 2026 On-Site Practical Pitfall Avoidance Guide
If you are about to perform this test, please pay attention to the following points:
Frequency Selection (0.1Hz vs 0.05Hz vs 0.02Hz):
0.1Hz: Most commonly used, fast test speed.
0.02Hz: When the cable is particularly long (capacitance is particularly large) and 0.1Hz exceeds the instrument's load capacity, switch to 0.02Hz. At this time, the load capacity is usually 5 times that of 0.1Hz, but the test time needs to be extended accordingly (usually recommended to extend to 2-3 times the time, or according to standards).
Waveform Requirements:
Be sure to use equipment that outputs sine waves or cosine square waves. Inferior square waves contain high-order harmonics, which may cause insulation breakdown or inaccurate test data.
Wiring and Grounding:
Non-tested phases grounded: When testing phase A, phases B and C and the shielding layer must be reliably grounded.
Grounding Resistance: The instrument's grounding terminal must have good contact with the earth, and the grounding resistance is recommended to be ≤4Ω; otherwise, high voltage back-strike may occur, damaging the instrument.
Discharge Safety:
After the test, a large amount of charge remains on the cable. Do not disconnect the wires directly! First, let the instrument automatically discharge, then use a discharge rod for manual discharge, and confirm no residual voltage before operation.
5. Result Judgment
Pass: During the test, the voltage is stable, no breakdown, no flashover, leakage current is stable and without periodic sudden changes.
Fail:
Sudden voltage drop and current rise (breakdown).
Leakage current increases significantly over time (insulation damp or aging).
Abnormal noise or burning smell.
In summary, the VLF withstand voltage test is currently the most cost-effective, portable, and non-destructive AC withstand voltage solution, especially suitable for your maintenance and acceptance work of distribution network cables in Yangzhou area.
