I. Working Principle and Technical Architecture
The complete operating process of a very low frequency (VLF) high voltage generator consists of three core stages: frequency conversion & inversion, voltage boost, measurement, control and protection. All modules work in coordination to ensure stable and accurate withstand voltage tests.
1. Frequency Conversion & Inversion
After connecting to standard power frequency supply, the device first converts 220 V / 50 Hz alternating current into steady direct current via rectification and filtering. The built-in inverter circuit then transforms DC power into adjustable VLF AC signals.
Three standard frequency options are available: 0.1 Hz, 0.05 Hz and 0.02 Hz. Among them, 0.1 Hz is the most widely used in engineering due to its optimal overall performance.
2. Voltage Boost
The low-voltage VLF signals from the inverter are transmitted to a dedicated step-up transformer to reach the rated test voltage. This transformer features a specially designed structure that fully adapts to the magnetic characteristics of VLF operation, ensuring stable voltage boosting.
3. Measurement, Control and Protection
Modern VLF high voltage generators adopt microcomputer control, enabling fully automatic operation including voltage rise, voltage drop, data measurement and recording with minimal manual intervention.
Equipped with overvoltage and overcurrent protection circuits with a response time within 20 ms, the unit can cut off abnormal operation instantly to protect both the test equipment and the device under test. Key voltage and current sampling points are arranged on the high-voltage side, which eliminates measurement errors caused by long leads and distributed parameters, and guarantees high data accuracy.
II. Key Technical Parameters
1. Rated Voltage
As a primary parameter for equipment selection, the rated voltage shall match the voltage class of the tested object. Common specifications include 30 kV, 50 kV, 60 kV and 80 kV.
- A 30 kV unit is applicable for withstand voltage tests on 10 kV power cables.
- A 60 kV or 80 kV unit is required for 35 kV power cables.
2. Output Frequency
The output frequency directly determines the load capacity, and different frequencies deliver distinct operating performance.
- At 0.1 Hz, the device features excellent output current and load capacitance performance, delivering the highest overall test efficiency.
- When the frequency is reduced to 0.05 Hz or 0.02 Hz, the unit can bear larger capacitive loads under the same output current, suitable for larger-capacity equipment, while the test duration will be extended accordingly.
In practical engineering, 0.1 Hz balances test efficiency and load capacity, covering the widest range of application scenarios.
3. Load Capacity
Load capacity is a critical indicator for the maximum testable cable length. For mainstream units operating at 0.1 Hz, the typical load capacity ranges from 0.5 μF to 1.1 μF.
The capacitance of 10 kV single-core XLPE cables is approximately 0.15 ~ 0.38 μF per kilometer. Calculation shows that a 0.5 μF VLF generator at 0.1 Hz can perform withstand voltage tests on cables with a length of 1 to 3 kilometers.
4. Measurement Accuracy & General Specifications
The voltage and current measurement error of mainstream products is controlled within ±3%, ensuring reliable data.
The equipment adopts a lightweight design: the control box generally weighs 4 ~ 6 kg for easy carrying. The weight of the high voltage booster varies by voltage class, ranging from 25 kg to 50 kg.
It is compatible with standard single-phase power supply: AC 220 V ± 5%, 50 Hz, and can be used reliably both indoors and outdoors.
III. Core Technical Advantages
Compared with traditional power frequency withstand voltage testers, VLF high voltage generators have prominent advantages in structure, operation, measurement accuracy and test performance, perfectly meeting the insulation detection requirements of modern electrical equipment.
1. Compact Size & Easy Transportation
VLF technology reduces the required power capacity to 1/500 of that of power frequency equipment, greatly downsizing the overall unit. For 35 kV cable testing, the booster weighs only around 40 kg. It solves the problems of bulky size and difficult transportation of conventional test devices, and adapts to various complex construction sites.
2. High Automation & Simple Operation
The intelligent control system executes the full test process automatically in accordance with preset procedures, including voltage rise, voltage holding and voltage drop. It displays real-time output waveforms, collects and records leakage current data, and generates and prints standard test reports upon test completion.
Full-automatic operation avoids manual errors and record omissions, lowers operation requirements and standardizes test procedures.
3. High Measurement Accuracy & Strong Anti-Interference
Sampling is implemented directly on the high-voltage side, which fundamentally eliminates measurement deviations caused by long connecting wires, ambient humidity and on-site electromagnetic interference. This advantage is particularly notable for tests on long-distance power cables and under complex working conditions, ensuring authentic and valid test data.
4. Superior Waveform Quality & Reliable Test Results
With digital frequency conversion and closed-loop negative feedback technology, the device outputs standard sine waves or cosine square waves featuring extremely low distortion and high stability.
In VLF operation, the capacitive reactance of the load increases significantly, which effectively suppresses the capacitive voltage rise commonly seen in power frequency withstand voltage tests. The test voltage remains compliant with standards, and the test results accurately reflect the actual insulation condition of equipment.
IV. Engineering Application Scenarios
As core equipment for insulation and withstand voltage tests, VLF high voltage generators are widely used for capacitive electrical equipment by virtue of light weight, high precision and low insulation damage.
1. Insulation Test for Power Cables
This is the primary application. It is mainly used for XLPE power cables of 10 kV, 35 kV and other common voltage classes.
- Handover test after installation: Verify construction quality and overall insulation condition of newly laid cables.
- Preventive maintenance test: Detect latent insulation defects such as water trees and electrical trees inside cables to prevent potential failures.
Different from DC withstand voltage tests, VLF AC tests will not cause space charge accumulation inside cables, resulting in negligible damage to insulation layers. The test results are more referential and help extend cable service life.
2. Insulation Test for Large Generators
The stator windings of large generators have relatively high capacitance. Traditional power frequency withstand voltage tests require huge power capacity, leading to high cost and difficult operation. VLF generators effectively solve this problem, and complete stator winding insulation tests efficiently with lower power demand and simpler configuration, delivering excellent cost performance and practicality.
3. Test for Other Capacitive Electrical Equipment
The device also serves as an auxiliary testing tool for power capacitors, composite insulators and other capacitive equipment. It can accurately detect hidden insulation risks and ensure the safe and stable operation of electrical systems.
Post time: May-26-2026