Very Low Frequency High Voltage Generator (Very Low Frequency High Voltage Generator, abbreviated as VLF HV Generator or 0.1Hz Withstand Voltage Tester) is a key piece of equipment in the power system specifically used for AC withstand voltage testing of high-voltage cables (especially long-distance cross-linked polyethylene XLPE cables).
It solves the problem that traditional power frequency (50Hz/60Hz) test equipment is bulky and heavy, making it impossible to test long cables in the field or in confined spaces, while also avoiding the risk of "space charge" accumulation damage to XLPE cable insulation caused by DC withstand voltage testing.
The following is an in-depth analysis of this equipment:
1. Core Working Principle
- Frequency Selection: The output frequency is typically 0.1Hz (with 0.05Hz or 0.02Hz options), which is only 1/500 of the power frequency (50Hz).
- Significant Capacity Reduction:
- A cable is equivalent to a large capacitor, and its charging current is I=2πfCUI=2πfCU .
- Since the frequency ff is reduced by 500 times, under the same test voltage UU and cable capacitance CC, the required output current and power are also reduced by approximately 500 times.
- Result: The equipment size and weight are only 1/10 to 1/50 of a power frequency test transformer of the same class, making it very suitable for field portability.
- Waveform Technology:
- Sine Wave: Complies with IEC 60060-3 and GB/T 18494 standards, with low waveform distortion and the best equivalence, making it the preferred choice for current mainstream high-end equipment.
- Cosine Square Wave: An earlier technology that, while passing some standards, has slight differences in electrical stress distribution on insulation compared to the actual power frequency, and is gradually being replaced by sine waves.
2. Main Application Scenarios
- High-Voltage Cable Acceptance Testing: Withstand voltage assessment of newly laid 10kV, 35kV, 66kV, 110kV, and even 220kV XLPE cables before commissioning.
- Preventive Testing: Periodic insulation condition assessment of cables in operation (replacing traditional DC withstand voltage, as DC withstand voltage accelerates XLPE cable aging).
- Long-Distance Cable Testing: Particularly suitable for submarine cables and tunnel cables several kilometers or even tens of kilometers long, which cannot be completed by power frequency equipment.
- Generator Stator Windings: VLF technology is also used for insulation testing of some large generators.
3. New Technology Trends for 2025-2026
With the development of smart grids, very low frequency high voltage generators are also continuously upgrading:
- Integration of "Withstand Voltage + Diagnosis":
- Modern equipment no longer merely judges "pass/fail" but integrates dielectric loss factor (tanδ) measurement functionality.
- During the voltage increase process, the curve of tanδ versus voltage is monitored in real time. If tanδ rises sharply, it indicates serious insulation defects (such as water tree aging), allowing early warning even without breakdown.
- Partial Discharge (PD) Linkage:
- Some high-end models reserve partial discharge detection interfaces, allowing partial discharge measurement simultaneously with 0.1Hz withstand voltage testing, more accurately locating insulation weaknesses.
- Fully Automatic Intelligent Control:
- Built-in expert system automatically recommends test voltage, frequency, and pressurization time based on cable length and type.
- Equipped with adaptive tuning function, automatically matching load capacitance to ensure continuous stable sine wave output.
- Lightweight and Modular Design:
- Using high-frequency inverter technology and new magnetic materials, the weight of a single unit is further reduced (for example, a 60kV/5μF device can be controlled within 30kg), supporting one-person handheld or small cart transportation.
- Cloud Data Management:
- Test data is automatically stored and electronic reports are generated, supporting WiFi/4G upload to the operation and maintenance cloud platform to establish cable insulation health records.
4. Key Parameters for Selection
When selecting a very low frequency high voltage generator, the following indicators should be given special attention:
Table
| Parameter | Description and Recommendations |
|---|---|
| Output Voltage | Select based on the cable's rated voltage. Common specifications: 30kV, 50kV, 60kV, 80kV, 100kV, 120kV, etc. (Note: 35kV cables typically require 50-60kV equipment; 110kV cables typically require 80-100kV equipment) |
| Load Capacitance | Most critical indicator. Determines the maximum cable length that can be tested. Formula: Lmax=Cdevice/(Cper_km×k)Lmax=Cdevice/(Cper_km×k) (Generally, XLPE cable capacitance is about 0.2~0.4 μF/km, with a 20% margin reserved) |
| Output Waveform | Strongly recommended to choose sine wave (Sinusoidal), which complies with the latest international standards and provides more authoritative test results. |
| Voltage Accuracy | Typically required to be ≤±3%≤±3% . |
| Frequency Range | The wider the adjustable range, the better (e.g., 0.01Hz ~ 0.5Hz), to accommodate different standards and special test objects. |
| Protection Functions | Must include overvoltage, overcurrent, flashover protection, zero-position start, grounding protection, etc. |
| Energy Feedback | High-end models feature energy feedback, recovering energy during capacitor discharge to reduce power consumption and heat generation. |
5. Brief Operating Procedure
- Wiring: Connect the high-voltage wire to one end of the cable, and reliably ground the other end and the metal shielding layer of the cable.
- Parameter Setting: Input the cable length and type; the instrument automatically calculates or manually sets the target voltage (e.g., 2U02U0 ), frequency (0.1Hz), and time (typically 60 minutes).
- Pre-check: The instrument automatically checks the grounding condition and circuit continuity.
- Voltage Increase: Press start, and the instrument automatically and smoothly increases the voltage to the target value.
- Withstand Voltage: Maintain the target voltage for the specified time, monitoring leakage current and tanδ (if available) in real time during this period.
- Voltage Decrease and Discharge: After the time is up, automatically reduce the voltage and discharge through internal resistors. Note: After the test, a dedicated discharge rod must be used to fully manually discharge the cable to prevent residual charge from causing injury.
- Report: Print or export the test report.
