Safety precautions (crucial): DC withstand voltage is strictly prohibited: For XLPE (cross-linked polyethylene) cables, it is absolutely forbidden to use DC high voltage for withstand voltage testing. The DC electric field will form space charges in the insulation layer, causing electric field distortion, which not only fails to detect defects but also induces breakdown accidents during operation. VLF is the best alternative. Full discharge: Cables have large capacitance and store a lot of energy. Strict discharge must be performed before and after testing, and the discharge time must be long enough. Safety distance: Fences and warning signs must be set up around the high-voltage output end to prevent personnel from entering by mistake. Reliable grounding: The instrument casing, non-test phases of the cable, and metal shielding layers must be reliably grounded at multiple points. 7. Reference for mainstream brands in the market: International brands: Haefely (Switzerland, now part of Megger), Baur (Austria, pioneer of VLF technology), Doble (USA). Leading domestic brands: Wuhan UHV Electric Power: complete product line, high cost performance, and wide after-sales network. Wuhan Musen Electric: performs well in intelligent control and tanδ integration. Shanghai Siyuan Hongrui: relies on Siyuan Electric Group, with strong technical strength. Baoding Tianwei/Xinnuo: relatively high market share in the northern market. If you have specific cable parameters (such as: voltage level 10kV, length 5 km, XLPE material), I can help you calculate the required equipment load capacitance specifications and recommend suitable model configurations.
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- Cosine square wave: Early technology, although it can pass some standards, the electrical stress distribution on the insulation is slightly different from the real power frequency, and it is gradually being replaced by sine wave.
2. Main application scenarios
- High-voltage cable acceptance test: Withstand voltage assessment of newly laid 10kV, 35kV, 66kV, 110kV and even 220kV XLPE cables before commissioning.
- Preventive test: Regular insulation status assessment of cables in operation (replacing traditional DC withstand voltage, because DC withstand voltage accelerates aging of XLPE cables).
- Long-distance cable testing: Especially 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 in 2025-2026
With the development of smart grids, very low frequency high voltage generators are also continuously upgrading:
- “Withstand voltage + diagnosis” integration:
- Modern equipment no longer only judges “pass/fail”, but integrates dielectric loss factor (tanδ) measurement function.
- During the voltage increase process, the curve of tanδ changing with voltage is monitored in real time. If tanδ rises sharply, it indicates serious defects in the insulation (such as water tree aging), and early warning can be given even if breakdown does not occur.
- Partial discharge (PD) linkage:
- Some high-end models reserve PD detection interfaces, allowing partial discharge measurement during 0.1Hz withstand voltage, 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 output of stable sine wave.
- Lightweight and modular:
- Using high-frequency inverter technology and new magnetic materials, the weight of a single box is further reduced (for example, 60kV/5μF equipment can be controlled within 30kg), supporting one-person carrying or small trolley 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, establishing cable insulation health files.
