Selection Guide for Very Low Frequency (VLF) Withstand Voltage Test Systems for 33kV Medium-Voltage Cables
To conduct acceptance or preventive withstand voltage testing on 33kV medium-voltage cables (with nominal voltage U0/U typically rated at 18/30kV or 19/33kV), 40kV VLF test sets must not be used as their output voltage is far below international test requirements.
A 80kV or 90kV VLF (Peak) test system is required to meet IEEE 400.2, IEC 60502-2, and relevant industry standards.
Why is 80kV / 90kV VLF Required?
According to standard testing specifications, XLPE cable test voltages are calculated based on multiples of the phase-to-ground voltage U0:
Acceptance Test (U0=19KV): The required RMS test voltage is 3*19KV =57KV (RMS). Converted to sinusoidal peak voltage 57KV*1.414=80.6KV
Maintenance / Preventive Test (2U0 ~2.2U0 RMS): The required test voltage ranges from 38kV to 42kV, which corresponds to a peak voltage of 54kV to 60kV Peak.
Conclusion: An 80kV VLF unit meets the baseline for standard acceptance testing, while a 90kV VLF unit provides optimal safety margin and flexibility.
Selection Comparison Matrix
When evaluating equipment specifications, ensure whether the manufacturer specifies output ratings in Peak or RMS values.
| Equipment Specification | Applicable Cable Rating | Suitability for 33kV Cables | Typical Application Scenario |
| 40kV VLF | 6kV ~ 10kV / 11kV | ❌ Not Suitable | Under-voltage for 33kV systems; cannot trigger insulation defects. |
| 50kV / 60kV VLF | 15kV ~ 22kV | ⚠️ Maintenance Only | Insufficient for acceptance testing; suitable only for low-standard diagnostic testing. |
| 80kV VLF | 33kV / 35kV | ✅ Recommended | Standard choice for 33kV cable commissioning and maintenance testing ($3U_0$). |
| 90kV VLF | 33kV / 35kV / 45kV | ✅ Optimal Choice | High safety margin; ideal for long-distance cables with higher capacitance load. |
Key Technical Parameters to Consider
Output Waveform and Peak Ratings
Ensure the rated output voltage reaches >80kV Peak (sinusoidal waveform).
Load Capacitance Capacity
Output current is limited by cable capacitance (I = 2ΠFCU). Single-core 33kV XLPE cable capacitance typically ranges from 0.15 to 0.25uf/km
For short to medium cable runs (< 1.5 km), select a system rated for >1.5uf at 0.1Hz.
For longer cable runs (1.5 km to 5 km), choose a high-capacity unit (>2.5UF~5.0UF) or a system equipped with automatic frequency reduction (down to 0.05Hz, 0.02Hz, or 0.01Hz).
Optional Diagnostic Features
Tan Delta (Dissipation Factor): Evaluates overall insulation aging and water tree degradation.
Partial Discharge (VLF-PD): Pinpoints localized defects in cable joints and terminations.
