Standard: 100Hz - 300Hz continuously adjustable (covering the 150Hz standard point).
High-end requirements: Support 45Hz - 65Hz (power frequency) and 0.1Hz (ultra-low frequency) expansion, achieving multi-purpose use of one machine.
Output voltage and capacity:
It needs to be calculated based on the no-load current of the test object and the test voltage multiple.
Empirical formula:
Note: The no-load current at 150Hz is more than 3 times that at 50Hz.
Partial discharge (PD):
If partial discharge testing is also required, the device must have a low background noise design, and the power supply waveform distortion rate must be <1% (for ordinary withstand voltage, it can be relaxed to 3%).
Compensation reactor:
For large-capacity transformers, it may be necessary to connect reactors in parallel or series to compensate for capacitive or inductive current and optimize the resonance point.
Intelligent control:
It has functions of automatic frequency search, automatic voltage boost, timed withstand voltage, automatic voltage reduction, and breakdown protection (<20ms cutoff).
Built-in data storage and report printing.
4. Brand and manufacturer recommendations for 2026
This field is highly concentrated, mainly dominated by leading domestic high-voltage testing enterprises.
First tier: Industry benchmarks (first choice for large main transformers and partial discharge testing)
Wuhan UHV Electric Power Technology Co., Ltd.
Status: A leader in domestic variable frequency resonance technology, its YDJ series and TF series are widely used in substations of 500kV and below.
Features: Provides an integrated solution of "induced withstand voltage + partial discharge measurement", with extremely high power purity, strong anti-interference ability, and software that complies with the latest national grid specifications.
Features: Leading in intelligent control and portable design, suitable for on-site mobile operations. Its variable frequency power module has high efficiency and good heat dissipation.
Baoding Tianwei/Xinnuo (relying on the transformer base)
Features: Provides customized high-power resonance systems for ultra-large capacity (such as 750kV, 1000kV UHV) transformers.
Second tier: High cost-effectiveness (distribution networks, small and medium transformers, PT testing)
Features: Products are sturdy and durable, with affordable prices. Suitable for routine induced withstand voltage testing of 10kV/35kV distribution transformers and instrument transformers by power supply bureau teams.
Hangzhou Gaodian / Ningbo Keyuan
Features: High market share in East China, with fast after-sales service response.
5. Typical test procedure
Wiring: Adopt the method of "pressurizing on the low-voltage side, inducing on the high-voltage side" (usually pressurizing from the low-voltage winding to induce high voltage in the high-voltage winding).
Frequency search: Start the variable frequency power supply and automatically search for the circuit resonance frequency (usually between 140Hz-160Hz).
Voltage boost: Boost to the target value (e.g., 2 times the rated voltage) at a uniform speed (e.g., 2kV/s).
Withstand voltage: Maintain for the specified time (usually 60 seconds; if partial discharge testing is performed, a longer time is required and the partial discharge amount must be monitored).
Voltage reduction: Automatically and quickly reduce to zero, then cut off the power supply.
Discharge: Fully discharge the test object.
6. Safety and precautions
Neutral point insulation: For graded insulated transformers (such as 110kV and above), attention should be paid to the insulation level of the neutral point during induced withstand voltage testing. If necessary, support the neutral point voltage or adopt special wiring methods.
Overexcitation protection: It is strictly forbidden to apply high voltage at low frequencies (<100Hz) to prevent core saturation and overheating.
Partial discharge background: If partial discharge testing is included, the background noise must be measured before the test, ensuring a quiet environment, single-point reliable grounding of the grounding system, and avoiding ground loop interference.
Temperature monitoring: During long-duration withstand voltage tests, monitor the transformer oil temperature and winding temperature.
7. Summary of purchase suggestions
Conventional 10-35kV distribution transformers/PTs: Choose a 5-10kVA fixed 150Hz triple-frequency or small variable frequency device, with brands such as Yangzhou Huimin, which are economical and practical.
110kV-220kV main transformers (including partial discharge): Must choose a variable frequency series resonance system, with capacity calculated (usually 30-100kVA), requiring waveform distortion rate <1%, and preferred brands are Wuhan UHV or Shanghai Shenggao.
500kV and above UHV: Customized ultra-large capacity resonance systems are required, usually specially designed after on-site survey by the manufacturer.
