For transformers with voltage levels of 110kV and above, before factory delivery, before commissioning, or after suffering a nearby sudden short circuit, relevant regulations require low-voltage short-circuit impedance testing or frequency response method winding deformation testing to retain original records as a basis for subsequent comparison. Such on-site tests have specific requirements for equipment output waveform and frequency, and very low frequency withstand voltage test devices are test equipment designed specifically for such scenarios.
From the perspective of design and selection, the working process of a very low frequency withstand voltage test device is usually as follows: 50Hz AC voltage is converted into DC through rectification and filtering, then inverted into 1kHz AC voltage, and finally amplitude-modulated by a very low frequency sine wave oscillator to output a 0.1Hz high-voltage sine wave. This output frequency is far lower than the power frequency, which can effectively reduce the test power supply capacity and equipment volume, making it suitable for on-site transportation and operation.
Point one: Cable sheath testing scenario. Very low frequency withstand voltage testing is suitable for performance detection of high-voltage cable outer sheaths, used to evaluate whether the cable sheath is damaged or damp during laying or operation. During the test, the connections at both ends of the cable need to be disconnected, a 0.1Hz sine wave voltage is applied, and changes in leakage current are observed. When selecting, attention should be paid to whether the device output voltage level covers the test requirements corresponding to the cable rated voltage.
Point two: Auxiliary judgment of transformer winding deformation. After a transformer suffers a short-circuit impact, low-voltage short-circuit impedance testing or frequency response method testing can provide a reference for winding condition. The high-voltage sine wave output by a very low frequency withstand voltage device can be used to assist in evaluating insulation performance, but attention should be paid to the difference between its output characteristics and power-frequency withstand voltage. When selecting, the device output waveform distortion and voltage stability indicators should be checked.
Point three: On-site adaptability. Design and selection personnel need to consider the device's anti-interference capability, working power supply conditions, and continuous working time at the substation site. The device usually has functions such as overcurrent protection and zero-position start, and the operation interface should facilitate on-site recording of test data.
Point four: Maintenance and calibration. Very low frequency withstand voltage test devices should be regularly calibrated for output voltage and checked for waveform to ensure traceability of test data. During routine maintenance, the high-voltage output end should be kept clean and dry to avoid moisture affecting insulating components and thereby affecting measurement results.
Overall, when selecting very low frequency withstand voltage test devices for different application scenarios, priority should be given to clarifying the type of test object, test voltage level, and on-site power supply conditions, and then matching the device's output frequency, voltage range, and protection functions to ensure that the test process meets the requirements of relevant regulations.
