The selection of AC/DC withstand voltage testing equipment is a systematic project, and it requires comprehensive consideration of the following key factors:
Determine the type of test (AC or DC)
It is selected based on the type of the tested equipment and the relevant standard requirements.
General principle: Preferentially select AC withstand voltage that can simulate the operating conditions. When the capacitance of the test sample is too large, causing the AC test equipment to be too cumbersome, consider DC withstand voltage or AC variable frequency resonance.
2. Key Performance Parameters
Rated output voltage (kV):
It must be greater than or equal to the maximum test voltage specified by the standard.
A certain margin (such as 20%) needs to be considered to accommodate future testing requirements for higher voltage levels.
Rated output capacity (kVA) or current (mA):
Communication equipment: Capacity P = ωCU², where C is the capacitance of the test sample and U is the test voltage. It is necessary to accurately estimate or consult the equipment manual to obtain the capacitance of the test sample, and then calculate the required capacity accordingly.
DC equipment: Output current I = 2πfCU. The capacitance also needs to be estimated. For cables and large generators, the leakage current may be relatively large, so equipment with sufficient output current should be selected.
AC/DC withstand voltage test device
Voltage measurement accuracy:
The general requirement is that the total uncertainty should be better than 3% (in accordance with the requirements of GB/T 16927.1).
Waveform distortion rate:
The output voltage of the communication should be approximately a sine wave, and the distortion rate is usually required to be less than 5%.
3. Equipment Type and Technology Selection
Traditional power frequency test transformer:
Advantages: Simple structure, mature technology, good waveform.
Disadvantage: When the capacity is large, the equipment becomes very heavy and is not suitable for on-site testing of large-capacity test samples.
Applicable to: Laboratory, factory and on-site tests for small-capacity samples (such as transformers, insulators).
Series resonance withstand voltage equipment:
Principle: By using an inductor and the capacitance of the test sample to form a resonant circuit, a high voltage is generated on the test sample. The power supply only needs to provide active power, thus significantly reducing the required power capacity.
Advantages:
Light in weight and small in size, it is highly suitable for on-site testing. The output voltage waveform is excellent.
When the test sample undergoes breakdown, the resonance condition is disrupted, the high voltage automatically disappears, the short-circuit current is small, and the damage to the test sample is minimal.
Type: Tuning type, frequency modulation type. Currently, variable-frequency resonance is the mainstream.
Applicable to: Almost all on-site AC withstand voltage tests for large-capacity test samples, such as long cables, GIS, large transformers, and generators.
Ultra-low frequency (0.1 Hz) pressure resistance equipment:
Principle: By replacing the 50Hz power frequency with a 0.1Hz alternating current, tests are conducted on capacitive loads such as cables. The equipment capacity can be reduced to 1/500 of the power frequency level.
Advantages: The equipment is lighter and does not cause cumulative damage to XLPE cables.
Disadvantage: The equivalence remains controversial, and there are specific requirements for voltage conversion in the standard.
Applicable to: Medium and low voltage cables, especially old cables and insulation diagnosis during operation and maintenance.
4. Functionality and Security
Overcurrent, overvoltage and flashover protection: It is necessary to have rapid and reliable protection functions to safeguard the equipment and the test samples.
Automatic voltage regulation and timing: Enhances test efficiency and accuracy, and reduces human errors.
Remote control: Ensures that operators maintain a safe distance.
Grounding and Shielding: The equipment must have a proper grounding design to prevent induced electricity and interference.
5. Portability and Usage Environment
For on-site use: Preferentially choose series resonance or ultra-low frequency equipment. Pay particular attention to the weight, size, and ease of transportation of the equipment due to its modular design.
For laboratory/factory use: You can choose either traditional test transformers or fixed resonant systems.
Environmental conditions: Consider factors such as temperature, humidity, and altitude (voltage correction is required for high altitudes).
