Principle of Ultra-Low Frequency Insulation Withstand Voltage Test
The ultra-low frequency insulation withstand voltage test is actually an alternative method to the power frequency withstand voltage test. As we know, when conducting power frequency withstand voltage tests on large generators, cables, and other test objects, due to their insulation layers exhibiting large capacitance, a test transformer or resonant transformer with a large capacity is required. Such huge equipment is not only bulky and expensive but also very inconvenient to use. To resolve this contradiction, the power sector has adopted a method of reducing the test frequency, thereby reducing the capacity of the test power supply. Years of theory and practice at home and abroad have proven that replacing the power frequency withstand voltage test with a 0.1Hz ultra-low frequency withstand voltage test not only provides the same equivalence but also significantly reduces the size and weight of the equipment. Theoretically, the capacity is about one five-hundredth of that at power frequency, and it is simple to operate, offering more advantages compared to power frequency testing. This is why developed countries widely adopt this method. The National Development and Reform Commission has formulated the industry standard "Test Method for Ultra-Low Frequency (0.1Hz) Withstand Voltage of 35kV and Below Cross-linked Polyethylene Insulated Power Cables." China is currently promoting this method, and this instrument has been developed based on this national need. It can be widely used in AC withstand voltage tests for cables and large high-voltage rotating motors.
Product Introduction
This product combines modern digital frequency conversion technology, adopts microcomputer control, and fully automates voltage boosting, voltage stabilization, measurement, and protection. Due to full electronization, it is small in size and light in weight, with a large-screen LCD display that is clear and intuitive, and can display output waveforms and print test reports. The design specifications fully comply with the power industry standard "General Technical Conditions for Special Testing Instruments for Power Equipment, Part 4: General Technical Conditions for Ultra-Low Frequency High Voltage Generators," and it is very convenient to use. Currently, both domestic and international methods use mechanical modulation and demodulation to generate ultra-low frequency signals, which results in non-standard sine waveforms, large measurement errors, spark discharge in the high-voltage part, bulky equipment, and the need for high-power high-voltage resistors for discharge in the second and fourth quadrants of the sine wave, leading to high overall power consumption. This product can overcome these shortcomings. In addition, the following features need special explanation:
1. Current, voltage, and waveform data are all sampled directly from the high-voltage side, ensuring accurate data and real-time display of dynamic voltage and current values.
2. It has overvoltage protection. When the output exceeds the set voltage limit, the instrument will shut down for protection, with an action time of less than 20ms.
3. It has overcurrent protection: designed with dual protection for high and low voltage. The high-voltage side can perform precise shutdown protection according to the set value; the low-voltage side will shut down for protection when the current exceeds the rated current, with action times less than 20ms.
4. The high-voltage output protection resistor is designed inside the voltage booster, so no external protection resistor is needed.
5. Due to the adoption of a high-low voltage closed-loop negative feedback control circuit, the output has no capacitive rise effect.
Technical Parameters
1. Rated output voltage: see Table 1
2. Output frequency: 0.1Hz, 0.05Hz, 0.02Hz
3. Load capacity: 0.1Hz max 1.1μF
0.05Hz max 2.2μF
0.02Hz max 5.5μF
4. Measurement accuracy: 3%
5. Voltage positive and negative peak error: ≤3%
6. Voltage waveform distortion: ≤5%
7. Operating conditions: indoor, outdoor; temperature: -10℃∽+40℃; humidity: ≤85% RH
8. Power fuse: see Table 1
9. Mains power: frequency 50Hz, voltage 220V±5%. If using a portable generator for power supply, the generator requirements are: frequency 50Hz, voltage 220V±5%, power greater than 3KW, and a resistive load (such as an electric furnace) with a power not less than 800W should be connected in parallel at the generator output to stabilize the generator speed. Otherwise, the instrument will not work properly.
