Principle of Very Low Frequency Insulation Withstand Voltage Test
The very low frequency insulation withstand voltage test is actually an alternative 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 large capacitance in the insulation layer, a test transformer or resonant transformer with large capacity is required. Such huge equipment is not only bulky and expensive, but also very inconvenient to use. To solve 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 using 0.1Hz very low frequency withstand voltage test instead of power frequency withstand voltage test not only has the same equivalence, but also greatly reduces the size and weight of the equipment. Theoretically, the capacity is about one five-hundredth of that at power frequency. The test procedure is greatly simplified, and it has more advantages compared to the power frequency test. This is why developed countries commonly adopt this method. The Ministry of Electric Power of China has commissioned the Wuhan High Voltage Research Institute to draft the industry standard "Test Method for Very Low Frequency (0.1Hz) Withstand Voltage of 35KV and Below Cross-linked Polyethylene Insulated Power Cables", and this method is being promoted in China. This instrument is developed according to the needs of our country. It can be widely used for AC withstand voltage tests of cables, large high-voltage rotating motors, and power capacitors.
Product Introduction
This product combines modern digital frequency conversion advanced technology, adopts microcomputer control, and the voltage boosting, voltage reduction, measurement, and protection are fully automated, and manual intervention can be carried out during the automatic voltage boosting process. Due to full electronization, it is small in size and light in weight, with a large screen liquid crystal display, clear and intuitive, and can display the output waveform, and the printer outputs the test report. The design indicators fully comply with the power industry standard DL/T849.4-2004 "General Technical Conditions for Special Testing Instruments for Power Equipment, Part 4: General Technical Conditions for Very Low Frequency High Voltage Generators", and it is very convenient to use. At present, mechanical methods are used at home and abroad for modulation and demodulation to generate very low frequency signals, so there are problems such as non-standard sine wave waveforms, large measurement errors, spark discharge in the high-voltage part, bulky equipment, and the second and fourth quadrants of the sine wave require high-power high-voltage resistors for discharge shaping, so the overall power consumption of the equipment is large. This product can overcome these shortcomings. In addition, it has the following features that need special explanation:
★ Current, voltage, and waveform data are all obtained directly through sampling on the high-voltage side, so the data is real and accurate.
★ Overvoltage protection: When the output exceeds the set voltage limit, the instrument will stop for protection, with an action time of less than 20 milliseconds.
★ Overcurrent protection: Designed with dual protection for high and low voltage. The high-voltage side can stop for protection according to the set value; when the current on the low-voltage side exceeds the rated current, it will stop for protection, with an action time of less than 20 milliseconds.
★ The high-voltage output protection resistor is designed inside the voltage boosting body, so no external protection resistor is needed.
★ Due to the adoption of high and low voltage closed-loop negative feedback control circuit, the output has no capacitive rise effect.
Note: The above information and technical parameters are compiled and released by Yangzhou Darui Electric Co., Ltd.
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