I.Overview

High voltage withstand test of electrical equipment is one of the most important items specified in insulation preventive test.

The withstand voltage test can be divided into ac withstand voltage test and dc withstand voltage test. The ac withstand voltage test can be divided into power frequency, frequency conversion and 0.1hz VLF test technology, of which 0.1hz ultra-low frequency technology is the latest technology recommended by the current international electrotechnical commission. Our company's new generation of ultra-low frequency and high voltage generator is the core product independently developed with the latest American technology. It adopts 7-inch touch screen, the latest arm7 single chip microcomputer, high-speed ad acquisition circuit, and is equipped with background management software. It overcomes many shortcomings of domestic similar products (see table 1), and its cost performance is much higher than that of similar imported products. It is especially suitable for withstand voltage test of electrical equipment with large insulation equivalent capacitance (such as power cable, power capacitor, large and medium-sized generator and motor, etc.), which conforms to the national standard of electric power industry, general technical conditions for ultra-low frequency and high voltage generator (d) newly issued in 2004 l / t849.4-2004.

Table 1 performance comparison between mechanical type and electronic type of 0.1Hz voltage withstand test equipment

II. Advantages Of 0.1Hz VLF Withstand Voltage Technology

In fact, the ultra-low frequency insulation withstand voltage test is an alternative to the power frequency withstand voltage test. In the power frequency withstand voltage test of large and medium-sized generators, motors, power cables and other test objects, due to their large capacitance in the insulation layer, a large capacity test transformer or resonance transformer is required. Some of these huge equipment is not only heavy and expensive, but also inconvenient to use. In order to solve this contradiction, the method of reducing the test frequency and the capacity of the test power supply is widely used in the world. It has been proved from many years of theory and practice at home and abroad that replacing power frequency withstand voltage test with 0.1Hz VLF withstand voltage test can not only have the same equivalence, but also greatly reduce the volume and weight of the equipment, theoretically, the capacity is about one fifth of the power frequency, and the operation is simple. This is the main reason why this method is widely used in developed countries.

According to the actual situation of China's power system, the national development and Reform Commission has formulated the industry standard of VLF (0.1Hz) withstand voltage test method for 35kV and below XLPE insulated power cables. In 2004, the industry standard DL / T 849.4-2004, general technical conditions for VLF and high voltage generators, which is being promoted in China. Although the DC voltage withstand test equipment has the advantages of small size, light weight and low cost, the DC voltage withstand test is also the most destructive to the insulation of the tested object. (see Table 2) therefore, the latest national regulations on preventive test of electrical equipment have expressly stipulated that DC high voltage is no longer used for voltage withstand test of electrical equipment, and AC voltage withstand test is recommended.

The new generation of 0.1Hz VLF high voltage generator developed by our company adopts the latest power electronic components and the latest ARM7 single-chip technology, further reducing the volume and weight of the equipment, eazy  operation, more stable performance, overcoming the shortcomings of the first generation of mechanical booster, such as short service life, high failure rate and large volume.

Table 2 performance comparison of various withstand voltage test equipment:

III.Technical Parameters Of Series Products

1. Output rated voltage： See table 3

2. Output frequency：0.1Hz、0.05Hz、0.02Hz、0.01Hz

3. Carrying capacity：   0.1Hz max 0.5µF

0.05Hz max 1.1µF

0.02Hz max 2.2µF

0.01Hz max 5.5µF

4. Measurement accuracy：3%

5. Voltage positive, negative peak error：≤3%±3kV

6. Voltage waveform distortion degree：≤5%

7. Conditions of use: indoor, outdoor; temperature：-10℃～+40℃；humidity：≤85%RH

8. Power supply fuse: See table 3

9. Power Supply: Voltage 220V±5%，50±5Hz

Note: If the portable generator is used for power supply, the generator output voltage and frequency are required to be stable (generally, the power is greater than 3kW, the frequency is 50Hz, and the voltage is 220V±5% 

10. The electrical capacity of the subject shall not exceed the maximum rated electrical capacity of the instrument, and the value is shown in the table 3；

1) Select products of appropriate specifications according to the subjects

When in use, the capacitance of the test object shall not exceed the rated capacitance of the instrument. If the capacitance of the test object is too small, the output waveform will be affected. If it is less than 0.05 μ F, the instrument will not output normally. At this time, 0.05 μ f capacitor (provided by the company) can be connected in parallel for auxiliary output. See Table 4 and table 5 for the estimation of capacitance of common electrical equipment.

Table 4 single phase earth capacity of different types of generators.

Table 5 capacitance of XLPE insulated single core power cable (μ f / km)

2) Estimation method of test object current:

Calculation formula: I = 2 π FCU