Keywords: voltage breakdown tester breakdown strength tester breakdown tester
The detection of the electrical performance of wires and cables requires the staff of the quality inspection department to do a lot of work. Among them, the following seven important measurement items play a key role in determining the quality of a wire and cable.
1 , the direct current resistance measurement of the wire:
For wire and cable, the conductor part is its most important part. Wire and cable conductive cores mainly transmit electrical energy or electrical signals.
The resistance of the wire is the main indicator of its electrical performance. Now the standard stipulates: whether the DC resistance or resistivity of the test wire core exceeds the value specified in the standard.
The main purpose of this test is to find out certain defects in the production process: if the wire breaks or some of its single wire breaks; the cross section of the wire does not conform to the standard; the length of the product is not correct.
In addition, for the power cable, this test can not only detect the condition of the conductor, but also detect whether the conductor has influence on the allowable ampacity in the operation of the wire and cable product.
2 , insulation resistance test:
The insulation resistance type reflects the important index of insulation properties of wire and cable products. It is closely related to the strength of the product, dielectric loss, and the gradual deterioration of the insulation material under working conditions.
The determination of insulation resistance can be found in the process of defects: If the insulation is dry or jacket moisture damage; insulation is contaminated and mixed with conductive impurities; various causes of insulation cracking.
For communication cables, too low insulation resistance between lines will also increase loop attenuation, crosstalk between loops, and long-distance power supply leaks on conductive cores. Therefore, the insulation resistance should be higher than the specified value. In the operation of wires and cables, insulation resistance and leakage current are often tested as the main basis for whether or not safe operation can continue.
3 , the measurement of capacitance and loss factor:
When the AC voltage is applied to the cable, current flows. When the voltage amplitude and frequency are constant, the capacitor current is proportional to the cable capacitance (Cx). For ultra-high voltage cables, the current of this capacitor may reach a value comparable to the rated current, which becomes an important factor limiting the cable capacity and transmission distance. Therefore, the capacitance of the cable is also one of the main electrical performance parameters of the cable.
In an AC electric field, insulators in the cable, due to leakage currents and various polarizations, generate dielectric losses, represented by the dielectric dissipation factor or loss tangent (TAN δ), which not only wastes power, but also makes the dielectric (insulator) ) Heating, accelerated insulation aging, so TANδ is also one of the main parameters of the cable.
The measurement of capacitance and loss factor can reveal that the insulation is damp, the insulation layer and the shielding layer fall off, and other insulation degradation phenomena. Therefore, both capacitance and loss tangent values ​​are measured during cable manufacturing or cable operation. For high-voltage cables, the measurement of capacitance and loss factor is carried out under its operating conditions, ie at high power frequency.
4 , insulation strength test: (including pressure test and breakdown test)
The insulation strength of wire and cable is the ability of insulation structure and insulation material to withstand the effect of electric field without breakdown. In order to check the quality of wire and cable products and ensure the safe operation of products, the insulation strength test is generally required for all insulation type wire and cable. .
Dielectric strength test can be divided into pressure test and breakdown test. The withstand voltage test is to apply a certain voltage to the sample under certain conditions. After a certain period of time, whether or not a breakdown occurs is used as a criterion for judging whether or not the sample is qualified. The voltage of time is generally higher than the rated working voltage of the test product. The specific voltage value and withstand voltage time are stipulated in the product standards.
Through the withstand voltage test can test the reliability of the product under the operating voltage and found serious defects in the insulation, but also can find some shortcomings of the production process, such as: insulation has serious external damage, the conductor has a serious distortion of the electric field to severe distortion Insulation in the production of penetrating defects or large conductive impurities.
The breakdown test is to measure the breakdown field strength or breakdown voltage by increasing the voltage until the breakdown of the sample under certain test conditions. The breakdown test can be used to assess the safety margin between the ability of the cable to withstand voltage and the operating voltage. One of the important parameters in the cable design when breaking the field strength. The cable is generally subjected to AC voltage in operation, but it also withstands DC voltage in DC transmission systems and certain special occasions. For high voltage cables, it may also be subject to atmospheric voltage (lightning) and operating overvoltage. Therefore, according to the different experimental voltage waveform, can be divided into 1. AC (power frequency) voltage, 2. DC voltage, 3 impulse voltage three dielectric strength test.
5 , partial discharge measurement:
This is mainly for extruded cables. Because the oil-filled cable has no local electricity generation at all; the oil-paper cable is usually very weak, such as several PCs, even if there is partial discharge, so these cables may not measure partial discharge in the factory test. The opposite is true of the extruded cable. Not only is there a high possibility of partial discharge, but also the damage of the partial discharge to the plastic and rubber is severe. With the increase of the voltage level and the improvement of the work field, this problem becomes even more serious. For high pressure extruded cables, partial discharge measurements are made during the factory test.
6 , aging and stability test:
Aging test is the stability test that can maintain stable performance under stress (mechanical, electrical, and thermal). The simple heat aging test is to test the aging of the sample under the effect of heat: put the sample in a certain temperature above the rated operating temperature, after certain time, measure some sensitive properties before and after aging Change to assess aging characteristics. Can also be used to increase the temperature to accelerate the aging test, coupled with moisture, vibration, electric field and other heat, machine, electricity and other stress to form an aging cycle, after each aging cycle, determine certain selected sensitive performance parameters. Until the performance drops to the value of the indicated lifetime. This results in a shorter lifetime (time for sample heating) at higher temperatures.
7 , thermal stability test:
The thermal stability test is that the wire and cable are heated by the current and also withstand a certain voltage. After a certain period of heating, certain sensitive performance parameters are measured to assess the stability of the insulation. The insulation stability test is divided into long-term stability test or short-term accelerated aging test.
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