Three important indicators of wire and cable quality inspection
The selection of qualified wire and cable products is related to the safety of people's production and life. For ordinary consumers, in addition to mastering how to select qualified wire and cable products, it is also necessary to understand the general monitoring knowledge of wire and cable. In the quality inspection of wires and cables, there are mainly the following important indicators.
Insulation resistance test:
Insulation resistance reflects an important indicator of the insulation properties of wire and cable products, which is closely related to the electrical strength of the product, dielectric loss, and the gradual deterioration of insulating materials under working conditions. For communication cables, if the insulation resistance between lines is too low, it will also increase loop attenuation, crosstalk between loops, and long-distance power supply leakage on the conductive core. Therefore, the insulation resistance should be higher than the specified value.
Determination of insulation resistance can find defects in the process, such as insulation dry and impervious or sheath damaged and damp; insulation is polluted and mixed with conductive impurities; insulation layer cracking caused by various reasons, etc. In the operation of wires and cables, it is often necessary to detect insulation resistance and leakage current as the main basis for continued safe operation.
At present, in addition to the ohmmeter (shake meter), the commonly used galvanometer comparison method is the high resistance meter method (voltage-current method) to measure the insulation resistance of wires and cables.
Measurement of capacitance and dissipation factor:
When the AC voltage is applied to the cable, a current flows. When the amplitude and frequency of the voltage are constant, the magnitude of the capacitor current is proportional to the capacitance (Cx) of the cable. 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 the AC electric field, the insulator in the cable will form dielectric loss due to leakage current and various polarizations, which is expressed by the dielectric loss factor or loss tangent (TAN ), which not only wastes electrical energy, but also causes dielectric (insulator) loss. ) generates heat and accelerates insulation aging, so TAN is also one of the main parameters of the cable.
Through the measurement of capacitance and loss factor, various insulation deterioration phenomena such as insulation damping, insulation layer and shielding layer falling off can be found, so the measurement of capacitance and TAN is carried out in cable manufacturing or cable operation. For high-voltage cables, the measurement of Cx and TAN is carried out under their working conditions, that is, power frequency and high voltage. Usually, high-voltage Xilin bridges are used. This year, current ratio transformer bridges have also been used.
Partial discharge measurement:
There is basically no local power generation for oil-filled cables; even if there is partial discharge in oil-paper cables, it is usually very weak, such as a few PCs, so these cables can be exempted from partial discharge in the factory test. For extruded cables, not only the possibility of partial discharge is high, but also the damage of partial discharge to plastic and rubber is also serious. With the increase of voltage level and the increase of working field strength, this problem becomes more serious. For extruded cables, partial discharge measurements are required in the factory test.
There are many measurement methods for partial discharge. The discharge pulse can be measured according to the instantaneous charge exchange generated by the discharge (electric measurement method); the voltage can also be measured according to the ultrasonic wave generated during the discharge (acoustic measurement method); it can also be measured according to the light generated by the discharge. , to measure the intensity of light (photometry). For cables, the electrical measurement method is basically used.