Insulation resistance test:
Insulation resistance reflects an important indicator of the insulation properties of wire and cable products. It is closely related to the electrical strength of the product, the dielectric loss, and the gradual deterioration of the insulation under working conditions. For the communication cable, the insulation resistance between the lines is too low, which also increases the loop attenuation, the crosstalk between the circuits, and the long-distance power supply leakage on the conductive core. Therefore, the insulation resistance should be higher than the specified value.
Determining the insulation resistance can reveal defects in the process, such as insulation drying or sheath damage and moisture; insulation is contaminated and conductive impurities are mixed; cracking of the insulation layer caused by various reasons. In the operation of wires and cables, it is often necessary to detect the insulation resistance and leakage current as the main basis for whether or not to continue safe operation.
At present, the measurement of the insulation resistance of wire and cable, in addition to the use of ohmmeter (shake table), is commonly used galvanometer comparison method high resistance meter method (voltage - current method).
Measurement of capacitance and loss factor:
When the cable is connected to the AC voltage, current flows. When the amplitude and frequency of the voltage are constant, the magnitude of the capacitor current is proportional to the capacitance of the cable (Cx). For ultra-high voltage cables, the current of such capacitors may reach a value comparable to the rated current, which is an important factor limiting cable capacity and transmission distance. Therefore, the capacitance of the cable is also one of the main electrical performance parameters of the cable.
Partial discharge measurement:
There is basically no local power generation for oil-filled cables; even if the oil-paper cable has partial discharge, it is usually very weak, such as several PCs, so these cables may not be tested for partial discharge during the factory test. For the extruded cable, not only the possibility of partial discharge is large, but also the damage of the partial discharge to the plastic and the rubber is serious. As the voltage level increases, the work field strength increases, and the problem becomes more serious, so the high voltage Extruded cables are tested for partial discharge during factory testing.
There are many methods for measuring partial discharge, which can measure the discharge pulse according to the instantaneous charge exchange generated by the discharge (electrical measurement method); it can also measure the voltage according to the ultrasonic wave generated during discharge (acoustic measurement method); , measuring the intensity of light (photometry). For the cable, basically, the electrical measurement method is used.