Test method for insulation strength of insulated wires and cables
The insulation strength of wire and cable refers to the ability of insulating structure and insulating material to withstand 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 of wire and cable refers to the ability of insulating structure and insulating material to withstand electric field without breakdown. In order to check the quality of wire and cable products and ensure the safe operation of products, insulation strength test is generally carried out for all types of insulated wires and cables. Insulation strength test can be divided into withstand voltage test and breakdown test.
The withstand voltage test is to apply a certain voltage to the test object under certain conditions. After a certain period of time, whether the test object is qualified or not is judged by whether the breakdown occurs. The time voltage is generally higher than the rated working voltage of the sample. The specific voltage value and withstand voltage time are specified in the product standard. Through the withstand voltage test, the reliability of the product under the working voltage can be tested, and serious defects in the insulation can be found. Some shortcomings of the production process can also be found, such as serious external damage to the insulation, There are serious defects on the conductor which make the electric field distort sharply; There are penetrating defects or large conductive impurities in the production of insulation.
The breakdown test is to measure the breakdown field strength or breakdown voltage by increasing the voltage until the breakdown occurs under certain test conditions. Through the breakdown test, the safety margin between the ability of the cable to withstand voltage and the working voltage can be evaluated. Breakdown field strength is one of the important parameters in cable design.
Cables generally bear AC voltage in operation, but they also bear DC voltage in DC transmission system and some special occasions. For high-voltage cables, they may be attacked by atmospheric voltage (lightning) and switching overvoltage. Therefore, according to the different experimental voltage waveform, it can be divided into three kinds of insulation strength tests: 1. AC (power frequency) voltage, 2. DC voltage and 3. Impulse voltage.
Partial discharge measurement
For oil filled cable, there is basically no local power generation; Even if oil paper cable has partial discharge, it is usually very weak, such as several PCs. therefore, partial discharge can not be measured in factory test of these cables. For extruded cable, not only the possibility of partial discharge is great, but also the damage of partial discharge to plastic and rubber is more serious. With the improvement of voltage level and working field strength, this problem becomes more serious. Therefore, for high-voltage extruded cable, partial discharge measurement should be done in factory test.
There are many methods to measure partial discharge, which can measure the discharge pulse according to the instantaneous charge exchange; The voltage can also be measured according to the ultrasonic wave generated during discharge (acoustic measurement method); The intensity of light can also be measured according to the light generated by discharge (photometry). For cables, the electrical measurement method is basically used.
Aging and stability test
Aging test is the stability test whether the performance can be kept stable under the action of stress (mechanical, electrical and thermal).
Thermal aging test
The simple thermal aging test is to test the aging characteristics of the test object under the action of heat. The test object is placed in the environment with a certain value higher than the rated working temperature. After a specified time, the changes of some sensitive properties before and after aging are measured to evaluate the aging characteristics. It is also possible to accelerate the aging of the test sample by increasing the temperature, and to form an aging cycle by adding thermal, mechanical and electrical stresses such as damp, vibration and electric field. After each aging cycle, some selected sensitive performance parameters are determined. Until the performance decreases to the value of recognized life. In this way, a shorter life l (heating time of sample) can be obtained at a higher temperature T.
Thermal stability test
The thermal stability test is that the cable is heated by electric current, and at the same time, it also bears a certain voltage. After a certain period of heating, some sensitive performance parameters are measured to evaluate the stability of insulation.
Insulation stability test can be divided into long-term stability test and short-term accelerated aging test.