Dielectric strength test method for insulated wires and cables

The insulation strength of wires and cables refers to the ability of the insulation structure and insulation materials to withstand the electric field without breakdown. In order to check the quality of wire and cable products and ensure the safe operation of the products, all insulation types of wires and cables are generally subject to insulation strength tests. The insulation strength test can be divided into a withstand voltage test and a breakdown test.

The withstand voltage test is to apply a certain voltage to the test product under certain conditions. After a certain period of time, whether a breakdown occurs is used as the standard to judge whether the test product is qualified. The voltage for the time is generally higher than the rated working voltage of the test product. The specific voltage value and withstand voltage time are specified in the product standard. The withstand voltage test can test the reliability of the product under the working voltage and find serious defects in the insulation. Some shortcomings of the production process can also be found, such as: serious external damage to the insulation, serious defects on the conductor that cause the electric field to distort sharply; penetrating defects or large conductive impurities in the insulation during production, etc.

The breakdown test is to increase the voltage under certain test conditions until the test product breaks down, and measure the breakdown field strength or breakdown voltage. The breakdown test can be used to assess the cable's ability to withstand voltage and the safety margin between the operating voltage. The breakdown field strength is one of the important parameters in cable design.

Cables are generally subjected to AC voltage during operation, but they are also subjected to DC voltage in DC transmission systems and some special occasions. High-voltage cables may also be subjected to atmospheric voltage (lightning) and operating overvoltage attacks. Therefore, according to the different experimental voltage waveforms, there are three types of insulation strength tests: 1. AC (power frequency) voltage, 2. DC voltage, and 3. impulse voltage.

Partial discharge measurement

There is basically no local generation for oil-filled cables; even if there is partial discharge for oil-paper cables, it is usually very weak, such as a few PCs, so these cables do not need to be tested for partial discharge in factory tests. For extruded cables, not only is there a high possibility of partial discharge, but partial discharge also causes serious damage to plastics and rubber. With the increase of voltage level and working field strength, this problem becomes more serious. Therefore, for high-voltage extruded cables, partial discharge measurements must be made in factory tests.

There are many methods for measuring partial discharge. You can measure the discharge pulse based on the instantaneous charge exchange generated by the discharge (electrical measurement method); you can also measure the voltage based on the ultrasonic wave generated during the discharge (acoustic measurement method); you can also measure the intensity of the light generated by the discharge (optical measurement method). For cables, the electrical measurement method is basically used.

Aging and stability test

Aging test is a stability test to determine whether the performance can be maintained stable under stress (mechanical, electrical, thermal).

Heat aging test

Simple thermal aging test is to test the aging characteristics of the sample under the action of heat. The sample is placed in an environment with a certain temperature higher than the rated working temperature. After a specified time, the changes of certain sensitive properties before and after aging are measured to evaluate the aging characteristics. The aging of the sample can also be accelerated by increasing the temperature, and the thermal, mechanical, and electrical stresses such as moisture, vibration, and electric field are added to form an aging cycle. After each aging cycle, some selected sensitive performance parameters are measured. Until the performance drops to the value of the recognized life. In this way, a shorter life L (the time the sample is heated) is obtained at a higher temperature T.

Thermal stability test

The thermal stability test is a test in which the cable is heated by electric current while being subjected to a certain voltage. After a certain heating period, certain sensitive performance parameters are measured to assess the stability of the insulation.

Insulation stability tests are divided into long-term stability tests or short-term accelerated aging tests.