Test method for insulating strength of insulated wires and cables

Insulation strength test methods for insulated wires and cables The insulation strength of wires and cables refers to the ability of the insulation structure and insulating materials to withstand the action of an electric field without breakdown damage. In order to check the quality of wire and cable products and ensure their safe operation, insulation strength tests are generally required for all types of insulated wires and cables. The 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, and after a certain period of time, determine whether the test object is qualified based on whether breakdown occurs. The voltage of the test object is generally higher than the rated working voltage of the test object. The specific voltage value and withstand time are specified in the product standard. Through the withstand voltage test, the reliability of the product operating at 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 and serious defects on the conductor that cause sharp distortion of the electric field; There are penetrating defects or large conductive impurities in insulation during production.
The breakdown test is to measure the breakdown field strength or voltage under certain test conditions by raising the voltage until the test object undergoes breakdown. The breakdown test can be used to assess the safety margin between the cable's ability to withstand voltage and the operating voltage. The breakdown field strength is one of the important parameters in cable design.
Cables generally bear AC voltage during operation, but they can also bear DC voltage in DC transmission systems and certain special occasions. High voltage cables may also be subject to atmospheric voltage (lightning) and switching overvoltage attacks. Therefore, according to different experimental voltage waveforms, insulation strength tests can be divided into three types: 1. AC (power frequency) voltage, 2. DC voltage, and 3. impulse voltage.
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 several PCs. Therefore, partial discharge may not be measured for these cables during factory tests. For extruded cables, not only is there a high possibility of partial discharge, but also the damage to plastics and rubber caused by partial discharge is relatively serious. As the voltage level increases and the working field strength increases, this problem becomes more serious. Therefore, for high-voltage extruded cables, partial discharge measurements are required during factory testing.
There are many methods for measuring partial discharge, which can measure the discharge pulse based on the instantaneous charge exchange generated by the discharge (electrical measurement); The voltage can also be measured based on the ultrasonic wave generated during discharge (acoustic measurement); The intensity of light can also be measured based on the light generated by the discharge (photometry). The electrical measurement method is basically used for cables.
Aging and stability test
The aging test is a stability test to determine whether the performance can be maintained stable under the action of stress (mechanical, electrical, and thermal).
Thermal aging test
A simple thermal aging test is to test the aging characteristics of the test object under the action of heat. Place the test object in an environment with a certain temperature higher than the rated operating temperature, and after a specified time, measure the changes in certain sensitive properties before and after aging to evaluate the aging characteristics. It is also possible to accelerate the aging of the test sample by increasing the temperature, plus thermal, mechanical, and electrical stresses such as moisture, vibration, and electric field to form an aging cycle. After each aging cycle, certain selected sensitive performance parameters are measured. Until the performance drops to the value of the recognized lifetime. This results in a shorter lifetime L (sample heating time) at a higher temperature T.
Thermal stability test
The thermal stability test refers to the measurement of certain sensitive performance parameters to assess the insulation stability of a cable heated by current and subjected to a certain voltage after undergoing a certain period of heating.
The insulation stability test is divided into two types: long-term stability test or short-term accelerated aging test.