1. Inspection method

Routine test: it refers to the test conducted by the manufacturer on all finished cables. The purpose is to check whether the product quality meets the requirements of the technical conditions, so as to find the accidental defects in the manufacturing process. It is a non-destructive experiment, such as the DC resistance of the conductor and the insulation resistance time. And voltage withstand test, partial discharge detection, etc.

Type test: it refers to the test conducted by the manufacturer to conduct a comprehensive performance inspection on products on a regular basis, especially before a new product is finalized for mass production, or when the structure, material and main process of a product are changed, which may affect the performance of the cable. Through type test, it can check whether the product can meet the operation requirements and compare with the old product. Such as thermal aging performance of insulation and sheath, long-term stability test of power cable, etc.

Acceptance test: it refers to the acceptance test of cables after installation and laying, so as to check the installation quality and find possible damage during construction. Such as voltage withstand test after installation.

2. Test items

2.1 Measurement of DC resistance of conductor

The conductive core of wires and cables mainly transmits electric energy or electrical signals. The resistance of the wire is the main indicator of its electrical performance. The resistance of the wire core under the action of AC voltage is larger than that under the action of DC voltage due to skin effect and proximity effect surface, but the difference between the two is very small when the eye frequency is 50Hz. Now the standard stipulates that only the DC resistance or resistivity of the wire core can be detected to see whether it exceeds the value specified in the standard, Through this inspection, some defects in the production process can be found, such as wire breakage or partial single wire breakage; The cross-section of the conductor does not conform to the standard; The length of the product is incorrect. For power cables, it can also be checked whether they will affect the allowable current carrying capacity of wire and cable products during operation.

The DC resistance of conductor can be measured by single arm DC resistance method and double arm DC bridge method, and the accuracy of the latter is higher than that of the former. The test steps are also more complex than the former.

2.2 Insulation resistance test

The insulation resistance type is an important indicator reflecting the insulation characteristics of wire and cable products. It is closely related to the electrical strength, dielectric loss, and the gradual deterioration of insulation materials under working conditions. For communication cables, if the insulation resistance between lines is too low, it will also increase circuit attenuation, crosstalk between circuits and leakage of long-distance power supply on conductive wire cores. Therefore, the insulation resistance should be higher than the specified value.

The measurement of insulation resistance can find defects in the process, such as dry insulation or damp sheath damage; The insulation is polluted and conductive impurities are mixed; Cracking of insulation layer caused by various reasons. During the operation of wires and cables, the insulation resistance and leakage current should always be detected as the main basis for continued safe operation.

At present, in addition to ohmmeter (megger), galvanometer comparison method and megger method (voltage current method) are commonly used to measure the insulation resistance of wires and cables.

2.3 Measurement of capacitance and loss factor

When AC voltage is applied to the cable, there will be current flow. When the amplitude and frequency of the voltage are fixed, the capacitance current is proportional to the capacitance (Cx) of the cable. For ultra-high voltage cables, the current of such capacitors may reach a value comparable to the rated current, which becomes 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.

In the AC electric field, the dielectric loss of the insulator in the cable will be formed due to the leakage current and various polarizations. The dielectric loss factor or loss angle tangent (TAN δ） It not only wastes electric energy, but also heats the medium (insulator) and accelerates insulation aging. Therefore, TAN δ It is also one of the main parameters of the cable.

Through the measurement of capacitance and loss factor, it can be found that insulation is affected with moisture, insulation layer and shielding layer fall off and other insulation deterioration phenomena. Therefore, capacitance and TAN are carried out no matter in cable manufacturing or cable operation δ Measurement of.

For HV cables, Cx and TAN δ The measurement of is carried out under its working conditions, that is, power frequency and high voltage. The high-voltage Xilin bridge is usually used. This year, the current ratio transformer bridge has also been used.

2.4 Insulation strength test

The insulation strength of wires and cables refers to the ability of insulating structures and insulating materials to withstand electric field without breakdown damage. In order to check the quality of wire and cable products and ensure safe operation of products, insulation strength tests are generally required for all insulated types of wires and cables. The insulation strength test can be divided into voltage withstand 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, take whether breakdown occurs as the standard to judge whether the test object is qualified. The time voltage is generally higher than the rated working voltage of the test object. 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, find serious defects in the insulation, and also find some defects in the production process, such as: the insulation has serious external damage, and the conductor has serious defects that cause sharp distortion of the electric field; The insulation has penetrating defects or large conductive impurities in production.

The breakdown test is to measure the breakdown field strength or breakdown voltage under certain test conditions by raising the voltage until the breakdown of the test object occurs. The breakdown test can be used to assess the safety margin between the ability of the cable to withstand the voltage and the working voltage. The breakdown field strength is one of the important parameters in cable design.

Cables generally bear AC voltage during operation, but they also bear DC voltage in DC transmission system and some special occasions. High voltage cables may also be attacked by atmospheric voltage (lightning) and switching overvoltage. Therefore, according to the different waveforms of the test voltage, it can be divided into three insulation strength tests: 1. AC (power frequency) voltage, 2. DC voltage, and 3. Impulse voltage.

2.5 Partial discharge measurement

There is basically no local power generation for oil filled cables; Even if there is partial discharge in the oil paper cable, it is usually very weak, such as several PCs. Therefore, partial discharge can not be measured in the factory test of these cables. For extruded cables, not only is it possible to produce partial discharge, but also the partial discharge damage to plastic and rubber is relatively serious. With the improvement of voltage level and working field strength, this problem becomes more serious. Therefore, for high-voltage extruded cables, partial discharge measurement is required in factory test.

There are many methods to measure partial discharge, and the discharge pulse can be measured according to the instantaneous charge exchange generated by discharge (electrometric method); The voltage can also be measured according to the ultrasonic wave generated during discharge (acoustic method); The intensity of light can also be measured according to the light generated by the discharge (photometry). Electric measurement is basically adopted for cables.

2.6 Aging and stability test

The aging test is the stability test of whether the performance can be stable under the action of stress (mechanical, electrical, thermal).

2.6.1 Thermal aging test

A simple thermal aging test is to test the aging characteristics of the test object under the action of heat. Put the test object in an environment higher than the rated working temperature by a certain value. After the specified time, measure the changes of some sensitive properties before and after aging to evaluate the aging characteristics. It is also possible to accelerate the aging of the test object by raising the temperature, and add 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 can be measured. Until the performance drops to the value of the recognized life. In this way, a shorter life L (heating time of the sample) can be obtained at a higher temperature T.

2.6.2 Thermal stability test

The thermal stability test is to measure some sensitive performance parameters to evaluate the stability of insulation after the cable is heated by current and also bears a certain voltage.

Insulation stability test is divided into long-term stability test or short-term accelerated aging test.