1, inspection method

Routine test: It is an experiment conducted by the manufacturer on all finished cables. The purpose is to check that the quality of the product meets the requirements of the technical conditions in order to identify accidental defects in the manufacturing process. It is a non-destructive experiment, such as the DC resistance of the wire and the insulation resistance time. And partial pressure test for withstand voltage test.

Type test: It is the manufacturer's regular performance inspection of the product, especially for a new product before the final batch production, or changes to the structure, materials and main processes of a product may affect the performance of the cable. Test conducted at the time. Type test: It can be tested whether the product can meet the operational requirements and can be compared with the old product. Such as insulation and sheath thermal aging performance, power cable long-term stability test.

Acceptance test: It is the acceptance test of the cable after the cable is installed, in order to check the installation quality and find the damage that may occur during the construction. Such as the pressure test after installation.

2. Test project

2.1 Measurement of wire DC resistance

The conductive core of the wire and cable mainly transmits electrical energy or electrical signals. The resistance of the wire is the main indicator of its electrical performance. When the AC voltage is applied, the wire core resistance is larger due to the skin effect and the adjacent effect surface is stronger than the DC voltage. However, when the electric eye frequency is 50 Hz, the difference between the two is small. That can only require that the DC resistance or resistivity of the test core exceeds the value specified in the standard. Through this inspection, some defects in the production process can be found: such as broken wires or partial single-wire breaks; Meet the standard; the length of the product is incorrect. For power cables, you can also check if it will affect the electricity.

Current carrying capacity is allowed during operation of the line cable product.

The measurement of the DC resistance of the conductor has a one-arm DC resistance method and a two-arm DC bridge method, and the accuracy of the latter is higher than that of the former. The test procedure is also more complicated than the former.

2.2 Testing of insulation resistance

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 communication cables, too low insulation resistance between lines will increase loop attenuation, crosstalk between circuits, and long-distance power supply leakage on conductive cores. Therefore, insulation resistance should be higher than the specified value.

Determining the insulation resistance can find 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 insulation resistance of wire and cable, in addition to the use of ohmmeter (shaking table), is commonly used galvanometer comparison method high resistance meter method (voltage - current method).

2.3 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.

In an alternating electric field, the insulator in the cable, due to leakage current and various polarizations, forms a dielectric loss, expressed as a dielectric loss factor or loss tangent (TAN δ), which not only wastes electrical energy, but also causes the dielectric (insulator) ) Heating, accelerating insulation aging, so TAN δ is also one of the main parameters of the cable.

Through the measurement of capacitance and loss factor, various insulation degradation phenomena such as insulation moisture, insulation layer and shielding layer dropout can be found, so capacitance and TAN δ measurement are performed in cable manufacturing or cable operation.

For high-voltage cables, the measurement of Cx and TANδ is carried out under the working conditions, that is, the power frequency high voltage. Usually, the high-voltage Xilin bridge is used. This year, the current ratio transformer bridge is also used.

2.4 Insulation strength test

The insulation strength of wire and cable refers to the ability of insulation structure and insulation material to withstand electric field without breakdown damage. In order to check the quality of wire and cable products and ensure the safe operation of products, all insulation type wires and cables are generally tested for insulation strength. . The dielectric 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 sample under certain conditions, and after a certain period of time, whether or not the breakdown occurs is used as a criterion for judging whether the test article is qualified. The voltage of time is generally higher than the rated working voltage of the sample, the specific voltage value and the withstand voltage time, which are specified in the product standard. Through the withstand voltage test, the reliability of the product operating under the working voltage can be tested and the seriousness of the insulation can be found. Defects can also find some shortcomings of the production process, such as: severe external damage to the insulation, serious defects on the conductor that cause the electric field to be sharply distorted; 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 increasing the voltage until the sample breaks down. Through the breakdown test, the safety margin between the cable's ability to withstand voltage and the operating voltage can be assessed. One of the important parameters in cable design when penetrating field strength.

Cables are generally subjected to AC voltage during operation, but they are also subjected to DC voltage in DC transmission systems and in certain special occasions. For high voltage cables, they may also be subjected to atmospheric voltage (thunder) and operating overvoltage. Therefore, according to the different experimental voltage waveforms, it can be divided into three kinds of dielectric strength tests: 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 the oil-paper cable has partial discharge, it is usually very weak, such as several PCs, so these cables may not be partially discharged 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.

2.6 aging and stability test

The aging test is a stability test that maintains stable performance under the action of stress (mechanical, electrical, and thermal).

2.6.1 Heat aging test

The simple heat aging test is to test the aging characteristics of the sample under the action of heat. The test sample is placed in an environment above a certain temperature of the rated working temperature. After a specified period of time, some sensitive properties are measured before and after aging. Change to assess aging characteristics. It is also possible to increase the temperature to accelerate the aging of the sample, and add heat, machine and electricity stresses such as moisture, vibration and electric field to form an aging cycle. After each aging cycle, certain selected sensitive performance parameters are determined. Until the performance drops to the value of the table life. Thus at a higher temperature T, a shorter life L (time of sample heating) is obtained.

2.6.2 Thermal stability test

The thermal stability test is that the cable is heated by the current while still receiving a certain voltage. After a certain period of heating, some sensitive performance parameters are measured to evaluate the stability of the insulation.

The insulation stability test is divided into two types: long-term stability test or short-term accelerated aging test.