What is an electrical performance test? What are the important electrical performance test items?

Date：2021-11-02 14:39:25 Views：5004

Electrical performance is usually defined in the rated voltage, current, active power, reactive power, resistance, capacitance, inductance, conductivity and other performance indicators of electrical components. Semiconductors cover a wider range, such as DC amplification, AC amplification, rectifier current, reverse breakdown voltage, forward conduction voltage, junction capacitance, noise figure, characteristic frequency, cut-off frequency, dissipated power, etc. The electrical performance test includes the basic parameter test of conductor or insulation quality, such as conductor resistance, insulation resistance, correct value of dielectric loss angle, capacitance, etc. The higher the working voltage of the cable, the stricter the requirements for its electrical performance.

（1） DC resistance test of conductive core

The resistance of the cable of each nominal section shall not exceed a certain value, otherwise it will increase the core loss of the cable during use, resulting in cable heating, which will not only consume material and electric energy, accelerate the aging of plastic cable, but also bring danger to the reliability and stability of cable operation.

At present, double arm measuring bridge is commonly used.

（2） Insulation resistance test

The ratio of DC voltage U applied to insulation to leakage current I is called insulation resistance R. the insulation resistance of cable is mainly to judge the humidity and insulation quality of cable insulation layer.

Among them, seven important measurement items in electrical performance testing play a key role in judging the quality of a wire and cable.

1. Measurement of DC resistance of conductor

For wires and cables, the conductor part is the most important part. The conductive core of wire and cable mainly transmits electric energy or electrical signal.

The resistance of the conductor is the main index of its electrical performance. Now the standard stipulates: check whether the DC resistance or resistivity of the wire core exceeds the value specified in the standard.

The main purpose of this test is to find some defects in the production process: for example, the conductor is broken or part of the single line is broken; The conductor section does not meet the standard; Incorrect length of product, etc.

In addition, for power cables, this test can not only detect the condition of conductors, but also detect whether conductors have an impact on the allowable ampacity during the operation of wire and cable products.

2. Insulation resistance test

Insulation resistance is an important index reflecting the insulation characteristics of wire and cable products. It is closely related to the electrical strength, dielectric loss and the gradual deterioration of insulating materials under working conditions.

Defects in the process can be found by measuring the insulation resistance: for example, the insulation is not dry or the sheath is damaged and damp; The insulation is polluted and mixed with conductive impurities; Insulation layer cracking caused by various reasons, etc.

For communication cables, too low insulation resistance between lines will also increase loop attenuation, crosstalk between loops and leakage of long-distance power supply on conductive cores. Therefore, it is required that the insulation resistance should be higher than the specified value. During the operation of wires and cables, the insulation resistance and leakage current should be often detected as the main basis for whether they can continue to operate safely.

3. Measurement of capacitance and loss factor

When the AC voltage is added to the cable, there will be current flow. When the amplitude and frequency of the voltage are constant, the capacitance current is directly proportional to the capacitance (Cx) of the cable. For EHV cables, the current of this capacitor may reach a value comparable to the rated current, which has become an important factor limiting the cable capacity and transmission distance. Therefore, the capacitance of the cable is also one of the main electrical performance parameters of the cable.

In AC electric field, the insulator in the cable will form dielectric loss due to leakage current and various polarization, which is measured by dielectric loss factor or loss tangent (Tan δ） It not only wastes electric energy, but also heats the medium (insulator) and accelerates insulation aging, so tan δ It is also one of the main parameters of the cable.

Through the measurement of capacitance and loss factor, various insulation deterioration phenomena such as damp insulation, falling off of insulation layer and shielding layer can be found. Therefore, the tangent values of capacitance and loss angle are measured whether in cable manufacturing or cable operation. The measurement of capacitance and loss factor of high-voltage cable is carried out under its working conditions, i.e. power frequency high voltage.

4. Insulation strength test (including withstand voltage test and breakdown test)

The insulation strength of wires and cables refers to the ability of insulating structures and insulating materials to withstand the action of electric field without breakdown damage. In order to check the product quality of wires and cables and ensure the safe operation of products, all insulation types of wires and cables generally need to undergo insulation strength test.

Insulation strength test can be divided into withstand voltage test and breakdown test. Withstand voltage test is to apply a certain voltage to the test object under certain conditions, and take whether breakdown occurs as the standard to judge whether the test object is qualified after a certain time. 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 and find serious defects in the insulation, as well as some shortcomings of the production process, such as serious external damage to the insulation and serious defects on the conductor that cause sharp distortion of the electric field; 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 breakdown of the test object occurs. Through the breakdown test, the safety margin between the voltage bearing capacity of the cable and the working voltage can be assessed. 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 also be attacked by atmospheric voltage (lightning) and operating overvoltage. Therefore, according to the different experimental voltage waveforms, it can be divided into 1 AC (power frequency) voltage, 2 Three insulation strength tests of DC voltage and 3 impulse voltage.

5. Partial discharge measurement

This is mainly for extruded cables. Because the oil filled cable basically has no local power generation; Even if there is partial discharge in oil paper cables, it is usually very weak, such as several PCs, so these cables can not measure partial discharge in factory test. On the contrary, extruded cables are not only more likely to produce partial discharge, but also have serious damage to plastics and rubber. With the improvement of voltage level and working field strength, this problem becomes more serious. Therefore, partial discharge measurement should be done for high-voltage extruded cables in factory test.

6. Aging and stability test

Aging test is the stability test of whether the performance can be stable under the action of stress (mechanical, electrical and thermal). 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 with a certain temperature higher than the rated working temperature. After a 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 sample by increasing the temperature, plus thermal, mechanical and electrical stresses such as damp, vibration and electric field to form an aging cycle. After each aging cycle, some selected sensitive performance parameters can be measured. Until the performance decreases to the value of the recognized life. In this way, a shorter life (heating time of the sample) is obtained at a higher temperature.

7. Thermal stability test:

Thermal stability test is that wires and cables are heated by current and bear 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 or short-term accelerated aging test.

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