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The electrical test of the current cable line generally includes: DC withstand voltage and leakage current test, power frequency withstand voltage test, measuring insulation resistance, insulating oil test, partial discharge test, 0.1 Hz ultra low frequency test, AC frequency conversion resonance test, and the like. At present, the power sector has different test methods and test contents for different voltage levels and different types of power cable lines.
1 DC withstand voltage test of oil-impregnated paper insulated cable The DC withstand voltage reflects the leakage and withstand voltage characteristics of cable insulation. Both theoretical analysis and practical effects show that the DC and AC withstand voltage characteristics of oil-impregnated paper dielectric cables, oil-filled cables or gas-filled cables are basically the same.
For the test of oil-paper insulated power cables, except for the AC voltage used by the manufacturer during routine tests, the installation and operation units use DC withstand voltage for the handover acceptance and preventive test or fault repair test. The DC withstand voltage test has the following advantages.
a. The DC test equipment is portable and suitable for on-site use. When the cable is subjected to DC withstand voltage test, the test voltage is generally obtained by half-wave rectification, and multi-voltage rectification technology is applied. Therefore, the test equipment (test transformer and rectification equipment) with small volume capacity can be used to obtain long cable lines. The voltage of the DC high voltage test.
b. AC withstand voltage test may cause free discharge in the insulation gap, resulting in permanent damage of the insulation. This is avoided by the DC withstand voltage test.
c. When conducting a DC withstand voltage test, the leakage current can be measured simultaneously. According to the value of the leakage current and its relationship with time, leakage current and test voltage, the insulation condition of the cable can be judged.
d. When performing DC withstand voltage test on the cable, use negative polarity wiring according to the regulations, that is, connect the conductor to the negative pole. The advantage of this connection is that if the paper insulation has been wet, due to the positive charge of the water, there is a significant "electroosmosis" under the DC voltage, which will cause the water molecules to move from the surface layer to the conductor (negative electrode), thus causing leakage current. Increasing, or even forming a penetrating passage, is advantageous for exposing defects that have been partially wetted in the paper insulation.
e. DC withstand voltage test pressurization time can be shorter, such as the regulation of 6 ~ 35 kV cable for the handover and preventive test, the pressurization time per phase is 5 min. This is because the DC breakdown voltage has little to do with the pressurization time. If there is a defect, it can be found within a few minutes under the DC voltage, and it is not necessary to press for a long time.
The voltage standards for DC test of oil-paper insulated power cables are shown in Table 1. Among them, when the cable is repaired and repaired, the 6-35 kV cable is tested with the preventive test, and the 110-220 kV cable is delivered in the same test. The voltage of the 110-220 kV cable outer sheath is tested at 10 kV DC. The time is 1 min.
Measuring the leakage current during the DC withstand voltage test is actually the same as measuring the insulation resistance of the cable with a megohmmeter. However, since the applied voltage and the accuracy of the instrument used in the DC withstand voltage test are higher than the megohmmeter, and the leakage current can be observed during the pressurization process, the leakage current test can more effectively detect the insulation defect than the measurement of the insulation resistance. .
When the cable is under DC voltage, the current flowing through the insulation is a superposition of the capacitor current, the sink current, and the conduction current. The leakage current flowing through the insulation changes with time. It is related to the quality of the cable insulation, impurities, bubbles, moisture, etc.: The cable with good insulation, with the prolonged pressurization time, the leakage current decreases, and tends to be stable. The value of the cable with poor insulation, the leakage current quickly reaches a stable value, and the value after stabilization is very close to the initial value; when there is serious defect in the insulation, the leakage current does not decrease with time, but instead rises, if the pressure is prolonged The time or increase in DC voltage, the trend of increased leakage current may continue to develop until insulation breakdown.
In order for the measured leakage current to reflect the true condition of the cable insulation, measures should be taken to eliminate the influence of external factors on the leakage current. If the measured leakage current value is unstable, the leakage current rises with time, or rises sharply as the test voltage increases, and the cause must be ascertained.
Generally, the ratio of the leakage current measured after the DC voltage withstand test of the cable and before the withstand voltage test is called the absorption ratio. The leakage current before the withstand voltage test refers to the leakage current I1 at 1 min after the DC withstand voltage test is applied to the specified voltage, and the leakage current after the withstand voltage test means that the withstand voltage lasts for 4 min (for a cable of 6 to 35 kV) ) or leakage current I2 for 14 min (for 110 to 220 kV cable). The regulations stipulate that the eligibility criteria for the cable leakage test is that the absorption ratio I2/I1 ≤ 1.
2 Withstand voltage test of XLPE cable In China, DC voltage is still the main power source for testing XLPE cable. The voltage standard of XLPE cable DC test is shown in Table 2. It is clearly stipulated in the IEC standard that the electrical test after installation of the XLPE cable and its accessories with a rated voltage of 150 kV or more is based on the AC voltage test, that is, the phase-to-phase voltage of the power system is applied, and the test is performed for 1 h or the normal operating voltage is applied. It is not recommended to use the DC voltage test.
China began using high voltage (110 ~ 220 kV) XLPE cable in 1984. With the development of urban power grid construction and transformation, since 1985, large cities such as Guangzhou, Shanghai, and Beijing have imported high-voltage XLPE cables and accessories from abroad. It is from this time that some countries have conducted research and analysis on the results of DC withstand voltage test and cable operation of high voltage XLPE cables, and reached a common conclusion that high voltage XLPE cables should not be tested with DC withstand voltage. When the cable is subjected to the DC withstand voltage test, the following problems mainly exist.
a. The XLPE cable insulation has a completely different internal electric field distribution under DC and AC voltages. Under DC voltage, the electric field is proportionally distributed according to the insulation resistance coefficient, while the XLPE insulation material has a non-uniformity of the resistivity, resulting in uneven distribution of the electric field under DC voltage. Under AC voltage, the electric field is distributed inversely proportional to the dielectric constant. XLPE is an integral insulation structure with a dielectric constant of 2.1 to 2.3 and is generally unaffected by temperature changes. Therefore, under AC voltage, the internal electric field distribution of XLPE insulation is relatively stable. In this way, it is often caused that the defective portion under the AC working voltage is not broken down during the DC test, and conversely, the portion that is broken during the DC test does not cause a problem under the AC operating voltage.
b. If there is a water branch inside the XLPE insulation, the development of the water branch is very slow under the AC working voltage, and the development of the water branch will be accelerated in the DC withstand voltage test, and even into the electric branch, that is, the DC test will This leads to the accumulation effect of XLPE insulation, accelerates insulation aging and shortens service life.
c. During the DC withstand voltage test, space charge will be formed in the insulation of the XLPE cable and its accessories. The continuous formation of space charge can cause the cable to break down under the AC working voltage, or flash along the interface due to accumulated charge at the accessory interface. .
In summary, the DC test voltage can not effectively find the insulation defects of the XLPE cable, and the DC test voltage may cause damage to the insulation of the XLPE cable, and even an insulation breakdown accident occurs at the AC working voltage when the operation is resumed after the test. . Therefore, it is necessary for the XLPE cable to use other test methods than the DC test.
2.1 Ultra-low frequency voltage test The output frequency of the ultra-low frequency withstand voltage test device is generally 0.01~0.1 Hz, and the output waveform is sine wave or cosine wave, so the ultra-low frequency test is also an AC withstand voltage test. The purpose of the ultra low frequency test is to satisfy the volume and weight of the test equipment as much as possible under AC voltage conditions.
The DC test can not effectively detect the defects of the XLPE cable line, and the injected space charge will affect its insulation performance. However, the AC voltage test requires high-voltage and large-capacity test equipment. Therefore, the ultra-low frequency voltage test can be used. From 50 Hz to 0.1 Hz, it is theoretically possible to reduce the test equipment capacity to 1/500. In this way, the 0.1 Hz test equipment can achieve the same capacity and light weight as the DC test equipment, and is suitable for field use.
For XLPE power cables, DC voltage should not be used for on-site withstand voltage test, but 0.1 Hz ultra-low frequency voltage test should be used. The 0.1 Hz ultra-low frequency voltage test project mainly includes the withstand voltage test and the dielectric loss measurement. At present, the 0.1 Hz test equipment developed internationally has voltages below 100 kV and is only suitable for medium voltage (6-35 kV) XLPE cable lines. The recommended test standard is 3U0/1 h.
2.2 AC frequency conversion series resonance test The power frequency withstand voltage test can best reflect the actual situation of cable insulation. This is because the cable is operated at the power frequency, and the test voltage and frequency are most reasonable under the power frequency, which can fully simulate the operation. In theory, the power frequency withstand voltage test can not only reflect the leakage characteristics of the cable, but also fully reflect the withstand voltage characteristics of the cable, and also reflect the local withstand voltage characteristics caused by the local dielectric loss of the cable.
For the power frequency AC withstand voltage test of XLPE cable, the biggest difficulty is to have a large capacity test equipment. The higher the voltage, the longer the line and the greater the capacity of the test equipment. In order to meet the needs of the XLPE cable AC withstand voltage test in the field, the key is to minimize the capacity of the test equipment. The application of series resonance technology is an effective measure to reduce the capacity of test equipment. Tests have shown that the variable frequency series resonant device can withstand the high test voltage of the cable insulation with a lower voltage, smaller capacity power supply device.
At present, the inverter series resonance test equipment imported from abroad includes a reactor with a fixed inductance of 10~100 H. It is mounted on a 20 t flatbed truck, and a container truck is used to install the frequency modulator and transformer. And equipment such as computer control systems. The inverter has a frequency conversion range of 30 to 300 Hz, an output voltage of up to 250 kV, and a current of 75 A, which can be applied to the AC withstand voltage test of a 220 kV XLPE cable.
3 Partial discharge test of plastic cable There is air gap, tidal water, etc. in the insulation of rubber and plastic cable. Under the rated DC voltage, there is generally only a short partial discharge process or no partial discharge. At rated AC voltage, partial discharge may or may not occur. If a partial discharge occurs, the discharge process is relatively short, and the partial discharge process does not cause the insulation of the cable to break down within a certain period of time, but the damage is great. Therefore, only partial discharge measurements are made on specific parts of the cable, such as suspected parts of the cable, intermediate joints, and terminal heads.
4 Conclusions a. For rubber-plastic cables, the DC withstand voltage test can only find that the cable insulation has been significantly deteriorated or broken down. Because of the “destructive” effect on the cable, it is only used when it is forced to do so and is for reference only. Information from: Transmission and distribution equipment network b. The ultra-low frequency test device consists of operation control and high-voltage power supply. The field operation is light and convenient, and there is no “destructive” effect on the cable. It can be used as a test method for rubber and plastic cables. It is mainly used for cable test of 35 kV or less.
c. The frequency conversion resonance test device is composed of variable frequency power supply, excitation transformer, resonant reactor and voltage divider. The cable of 35 kV or less is more troublesome to operate on site. The cable of 66 kV or more can be used as a kind of site. experiment method.
d. For cables greater than or equal to 110 kV, the oscillating voltage method is small in size and convenient for field operation, but whether it can have the effects of ultra-low frequency and frequency conversion resonance test has yet to be verified.
e. The partial discharge test can only detect special parts of the cable (intermediate joints, terminal heads, etc.). For cables of 110 kV or more, it is necessary to carry out this test on site.
f. Dielectric loss measurement method, because of its limitations, is almost meaningless in the field.
June 28, 2024
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June 28, 2024
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