Abstract: This paper analyzes the shortcomings and problems of DC withstand voltage test on XLPE cable, and selects frequency conversion resonance device for AC withstand voltage test on site by comparison;
Key words: high voltage cable, DC withstand voltage test, AC withstand voltage test
preface:
Power cables are often used as power lines for power plants, substations and industrial and mining enterprises. They are also commonly used when crossing rivers and railways. Power cable can be used as urban power transmission and distribution lines and trunk lines in industrial and mining enterprises to reduce land occupation and beautify the environment. The development of electric power construction directly drives the development of the country. In electric power construction, power cable plays an important role. It is loved by electric power users because of its small influence of external climate, concealment, durability, high insulation performance, good waterproof and acid proof performance, strong tensile and compression resistance. However, it is easy to have some faults in the process of use, such as mechanical damage, lead package corrosion and excessive corrosion Heat aging, etc. Therefore, the power cable must be tested for hidden faults by routine preventive test to ensure the normal operation of the power system.
According to iec840 or cigrewg21.03, the purpose of field test is not to inspect the manufacturing quality of cable or cable accessories, which has been confirmed in type test and factory test. The purpose of site completion acceptance test is to check whether the cable laying and accessories installation are correct. The cable may be damaged accidentally in the process of transportation, handling, storage, laying and backfilling. According to iec229, for cables with outer sheath thickness of 2.5mm or more, 10kV DC voltage is applied between the cable shield and the ground for 1 minute. For the withstand voltage test of cable main insulation, IEC recommends two methods:
DC withstand voltage: 3u015min; AC withstand voltage: u05min.
The traditional DC withstand voltage test equipment has the advantages of light weight, good mobility and low capacity. It has a good application effect for oil paper insulated cable, but for XLPE cable, it is proved that DC withstand voltage method is not suitable in theory and practice.
The test items of high voltage cable specified in article 18.0.1 of national standard are as follows:
1. Measure the insulation resistance;
2. DC withstand voltage test and leakage current measurement;
3. AC withstand voltage test;
4. Measure the resistance ratio of metal shield and conductor;
5. Check the phase at both ends of the cable line;
6. Insulating oil test of oil filled cable;
7. Cross interconnection system test.
There is no test item in the national standard to detect the water entering into the inner lining and outer sheath of the cable
1. Since it is impossible to detect whether there is water in the inner layer of cable outer sheath according to the national standard, the additional test items in each province are as follows:
1.1. Judge by the ratio of copper cover resistance to conductor resistance. The step is to measure the DC resistance of copper shield and conductor at the same temperature with double wall bridge. When the ratio of the former to the latter is higher than that before operation, it indicates that the DC resistance of the copper shield layer increases, and the copper shield may be corroded; when the ratio is lower than that before operation, it indicates that the contact resistance of the conductor connection point in the accessory may increase. Generally, in the field test, the insulation resistance value of steel armor and shield is measured, and the resistance ratio is used to judge whether the outer sheath and inner lining of the cable are flooded.
1.2. Use a megohmmeter to measure the insulation resistance. The steps are as follows: use a 500V megohmmeter to measure the insulation resistance of the outer sheath of the inner lining of the rubber and plastic cable. When the insulation resistance per kilometer is less than 0.5 megohm, use the following methods to further judge. Use a multimeter to measure the insulation resistance. According to the principle of primary battery, the metal layer, armor layer and coating materials of the rubber and plastic cable are copper, lead, iron, zinc and aluminum The electrode and potential of these metals are + 0.334, -0.122, -0.44, -0.76v and -1.33v respectively after the water enters into the inner layer of the outer sheath of the rubber plastic cable. The principle is that when the outer sheath of the rubber plastic cable is damaged and the water enters into the inner layer, the potential to the ground of -0.76v will be generated on the galvanized steel strip of the armor layer because the groundwater is electrolyte. When the outer sheath or inner liner is damaged and water enters, when the insulation resistance per kilometer is lower than 0.5 megohm, use the positive and negative probes of the multimeter to measure the insulation resistance of the armor layer to the ground or the armor layer to the copper shielding layer alternately. At this time, the primary battery formed in the measurement circuit is connected in series with the dry battery in the multimeter. When the polarity combination makes the voltage add, the measured resistance value is smaller; on the contrary, the measured resistance value is larger. Therefore, when the difference between the above two measured insulation resistance values is large, it indicates that the primary battery has been formed, and it can be judged that the outer sheath and inner lining have been damaged.
For example, after a rubber and plastic cable sheath is damaged and damped, the measured resistances are 7 Ka ohm and 55 Ka ohm respectively.
2. For the voltage withstand test of cables, the national standard stipulates that DC voltage withstand test and AC voltage withstand test should be conducted, but local provinces choose one of them according to their own actual situation. Now the advantages and disadvantages of the two are compared as follows: XLPE cables should not be subject to DC voltage withstand test, but should be subject to AC voltage withstand test.
2.1 DC withstand voltage test:
It is a general principle of high voltage test that the test voltage field applied on the tested object should simulate the operation condition of high voltage apparatus. The DC withstand voltage test is very effective to find the defects of paper insulated cables, but it may not be effective to XLPE insulated cables, and it may also have negative effects, mainly in the following aspects:
2.1.1 the electric field distribution of XLPE cable under AC and DC voltage is different. The XLPE insulation layer is made of polyethylene through chemical crosslinking, which belongs to integral insulation structure, and its dielectric constant is 2.1-2.3, which is less affected by temperature change. Under AC voltage, the electric field distribution in the insulation layer of XLPE cable is determined by the dielectric constant of each medium, that is, the electric field intensity is distributed in inverse proportion to the dielectric constant, which is relatively stable. Under DC voltage, the electric field distribution in the insulation layer is determined by the volume resistivity of the material, and it is distributed in a positive proportion, and the distribution coefficient of the insulation resistance is not uniform. In particular, the distribution of AC electric field strength in cable accessories such as cable terminal and junction box is completely different from that of DC electric field strength, and the aging mechanism of insulation under AC voltage is different from that under DC voltage. Therefore, DC withstand voltage test can not simulate the operation condition of XLPE cable.
2.1.2 XLPE cable will produce "accumulation" effect under DC voltage to store and accumulate unipolar residual charge. It takes a long time to release the residual charge due to the charge accumulation during DC withstand voltage test. If the cable is put into operation before the DC residual charge is fully released, the DC residual voltage will be superimposed on the peak value of power frequency voltage, making the voltage value on the cable exceed the rated voltage under operating conditions, which will accelerate the insulation aging, shorten the service life of the cable, and even lead to insulation breakdown.
2.1.3 a fatal weakness of XLPE cable is that water branches are easy to form in the insulation. Under DC voltage, water branches will quickly change into electrical branches and form discharge, which accelerates the deterioration of insulation and causes breakdown under power frequency voltage. However, the pure water branch can maintain a considerable withstand voltage value under AC working voltage for a period of time.
2.1.4 flashover or breakdown during DC high voltage test on site may cause harm to normal cable and joint insulation. Moreover, DC withstand voltage test can not effectively find some defects under AC voltage, such as mechanical damage or misplaced stress cone in cable accessories. The place where the insulation is most prone to breakdown under AC voltage is often unable to breakdown under DC voltage. Under DC voltage, the insulation breakdown often occurs at the place where the insulation normally does not break down under AC working conditions.
2.2 AC withstand voltage test:
Since the DC withstand voltage test can not simulate the operating field strength of XLPE insulated cable and can not achieve the expected test effect, we consider using AC high voltage test. Due to the different capacitance values of the cables, we should first measure the capacitance value of the power cable before the test, and calculate the capacitive current under the test voltage according to the capacitance value, so as to select the appropriate test instrument.
2.2.1 it is understood that the rated voltage of cables in most power plants is 6kV, and the length is mostly within 1.5km, so we can adopt the conventional AC withstand voltage test method. If a 50kV, 20KVA test transformer is used, its maximum output current is 1000mA. According to I = 2 π fuc, taking 6kV cable as an example, the maximum capacitance of the cable that can be tested by this test transformer is 265nf (F = 50Hz, u = 12kV).
2.2.2 for some large capacity cables, if the conventional AC withstand voltage test method is adopted, large capacity test transformer is required, and the capacity of voltage regulator and power supply is also required. It is often difficult to do on site, and it is time-consuming and laborious to transport and place test instruments by using large vehicles and cranes. Therefore, we use frequency conversion test, series or series parallel resonance method to carry out the voltage withstand test of the cable according to the specific situation.
2.2.3 ultra low frequency 0.1Hz withstand voltage test:
According to the test capacity (formula s = wcus2 = 2 Π fus2kva, where C-test cable capacitance, US – test voltage, f-power frequency, China's 50 Hz), it can be seen that compared with 50 Hz voltage, 0.1 Hz AC voltage needs 1 / 500 of the power of the latter, so it can produce portable equipment for on-site use without any problem. At present, this method is mainly used in the test of medium and low voltage cables.
The field practice demonstrates that the withstand voltage test of XLPE cable with 0.1 Hz ultra-low frequency voltage can be 1.5-1.8 times of 50 Hz voltage, which is easier to find cable insulation defects than DC withstand voltage, and easier to expose insulation defects than 50 Hz AC voltage.
2.2.4 variable frequency resonance withstand voltage test:
Frequency conversion resonance test system can not only meet the requirements of high voltage XLPE cable, but also has the advantages of light weight and good mobility, which is suitable for field test. The fixed reactor is used as resonance reactor to realize resonance by frequency modulation. The frequency range is 30-300hz, which conforms to cigrewg21.09 "recommended guide for completion test of high voltage extruded insulated cables". AC voltage of power frequency and approximate power frequency (30-300hz) is recommended. This kind of AC voltage can reproduce the same field strength as that under the operation condition. It has the advantages of good equivalence, high efficiency, light equipment and almost unlimited sample length.
To sum up, in view of the small capacity and volume of power frequency test equipment on the cable site, easy to carry and operate, it is more effective to find cable defects than the conventional DC withstand voltage, so the power frequency or frequency conversion resonance test method should be used for the cable site completion acceptance test. Moreover, the frequency conversion resonance device can meet the requirements of cross-linked polyethylene cable handover test of 10kV and 220kV and above, so it is recommended that frequency conversion resonance withstand voltage is preferred.