Power transformer high voltage bushing maintenance, test and test method
Power Transformer high voltage bushing maintenance, test and test method
This paper mainly discusses the routine maintenance, test and testing methods of oil-paper capacitor type casings, and introduces the power-off test, inspection contents and attention problems from the aspects of preventive test technology, professional inspection technology and online monitoring technology. Key areas and projects, as well as applications and recommendations for online monitoring techniques.
In recent years, the failure of the high-voltage bushing of power transformers has occurred. The power unit attaches great importance to the operation of the casing and formulates various anti-accident measures to ensure the safe operation of the casing. The author combines the actual field work experience over the years to talk about the field test monitoring technology of casing.
Second, the structural principle of oil-paper capacitor type casing
Most of the high voltage bushings of 110kV and above are oil-paper capacitor type bushings, which rely on the capacitor core to improve the electric field distribution. The capacitor core is composed of multi-layer insulating paper, and the aluminum foil is sandwiched between the layers according to the design requirements. A series of coaxial cylindrical capacitors are insulated with mineral oil by insulating paper.
Third, preventive testing technology
The preventive test of the oil-paper capacitor type bushing is to perform regular power-off test and inspection on the bushing, mainly the main insulation test and the final screen test, as well as other parts of the inspection.
(1) Main insulation test. The main insulation dielectric loss measurement is positively connected. The increase in the dielectric loss value is likely to be caused by deterioration of the casing itself and moisture. However, if the dielectric loss value is abnormally small or negative, it may be caused by poor grounding of the flange of the casing base, moisture on the surface of the casing, moisture in the casing, and moisture in the final screen. It may also be caused by the dielectric capacitor of the dielectric loss meter. Etc.
The increase in capacitance may be due to poor sealing of the equipment, moisture ingress, or free discharge inside the casing, which may cause insulation of some of the insulation layers, resulting in short circuits between the electrodes. The decrease in capacitance may be caused by oil leakage from the casing and some air is inside.
(2) The final screen test. When measuring the insulation resistance, less than 1000MΩ, the final screen to ground tgδ should be measured, and its value is not more than 2%. The final screen dielectric loss measurement is shielded and reversed. The insulation of the final screen reflects the level of insulation of the outer layer, and the outer insulation is damp, which will cause the main insulation to gradually get wet.
(3) Inspection of the sealing performance of the cap and the contact with the conductive rod. When the sealing ring outside the general cap is poorly sealed, the humid air enters the cavity inside the general cap, oxidizing the internal thread connecting the cap and the conductive core rod, resulting in poor contact between the military cap and the conductive core rod, which easily causes the casing to be operated by the military cap. Abnormal fever. Some unreasonable rain cover is in a "suspended potential" due to poor contact with the conductive core fixing pin, and a high-frequency discharge is generated on the porcelain sleeve, causing the main insulation dielectric loss test value to become abnormally large.
During the inspection, pay attention to whether there is patina or oil leakage near the seal ring. Also, use a universal meter to measure whether the resistance of the cap and the conductive rod is zero. If necessary, perform three-phase DC resistance test of the transformer before and after the repair. Is the resistance value and balance factor Excessive.
(4) Check the casing oil level and oil leakage. The oil level is abnormally high, and the main insulation insulation test must be carried out after power failure. If necessary, the casing insulating oil is subjected to oil gas chromatographic analysis to check whether the content of hydrogen, acetylene and total hydrocarbons exceeds the standard; if the casing oil level is abnormally low, then Check the casing for oil leakage, usually at the cap and the end screen. If necessary, take an oil sample for moisture content testing. Also note that a false oil level will occur when the oil standard tube is blocked.
(5) Check the grounding of the final screen. When the final screen is in normal operation, it must be well grounded.
There are probably three ways to ground the casing at the end of the casing:
1. External type: The final screen is connected to the sleeve base through an external copper or copper wire, tightened with screws, and the base is grounded. It is easier to see the grounding condition when the external connection is used. When the insulation test is performed, it is best not to move the end of the screen, and only remove the grounding screw at the end of the base. Pay attention to the strength of the screw tightening to avoid breaking the metal rod at the end screen. After returning to grounding, it is recommended to check the resistance of the final screen and the transformer case with a universal meter. The value should be zero.
2. Internal connection: The final screen is grounded through the grounding cap. The grounding cap is tightened on the sleeve base by the thread. The grounding cap is pressed inside the end screen and the base is grounded. Observe the presence of spark discharge marks inside the grounding cap. Pay attention to the force when unscrewing the grounding cap to avoid breaking the metal rod at the end of the screen; do not use a wrench when tightening, and tighten the grounding protective cap with the hand. The grounding cap should be tightened to avoid moisture and corrosion inside.
3. Push out the normal connection type: the final screen directly presses the outer copper sleeve against the inner wall of the sleeve base through the spring, and the base is grounded. Open the protective cap and check if the outer copper sleeve has spark discharge marks or the copper sleeve is discolored. When the insulation test is restored to the grounding state, check whether the copper sleeve is freely movable, and there is no jam. Use a multimeter to measure the resistance value of the transformer screen (ground) at the end screen. The protective cap should be tightened to avoid moisture at the end screen, which causes the metal parts in the grounding device of the final screen to rust, which causes the contact between the outer copper sleeve and the flange due to the presence of copper rust.
The above is the test and inspection items at the time of power failure. If you need to perform chromatographic analysis of dissolved gases in oil and moisture content testing, you must consult the casing manufacturer. Fourth, professional inspection technology
Professional inspection is a professional technician to carry out targeted inspection and testing of certain items of equipment in operation. Generally equipped with telescope and infrared camera
(1) Check the oil level and oil leakage of the casing. Check carefully with a telescope, and the inspection site is the same as above.
(2) Infrared inspection. Infrared technology is used to detect and diagnose live equipment with current, voltage heating or other heating effects in the power system.
1. Selection of instruments. For professional infrared detection, it is not advisable to use an infrared thermometer (point thermometer) instead of an infrared camera.
2. Selection of test conditions. It is better to use 2 days after sunset on cloudy, cloudy, nighttime or sunny days. It is best at night and should not be tested under thunder, rain, fog and snow meteorological conditions.
3. Instrument settings. The radiance of the device is 0.9, and the color scale temperature range should be set within the ambient temperature plus a temperature rise range of about 10K-20K.
4. Measurement method. First, a full scan of the three-phase bushing is performed. Then focus on the test and analysis of abnormal hot spots and key parts. The key scanning area of the casing is the top wire joint of the three-phase casing, the column head (including the general cap), the porcelain bottle column and the end screen.
5. The result is judged. The bushing is a comprehensive heating type device with both current heat loss and voltage heat loss. Firstly, a comparatively straightforward comparison method is used to compare and analyze the temperature difference of the corresponding parts between the three-phase bushings to find the abnormal part. Then judge according to the following method.
6. The treatment of three types of defects. For general defects, use the motor to be overhauled, plan to arrange for inspection and repair to eliminate defects; should be arranged within 6 months; for serious defects, it should be arranged within 7 days, for the top wire joints, stigma defects, should Take measures to reduce the load current immediately; take measures to eliminate the defects of the porcelain bottle column and the final screen; for critical defects, arrange the treatment immediately (eliminate the defects or take temporary measures to limit its continued development), and should not exceed 24 hours. In general, the voltage difference between the voltage-heated porcelain bottle column and the final screen defect position is 2-3K, which is a serious defect, which is not easy to find. It must be carefully compared when testing. Fifth, online monitoring technology
(1) Improve the system defect handling measures to eliminate the failure recovery system operation as soon as possible. In practical applications, the system often has hardware, software, communication problems, etc. These faults often need to be solved by the manufacturer's technicians, and the reasons are not easy to find and take a long time. It is recommended to improve the countermeasures for defect handling, and continuously improve the ability of system management personnel and on-site inspection personnel to handle abnormal faults and make the monitoring system work normally.
(2) The data of online monitoring differs from the judgment of the traditional preventive test experience data, and the specificity of online monitoring should be comprehensively considered to improve the judgment ability.
1. Comprehensive consideration of test conditions. When the same bushing is powered off, the main insulation dielectric loss value during operation should not be simply compared. Because the operating voltage applied to the device is not single-phase but three-phase voltage during on-line monitoring, and the voltage value is also compared with the power failure pre-test. Very different; there are also the influence of neighbors and spurious interference, and the temperature, humidity, surface contamination, etc. will also change, which is much more complicated than when the power is off.
(3) Pay special attention to the comparison of online three-phase data and online historical data. When there is an abnormality, increase the number of professional inspections, and try to conduct tests and inspections of preventive test items when there is a motor stop. If necessary, immediately stop the power supply for preventive testing.
(4) Strengthen basic research work. At present, most of the online monitoring technology is still at the level of providing only monitoring data, and the relationship between the change of the online monitoring parameters of the casing and the degree of insulation degradation still lacks judgment experience. The historical data of online monitoring data and the casing data of the same model were compared and analyzed, and the relationship between the monitoring parameters and their changes and the insulation aging of the tested casing was studied to find out the law.
In general, during the normal operation of the casing, the above three test techniques should be considered comprehensively, complement each other and complement each other. In the daily casing maintenance work, professional inspections should be strengthened, especially in the critical power supply period, the number of professional inspections must be increased. If the online monitoring system is installed and the stability is good, the preventive test cycle of the casing can be appropriately delayed, and even the test work requiring the disconnection line is considered. However, a comprehensive inspection of the power outage is necessary.