Maintenance, testing and inspection methods for high voltage bushings of power transformers

In recent years, failures of high-voltage bushings of power transformers have occurred frequently. Power companies attach great importance to the operation of bushings and formulate various anti-accident measures to ensure the safe operation of bushings. Based on years of practical work experience on site, the author discusses the field test monitoring technology of bushings.

2. Structural principle of oil-paper capacitor bushing

Most of the high-voltage bushings of power transformers of 110kV and above are oil-paper capacitor bushings, which rely on capacitor cores to improve the electric field distribution. The capacitor cores are composed of multiple layers of insulating paper, with aluminum foil sandwiched between the layers at positions required by the design, forming a string of coaxial cylindrical capacitors, with insulating paper impregnated with mineral oil as insulation.

3. Preventive testing technology

The preventive test of oil-paper capacitor type bushing is to conduct regular power outage test and inspection on the bushing, mainly the main insulation test and the end screen test, as well as the inspection of other parts.

(I) Main insulation test. The main insulation dielectric loss measurement uses the positive connection method. The increase in dielectric loss value is likely caused by the deterioration of the bushing itself or moisture. The abnormal decrease or negative value of dielectric loss value may be caused by poor grounding of the bushing base flange, dirt and moisture on the bushing surface, moisture on the end screen, etc., forming a "T"-shaped network interference, or it may be caused by moisture on the standard capacitor of the dielectric loss meter.

The increase in capacitance may be due to poor sealing of the equipment, water ingress and moisture, or free discharge inside the casing, burning out the insulation of part of the insulation layer, resulting in a short circuit between electrodes. The decrease in capacitance may be caused by oil leakage from the casing, which allows some air to enter the interior.

(II) End screen test. When measuring insulation resistance, if it is less than 1000MΩ, the end screen to ground tgδ should be measured, and its value should not exceed 2%. The end screen dielectric loss measurement uses the shield reverse connection method. The insulation condition of the end screen reflects the insulation level of the outer layer. If the outer layer insulation is damp, the main insulation will gradually be damp.

(III) Check the sealing of the cap and its contact with the conductive rod. When the sealing ring outside the cap is not sealed well, moist air will enter the cavity inside the cap, causing oxidation of the internal thread connecting the cap and the conductive core rod, resulting in poor contact between the cap and the conductive core rod, which can easily cause abnormal heating during the operation of the cap. Some improperly designed rain covers are in a "floating potential" due to poor contact with the conductive core fixing pin, which generates high-frequency discharge to the porcelain sleeve, causing the main insulation dielectric loss test value to become abnormally large.

When checking, pay attention to whether there is verdigris rust or oil leakage near the sealing ring; in addition, use a multimeter to measure whether the resistance between the general cap and the conductive rod is zero; if necessary, perform a three-phase DC resistance test on the transformer before and after maintenance to see if the resistance value and balance coefficient exceed the standard.

(IV) Check the oil level and oil leakage of the casing. If the oil level becomes abnormally high, the power must be shut down to perform the main insulation test. If necessary, the dissolved gas chromatography analysis of the casing insulation oil should be carried out to check whether the content of hydrogen, acetylene and total hydrocarbons exceeds the standard. If the oil level of the casing becomes abnormally low, check whether the casing has oil leakage, generally at the general cap and the end screen. If necessary, take oil samples for moisture content test. In addition, please note that false oil level will appear when the oil gauge tube is blocked.

(V) Check the grounding condition of the terminal screen. When the terminal screen is operating normally, it must be well grounded.

There are three ways to ground the end screen of the bushing:

1. External connection: The end screen is connected to the bushing base through an external copper sheet or copper wire, tightened with screws, and the base is grounded. The external connection makes it easier to see the grounding situation. During the insulation test, it is best not to move the end screen end, and only remove the grounding screw at the base end. Pay attention to controlling the force of tightening the screw to avoid breaking the metal rod of the end screen. After restoring the grounding, it is recommended to use a multimeter to check the resistance between the end screen and the transformer casing, and the value should be zero.

2. Internal connection: The end screen is grounded through the grounding cap, which is screwed onto the base of the casing. The inside of the grounding cap presses the end screen tightly, and the base is grounded. Pay attention to whether there are spark discharge marks inside the grounding cap. Pay attention to the strength when unscrewing the grounding cap to avoid breaking the metal rod of the end screen; do not use a wrench when tightening, but tighten the grounding protective cap by hand. The grounding cap should be tightened to avoid moisture, oxidation and corrosion inside.

3. Push-pull normal connection type: The end screen directly presses the outer copper sleeve against the inner wall of the bushing base through the spring, and the base is grounded. Open the protective cap to check whether there are spark discharge marks on the outer copper sleeve or discoloration of the copper sleeve. When the insulation test is restored to the grounding state, check whether the copper sleeve is free to move and cannot be stuck. Use a multimeter to measure the resistance value of the end screen to the transformer casing (ground). If abnormal, it should be handled. The protective cap should be tightened to prevent moisture from getting on the end screen, causing rust on the metal parts in the end screen grounding device, and then causing the contact surface between the outer copper sleeve and the flange to have poor grounding of the end screen due to the presence of copper rust.

The above are the test and inspection items during power outage. If it is necessary to conduct oil dissolved gas chromatography analysis and water content test, the casing manufacturer must be consulted.

Professional inspection is a targeted inspection and test of certain items of running equipment by professional technicians. It is usually equipped with a telescope and an infrared thermal imager.

(I) Check the oil level and oil leakage of the casing. Use a telescope to carefully check the same parts as above.

(ii) Infrared inspection: Use infrared technology to detect and diagnose live equipment in the power system that has current, voltage or other heating effects.

1. Instrument selection. When performing professional infrared testing, it is not appropriate to use an infrared thermometer (spot thermometer), but an infrared thermal imager.

2. Selection of test conditions: It is best to test on cloudy days, at night, or 2 hours after sunset on a sunny day. Night is best. Testing should not be carried out under thunder, rain, fog, or snow.

3. Instrument settings. The emissivity of the equipment is 0.9, and the color scale temperature range should be set within the temperature rise range of about 10K-20K plus the ambient temperature.

4. Measurement method. First, conduct a comprehensive scan of the three-phase bushing. Then conduct key test and analysis on abnormal heating points and key parts. The key scanning parts of the bushing are the top wire joint, column head (including the general cap), porcelain bottle column and end screen of the three-phase bushing.

5. Result judgment. The bushing is a comprehensive heating device, which has both current-induced heat loss and voltage-induced heat loss. First, use the more intuitive similar comparison judgment method to compare and analyze the temperature difference of the corresponding parts between the three-phase bushings to find the abnormal parts. Then judge according to the following method.

6. Treatment methods for three types of defects. For general defects, use the power outage opportunity for maintenance, and arrange test maintenance in a planned manner to eliminate defects; treatment should be arranged within 6 months; for serious defects, treatment should be arranged within 7 days, and for defects at the top wire joints and column heads, measures should be taken immediately to reduce the load current; for defects in porcelain bottle columns and end screens, measures should be taken immediately to eliminate the defects; for critical defects, treatment should be arranged immediately (eliminate the defects or take temporary measures to limit their continued development), and it should not exceed 24 hours. Generally speaking, voltage-induced heating type porcelain bottle columns and end screen defects have a temperature difference of 2-3K, which is a serious defect and is not easy to find. During testing, you must be particularly careful to compare to find it. 5. Online monitoring technology

(I) Improve the system defect handling measures to eliminate faults and restore system operation as soon as possible. In actual applications, the system often has hardware, software, communication problems, etc. These faults often require the manufacturer's technicians to solve, and the causes are not easy to find and take a long time. It is recommended to improve the defect handling measures and continuously improve the abnormal fault handling and response capabilities of system managers and on-site inspectors to ensure the normal operation of the monitoring system.

(II) The judgment of insulation defects based on online monitoring data is different from that based on traditional preventive test experience data. The particularity of online monitoring should be comprehensively considered to improve the judgment ability.

1. Comprehensive consideration of test conditions. The main insulation dielectric loss values ​​of the same bushing during power outage and operation should not be simply compared, because during online monitoring, the operating voltage applied to the equipment is not single-phase but three-phase voltage, and the voltage value is also very different from that during power outage pre-test; in addition, there are the influence of adjacent phases and stray interference, and the temperature, humidity, surface contamination and other conditions will also change, which are much more complicated than during power outage.

(III) 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 strive to conduct tests and inspections of preventive test items when there is a power outage opportunity. If necessary, immediately shut down the power to conduct preventive tests.

(IV) Strengthen basic research. At present, most online monitoring technologies are still at the level of providing only monitoring data, and there is still a lack of experience in judging the relationship between the changes in the online monitoring parameters of the bushing and the degree of insulation degradation. Compare and analyze the historical data of online monitoring data and the data of bushings of the same model, study the relationship between the monitoring parameters and their changes and the insulation aging of the measured bushings, and find out the rules.

In general, during the normal operation of the bushing, the above three test technologies should be implemented comprehensively, taking advantage of each other's strengths and weaknesses. In the daily bushing maintenance work, professional inspections should be strengthened, especially in the critical power supply period, the number of professional inspections must be increased. If an online monitoring system has been installed and has good stability, the preventive test cycle of the bushing can be appropriately delayed, and even the test work that needs to be connected and removed can be considered to be reduced, but a comprehensive inspection during power outage is necessary.