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Zhejiang ERG Technology Joint Stock Co.,Ltd

Mobile: 86-18257346462

Tel: 86-576-83200088

E-mail: nick@erg.cn   bella@erg.cn   blancachen@erg.cn

Contacts: Wu Nick/Bella Zheng/Blanca Chen

Add: 135 Haisha Road, Shaliu Economic DevelopmentZone, Sanmen County, Zhejiang, P.R.China

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Power Transformer High-voltage Bushing Maintenance, Test And Detection Method-ERG
- May 11, 2018 -


Power transformer high-voltage bushing maintenance, test and detection method-ERG


This paper mainly discusses the daily maintenance, testing and inspection methods of oil-paper capacitive casing, and introduces the power cut test, inspection contents and attention problems from three aspects: preventive testing technology, professional inspection technology and online monitoring technology. Key areas and projects, as well as online monitoring technology applications and recommendations.

I. Introduction


In recent years, failures of high-voltage bushings of power transformers have occurred from time to time. The power unit has attached great importance to the operation of the bushings, and formulated counter-accident measures to ensure the safe operation of the bushings. The author combines the actual field experience over the years to talk about casing field test monitoring technology.


Second, the structure of oil-paper capacitor sleeve


Most high-voltage bushings for power transformers of 110kV and above are oil-paper capacitor bushings. They rely on a capacitor core to improve the electric field distribution. The capacitor core consists of multiple layers of insulating paper. Aluminum foil is sandwiched between the layers according to the design requirements. A series of coaxial cylindrical capacitors insulated with insulation paper impregnated with mineral oil.


Third, preventive testing technology


The preventive test of the oil-paper-capacity type bushing is the periodic power-off test and inspection of the bushing, mainly the main insulation test and the end-screen test, as well as other parts of the inspection.


(a) The main insulation test. The main insulation dielectric loss measurement using positive connection. Dielectric loss increases, most likely due to deterioration of the casing itself and moisture. The abnormally small dielectric loss value or negative value may be caused by the “T” shaped network interference caused by poor grounding of the flange of the bushing base, dirt on the surface of the bushing, moisture on the surface of the bushing, etc., and it may be that the standard capacitor of the dielectric loss meter is damp. Caused.


The increase in capacity may be due to poor sealing of the equipment, moisture in the incoming water, or internal free discharge of the bushing, which may burn insulation of part of the insulation layer, resulting in a short circuit between the electrodes. The decrease in the electric capacity may be caused by the oil leakage from the casing, and the internal part of the air enters.


(B) the final screen test. When measuring the insulation resistance, less than 1000MΩ, the tgδ of the end screen on the ground shall be measured, and its value shall not exceed 2%. The screen dielectric loss measurement using shielded reverse connection method. The insulation of the final screen reflects the insulation level of the outer layer, and the outer layer of the insulation is damp, which will cause the main insulation to gradually damp.


(c) Inspection of the seal of the general cap and its contact with the conductive rod. When the outside seal of the general cap is not well sealed, the humid air enters the cavity of the general cap, which causes oxidation of the internal thread connecting the general cap and the conductive core rod, resulting in poor contact between the general cap and the conductive core rod, which easily leads to running of the sleeve general cap. Abnormal fever. Some rain hoods with unreasonable designs are at “suspension potential” due to poor contact with the conductive core fixing pins, causing high-frequency discharges to the porcelain sleeves, causing the measured value of the main insulation dielectric loss to become abnormally large.


During the inspection, pay attention to whether there is patina rust or oil leakage near the sealing ring; in addition, use a universal meter to measure whether the resistance of the general cap and the conductive rod is zero; if necessary, perform a transformer three-phase DC resistance test before and after repair, and check whether the resistance value and balance coefficient are correct. Excessive standards.


(d) Check casing oil level and oil leakage. If the oil level becomes abnormally high, the main insulation insulation test must be conducted to cut the power. If necessary, the gas chromatography of the casing insulating oil should be analyzed to check whether the contents of hydrogen, acetylene and total hydrocarbons are excessive. If the oil level of the casing becomes abnormally low, then Check the casing for oil leakage, which is generally at the general cap and the final screen. Take oil samples when necessary for moisture content testing. Also note that a false oil level may occur when the oil standard tube is clogged.


(e) Check the grounding of the screen. When the screen is operating normally, grounding must be ensured.


There are about three types of grounding methods for the casing:


1. External type: The last screen is connected to the bushing base through an external copper or copper wire, tightened with a screw, and the base is grounded. It is easy to see the grounding situation in the external type. In the insulation test, it is better not to move the end of the screen, just disassemble the grounding screw at the end of the base. Pay attention to control the strength of the screw, to avoid breaking the screen metal rod. After returning to ground, it is recommended to use a universal meter to check the resistance of the final screen and the transformer case. The value should be zero.


2. Integral: The screen is grounded through the grounding cap. The grounding cap is tightened on the sleeve base through the thread. The grounding cap presses the end screen and the base is grounded. Observe that there are traces of spark discharge inside the grounding cap. When unscrewing the grounding cap, pay attention to the strength and avoid breaking the metal bars at the end of the screen. When tightening, do not use wrenches, and use the hand to tighten the grounding protection cap. The grounding cap should be tightened to avoid moisture oxidation corrosion inside.


3. Push and pull the regular connection type: the end 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 to check whether there is trace of spark discharge or discoloration in the copper sheath. Insulation test to restore the grounding state should check whether the copper sleeve is free, can not jam, and use a multimeter to measure the resistance value of the terminal screen on the transformer shell (ground), such as abnormal should be handled. The protective cap should be tightened to avoid moisture at the end of the screen, resulting in corrosion of the metal components in the final screen grounding device, which in turn causes the end of the screen to be grounded due to the presence of copper rust.


The above are the tests and inspection items during the power outage. If you need to perform gas dissolved gas chromatographic analysis, moisture content test, you must seek the advice of the casing manufacturer. Fourth, professional inspection technology


Professional patrols are professional technicians who carry out targeted inspections and tests on certain items of equipment in operation. General equipped with telescope and infrared camera


(a) Check the oil level and leakage of the casing. Use a telescope to perform a careful inspection. The inspection site is the same as above.


(b) Infrared inspection. Infrared technology is used to detect and diagnose charged devices with current, voltage heating, or other heating effects in the power system.


1. The choice of instrument. Professional infrared detection, infrared thermometers (point thermometers) should not be used, and infrared thermal imager.


2. Selection of test conditions. It is advisable to take a cloudy day, a cloudy night, or a sunny day after sunset for 2 hours. It is advisable to perform the test at night, and it should not be detected under the meteorological conditions of thunder, rain, fog and snow.


3. Instrument settings. The emissivity of the equipment is taken as 0.9. The temperature range of the color mark should be set within the range of ambient temperature plus 10K-20K.


4. Measurement method. First a full scan of the three-phase bushings. Then focus on the abnormal hot spots, key parts of the test analysis. The key scanning positions of the bushing are the top wire joints of the three-phase bushings, the column heads (including the general caps), the porcelain bottle columns, and the end screens.


5. The result is judged. Casings are comprehensively heat-generating devices that have both current-induced heat loss and voltage-induced heat loss. First, compare and analyze the temperature differences of the corresponding parts of the three-phase bushings to find out the abnormal parts with a comparatively intuitive method of comparison and judgment. Then judge according to the following method.


6. The treatment of three types of defects. For the general defects, the use of stopping the motor will be overhauled, and the planned repairs will be arranged to eliminate the defects; the treatment shall be arranged within 6 months; for severe defects, the treatment shall be arranged within 7 days, and the top wire joints and column head defects shall be Immediately take measures to reduce the load current; for porcelain bottle columns and screen defects, they shall immediately take measures to eliminate them; for critical defects, they shall be immediately disposed of (eliminate defects or take temporary measures to limit their continued development), and shall not exceed 24 hours. In general, a temperature difference of 2-3K between the voltage-heated porcelain bottle column and the endplate defects is a serious defect. It is not easy to find, and it must be carefully and carefully compared before testing. V. Online monitoring technology


(I) Improve the system defect management response measures to eliminate the fault recovery system operation as soon as possible. In practical applications, hardware, software, communication problems, etc. often occur in the system. These failures often require the technical personnel of the manufacturer to solve the problems. It is not easy to find the reasons and it takes a long time. It is recommended to improve the response measures for defect management and continuously improve the ability of the system administrators and on-site inspection personnel to deal with abnormal fault handling so that the monitoring system can work normally.


(2) The data of online monitoring The difference between the judgment of insulation defects and the empirical data of traditional preventive tests is different, and the specificity of on-line monitoring should be comprehensively considered to improve the judgment ability.


1. Comprehensive consideration of test conditions. The main insulation dielectric loss value of the same bushing power failure and runtime is not a simple equivalent comparison, because on-line monitoring, the operating voltage applied to the equipment is not a single-phase but three-phase voltage, and the voltage value is also pre-tested with the power failure. It is very different; in addition there is the influence of the neighboring phase and stray interference, and the conditions such as temperature, humidity and surface contamination will also change, which are more complex than the power outage.


(3) Pay special attention to the comparison of on-line three-phase data and on-line historical data. If there is any abnormality, increase the number of professional inspections and strive to have the test and inspection of preventive test items stopped when the motor is stopped. When necessary, power outage immediately for preventive testing.


(d) Strengthening basic research work. At present, most online monitoring technologies still remain at the level of providing only monitoring data. The relationship between changes in the on-line monitoring parameters of bushings and the degree of insulation degradation still lacks judgment experience. Compare and analyze the historical data of online monitoring data and casing data of the same type, and study the relationship between the monitoring parameters and their changes and the insulation age of the tested casing, and find out the rules.


In general, during the normal operation of the casing, the above three test technologies should be comprehensively considered and implemented to complement each other and complement each other. In routine casing maintenance work, professional patrol inspections should be strengthened. In particular, during the critical period of power supply maintenance, the number of professional inspections must be increased. If an on-line monitoring system has been installed and the stability is good, the preventive testing period of the casing can be appropriately delayed, and even considering the reduction of the test work required to remove the disconnection, a complete inspection of the power outage is necessary.


Thank you for your kindly attention. Hope everything goes well with you.

This is Blanca Chen, from Zhejiang ERG Technology Joint Stock Co.,Ltd. We manufacturer and design transformers accessories. 

 

Products: (Sorry that for the e-mail not to be sent back, our catalogue and web are not attached for our first contact.)
1.
All kinds of transformer cooler
2.
Oil pump
3.
Butterfly valve
4.
Oil flow relay 
5.
Control cabinet

6. Terminal box etc.
ERG join the draft of the standard for chinese government in transformers accessories, with certifications:CE, CRCC,IRIS,EUP,TS,CNAS,BV etc. 

 

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Whatsapp/Wechat/Tel: 86 13626615457

E-mail: blancachen@aliyun.com

 

Best Regards,

Blanca Chen