The status of the transformer is based on the test data obtained from the test. After the test data is obtained, the correctness of the test results must first be determined, and the principle errors of the test method and the influence of the environment and human factors should be excluded; then the test results should be compared with the procedures and standards, compared with the historical data, and other Comparing with similar products, the test data of multiple test methods are comprehensively used for joint analysis; finally, the status of the transformer is judged according to the analysis.
I. Transformer fault types and analysis
Insulation structure
35kV and above voltage level transformers are generally oil-paper insulated structures. The main insulation structure of the 220kV voltage transformer is a capacitive structure, that is, a capacitive core composed of a series of capacitive screens. The capacitive screen is made of aluminum foil or semiconductor paper. The cable paper is continuously wound to form the inter-panel insulation. The end of the main screen of the capacitive screen is attached with a secondary screen to improve the electric field at the end. The capacitor core is vacuum-dried and then assembled in a porcelain sleeve.
2. Cause of failure
2.1 insulation thermal breakdown
High-voltage current transformers not only withstand high voltage, but also pass large currents. The dielectric loss under the action of high voltage and the thermal effect of current in the insulating medium increase the insulation temperature. If there is a defect, there will be an increase in heat loss, an increase in the temperature of the insulation, and a long-term operation above the operating temperature of the insulating material will cause insulation thermal breakdown.
2.2 partial discharge damage
220kV current transformer main capacitor voltage distribution in the normal state, if the process is not qualified in the manufacturing process, it will cause the capacitor plate is not smooth, insulation wrapped around uneven tightness, external tight within the loose, paper wrinkles, capacitance The screen is misplaced and broken, and the insulation and other defects are caused when the “legs†are clamped; the insulation of the lower U-shaped clips is too tightly clamped, and the air bubbles accumulate between the capacitive screens due to the fact that the end-panel aluminum foil has no holes and is not filled with vacuum. In this way, the voltage distribution between the capacitive screens is changed, so that individual capacitive screens can withstand high field strengths, severe corona discharges or strong partial discharges, and if not found or handled in time, it will lead to insulation cracking breakdown of the entire capacitor core rods. ACCIDENT.
2.3 Damp
Due to the non-strict sealing of the inlet and the moisture in the water, the free discharge inside the transformer is increased and the internal creeping discharge is one of the main causes of the insulation deterioration of the current transformer. The bottom of the U-capacity mandrel of the current transformer is close to the bottom of the tank. The water that enters the transformer is deposited on the bottom of the capacitor mandrel. The insulation of the mandrel is severely damped and is the weakest part of the insulation. It is strong in the working field. Under the long-term effect, one or several pairs of main capacitance screens are broken down, and even the entire capacitor core rod breaks down, thereby causing an explosion accident.
2.4 Insulation drying and degassing are not complete
If 220kV current transformer is not filled with vacuum, the internal gas cannot be discharged, or although vacuum injection is performed, high vacuum cannot be maintained, degassing processing time is not enough, and drying is not complete. Under the action of operating voltage and temperature, Thermal and/or electrical aging breakdowns occur.
2.5 Staff Negligence
The common faults include a loose lead connector, a poor oil injection process, an open secondary winding, and a bad grounding capacitor. Because these errors often lead to local overheating or discharge, the chromatographic analysis results of dissolved gases in oil are abnormal.
Second, transformer fault diagnosis method
1. Conduct preventive tests
DL/T 596-1996 “Preventive Test Regulations for Electrical Equipment†provides preventive test items for current transformers: Insulation resistance of measuring windings and screens, tanδ and capacitance of measured dielectric loss, and chromatographic analysis of dissolved gases in oil. Such as, through the comprehensive analysis of the test results of these projects, it can be found that the incoming water is wet and the manufacturing process is poor.
2. Partial discharge measurement
The conventional insulation test cannot detect the partial discharge type defect of the current transformer, and the partial discharge measurement can sensitively detect this type of defect. The "Regulations" stipulate that the current transformers perform partial discharge measurements after overhaul. 220kV oil-immersed transformer at a voltage of 1.1Um/√3, the discharge capacity is not greater than 20pC.
3. Online monitoring and infrared temperature measurement
The on-line monitoring projects carried out by high-voltage current transformers mainly include measuring the dielectric loss factor tan δ, capacitance and capacitance current of the main insulation. Field tests show that it is very effective in detecting insulation defects.
Infrared temperature measurement is based on the internal structure and operating state of the current transformer, and based on the theory of heat transfer, the conduction and convection caused by the metal conductive circuit, insulating oil and gas, etc. are analyzed, and the temperature distribution thermal image appearing from the outside of the current transformer is analyzed. Determine internal faults. Test results show that it is effective for detecting looseness of the current transformer's internal joints.
4. Visual inspection method
4.1 Unusual "click" caused by open circuit of secondary circuit
The current transformer of a certain phase in operation emits a loud “buzz†sound. There is no indication in the ammeter of the loop, and there is no indication or indication of power meter, power meter, etc. being too small. The cause of this anomaly is the open circuit on the secondary side (open circuit caused by poor contact of the secondary circuit terminal and accompanied by spark discharge. The sound of the current transformer with no open circuit is normal and the relevant instrument indicates correct).
When the secondary circuit is opened, an overvoltage will be generated at its port. In severe cases, it can cause insulation breakdown, resulting in grounding or burning of the secondary port. Overvoltage can also cause great harm to the human body. Therefore, the relevant regulations stipulate that the secondary side of the current transformer is strictly prohibited open circuit, resulting in the following three main reasons for the above failure: First, the secondary side connection screw loose or crimping is not tight; Second, there is a broken line in the secondary circuit; It is the open circuit of the secondary circuit caused by the caretaker’s careless work.
The method of processing is to check the secondary circuit wiring after the equipment is powered off, find the open point and process it.
4.2 Unusual "click" sound caused by iron loosening
The current transformer emits a large and even “click†sound, but the connected instrument indicates normal. This is caused by the loosening of the iron core screw and the increase in the vibration amplitude of the iron core, and the size of the sound changes with the load and the load is increased. The louder the sound is. Under long-term operation in this case, the current transformer will be severely heated, causing insulation aging, resulting in grounding, insulation breakdown and other consequences.
In this regard, in addition to strengthening surveillance, it should also apply for blackouts. Take a thorough check when handling, find loose screws and tighten them.
4.3 Abnormal sound caused by surface dirt or dust
The surface of the current transformer has dirt or dust. In the rainy and cloudy days, it will cause a "panic" discharge sound, and there may be corona. If the discharge is severe, surveillance should be strengthened and blackouts must be applied.
When there is a severe discharge sound inside the current transformer, the main reason is that the internal insulation is reduced, causing a secondary winding or core discharge. When this situation is discovered, it should be immediately powered off.
Third, the diagnosis of
1. Hydrogen and total hydrocarbon content exceeded
A U-phase current transformer (model: LB9220W) of a power station 220kV booster station is in operation. The gas expander is in operation. The content of hydrogen and total hydrocarbons in the oil is higher than the standard by gas dissolved gas analysis. The data of hydrogen and total hydrocarbons increase rapidly on the next day. , and then exit the device.
The test data shows that the content of methane is relatively high, hydrogen and total hydrocarbons are excessively exceeded, acetylene is not present, and the amount of micro-water is significantly increased. The reason for this is that partial discharge within the transformer causes the breakdown of insulating oil.
From the electrical test results, the test voltage is from 10kV to Um / 3, and the dielectric loss factor tan δ change exceeds ± 0.3%, indicating that the current transformer is seriously damp.
After investigation, the current transformer is a spare part. Due to the unreasonable end seal structure, the aging of the gasket loses its elasticity. Not only is the phenomenon of serious oil leakage, but also the sealing is not strict, resulting in moisture intrusion, and the transformer is seriously affected by moisture. Before putting into operation, during the process of drying and oil-filling the transformer, no vacuum treatment was performed. This not only had bad drying effect, but also accumulated a large amount of gas in the interlaminar and oil, and the insulation was not obtained after maintenance. After a fundamental change, partial discharges will occur after being put into operation, and the insulation will continue to deteriorate. Eventually, it will develop into a discharge failure. Due to the trace analysis and treatment of dissolved gas chromatograms in oil, an accident is avoided.
2. Abnormal infrared temperature measurement
When a substation was measuring temperature on the 220kV side current transformer of the No. 1 main transformer, the three-phase overall temperature of the equipment was found to be different. The three-phase temperatures were U-phase 27.4°C, V-phase 27.4°C, and W-phase 29.2°C. At the same position on the surface), when the 6th day was continuously monitored, the three-phase temperature difference had reached 2.9°C.
After the power failure of the equipment, the three-phase equipment was electrically and chromatographically tested. From the results of the electrical test, the insulation medium in the W-phase current transformer was deteriorating or aged, and the dielectric loss increased. The chromatographic analysis results of dissolved gases in the oil were observed. The content of methane and ethylene is relatively high. Hydrogen and total hydrocarbons are severely exceeded and acetylene occurs. The analysis is caused by the overheating of bare metal inside the W-phase current transformer and the insulation oil cracking.
When the manufacturer dissects the W-phase current transformer, it is found that there is a clear trace of electric arc burn at the lead joint of the primary winding. It is considered that this burn injury trace is caused by the lightning overvoltage invading the inside of the transformer.
Fourth, transformer fault prevention measures
1. First terminal lead connector to contact well
The terminals of the primary terminals of current transformers must be well-contacted and have sufficient contact area to prevent overheating faults. L2 terminal and expander cover should pay attention to make equipotential connection to prevent potential suspension. In addition, the secondary wire lead-out terminal should have anti-rotation measures to prevent external operation caused by the internal lead wire twisted.
2. Abnormal test values ​​should be identified
When the tanδ value of the dielectric dissipation factor measured before and during the operation is abnormal, the relationship between tan δ and temperature and voltage shall be comprehensively analyzed; when the accompanying temperature changes significantly or the test voltage rises from 10 kV to Um/√3, the tan δ variation exceeds When ±0.3%, it should be out of operation. When the chromatographic analysis result of dissolved gas in oil is abnormal, follow-up analysis should be conducted to examine its growth trend. If the data grows faster, attention should be paid to it. The test shows that the faults such as the discharge of the current transformers by the chromatography can be found completely and the accident can be eliminated in the bud.
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Main technology parameter
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specification
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2800 size
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Max size(length+width)*2
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2800mm
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Min size(length+width)*2
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340mm
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Max size(width+height)*2
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1515mm
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Min size(width+height)*2
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260mm
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Paper height
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1000mm
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Engine power
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4.0kw
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Machine length
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5300mm
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Machine weight
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2500kg
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Machinery speed
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60m/min
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