The common faults and diagnosis technology of power transformer are discussed
Abstract: This paper introduces the common defects and faults of power transformers, analyzes the harm of these faults to transformers, and summarizes the methods of eliminating faults. In addition, it also analyzes the online monitoring technology commonly used in transformers, which has certain engineering practical value.
2 Common faults and diagnostic measures
2.1 Transformer oil seepage
Transformer oil leakage will not only bring greater economic losses and environmental pollution to power enterprises, but also affect the safe operation of transformers, which may cause unnecessary outage and even transformer damage accidents, and bring production losses and life inconvenience to power customers. Therefore, it is necessary to solve the problem of leakage of transformer oil.
Oil seepage from tank welds. The oil seepage at the plane joint can be welded directly, and the oil seepage at the corner and reinforcement joint is often not allowed to find the leakage point, or the leakage again due to internal stress after welding. For such penetration points, an iron plate can be added for repair welding, and the iron plate can be cut into a spindle shape for repair welding at the joint of both sides; The joints on the three sides can be cut into triangles according to the actual position for repair welding; The method is also suitable for leakage welding of corner weld of secondary lead box of bushing current transformer.
Oil seepage of high pressure casing elevating seat or manhole flange. These parts are mainly due to the improper installation of rubber pads, and the flange can be sized and sealed during operation. Before plugging, use plugging glue to plug the gaps between flanges. When the plugging glue is fully cured, exit one flange fastening screw, screw the glue gun nozzle into the screw hole, and then inject the sealant into the flange gap with high pressure until the glue is extruded from the screw caps of each flange.
Low pressure side casing leakage. The reason is that the busbar is stretched and the lead on the low pressure side is short, and the rubber bead is pressed on the thread. When stretched by the busbar, the busbar can be connected with expansion joints according to the regulations; If the lead is too short, the lead length can be re-adjusted; If it is difficult to adjust the lead wire, sealant can be added to each sealing surface of the rubber bead. In order to increase the compression force, the porcelain pressure cap can be replaced by a copper pressure cap.
Because the clamp limb plate is too close to the core, so that the warped lamination collide with it, the clamp limb plate and the straightened warped lamination should be adjusted to make the distance between them meet the insulation gap standard.
Remove metal foreign bodies, metal particles and impurities in the oil, remove the sludge in all parts of the tank, and vacuum dry the transformer oil if possible to remove water.
2.3 Connector Overheating
The current-carrying joint is an important part of the transformer itself and its connection to the power grid. If the joint is not well connected, it will cause heat and even burn off, which seriously affects the normal operation of the transformer and the safe power supply of the power grid. Therefore, the overheating problem of the joint must be solved in time.
Copper-aluminum connection. The leading end of the transformer is made of copper, and the aluminum conductor cannot be bolted to the copper end outside the house and in wet places. When the water containing dissolved salt is infiltrated between the contact surface of copper and aluminum, that is, the electrolyte, under the action of electric coupling, electrolytic reaction will occur, and aluminum is strongly electrocorroded. As a result, the contacts are quickly damaged, leading to heat and possibly even major accidents. In order to prevent this phenomenon, when it is necessary to connect the aluminum conductor to the copper conductor in the above device, a special transition contact with one end of aluminum and the other end of copper is used.
Normal connection. Ordinary connection on the transformer is quite a lot, they are the key parts of overheating, the plane joint, the butt surface is processed into a plane, remove the impurities on the plane, it is best to evenly coated with conductive paste to ensure good connection.
Oil immersed capacitive bushing overheats. The treatment method can be used to fix the heating sleeve in the way of positioning sleeve, first disassemble the military cap, if the military cap and the lead connector are burned, the application of dental tap repair to ensure that the wire buckle is well matched, and then put a thin gasket with the same section size and appropriate thickness between the positioning sleeve and the military cap, and reinstall the military cap, so that the military cap can be tightened. It can be fixed to the top flange of the casing.
The lead joint and the military cap wire should be in good alignment with the tolerance, otherwise it should be replaced to ensure that there is sufficient pressure between the wires to reduce the contact resistance when tightened.
3 Transformer online monitoring technology
The purpose of transformer online monitoring is to identify the status of the transformer through the collection and analysis of the characteristic signal of the transformer, in order to detect the initial failure of the transformer, and monitor the development trend of the fault state. At present, the on-line monitoring of power transformers is one of the most studied objects in the world, and many different methods have been proposed.
Analytical technique for dissolved gases in oil. Because different faults in the transformer will produce different gases, the purpose of transformer insulation diagnosis can be achieved by analyzing the composition, content, gas production rate and relative percentage of the gas in the oil. Several typical dissolved gases in oils, such as H2, CO, CH4, C2H6, C2H4, and C2H2, are often used as characteristic gases for analysis. After detecting the composition and content of each gas, the characteristic gas method or ratio method is used to judge the internal fault of the transformer.
Partial discharge on-line monitoring technology. Partial discharge (PD) will be generated due to high local field strength when the transformer has internal failure or bad operating conditions. The obvious change in PD level and its growth rate can indicate the changes taking place inside the transformer or reflect the voids, metal particles and bubbles of solid insulation in the insulation due to certain defect states.
Vibration analysis. Vibration analysis is an effective method widely used to monitor such transformer faults. Through the monitoring and analysis of the transformer vibration signal, the purpose of monitoring the transformer state is achieved.
Infrared temperature measurement technology. Infrared thermal image technology is the use of infrared detector to receive the infrared radiation signal of the measured target, amplified processing, converted into standard video signal, and then through the TV screen or monitor to display infrared thermal image. When the transformer lead contact is poor, overload operation, etc., it will cause local overheating of the conductive circuit, and multi-point grounding of the iron core will also cause overheating of the iron core.
Frequency response analysis. Frequency response analysis is an effective method for judging whether the winding or lead structure of transformer is offset. The mechanical displacement of the winding will produce a subtle change of inductance or capacitance, and the frequency response method is just to measure this subtle change to monitor the state of the transformer winding.
Winding temperature indication. The winding temperature indicator is used to monitor the temperature of the transformer winding, give an out-of-limit alarm, and start a protective trip when necessary. At present, a new technology for temperature monitoring of large transformer winding has been developed, that is, an optical fiber is embedded in the transformer winding to directly measure the real-time temperature of the winding, so as to improve the predictive modeling technology of the transformer and achieve the purpose of real-time monitoring of the temperature state of the transformer winding.
Other condition monitoring methods. The Low Voltage Impulse Response (LVIR) test is also an effective method for monitoring the condition of a transformer and has become an accepted method for determining whether a transformer can pass a short circuit test. In addition, the leakage sensing test between windings, the relative humidity test of oil, and the insulation resistance test are also common methods for transformer condition monitoring.
Conclusion
In the 21st century, the power industry will have greater development, and the fault diagnosis and maintenance of power transformers as a powerful measure to achieve the system transformation of China’s power system and improve the scientific management level of power equipment, is the future direction of efforts and development in power production.