In the process of energy transmission and distribution,power transformers are the core of energy conversion and transmission,and are the most important and critical equipment in the power grid.The safe operation of power equipment is the first defense system to avoid major wind accidents,and power transformers are the most critical equipment in this defense system.Serious accidents of transformers not only cause damage to themselves,but also interrupt power supply,causing huge economic losses to society.
1.Common causes,inspection methods,and troubleshooting methods for power system transformers
1.1 Insulation Failure
The main factors affecting transformer insulation faults include temperature,humidity,oil protection methods,and overvoltage effects.
The discrimination method for insulation aging of transformers is as follows:
(1)Hanging core inspection:Lift out the transformer winding,observe the color of the winding,and judge the insulation strength based on the elasticity,density,mechanical strength,and damage of its insulation layer.
(2)Testing insulation resistance:The purpose of testing insulation resistance is to grasp the trend of insulation aging.Due to the high sensitivity of insulation resistance to moisture absorption,its value can serve as a rough standard for understanding the degree of insulation aging.
(3)Absorption current test:Apply DC high voltage to the transformer,measure the relationship between current and time,and then compare it with the original value at the beginning of operation to determine the quality status of the insulation.The absorption current will increase with the continuous aging of the insulation.
1.1.2 Measures to prevent insulation aging
(1)Prevent transformer overload and overcurrent.(2)Prevent sudden short circuits at the transformer outlet.
1.2 Transformer discharge fault
The discharge faults of transformers are often divided into three types:partial discharge,spark discharge,and high-energy discharge.Effective control and elimination of partial discharge cannot be achieved.Firstly,the insulation structure should be determined based on the electric field strength in design and manufacturing.It is not entirely reasonable to determine the insulation structure solely based on the test voltage.With the continuous development of electrical strength testing research on insulation structures and the application of electric field numerical calculation methods and computers,a basis has been provided for the rational selection and determination of transformer insulation structures.Secondly,strict control should be exercised over materials.Among the factors affecting partial discharge,bubbles and acrobatics in suspended potential metal bodies and insulating materials are quite common,which requires strict process requirements in the material manufacturing process.For purchased parts,it is very important to ensure stable performance.Thirdly,in the structural design and manufacturing process of transformers,it is extremely important to avoid and eliminate protruding metal electrodes as much as possible.Shielding measures should be taken at the iron core columns,tap changer nuts,and lead welding points.Sharp corners and burrs formed during the manufacturing process should be polished to improve the electric field and make it as evenly distributed as possible.
1.3 Multi point grounding of iron core
The transformer core can only have one grounding point.If there are two or more grounding points,it is considered as multi-point grounding.The operation of the transformer core with multiple grounding points will cause core faults,endangering safe operation,and should be dealt with in a timely manner.The reason for multiple grounding points in the iron core:The fault of multiple grounding points in the iron core of transformers ranks third in the total number of transformer accidents.The main reason is that the transformer was accidentally assembled and constructed on site,leaving metal foreign objects,causing multiple grounding points or short circuits between the iron yoke and the clamp,as well as collisions between the core column and the clamp.
1.4 Overheated pick-up and drop off
The current carrying joint is an important component of the transformer itself and its connection to the power grid.Poor connection of the joint can cause heating or even burning,seriously affecting the normal operation of the transformer and the safe power supply of the power grid.Therefore,the problem of joint overheating must be solved in a timely manner.
1.5 Transformer oil leakage
1.5.1 Oil tank weld leakage
For oil leakage at flat joints,welding can be directly carried out.For oil leakage at corners and reinforcement connections,the leakage points are often difficult to locate,or leakage occurs again due to internal stress after welding.For such leakage points,an iron plate can be used for welding repair.For double-sided connections,the iron plate can be cut into a spindle shape and welded to the ground.For three sided connections,the iron plate can be cut into a triangle according to the actual position for welding repair.This method is also suitable for welding leakage at the corner weld of the secondary lead box of bushing current transformers.
1.5.2 Leakage of Low Voltage Side Sleeve
The reason is due to the stretching of the busbar and the short lead wire on the low-voltage side,causing the adhesive beads to press against the threads.When the busbar is stretched,expansion joints can be used to connect the busbar according to regulations.If the lead is too short,the lead length can be readjusted.If it is difficult to adjust the lead,sealant can be added to the sealing surfaces of the installation beads.To increase the clamping force,ceramic pressure caps can be replaced with copper pressure caps.
1.5.3 Explosion proof pipe oil leakage
Explosion proof tube is a safety measure to prevent excessive pressure inside the transformer caused by internal faults,and to avoid the rupture of the transformer oil tank.However,the glass film of the explosion-proof tube is prone to rupture during transformer operation due to vibration,and the glass cannot be replaced in a timely manner.Moisture enters the oil tank,causing the insulation oil to become damp and the insulation level to decrease,endangering the safety of the equipment.Therefore,the explosion-proof tube can be removed and a pressure relief valve can be modified.
2.Preventive measures for transformer faults
2.1 Strictly follow the relevant maintenance technical standards to conduct pre operation inspections and tests on transformers to prevent potential problems.
2.2 Operation and maintenance
2.2.1 Keep the porcelain bushing and insulator clean.Regularly clean the dirt on the transformer,check whether the bushing has flashover discharge,whether the grounding is good,whether there is wire breakage,desoldering,or fracture phenomenon,regularly telemetry the grounding resistance not exceeding 4,or take anti fouling measures and install anti fouling caps on the bushing.
2.2.2 In the oil cooling system,check the radiator for leakage,rust,accumulation of dirt,and any mechanical damage caused by the free flow of oil.At the same time,regularly check the oil level,color,and leakage of the transformer,and promptly eliminate any defects found.
2.2.3 Ensure the firmness and reliability of electrical connections.
2.2.4 Regularly inspect the tap changer and check the firmness,burns,scars,flexibility of rotation,and positioning of the contacts.
2.2.5 Dielectric loss testing should be conducted on transformer coils,bushings,and lightning arresters every three years.
2.2.6 Check the reliability of lightning arrester grounding every year.The grounding must be reliable,and the lead wire should be as short as possible.The lead wire should comply with regulations and have no broken strands.During the dry season,the grounding resistance should be tested,and its value should not exceed 5.Annual preventive tests should be carried out to replace unqualified lightning arresters and reduce transformer damage caused by lightning overvoltage.
2.2.7 Transformers should undergo regular major and minor repairs,and can be promptly overhauled during operation or in case of abnormal situations.
2.2.8 It should be considered to use online detection systems on the most critical transformers.Large transformer online monitoring systems can detect abnormal states of transformers in operation in advance,and combine online monitoring with expert systems to predict transformer insulation and detect transformer abnormalities at the very beginning.
The fault diagnosis and condition based maintenance of power transformers are powerful measures for China's power system to achieve institutional transformation and improve the scientific management level of power equipment.They are the direction for future efforts and development in power production.
