Winding and iron core are important components for transmitting and transforming electromagnetic energy in transformers.Accurately evaluating,detecting,and correcting iron core faults is the key to ensuring the normal operation of transformers.

　　1、The hazards,causes,and types of multi-point grounding faults in transformer iron cores

　　The hazards of multi-point grounding faults in transformer iron cores:Transformer iron cores should not experience multi-point grounding faults during normal operation.When a transformer is operating normally,there is an alternating magnetic field around the winding.Due to electromagnetic induction,parasitic capacitance exists between the high-voltage winding and the low-voltage winding,between the low-voltage winding and the iron core,and between the iron core and the shell.Due to the coupling effect of parasitic capacitance,the energized winding generates a floating potential between the iron core and ground.Due to the unequal distance between the iron core and other metal components around the winding,there is a potential difference between these components.When the potential difference between two points reaches the level of insulation breakdown,spark discharge will occur.This discharge is intermittent and over time,it can have adverse effects on transformer oil and solid insulation.To eliminate this phenomenon,the iron core should be reliably connected to the shell so that they are at the same potential.But when there are two or more grounding points on the iron core or other metal components,the grounding points will form a closed circuit,causing circulating current,local overheating,oil decomposition,and decreased insulation performance.In severe cases,it can lead to major accidents such as burning of the iron core silicon steel sheets.Therefore,only single point grounding is allowed for the iron core of the main transformer.

　　Reasons for iron core grounding faults:Transformer iron core grounding faults mainly have the following reasons:short circuits caused by poor construction technology and design of grounding strips;Multiple grounding points caused by attachments and external factors;Grounding faults caused by metal foreign objects left inside the main transformer and rough iron core processes such as burrs,rust,welding slag,etc.

　　There are six common types of iron core faults in transformers:(1)When installing,the iron core comes into contact with the shell or clamp.This situation is caused by negligence or inversion of the stabilizing pin used for transportation on the fuel tank cover during disassembly,resulting in contact between the iron core and the shell.Other types include iron core clamps that come into contact with iron core columns,silicon steel sheet curled contact clamps,paper detachment between the lower clamp foot of the iron core and the iron yoke,and temperature gauge seat sleeves that are too long and come into contact with clamps,iron yokes,or iron core columns.(2)The steel seat sleeve of the through bolt is too long,causing a short circuit in the silicon steel sheet.(3)There is a foreign object inside the fuel tank,causing a local short circuit in the silicon steel sheet.For example,a screwdriver was found between the clamp and the iron yoke of a 31500/110 power transformer at a power plant in Shanxi,and a 120mm long copper wire was found during the hood inspection of a 60000/220 power transformer at another substation.(4)The insulation of the iron core is affected by moisture or damage,such as sedimentation of mud and moisture,which leads to a decrease in insulation resistance.The insulation of clamps,iron pads,and iron boxes(paper or wooden blocks)may become damp or damaged,which can also form high resistance multi-point grounding points inside the iron core.(5)The pump shaft bearing is worn,and metal powder enters the oil tank,accumulating at the bottom.Under the action of electromagnetic force,a bridge circuit is formed,which connects the lower rail with the pad or the bottom of the oil tank,resulting in multi-point grounding.(6)Poor operation and maintenance,failure to conduct regular inspections as planned.

　　2、Transformer iron core fault detection and handling methods

　　Method for detecting faults in transformer iron cores

　　(1)Clamp on ammeter method(online measurement):For transformers with iron cores externally connected,the clamp on ammeter method can accurately and continuously detect multiple grounding faults in the iron core.Regularly measure the grounding lead current every year,and the current should generally be below 100 milliamps.If it exceeds this value,monitoring should be strengthened.After the transformer is put into operation,the grounding wire resistance is measured continuously multiple times as the initial value.If the initial value is already very large,it indicates that the transformer itself has obvious leakage flux,and the subsequent measured values are not significantly different,it can be considered that there is no grounding fault point.If the current of the grounding wire is greater than 1 ampere and significantly increases compared to the initial value,it may be a low resistance grounding or metal grounding fault,which should be dealt with in a timely manner.

　　(2)Chromatographic analysis method(energized oil extraction):Perform chromatographic analysis on the oil sample.If the total hydrocarbons increase significantly and the gas composition is mainly methane and ethylene,while the carbon monoxide and carbon dioxide gases remain basically unchanged or change very little compared to before,it is due to overheating of the bare machine,which may be caused by multiple grounding faults in the iron core or insulation damage between the iron core and the silicon steel sheet.Further inspection is needed.If acetylene appears in the total hydrocarbons,it may be due to intermittent instability of the iron core and multiple grounding faults.

　　(3)Insulation resistance method(power-off test):Use a 2500 volt megohmmeter to measure the resistance between the iron core and the shell.If the insulation resistance is above 200 megohms,it indicates good insulation of the iron core.If the megohmmeter indicates conductivity between the iron core and the outer shell,use an ohmmeter to measure the resistance between the iron core and the outer shell.If the measured value is between 200 and 400 ohms,it indicates that there is a high resistance grounding point inside the iron core,and multiple grounding faults in the transformer iron core need to be addressed.

　　If the measured value is above 1000 ohms,the ground current is small,and the fault is difficult to eliminate,it can continue to operate without further processing.Regular online monitoring should be conducted,such as using clamp current meter method(for transformers with iron core external)or oil chromatography analysis.If any abnormalities are found,further processing is required.If the measured value is between 1-2 ohms,it indicates that there is a metal grounding point inside the iron core,and the transformer needs to be treated.

　　2.Multi point grounding treatment method for transformer iron core:

　　(1)For transformers with an external grounding wire on the iron core,a resistor can be connected in series on the iron core grounding circuit to limit the current in the iron core grounding.This method is only suitable as an emergency measure.

　　(2)For iron core grounding faults caused by metal foreign objects,shell inspection is generally used to detect the problem.

　　(3)For grounding faults caused by iron core burrs or metal powder accumulation,methods such as capacitor discharge impact,AC arc,and high current impact are more effective.

　　3、Quality standards for transformer iron core maintenance

　　The iron core should be flat,without insulation paint peeling off,the laminations should be tight,and the side silicon steel sheets should have no protrusions or waves.The surface of the iron core is free of oil stains and impurities,and there is no short circuit or overlap between the laminations.The gap at the joint should meet the requirements.

　　Good insulation should be maintained between the iron core and the upper and lower clamps,yoke,pressure plate,and bottom plate;

　　There should be a clear and uniform gap between the steel pressure plate and the iron core;The insulation pressure plate is intact,without damage or cracks,and is properly fastened;

　　The steel pressure plate should not form a closed loop and should have a single point grounding;

　　After disconnecting the connection between the upper clamp and the iron core,as well as the connection between the steel pressure plate and the upper clamp,measure the insulation resistance between the iron core and the upper and lower clamps,and between the steel pressure plate and the iron core.Compared with the previous test,there should be no significant changes;

　　The bolts should be securely fastened,and there should be no looseness in the positive and negative pressure pins and locking nuts on the clamp.They should have good contact with the insulation gasket and no discharge burn marks.The back pressure pin should have sufficient distance from the upper clamp;

　　The iron core bolts should be securely fastened,and their insulation resistance should not show significant changes compared to the previous test;

　　The oil passage should be unobstructed,and the oil passage gasket should not be loose or blocked,arranged neatly;

　　The iron core should have only one grounding point,and the grounding plate should be made of copper with a thickness of 0.5mm and a width of not less than 30mm,inserted into 3-4 layers of iron core.For large transformers,the insertion depth should not be less than 80mm,and the exposed part should be wrapped with insulation to prevent short circuits inside the iron core;

　　It should be securely fastened,have sufficient mechanical strength,have good insulation,not form a closed loop,and not come into contact with the iron core;

　　Good insulation and reliable grounding.