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The operation department has also raised the detection and discovery of core failures to a high level. However, transformer core faults still occur frequently, mainly due to multi-point grounding of the core and poor grounding of the core. The judgment and processing methods of the two fault conditions are now introduced.

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I. 

The reason why the iron core needs a little grounding when it is normal




During normal operation of the transformer, there is an electric field between the live windings and the tank, and the iron core and other metal components are in this electric field. Due to uneven distribution of capacitance and different field strengths, if the metal hose and stainless steel hose of the iron core cannot be grounded, charging and discharging will occur, which will destroy the insulation strength of solid insulation and oil, so the iron core must have a point that can be grounded.




The iron core is composed of silicon steel sheets and metal hoses and stainless steel hoses. In order to reduce eddy currents, there is a certain insulation resistance between the sheets (generally only a few ohms to several tens of ohms). It is regarded as a path, so only one point of grounding in the iron core can clamp the potential of the whole stack of iron core laminations to the ground potential.




When the iron core or its metal components are grounded at two or more points (multi-point), a closed loop will be formed between the grounding points, and the magnetic flux of the bond chain will induce electromotive force and form a loop, resulting in a local Overheating and even burning the iron core.




Only one point of the transformer core is grounded, which is normal grounding. That is, the metal hose of the iron core must be grounded, and it must be grounded at one point.




The iron core failure is mainly caused by two reasons, one is short circuit caused by poor construction process, and the other is multi-point grounding caused by metal hose stainless steel hose accessories and external factors.




II.


Iron core multi-point grounding type


(1) After the installation of the transformer is completed, the positioning pins transported on the top cover of the fuel tank are not turned over or removed to form multi-point grounding of the metal hose and the stainless steel hose.




(2) Due to the fact that the iron core clamp limb is too close to the core column, and the iron core lamination is lifted up for some reason, it touches the clamp limb, forming a multi-point grounding of the metal hose and the stainless steel hose.




(3) The bushing of the iron yoke screw is too long and collides with the iron yoke lamination, forming a new grounding point.




(4) The insulating cardboard between the foot of the clip under the iron core and the iron yoke falls off or is damaged, causing the laminations at the foot and iron yoke to collide to cause grounding.




(5) For large and medium-sized transformers with submersible pump devices, due to the wear of the submersible pump bearings, metal powder enters the oil tank and accumulates at the bottom of the oil tank, forming a bridge under the action of electromagnetic force, connecting the lower iron yoke to the foot or the bottom of the box to form metal Hose Stainless steel hose is grounded at multiple points.




(6) The thermometer seat cover on the oil tank cover of the oil-immersed transformer is too long, and it collides with the upper clip or the iron yoke and the edge of the side column, forming a new grounding point.




(7) Metal foreign objects fall into the oil tank of the oil-immersed transformer, and such metal foreign objects make the iron core lamination and the box body connect to form grounding.




(8) The wooden block between the lower clamp and the iron yoke ladder is damp or the surface is not clean, and there is a lot of sludge attached, so that when the insulation resistance value drops to zero, it constitutes a multi-point grounding.





III.


Abnormal phenomenon in multi-point grounding




(1) Eddy currents are generated in the iron core, the iron loss increases, and the metal hose of the iron core is partially overheated.




(2) When the multi-point grounding is serious and it is not handled for a long time, the continuous operation of the transformer will lead to overheating of the oil and windings, and the oil-paper insulation will gradually age. If the large iron core is overheated, the iron core will burn out.




(3) Multi-point grounding for a long time will deteriorate the oil-immersed transformer oil and generate flammable gas, which will make the gas relay act.




(4) Due to the overheating of the iron core, the wooden blocks and clips in the body are carbonized.




(5) Serious multi-point grounding will cause the grounding wire to burn out, so that the transformer loses the normal one-point grounding, and the consequences are disastrous.




(6) Multi-point grounding can also cause discharge.




IV.


Detection of multi-point ground faults




The iron core multi-point grounding fault judgment method is usually detected from two aspects:




(1) Carry out gas chromatographic analysis. In the chromatographic analysis, if the content of methane and olefin components in the gas is high, and the content of carbon monoxide and carbon dioxide gas does not change much compared with the past, or the content is normal, it means that the iron core is overheated and the iron core is overheated. Overheating can be caused by grounding at multiple points.




When acetylene gas appeared in the chromatographic analysis, it indicated that the iron core had intermittent multi-point grounding.




(2) Measure whether there is current in the grounding wire. You can use a clamp meter to measure whether there is current on the grounding lead of the grounding bushing outside the transformer core. When the transformer core is normally grounded, there is no current loop formed. Grounding The current on the line is very small, at the milliamp level (generally less than 0.3A). When there are multiple grounding points, there is a short-circuit turn around the main magnetic flux of the iron core, and the circulating current flows in the turn, and its value depends on the fault point and the The relative position of the normal grounding point, that is, the amount of magnetic flux enclosed in the short-circuit turn. Generally, it can reach several tens of amperes. By measuring whether there is current in the grounding lead, it is very accurate to determine whether there is a multi-point grounding fault in the iron core.




V.


Troubleshooting of multi-point ground faults




(1) Temporary elimination method when the transformer cannot be shut down:




①There is an external ground wire. If the fault current is large, the ground wire can be temporarily opened for operation. However, monitoring must be strengthened to prevent the iron core from appearing floating potential after the fault point disappears.




②If the multi-point grounding fault is of unstable type, a sliding wire resistance can be connected in series with the working ground wire to limit the current to less than 1A. The choice of the sliding wire resistance is to divide the voltage measured when the normal working ground wire is opened and divided by current on the ground wire.




③ Use chromatographic analysis to monitor the gas production rate at the fault point.




④ After finding the exact fault point through measurement, if it cannot be dealt with, the normal working grounding piece of the iron core can be moved to the same position of the fault point to greatly reduce the circulating current.




(2) Complete overhaul measures. After monitoring finds that there is a multi-point grounding fault in the transformer, the transformer that can be shut down should be shut down in time, and the multi-point grounding fault should be completely eliminated after exiting. To eliminate such faults, corresponding maintenance measures should be taken according to the type and cause of multi-point grounding. However, in some cases, the fault point cannot be found after the power outage is suspended. In order to find the grounding point exactly, the following methods can be used on site .




① DC method. Open the connecting piece of the iron core metal hose stainless steel hose and the clamp, and pass 6V DC to the silicon steel sheets on both sides of the yoke, and then use a DC voltmeter to measure the voltage between all levels of silicon steel sheets in turn. When the voltage is equal to zero or When the indication of the meter is reversed, it can be considered as the fault grounding point.




② communication method. Connect the low-voltage winding of the transformer to the AC voltage of 220~380V. At this time, there is a magnetic flux in the metal hose and stainless steel hose in the iron core. If there are multiple ground faults, use a milliammeter to measure the current (iron core and clamps). The connecting piece should be opened). Use the milliammeter to measure point by point along the iron yoke at all levels. When the current in the milliampere is zero, it is the fault point.


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