The windings and cores of the transformer are the main components that transmit and transform electromagnetic energy. Ensuring their reliable operation is a concern. The statistics show that the fault caused by the core problem is the third in the total transformer accident. The manufacturing department has paid attention to the defects of the transformer core, and has made technical improvements in the core can be grounded, the core grounding monitoring, and ensuring a grounding. The operating department also mentioned the detection and discovery of core failures to a considerable height. However, transformer core failures still occur frequently, mainly due to the multi-point grounding of the core and the poor grounding of the core. An introduction to the determination and treatment of two fault conditions is given.
1. When the iron core is normal, it needs a little grounding. When the transformer is in normal operation, there is an electric field between the live winding and the oil tank, and the iron core and other metal components are in the electric field. Because the capacitance distribution is uneven, the field strength is different. If the iron core cannot be grounded, the charging and discharging phenomenon will occur, and the insulation strength of the solid insulation and oil will be destroyed. Therefore, the iron core must have a reliable grounding.
The iron core is composed of silicon steel sheets. In order to reduce the eddy current, there is a certain insulation resistance between the sheets (generally only a few ohms to several tens of ohms). Since the inter-chip capacitance is extremely large, it can be regarded as a path in the alternating electric field, and thus the iron core The stack of iron core laminations can be clamped to ground potential with a single grounding.
When the iron core or its metal member is grounded at two or more points (multiple points), a closed loop is formed between the grounding points, and the magnetic flux of the key chain portion induces an electromotive force and forms a loop to generate a local portion. Overheating, even burning the iron core.
The transformer core is only grounded at one point, which is the normal grounding that can be*. That is, the core must be grounded and must be grounded at all.
The core failure is mainly caused by two reasons. One is the short circuit caused by poor construction process, and the other is caused by multiple points of grounding due to accessories and external factors.
2. Iron core multi-point grounding type
(1) After the completion of the installation of the transformer, the positioning pin transported on the top cover of the fuel tank is not turned over or removed to form a multi-point grounding.
(2) Since the core clip is too close to the stem and the core lamination is lifted for some reason, it touches the clip limb to form a multi-point grounding.
(3) The bush of the iron yoke screw is too long and collides with the lamination of the iron yoke to form a new grounding point.
(4) The insulating paperboard between the lower leg of the iron core and the iron yoke is broken or broken, so that the laminations at the iron yoke of the foot are collided to cause grounding.
(5) Large and medium-sized transformers with submersible pump devices. Due to the wear of the submersible pump bearings, the metal powder enters the fuel tank, deposits the bottom of the fuel tank, forms a bridge under the action of electromagnetic force, and connects the lower iron yoke with the foot or the bottom of the tank to form a multi-layer transformer. Point to ground.
(6) The thermometer seat cover on the oil-immersed transformer tank cover is too long, and it collides with the upper clamp or the iron yoke and the side of the side column to form a new grounding point.
(7) Metal foreign matter falls into the oil-immersed transformer oil tank. This kind of metal foreign matter allows the iron core lamination and the box body to pass through to form a ground.
(8) The wooden block between the lower clamp and the iron yoke step is damp or the surface is not clean, and more oil is attached to make the insulation resistance value drop to zero, which constitutes multi-point grounding.
3. Anomalies that occur when multiple points are grounded
(1) A vortex is generated in the iron core, the iron loss is increased, and the iron core is locally overheated.
(2) When the multi-point grounding is serious, it will not be processed for a long time. The continuous operation of the transformer will cause the oil and windings to overheat, which will gradually deteriorate the oil-paper insulation. It will cause the two insulation layers of the iron core laminate to aging and fall off, which will cause the larger iron core to overheat and the iron core will burn out.
(3) Grounding at a multi-point for a long time, causing the oil-immersed transformer oil to deteriorate and generating a flammable gas to operate the gas relay.
(4) The wooden blocks and clips in the body are carbonized due to overheating of the iron core.
(5) Severe multi-point grounding will cause the grounding wire to blow, causing the transformer to lose its normal grounding, and the consequences are unimaginable.
(6) Multi-point grounding can also cause discharge.
4. Multi-point ground fault detection The core multi-point ground fault determination method is usually tested from two aspects:
(1) Perform gas chromatography 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 previous one, or the content is normal, the iron core is overheated, and the iron core overheating may be due to multi-point grounding. To.
When acetylene gas is present in the chromatographic analysis, it indicates that the iron core has intermittent multi-point grounding.
(2) Measure the current of the grounding wire. A clamp meter can be used to measure the current on the lead wire on the ground lead of the grounding bushing of the transformer core. When the transformer core is normally grounded, no current loop is formed. The current on the ground line is very small and is milliampere (generally less than 0.3A). When there is multi-point grounding, the main magnetic flux of the core is equivalent to the presence of a short-circuit åŒ, and the turbulent flow flows through the circumstance. The value depends on the relative position of the fault point and the normal grounding point, that is, the amount of magnetic flux enclosed in the short-circuit åŒ. Generally up to tens of amps. By measuring the presence or absence of current in the ground lead, it is very accurate to determine whether the core has multiple ground faults.
5. Elimination of multi-point ground fault
(1) Temporary elimination method when the transformer cannot be stopped:
1 There is an external grounding wire. If the fault current is large, the grounding wire can be temporarily opened. However, monitoring must be intensified to prevent the core from appearing in a floating potential after the fault point has disappeared.
2 If the multi-point ground fault is unstable, a slip line resistor can be placed in the working ground wire to limit the current to less than 1A. The choice of the slip line resistance is the voltage measured by turning on the normal working ground line divided by the current on the ground line.
3 Use chromatographic analysis to monitor the gas production rate at the point of failure.
4 After the exact fault point is found by measurement, if it cannot be processed, the normal working grounding piece of the iron core can be moved to the same position of the fault point to greatly reduce the circulation.
(2) Thorough maintenance measures. After the monitoring finds that there is a multi-point ground fault in the transformer, the transformer that can be stopped should be stopped in time, and the multi-point ground fault is completely eliminated after exiting. For the method of eliminating such faults, according to the type and reason of multi-point grounding, corresponding maintenance measures should be taken. However, there are some cases where the fault point cannot be found after the power failure of the suspension core. In order to find the grounding point accurately, the following method can be used on the site.
1 DC method. Open the connecting piece of the iron core and the clamp member, pass 6V DC on the silicon steel sheet on both sides of the yoke, and then sequentially measure the voltage between the silicon steel sheets at each level by using a DC voltmeter. When the voltage is equal to zero or the meter indicates reverse, then This can be considered as the fault ground 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 core. If there is a multi-point ground fault, the current will be measured with a milliampere meter (the connecting piece of the core and the clamp should be opened). Use the milliampere meter to measure point by point along the iron yoke. When the current in the milliampere meter is zero, then it is the fault point.
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