What Makes A Transformer Overheat

Which parts of the transformer may have high heat during operation? What is the reason? How to judge?

 

There are various types of transformer failures during operation, which pose a serious threat to the transformer. Several major components may have local high heat. In addition to the hottest spot of the coil during normal operation, we talk about the local high heat problem under abnormal conditions of each part.

 

1. The tap changer is in poor contact

 

The local high temperature caused by the poor contact of the tap changer is a relatively common problem at present. In fact, since the transformer is a static electrical appliance, in addition to the hidden dangers of the iron core and coil caused by poor manufacturing quality, as long as attention is paid during operation, the transformer iron core and coil are faulty. Still relatively few. But the tap-changer is switched frequently, and the chance of problems may be more.

 

The heating of the tap changer is mainly caused by the poor contact, which increases the contact resistance. Since the loss is equal to I²R, if R is large, the loss will be large and the heat will be severe. This is especially likely to happen after reverse splits and when the transformer is overloaded. Poor contact can be caused by:

(1) The contact point pressure is not enough;

(2) There is sludge accumulation in the contact of the switch, so that there is a layer of sludge film between the dynamic and static contacts;

(3) The contact surface is small so that the contacts are melted;

(4) The positioning indication and the contact position of the switch are not quite aligned (the positioning point has been indicated correctly but the close contact point of the switch has moved away);

(5) Several contact rings of DW type drum tap changer are not in contact with the contact column at the same time, etc.

 

The local high temperature of the tap changer has endangered the safe operation of the transformer, some caused the transformer to burn out, and some caused the rapid deterioration of the oil quality, which forced emergency shutdown for maintenance.

 

So how to judge the poor contact of the tap changer during operation? This kind of fault is most likely to occur after overhaul or after switching the split head, and the contact surface may also be burned after the traversing fault. The current chromatographic analysis basically correctly judges it. When the chromatogram determines that the bare metal is overheated, the transformer can be stopped, and the DC resistance of the three-phase tap can be measured to determine the contact condition of the tap changer. During chemical analysis, the flash point drops rapidly, it may also be a poor contact of the tap changer. High pressure testing, chromatography and chemical analysis can all monitor it from different angles. In addition, strengthen the observation during operation, especially the frequent movements of light gas may also be caused by poor contact.

 

2. Short circuit between turns of coil

 

The so-called inter-turn short circuit is the insulation damage between several adjacent turns. This will create a closed short-circuit loop and, at the same time, reduce the number of turns in the coil for that phase. The short-circuit current induced by the alternating magnetic flux flows in the short-circuit loop, which will generate high heat and may cause the transformer to burn out.

 

According to statistics, transformer damage caused by inter-turn short circuit is more, accounting for about 77% to 80% of the total damage. Therefore, enough attention should be paid to this issue.

 

There are many reasons for the short circuit between turns, such as:

 

(1) Mechanical damage to insulation caused by beating, bending, pressing and other processes during coil manufacturing, or some copper and iron puncture insulation leaves hidden dangers;

 

(2) After running for a long time, the insulation ages and becomes brittle, so that the wires are connected;

 

(3) The local high temperature during operation causes the insulation to age rapidly (these local high temperatures such as "dead corners" of oil flow, blockage of oil passages, etc.);

 

(4) In the case of a penetrating short circuit, the axial or radial displacement of some turns under the action of electrodynamic force will cause the insulation to wear out;

 

(5) The oil level of the transformer drops, so that the coil is exposed to lose cooling;

 

(6) Long-term overload operation and unscientific temperature control make the copper wire temperature too high and the insulation becomes brittle quickly. Of course, the real development of inter-turn short circuit often occurs after over-voltage and over-current.

 

Less serious inter-turn short circuits are difficult to find, even by conventional insulation tests, but chromatographic analysis is more sensitive and may be found. The more serious inter-turn short circuit can also be found during operation. Due to severe heating, the oil temperature rises, and the current increases to some extent;

 

Light gas action. It is particularly worth mentioning that when the short-circuit turns are hot, the oil is like boiling, and the sound of "gurgling" can be heard next to the transformer. Before the development of heavy gas operation, take oil samples for testing, the oil quality will change, and chromatographic analysis is an obvious fault, which is easy to confirm, and gas sample analysis can also find the problem.

 

A stopped transformer can sometimes find inter-turn short circuits from the ratio and DC resistance tests.

 

3. There is a short circuit in the iron core silicon steel sheet

 

The iron core is made of silicon steel sheets that are insulated from each other. Due to external force damage or insulation aging and other reasons, the insulation of the paint skin between the silicon steel sheets is damaged, which will increase the eddy current, cause local overheating, and even melt in severe cases, which is the so-called "iron core fire".

 

In addition, the insulation damage of the feedthrough screw is also one of the reasons for the circulation. The feedthrough screw generally has an insulating sleeve to insulate it from the silicon steel sheet, and there are insulating washers at both ends to insulate it from the clip. The above-mentioned insulation damage may be caused by the damage to the insulation when the nut is tightened or the eddy current heating in the screw itself, which often makes the insulation brittle at high temperatures. If the insulation of several screws is damaged, a short circuit will be formed between the screws and the iron core, and the circulating current will flow through, causing the iron core to be partially overheated and damaged.

 

Similar to the above-mentioned short-circuit circuit, if the grounding of the transformer iron core silicon steel sheet is incorrectly installed (for example, there are several artificial grounding points, or the iron core is grounded at several points for some reason), it may also occur. This will cause circulation, local overheating and lead to serious accidents.

 

What will happen to the local heating of the iron core during operation? According to the operating experience, if there is a slight local overheating, the oil temperature of the transformer cannot even be observed, and the protection will not act, because the gas generated by the decomposition of the oil is dissolved in the oil at this time. went. If the iron core is overheated locally, the oil temperature will rise, the light gas will operate frequently, flammable gas will be precipitated, the flash point of the oil will drop (this is a unique phenomenon in the oil deterioration process in the transformer), and the oil color will become darker. And may smell burnt. In more serious cases, heavy gas will act.

 

The phenomenon of short circuit in the iron core can be found by chromatographic analysis. After the transformer is stopped, it can also be used to measure the no-load loss (the no-load loss is increased compared with the original when there is a short circuit), the insulation resistance (such as the through-core screw to the iron core) and other tests to find out initially, and then hang the core to find the fault point.

 

For the parts and causes of high heat in the transformer, only the main and possible points have been mentioned above. Of course, there are other parts and causes, such as joint heating (the welding place between the lead and the coil, the lead and the middle conductor of the bushing, etc.). The nut connection of the rod, the internal welding head of the coil, etc.); the insulation of the pressure ring screw is damaged or the pressure ring touches the iron core to cause a circulating current; a screw or iron piece causes overheating through a large amount of magnetic leakage and a large eddy current.

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Transformer oil performs important role in power transformers, it works as cooling and insulation material. Therefore, the properties of transformer oil must be ensured by transformer oil treatment, that is, transformer oil filtration, vacuum dehydration and transformer oil degassing, as well as Transformer Oil Regeneration.

More information about Transformer Oil Filtration Machines, and power transformer maintenance equipment, please don't hesitate to contact us.

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