Power Transformer Cabinet Typical Problem Analysis and Processing Measures

In this paper, I introduced the common types of sealed transformer cabinet, and according to its structural characteristics, combined with transformer design, manufacturing and installation practice, illustrated the sealed transformer cabinet sealing material breakage, elastic parts expansion obstruction, false oil level and other typical problems, explained the transformer cabinet in the selection, installation, oil replenishment, operation should be concerned about the focus of each link, and put forward the relevant problems of prevention and treatment measures.

Transformer Cabinet1

Transformer cabinet is one of the important components of large power transformer. Power transformer cabinet can generally be divided into open and sealed two categories, because the insulating oil in the open transformer cabinet is directly connected to the atmosphere through the pipeline and breather, it is easy to moisture and oxidation, the safety of the transformer, economic operation, reliable operation, etc. caused a great threat, so 35kV and above voltage level of transformers are no longer allowed to be used.According to the different sealing materials, sealed transformer cabinet has rubber type and metal bellows type, according to the structure of the oil chamber, rubber type can also be divided into diaphragm type, capsule type.

According to the use of transformer cabinet elastic material, it can be divided into internal oil type and external oil type, when the elastic part is used as oil capsule is called internal oil type, and when the elastic part is used as air capsule is called external oil type.

Diaphragm sealing type transformer cabinet by two half-cylinder put together, transformer cabinet in the middle is the diaphragm, the perimeter of the diaphragm fixed in the lower half of the cylindrical cabinet along the bottom, pressed with a gasket, the diaphragm floating on the liquid surface, with the liquid surface of the lift and float. Diaphragm material is generally nylon cloth, neoprene, nitrile rubber.

Because the diaphragm sealing type transformer cabinet sealing area is large, flatness is not easy to ensure, and installation, maintenance process is more complex, easy to cause oil leakage, diaphragm type transformer cabinet use less and less, and therefore this paper does not analyze.

At present, in the power transformer is widely used in the metal corrugated transformer cabinet (including external oil vertical, external oil horizontal, internal oil vertical three types) and capsule sealed transformer cabinet, this paper, the author introduced the structural characteristics of these four transformer cabinet, combined with transformer design, manufacturing, installation In this paper, I introduced the structural characteristics of these four types of transformer cabinets, combined with transformer design, manufacture, installation, operation practice, analyzed the causes of typical problems such as broken sealing materials, elastic parts expansion and contraction obstruction, false oil level, and summarized the corresponding preventive and treatment measures.

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Frequency: 50/60Hz Rated voltage:10kv, 20kv,30kv Rated Power: 400~2500kva

Table of Contents

Common Sealed Transformer Cabinet Structure

The rated compensation volume of transformer cabinet should meet the maximum volume expansion required within the temperature variation of insulating oil under various load conditions and operating environment conditions of transformer, and its size is generally about one tenth of the total oil volume of transformer tank, and the four types of sealed transformer cabinet structures commonly used are described below.

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Capsule sealed transformer cabinet structure

General transformer cabinet is tube-shaped or hexagonal (octagonal) column, its internal is a kind of nitrile rubber and nylon reinforced cloth by the composition of the capsule, the inner cavity of the capsule is connected to the moisture absorber with the tube, and through the moisture absorber and the atmosphere directly connected, the capsule is used for the airbag.

The bottom surface of the capsule is pressed against the upper surface of the insulating oil in the transformer cabinet, and the capsule acts as a barrier between the insulating oil and the air to avoid deterioration of the insulating oil. When the temperature of the oil surface rises or falls, the capsule expands or shrinks accordingly, acting as a respirator.

The float ball is set between the capsule and the oil level meter connecting rod, it relies on buoyancy and the bottom of the capsule in close contact, and relies on the rotation of the pendulum and the magnetic coupling drive to achieve oil level indication.

Metal corrugated external oil vertical transformer cabinet structure

current transformer cabinet3

The transformer cabinet is similar to the capsule type structure, consisting of cabinet, corrugated core, pipeline, oil level meter, etc. The cabinet and the core system is formed into a runway shape, the core material is stainless steel sheet, the cabinet is equipped with an oil level meter, the transformer cabinet main pipeline, oil injection and discharge pipe, exhaust pipe to the oil chamber inside the cabinet, breathing pipe directly to the core body of the air chamber inside the core. The upper end of the core body is fixed in the upper part of the cabinet, while the lower end is free to expand the activity end.

When the transformer insulating oil with the load and the external environment temperature changes and volume changes, the core body also follows the up and down expansion, the volume of the oil cavity inside the cabinet also make certain changes, in completely sealed conditions can be carried out on the transformer insulating oil volume compensation.

The oil level meter is synchronized with the up and down retractable structure of the core, so that the change of the oil level can be observed and the corresponding alarm signal can be output when the low or high oil level is reached.

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Horizontal Transformer Cabinet Structure with Corrugated Metal External Oil

400 amp ct cabinet4

The core of the horizontal transformer cabinet is connected to the atmosphere through a breather tube, the outer wall of the bellows and the inside of the transformer cabinet are filled with insulating oil.

If the transformer load changes or external temperature changes cause oil temperature rise and fall, resulting in transformer oil volume contraction or expansion, the corrugated core body also followed by left and right expansion, transformer cabinet oil cavity volume also changed accordingly, the volume of insulating oil in the fully sealed conditions to achieve compensation.

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Metal corrugated internal oil vertical transformer cabinet structure

The oil container of the metal corrugated internal oil vertical transformer cabinet is a corrugated core body, the core body material has anti-atmospheric corrosion and high temperature resistance, according to the size of the compensation oil volume can be used one or more corrugated core body parallel vertical installation on the same chassis, the external arrangement of dust cover, relying on the corrugated core body up and down expansion of the volume of insulating oil compensation, its appearance is generally rectangular in shape. Its appearance is generally rectangular in shape.

The outer cover is equipped with an oil level indicator window, the transformer cabinet is connected to the inner cavity of the core, the oil injection and exhaust piping is led from the main piping, and the exhaust pipe is led from the top of the core for vacuuming and exhausting.

The lower end of the core is fixed to the base and the upper end is free to extend and retract, and the transformer insulating oil is connected to the oil tank through the lower main line. As the transformer load or ambient temperature rises and falls, the insulating oil volume expands or contracts, the core body also expands and contracts, and the volume of the oil cavity inside the core body changes accordingly.

The oil level indicator plate mounted on the core body moves synchronously with the up and down expansion and contraction of the core body, and the change of oil level can be observed through the oil level indicator window.

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Typical problems of sealed transformer cabinet and handling

ct cabinet vs transformer5

Capsule aging, wear

Due to the low mechanical strength of the capsule transformer cabinet capsule itself, in the process of transformer installation, operation and maintenance, it is easy to rupture oil leakage, and not easy to be found. In the transformer operation, the capsule transformer cabinet oil level meter float of the connecting rod half shaft against the lower part of the capsule, and with the expansion and contraction of the capsule appears to move, if the float is not smooth or have scratches and other small defects, it is very easy to puncture the capsule, the operator is difficult to find.

In addition, the capsule ends of the existence of seams and uneven surface, when the transformer cabinet wall on both sides of the existence of uneven welds, burrs, in the transportation and operation process of the two constantly friction, will also lead to damage, the capsule damage location is mostly concentrated in the ends of the capsule. There is also a possibility that, in the factory or on-site installation process, if the oil injection operation is improper will lead to the float rod fixed parts of the four corners of the tip cut the capsule.

Large power transformers need to use full vacuum oiling, when the transformer cabinet has not been filled with enough insulating oil, too early or too fast to open the broken vacuum valve is likely to cause rapid expansion of the capsule, if and float steering plate tip of the violent collision, easy to produce a rupture.

Therefore, make sure the transformer oil has entered the transformer cabinet and to a certain oil level before breaking the vacuum, and the vacuum breaking process should be implemented slowly.

If there is water in the capsule transformer cabinet, it can be discharged only when the transformer cabinet is dismantled for maintenance. The water condensed on the surface of the capsule will lead to rapid cracking and aging of the capsule, permeability and short service life.

Capsule due to deterioration, wear, breakage, lap seam cracking, flange root fracture, shedding will lead to leakage, transformer cabinet sealing material aging, will lead to a decline in sealing performance, water into the transformer oil. In order to slow down the aging of the material of the capsule transformer cabinet, you need to configure the corresponding capacity of the moisture absorber to ensure that the air enters the transformer cabinet after drying and filtering, and need to regularly check and replace the moisture absorber silica gel.

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Metal bellows damage, welding seam leakage

800 amp ct cabinet6

Metal corrugated transformer cabinet is strictly prohibited to carry oil transportation, because its core body for the elastic welding body, the corrugated core body should be pumped or inflated to completely shrink, is a negative pressure state, in order to prevent the corrugated tube in transit bumps, shaking, expansion, otherwise the core body operating life will be affected, and even cracking leakage phenomenon.

Metal corrugated internal oil transformer cabinet can withstand negative pressure of not more than 50Pa duration of 30min sealing test, while the external oil type before the vacuum test, the exhaust pipe and breathing tube should be connected. The corrugated core can be inflated and pressurized to 50kPa for leak test only if it is limited in height or length. During lifting and installation, the corrugated metal transformer cabinet should be properly protected from damage by bumping.

In addition, if there are some defects in the main transformer, or if the internal pressure changes suddenly during the test or live operation, causing the oil level of the main transformer to rise suddenly, and the corrugated core body is deformed and damaged by the huge oil pressure shock in a short period of time, necessary inspection is needed.

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Elastic parts expansion obstruction

For the capsule transformer cabinet, due to the winter moisture absorber frozen and poor breathing, capsule stretching poor. Once the freeze is lifted, the transformer oil flows too fast and may cause the gas relay to operate.

Transformer Cabinet7

Therefore, in the rainy environment need to pay attention to prevent moisture absorber silica gel moisture caking phenomenon, when two-thirds of the moisture absorber moisture discoloration should be replaced; if the moisture absorber moisture rate is abnormal too fast, should check the sealing condition, and take insulation oil samples for analysis.

For metal corrugated external oil horizontal transformer cabinet, when the oil level decreases, the upper end of the transformer cabinet form a vacuum area, the top of the expander to the vacuum area movement until there is a tilt, the bellows in the expansion process is easy to jam, the oil flow too fast, the serious situation will trigger the gas relay action, while the metal corrugated internal oil type transformer cabinet does not have this problem.

False oil level

For the capsule type transformer cabinet, most of them use the connecting rod type pointer type oil level meter, but because of the low accuracy of the pointer itself, plus the long connecting rod is easy to deform, it is very easy to appear false oil level phenomenon.

There is another situation, when the transformer oil injection after the static time is not enough, the gas in the oil is not fully precipitated, the gas in the oil can not be exhausted, the residual gas outside the capsule exists, with the oil temperature rises, these gases due to heat and expansion, as the air pressure continues to rise, the capsule will be further compressed will lead to breathing tube and transformer cabinet interface is blocked, if If the gas pressure in the upper part of the transformer cabinet exceeds the atmospheric pressure and the interface between the breathing tube and the transformer cabinet suddenly leaks, the oil level indicated by the oil gauge will rise rapidly and the phenomenon of false oil level will appear.

One of the effective ways to deal with false oil level for the oil injection method, the specific operation steps are: the transformer cabinet top two ends of the exhaust plug open, to ensure that the sealing surface here is good, sealing ring intact, and then injected from the transformer qualified insulating oil, the capsule will slowly rise, the transformer oil will transformer cabinet gas from the exhaust plug hole top, until the transformer oil from the exhaust plug overflow. Until the transformer oil from the exhaust plug overflow, the residual gas is drained.

At this time should tighten the exhaust plug, turn off the oil filter, so as to avoid air being sucked into the transformer cabinet, after which the transformer oil will be put to the normal operating oil level position. Although the oil injection method can be more thorough exhaust, but the operation steps are complicated.

In the field of false oil level processing, usually use a more simple compressed air method, that is, first remove the moisture absorber, and then to the capsule filled with dry gas, so that the capsule expansion, so that the residual gas in the transformer cabinet from the exhaust plug out of the top, when the capsule all stick to the transformer cabinet liquid level, promptly screw the exhaust plug, and stop filling To prevent the air from entering the transformer cabinet.

Because the metal corrugated external oil type transformer cabinet and the capsule transformer cabinet structure is similar, the causes of false oil level and the way to deal with it is also similar. The end of the corrugated metal core is connected to the oil level meter through a connecting rod or a metal tie, and a mechanical failure in this part will directly affect the accurate indication of the oil level.

In the actual operation process, the corrugated core or chute may be deformed due to improper treatment of the corrugated core weld, and the roller may be worn out, thus causing stagnation and incorrect oil level indication.

When the corrugated core body itself has cracks aging phenomenon, transformer cabinet transformer oil and bellows core body air may penetrate each other, which will lead to transformer oil volume change, corrugated core body can not be normal contraction or extension, thus causing false oil level phenomenon, resulting in abnormal oil level display. Metal corrugated external oil transformer cabinet oil filling, should be completely drained of the oil cavity gas.

Metal corrugated internal oil transformer cabinet core body full of transformer insulating oil, transformer cabinet shell cabinet cover with oil level observation window, can directly observe the transformer cabinet oil level, without trip conversion, can also be indirectly displayed through the oil level meter, usually no false oil level phenomenon. This type of transformer cabinet is more convenient for field operation.

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Conclusion

The transformer cabinet is one of the important components of a transformer, not only can automatically compensate for the expansion or contraction of transformer oil due to temperature changes, but also effectively prevent the transformer insulating oil from moisture deterioration.

Transformer designers should combine with customer requirements, project characteristics, maintenance feasibility, etc., to recommend the appropriate type, choose the appropriate volume of transformer cabinet; transformer in the vacuum oil filling, sealing test, should strictly comply with the transformer cabinet installation instructions for related operations; operation personnel in the daily inspection and maintenance.

Operating personnel in the daily inspection and maintenance to pay special attention to the transformer cabinet status, check whether the oil level is normal, transformer cabinet whether there is oil seepage phenomenon.

The institutions or units that are in a position to do so can adopt more advanced methods for overhaul and maintenance, and scientifically formulate response plans and strategies to avoid unplanned shutdowns as much as possible.

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