Seven reasons why transformers burn out

(1) Being struck by lightning. Most of the high and low voltage lines of power transformers are introduced by overhead cables. Because they are located in mountainous forests, the probability of lightning strikes is high. Therefore, during the thunderstorm season each year, the proportion of distribution transformers damaged by lightning strikes accounts for more than 30% of the overhaul.
(2) Ferroresonance occurs in the system. 10kV distribution lines in rural areas have conditions for overvoltage. When the system resonant overvoltage occurs, the primary current of the transformer surges. At this time, in addition to the fuse on the primary side of the transformer, the transformer windings will also be damaged. In some cases, it can also cause flashover or explosion in the bushing of the transformer.

2. Insulation damage
(1) The short-circuit fault of the low-voltage cable and the sharp increase of the load make the current of the transformer more than tens of times the rated current. At this time, the winding is affected by a large electromagnetic torque and shifts and deforms. Due to the sharp increase in current, the temperature rises rapidly, leading to accelerated aging of the insulation.
(2) The winding insulation is damp. This is caused by poor insulating oil or lowered oil level. One is that in the storage, transportation, or operation and maintenance of transformer insulating oil, moisture, impurities or other oils are accidentally mixed into the oil, which greatly reduces the insulation strength. The second is that the inner layer of the winding is impervious to impregnation, incomplete drying, poor welding of winding lead joints, and incomplete insulation, resulting in short circuits between turns and layers. Third, the lowering of the oil level increases the contact surface between the insulating oil and the air, and accelerating the entry of moisture in the air into the oil will also reduce its insulation strength. When the insulation is reduced to a certain value, a short circuit will occur.
3.tap switch
(1) Transformer oil leaks, so that the tap changer is exposed to the air, and the insulation performance decreases when the insulation is damp, resulting in a short circuit of discharge and damage to the transformer.
(2) The oil temperature is too high. The oil in the transformer is mainly used to insulate the windings, dissipate heat and prevent moisture. The oil temperature in the transformer is too high, which will directly affect the normal operation and service life of the transformer.
(3) The quality of the tap changer is poor, the structure is unreasonable, the pressure is not enough, the contact is unreliable, the position of the external character wheel is not completely consistent with the actual position of the internal, resulting in incomplete contact of the star moving contact position, dislocation of the dynamic and static contacts The tip makes the insulation distance between the two taps smaller, and short-circuits or discharges to the ground under the action of the potential between the two taps. The short-circuit current quickly burns the tapped turns and even damages the entire winding.

4. oil seepage
Oil seepage is the most common appearance abnormality of transformers. Since the transformer body is filled with oil, there are rubber beads and rubber pads at each connection part to prevent leakage. After a long time operation of the transformer, the rubber beads and rubber pads will age. Crack and cause oil seepage. Of course, if the screw is loose or the oil drain valve is not closed tightly, there will be blisters or poor welding quality during manufacturing, which will also cause leakage.
5. the iron core is grounded at multiple points
(1) It is not easy to find and test the multi-point grounding of the iron core of a 10kV distribution transformer. This is because the iron core grounding of the distribution transformer is internally sandwiched between the iron core (silicon steel sheet) with a thin copper sheet. The other end is pressed on the iron core splint and directly connected to the transformer shell.
(2) Short circuit between iron core silicon steel sheets. Although the silicon steel sheets are coated with insulating paint, their insulation resistance is very small, which can only block eddy currents but cannot prevent high-voltage induced currents. If the insulating paint on the surface of the silicon steel sheet is naturally aging, it will produce a large eddy current loss and increase the local overheating of the iron core.
6. overload
The uneven distribution of the three-phase load of the distribution transformer results in asymmetrical three-phase currents. The asymmetrical currents make the impedance drop of the transformer asymmetrical, so the low-voltage three-phase voltage is unbalanced, which is detrimental to the transformer and the user’s electrical equipment.
7.silica gel discoloration
Old-fashioned silicone is blue before being damp, and pink after being damp. However, this type of silica gel contains cobalt, which has an impact on human health, and its use has been banned in Europe. There is currently no cobalt-free silica gel in China, which is pink before being damp, and dark green after being damp.sformers burn out

Discussion on cable process: cable forming

1、 What is cable formation
Definition of cable forming: the process of twisting multiple insulated cores into AAC Cables according to certain rules.
2、 The role of cable forming in cable production
Cable forming is one of the important processes in the production of multi-core cables. The three core, four core (one core is the ground wire) and five core (one core is the ground wire and the other is the neutral wire) of the three-phase power supply is commonly used for power cables. The number of control cable cores is more (more than 2 cores to 61 cores). During the cable forming process, several insulated cores are twisted together according to certain rules, The process of forming a multi-core cable. In addition to the stranding, the process of cable forming includes filling of the gap between the cores of the insulated wires, wrapping and shielding on the core after the cable forming.
When forming cables, the twisted form of insulation core adopts concentric normal stranding. If the diameter of the insulated core is identical, it is called symmetrical cable forming. If the diameter of the insulation core is different, the cable formation is called asymmetric cable formation. In order to avoid the influence of torsional stress on the core during the process of cable forming, the cable forming machine with the torsion device or bow shaped cable forming machine is used for the cable forming of circular insulated core to conduct the twisting and twisting.
3、 The way of cable forming
There are two ways to twist the strand and the cable, one is to twist back and the other is not to twist.
The rewinding and twisting is that the cable tray frame equipped with the setting out plate keeps the setting out plate at all times horizontal position when the machine rotates by means of the special device (the torsion device) on it. When the cable is formed, the insulation core is only subjected to the bending action, but does not twist. The back twisting is often used in the process of forming the circular insulated core. The core has no rebound stress after the cable forming, which can ensure the accuracy of the roundness and diameter of the cable.
The non twisted twisted cable is mostly used for the formation of fan-shaped lines. After the compression of the die, it becomes plastic deformation, thus eliminating the original torsional stress and ensuring the round after the cable is formed.


4、 Cable forming direction and pitch diameter ratio

The cable forming direction is generally right. The confirmation of cable forming direction is in the direction of ACSR Cable core forward, if the cage turns left, it is right direction, otherwise, otherwise, it is opposite.
The ratio of cable pitch diameter is different according to different types of cables. The circular core insulated by cross-linked polyethylene is hard and the diameter of cable forming is larger, with a disk of 30-40; The pitch diameter ratio of PVC insulated power cable is 30-40, and that of sector line is 40-50; The diameter ratio of the cable core of plastic insulated control cable is specified in the national standard, and generally it shall not be more than 16-20.
5、 Stranding coefficient and twist rate
In a pitch of the cable forming, the ratio of the actual length of the insulation core to the length of the pitch of the cable is called the stranding coefficient (k=l/h);
The stranding rate is the ratio between the difference between the actual length of the insulating core and the length of the cable forming pitch and the length of the formed pitch within a cable forming pitch λ=( L-H)/H × 100%
It is convenient to adopt the coefficient K, and the K value is always greater than 1. Thus, the actual value of K is increased by one K value for the single insulated core after the cable forming. The resistance of the core is proportional to the core, that is, the resistance of the core is also increased by a k value. If the insulation resistance is inversely proportional to the length of the insulating core, the insulation resistance of each core will be reduced by a k value. From the angle of reducing the core resistance and increasing the insulation resistance value, it is hoped that the smaller the coefficient of cable winding is, the better.
The ratio of the cable formation to pitch ratio is inversely proportional to the square of the pitch ratio. Therefore, the smaller the pitch ratio, the greater the coefficient of the cable formation, the greater the amount of insulating core material is, otherwise, from the perspective of saving the material consumption, the smaller the coefficient of cable formation is, the better.
6、 Wrapping process
The production process of wrapping all kinds of metal or non-metallic materials on the core or core of the guide cable is covered with the specified pitch spiral in the form of ribbon or wire.
7、 Non armored cable wrapping
In order to prevent the cable core from deformation after the cable forming and to prevent the adhesion with sheath, the insulating core shall be wound with the wrapping layer while forming and filling on the cable forming machine. For the non armored plastic insulated cable, the 1-2-layer non-woven fabric belt is usually wrapped by covering (the first layer or two layers are used for the specific purpose, and the principle of cable forming and tightening is adopted), The covering size is 10% – 15% of the bandwidth, and the wrapping angle is 25% °~ forty ° Within the scope.


8、 Armouring of cables
Steel belt armored cable is mainly suitable for underground direct burial, and can bear certain mechanical pressure; Steel wire armored cable is mainly suitable for laying with drop or vertical, and can bear large mechanical tension. Armored cable is mainly divided into steel strip armor and steel wire armour, and their combination armouring mode.
If there is shielding material (including unified shielding) on the core of the steel belt armored cable, the cable core shall be replaced by the extruded insulating sleeve instead of the inner layer. If the insulation core has no metal shielding layer, the inner layer of the insulation wire can be extruded or wrapped. The inner layer of the wrapping is generally PVC or PE and other similar strip.
Generally, the steel wire armored cable adopts the extruded inner layer.
For the model of armored cable, such as yjv22, there will be two numbers, the first number is the armor Code: generally, there are 2, 3 and 4 numbers:
2 – indicates double layer steel belt armor
3 – indicates the thin steel wire armouring
4 – indicates the thick steel wire armouring
The steel strip thickness and copper clearance of armored cable, wire diameter and clearance of steel wire armored cable shall meet the relevant standards.

Factors Affecting XLPE Insulation Thermal Shrinkage

The cross-linked polyethylene cable material (XLPE) can make the aggregate structure of the insulating material in a reasonable state through the cross-linking process, and can increase the long-term working temperature of the power cable to 90 ℃, and the instantaneous short-circuit temperature is 170 ℃ ~ 250 ℃. The performance remains unchanged while other performances are improved and enhanced. Therefore, the use of cross-linked polyethylene (XLPE) insulated cables is becoming wider and wider. However, the author found in the experiment that due to the relatively small contact area between the insulating material and the conductor of the small-area cable, especially when the surface of the single-core conductor is smooth and rounded and the adhesion is insufficient, the thermal shrinkage of the insulation is large, and it is difficult to reach the national standard GB/ T12706-2008 “Rated Voltage 1kv (Um=1.2kV) to 35kV) Extruded Insulated Power Cable,such as 1-35kV underground armoured power cable , and Accessories” stipulates the requirement of not more than 4%, and the cable with larger area and higher voltage level is due to insulation and conductor It is easier to pass the test if the contact area is larger and the insulation thickness is larger.
Cables that fail the insulation heat shrinkage test are prolonged during use. Because of the excessive shrinkage caused by the insulation, the conductors may be exposed, which may cause the danger of telephone calls. Therefore, we must try our best to solve the problems in the production and improve the product quality of the cable.
So what factors will affect the insulation thermal shrinkage, and what causes the insulation thermal shrinkage test to fail?

PE is a crystalline polymer that is subjected to shearing and traction stretching under a heated environment (melting temperature), which makes the crystal grains of PE molecules increase in size along the stretching direction (longitudinal) and decrease in lateral size. The ordering is improved, that is, PE molecules are oriented, which increases the number of crystal nuclei, shortens the crystallization time, and increases the crystallinity and strengthens the orientation. However, when the finished XLPE insulated cable is placed at room temperature, the internal stress (shrinkage stress) generated during the extrusion of the XLPE insulation increases, which makes the crystallized XLPE molecules easy to de-orientate (the trend of shrinking). These factors, and Mainly related to the melting temperature and time, cooling rate, and external force (traction and stretching) in these three aspects:

1) Melting temperature and time
At high melt temperature, the crystalline polymer is a melt containing crystal nuclei, and the longer the melting time, the fewer the number of crystal nuclei. Therefore, during the cable insulation extrusion process, the higher the heating and melting temperature of the XLPE insulation material, the longer the residence time (holding time) at the heating temperature, the less the number of crystal nuclei, and the lower the crystallization performance of PE Cable, which is beneficial to reduce The crystallinity of the insulation can make the insulation thermal shrinkage meet the standard requirements;

2) Cooling rate
The temperature decrease rate of the polymer melt from above the melt temperature to below the glass transition temperature is called the cooling rate, and the cooling rate is the key to the crystallization of the polymer. The cooling rate is not only related to the melt temperature and room temperature, but also related to the crystallization rate and thermal properties of the polymer itself. The crystallization rate of PE itself is very large, and PE insulation can also get a higher crystallinity under extremely fast cooling conditions. Therefore, this situation is especially obvious in winter, and special attention should be paid to the control of the cooling rate during the extrusion process of XLPE cable insulation. The specific heat capacity of PE is large and the thermal conductivity is small. If the PE melt has a slow cooling rate and sufficient cooling is obtained, the relaxation process of the PE molecules will be prolonged, the orientation can be easily de-orientated, the degree of orientation can be reduced, and the generation of PE nuclei can be controlled and the grains can be delayed Grow up;
In addition, the conductor temperature also affects the cooling rate of the XLPE insulation. The conductor temperature is too low. When the high-temperature PE melt at the die of the extruder is coated on the surface of the conductor, the LPE insulation will shrink and shrink due to contact with the low-temperature conductor to produce shrinkage stress, and reduce the adhesion between the XLPE insulation and the conductor. Focus on reducing the resistance to heat shrinkage, and ultimately affect the heat shrinkage performance of the insulation layer of the XLPE insulated cable.


3) External force (traction and stretching)
In the cable insulation production process, the molecules are oriented along the force side under the action of external force (traction and stretching), which will promote the formation of nuclei, increase the speed of crystal nucleus generation, increase the number of crystal nuclei, and shorten the knot time and crystal Degree increases. Extrusion tube extrusion is commonly used in current electric winding production enterprises. Compared with extrusion molds, the insulation heat recovery of Si-XLPE insulated electric windings produced by extrusion tube molds that must be stretched during the extrusion process It is much larger. The disadvantage of the poor compactness of the plastic layer of the extrusion tube extrusion is likely to cause the marginal heat shrinkage test to fail, but because the process inspection and final inspection do not do the type test item, the insulation heat shrinkage test is ignored, even if it is extrusion In order to improve the production speed and the smoothness of the extrusion surface, the compression mold is generally several millimeters larger than the insulation outer diameter of the cable. In this way, the insulation outer diameter is ensured during the insulation production process, and insulation is inevitable. It will be stretched, and the molecules are still affected by external forces during the stretching process, resulting in orientation, which makes the insulation thermal shrinkage of the produced cable also larger, even far exceeding the standard requirements.

Cable Insulation Resistance Decreases Due to Moisture

The insulation resistance of directly buried power cables is reduced and the phenomenon of cable line failures often occurs, especially when the cable insulation is damp, it is easy to cause the insulation resistance of directly buried power cables to decrease and substandard

1. The cable raw materials are damp

The raw materials used for cable insulation and sheathing are mainly plastic and rubber materials, and many kinds of materials with special functions are derived from this modification. When manufacturing materials, material manufacturers go through the processes of compounding, mixing, granulation, cooling and drying, as well as during the transportation and storage of the materials, dampness of varying degrees often occurs, resulting in varying degrees of material content. Moisture. Therefore, the cable manufacturer must dry the material before squeezing the material on the cable conductor. The extrusion unit is equipped with a material drying device to prevent the extruded insulation layer and sheath from happening. No defects such as bubbles and blisters, and no bubbles on the surface. This is the rigid process regulation of the cable manufacturer, otherwise the finished cable will not pass the factory withstand voltage test.

2. the cable manufacturing process is damp

In the insulation extrusion process, the insulation layer is scratched, causing holes or degumming of the insulation layer, and the insulation core enters the cooling water tank, which causes the insulation resistance to drop. Or when the protective layer is squeezed, the protective layer is damaged and water enters, so that the insulating layer is damp and the insulation resistance decreases. When manufacturing a multi-core cable, even if the insulation layer is extruded intact, when the insulated core is twisted into a cable, and when the sheath is extruded, damage may occur and the water may enter and become damp, so the finished cable cannot pass the factory withstand voltage test .

3. Moisture during cable construction

In the process of direct buried cable construction, if the cable trench is excavated, the cable burying operation, the cable intermediate joint and the terminal joint are not made standardly, it is very likely to damage the cable sheath and insulation layer. If the soil is wet or the cable trench is filled with water, water in the cable will definitely occur. After the insulation is damp, the surface resistance of the cable insulation will decrease and the surface leakage current will increase, and the insulation resistance will decrease, which will also cause the electric field distortion between the conductor and the insulation layer. The uneven electric field distribution in the insulation will cause free discharge inside the insulation and even lead to cable breakdown. After-sales service practice has proved that more than 95% of the direct-buried cable insulation resistance drop accidents are caused by improper construction.

 

As we all know, water and damp in the process of cable manufacturing and laying operation are the main factors that endanger the electrical performance and service life of the cable. Both cable manufacturers and users attach great importance to this.
Practical experience has proved that the main reasons for the ingress of water and moisture in the cable are as follows.

1) Material purity
If the cable insulation material is mixed with impurities, especially metal impurities, even the pigments of different colors used by China Wire and Cable manufacturer  will directly affect the electrical performance of the insulation and reduce the insulation resistance. The reason is that the non-metallic impurities in the insulating layer will absorb moisture when the cable is damp, and form numerous conductive points; second, the m

etallic impurities in the insulating layer are directly conductive points. Under the combined action of the conductor operating temperature and the external ambient temperature, these conductive points form conductive channels in the insulating layer, resulting in a decrease in insulation resistance and an increase in leakage current, which in turn leads to insulation breakdown.

2) The material is damp
If the cable insulation material has been damp and is not dried before being squeezed on the conductor, there will be quality defects such as a large number of pores in the insulation layer, an unsmooth extrusion surface, reduced mechanical strength, and even cracking. Therefore, when extruding the cable insulation layer, the cable manufacturer must dry the material. When extruding low-smoke and halogen-free materials, more attention should be paid to drying. These are the basic technical knowledge of cable manufacturers.