Power transformer burnout causes and preventive measures - News - Global IC Trade Starts Here Free Products

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First, overvoltage
(1) Suffering from lightning strikes. Most of the high and low voltage lines of power transformers are introduced by overhead lines. Due to the high probability of lightning strikes in mountainous forests, the proportion of distribution transformers damaged by lightning strikes accounts for more than 30% of overhaul during the annual thunderstorm season.
(2) Ferromagnetic resonance occurs in the system. The rural 10kV distribution line has the condition of overvoltage. When the system resonates overvoltage, the primary current of the transformer increases sharply. In addition to causing the primary side fuse of the transformer to be blown, the transformer winding will be damaged. In some cases, it can also cause the casing of the transformer to flash or explode. 
Second, insulation damage
(1) The short-circuit fault of the low-voltage line and the sharp increase of the load cause the current of the transformer to exceed several dozens of the rated current. At this time, the winding is displaced and deformed by the influence of a large electromagnetic torque. Due to the sharp increase in current, the temperature rises rapidly, causing the insulation to accelerate aging.
(2) The winding insulation is damp. This is caused by poor insulation oil or reduced oil level. First, in the storage, transportation or operation and maintenance of transformer insulating oil, water, impurities or other oils are inadvertently mixed into the oil, so that the insulation strength is greatly reduced. Second, the inner layer of the winding is impervious to the immersion paint, the drying is not thorough, the winding lead joints are poorly welded, and the insulation is incomplete, resulting in short-circuit between the turns and the layers. Third, the oil level is lowered to increase the contact surface between the insulating oil and the air. Accelerating the moisture in the air into the oil also reduces the dielectric strength. When the insulation is reduced to a certain value, a short circuit occurs.
Third, tap changer
(1) The oil leakage of the transformer makes the tap changer exposed to the air, and the performance of the insulation is degraded after the damp, resulting in short circuit of the discharge and damage to the transformer.
(2) The oil temperature is too high. The oil in the transformer is mainly used to insulate, dissipate heat and prevent moisture from the windings. 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 internal position, causing the position of the star-shaped moving contact to be incomplete contact, the misaligned movement and static contact The head makes the insulation distance between the two taps smaller, and short-circuit or discharge to the ground under the potential between the two taps, the short-circuit current quickly burns the tap wire, and even causes the entire winding to be damaged.
4. Oil seepage is the most common abnormal phenomenon of transformers. Since the transformer body is filled with oil, rubber beads and rubber pads are placed at each joint to prevent leakage. After long-term operation of the transformer, the beads will be glued. The rubber pad is aged and cracked to cause oil leakage. Of course, the loose screws or the oil drain valve are not tightly closed, and there are blisters or poor welding quality during manufacturing.
Five, iron core multi-point grounding
(1) Multi-point grounding of 10kV distribution transformer core is not easy to find and test. This is because the core grounding of the distribution transformer is internally clamped between the iron core (silicon steel sheet) with a very thin piece of copper. The other end is pressed against the core clamp and directly connected to the transformer housing.
(2) Short circuit between iron core silicon steel sheets. Although the silicon steel sheets are coated with an insulating varnish, the insulation resistance is small and can only block the eddy currents and cannot prevent the high voltage induced current. If the insulating paint on the surface of the silicon steel sheet is naturally aged, it will cause a large eddy current loss and increase the local overheating of the iron core. 
6. Overloading
(1) The three-phase load distribution of the distribution transformer is unevenly distributed, resulting in asymmetry of the three-phase current. The asymmetric current makes the transformer impedance step-down and asymmetrical, so the low-voltage three-phase voltage is unbalanced. This is for the transformer and the user's electrical equipment. Adverse.
(2) When the grounding and phase-to-phase short circuit occurs on the low-voltage side of the transformer, a short-circuit current of 20 to 30 times higher than the rated current will be generated. Such a large current acts on the high-voltage winding, and a large mechanical stress will be generated inside the coil. The mechanical stress will cause the coil to compress, and the stress will disappear after the short-circuit fault is removed. If the coil is repeatedly subjected to mechanical stress, the insulating gasket and the pad will loosen off; the core splint will also loosen, the high-voltage coil Distortion or cracking. In addition, it also produces a temperature that is several times higher than the allowable temperature rise, causing the transformer to burn out in a short period of time.
Seven, man-made damage
(1) The lead wire of the transformer is a copper screw, and the overhead wire is generally made of an aluminum core rubber wire, so that galvanic corrosion is easily generated between copper and aluminum. Under the action of ionization, an oxide film is formed between the copper and aluminum to increase the contact resistance. Large, burnt or fuse the screw, nut and lead at the lead.
(2) Casing flashover discharge is also one of the common appearance abnormalities of transformers. Metal dust with conductive properties in the air is attached to the surface of the casing. In the case of rain and snow, the grid system resonates and is subjected to lightning overvoltage, and a casing flashover discharge or explosion occurs.
(3) During the process of tightening or loosening the lead nut of the transformer, the conductive screw rotates, causing the internal high voltage coil lead of the transformer to be twisted or the soft copper piece pulled out by the low voltage to cause a phase short circuit.
(4) When the core is overhauled, it is not carried out according to the maintenance rules and process standards. Insufficient insulation of coils, leads, tap-changers, etc. is often inadvertently destroyed or the tool is forgotten in the transformer, and flicker occurs, and the voltage is short-circuited. Ground. 
Eight, matters needing attention
(1) Regularly check whether the three-phase current is balanced or exceeds the rated value. If the three-phase load current is seriously unbalanced, measures should be taken to adjust it in time; check whether the oil level of the transformer is normal, whether there is leakage, and replenish oil as soon as possible after the discovery. The switch and coil are exposed to moisture in the air.
(2) Regularly clean the surface of the distribution transformer bushing: check the casing for flash marks, whether the grounding is good, whether the grounding lead has broken strands, desoldering or breaking; use a megger to check that the grounding resistance is not more than 4Ω.
(3) Reasonably select the wiring method of the low-voltage side conductor, such as copper-aluminum transition cable clamp or terminal block. The contact surface is coated with a conductive paste to increase the contact area and prevent heat generation and oxidation.
(4) According to the technical requirements of 10/0.4kV distribution transformers, a set of lightning arresters are installed near the high and low voltage sides of the transformer.
(5) When switching the tap changer, the DC resistance values ​​of the front and rear must be measured by the bridge, and the record should be made to compare whether the three-phase DC resistance is balanced (the phase difference is not more than 4%, and the line difference is not more than 2%). ), the DC resistance value after the switching is compared with the previous records before the switching, and the operation can be put into operation after the switching is determined to be normal.