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Causes and Preventive Measures of Power Transformer Burning

(1) Lightning strike. Most of the high-voltage and low-voltage lines of power transformers are introduced by overhead lines. Because they are located in mountainous forest land, the probability of lightning stroke is high. Therefore, in the annual thunderstorm season, the proportion of distribution transformers damaged by lightning stroke accounts for more than 30% of the overhaul.

(2) Ferromagnetic resonance occurs in the system. The rural 10kV distribution line has the condition of overvoltage. When the system resonance overvoltage occurs, the primary current of the transformer will surge. At this time, in addition to fusing the fuse at the primary side of the transformer, it will also damage the transformer winding. In some cases, it will also cause flashover or explosion of transformer bushing.

(1) The short-circuit fault of low-voltage line and the sharp increase of load make the current of transformer exceed dozens of times of rated current. At this time, the winding is displaced and deformed due to the influence of great electromagnetic torque. Due to the sharp increase of current, the temperature rises rapidly, resulting in accelerated aging of insulation.

(2) The winding insulation is damp. This is caused by poor insulation oil quality or low oil level. First, during the storage, transportation or operation and maintenance of transformer insulating oil, water, impurities or other oil stains are inadvertently mixed into the oil, which greatly reduces the insulation strength. Second, during manufacturing, the inner layer of the winding is not fully impregnated and dried, the winding lead joint is poorly welded, and the insulation is incomplete, resulting in inter turn and inter layer short circuit. Third, the decrease of oil level will increase the contact surface between insulating oil and air, accelerate the water in the air into the oil, and also reduce its insulation strength. When the insulation is reduced to a certain value, a short circuit will occur.

(1) Oil leakage of transformer makes the tap changer exposed in the air, and the performance decreases after the insulation is damp, resulting in discharge short circuit and damage to the transformer.

(2) The oil temperature is too high. The oil in the transformer mainly plays the role of insulation, heat dissipation and moisture-proof for the winding. If the oil temperature in the transformer is too high, it will directly affect the normal operation and service life of the transformer.

(3) The tap changer has poor quality, unreasonable structure, insufficient pressure and unreliable contact. The position of the external word wheel is not completely consistent with the internal actual position, resulting in incomplete contact of the star moving contact. The misplaced moving and static contacts reduce the insulation distance between the two taps, and short circuit or ground discharge occurs under the action of the potential between the two taps, The short-circuit current will soon burn out the tap turns and even damage the whole winding.

Oil leakage is the most common appearance abnormality of the transformer. Because the transformer body is full of oil, rubber beads and rubber pads are clamped at each connecting part to prevent leakage. After long-term operation of the transformer, the rubber beads and rubber pads will age and crack, resulting in oil leakage. Of course, if the screw is loose or the oil drain valve is not closed tightly, there will be sand holes or poor welding quality during manufacturing.

(1) The multi-point grounding of 10kV distribution transformer iron core is not easy to find and test, because the iron core grounding of distribution transformer is internally clamped with a very thin red copper sheet, one end is clamped between the iron core (silicon steel sheet), and the other end is pressed on the iron core clamp plate and directly connected with the transformer shell.

(2) Short circuit between iron core and silicon steel sheet. Although the silicon steel sheets are coated with insulating paint, the insulation resistance is very small, which can only isolate the eddy current and can not prevent the high-voltage induced current. If the insulating paint on the surface of silicon steel sheet is naturally aged, it will produce great eddy current loss and increase the local overheating of the iron core.

(1) The uneven three-phase load distribution of distribution transformer leads to the asymmetry of three-phase current. The asymmetric current makes the transformer impedance step-down asymmetric, so the low-voltage three-phase voltage is unbalanced, which is unfavorable to the transformer and users' electrical equipment.

(2) When grounding and phase-to-phase short circuit occur at the low-voltage side of the transformer, a short-circuit current 20 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. This 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 the mechanical stress, The insulating pad and backing plate will loosen and fall off; The iron core clamping plate screws will also be loose, and the high-voltage coil will be distorted or cracked. In addition, the temperature will be several times higher than the allowable temperature rise, resulting in the burning of the transformer in a very short time.

(1) The outgoing line of the transformer is a copper screw, while the overhead line generally adopts aluminum core rubber wire, so it is easy to produce electrochemical corrosion between copper and aluminum. Under the action of ionization, an oxide film is formed between copper and aluminum, increasing the contact resistance, burning or melting the screw, nut and lead at the lead.

(2) Bushing flashover discharge is also one of the common appearance abnormalities of transformer. Metal dust with conductive properties in the air is attached to the surface of the bushing. In case of rain, snow and humid weather, resonance of the power grid system and lightning overvoltage, the bushing flashover discharge or explosion will occur.

(3) In the process of fastening or loosening the lead nut of the transformer, the conductive screw rotates, resulting in the torsion of the lead of the high-voltage coil inside the transformer or the collision of the soft copper strip led out by the low-voltage, resulting in phase to phase short circuit.

(4) When overhauling the hanging core, it is not carried out according to the maintenance procedures and process standards. It is often careless to damage the insulation of coils, leads, tap changers, etc., or leave tools in the transformer. In some cases, flickering occurs, or short-circuit grounding occurs.

(1) Regularly check whether the three-phase current is balanced or exceeds the rated value. If the three-phase load current is seriously unbalanced, take measures to adjust it in time; Check whether the oil level of the transformer is normal and whether there is leakage. If any, replenish the oil as soon as possible to avoid the tap changer and coil exposed to moisture in the air.

(2) Regularly clean the dirt on the surface of distribution transformer bushing: check whether the bushing has flashover marks, whether the grounding is good, and whether the grounding lead has broken strands, desoldering and fracture; The grounding resistance detected with a megger shall not be greater than 4 .

(3) Reasonably select the wiring mode of low-voltage side conductor, such as copper aluminum transition clamp or terminal block. Apply conductive paste on the contact surface to increase the contact area and prevent heating and oxidation.

(4) According to the technical requirements of 10 / 0.4kV distribution transformer, a set of lightning arresters shall be installed at the high and low voltage sides close to the transformer.

(5) When switching the tap changer, the DC resistance values of the first and second times must be measured with an electric bridge, and records must be made to compare whether the three-phase DC resistance is balanced (the phase to phase difference is no more than 4%, and the line to line difference is no more than 2%). Compare the DC resistance value after switching with the previous records before switching, and put into operation after confirming that the switching is normal.

Causes and Preventive Measures of Power Transformer Burning 1

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