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Several reasons for transformer burning

time:2020/12/8   source:华天电力  reading:248 time

1. Overvoltage


1. Suffered from lightning, most of the high and low voltage control circuit designs of power system transformers are introduced by overhead lines. Because my country is located in the mountainous forests of China, the probability of lightning strikes is relatively high, so in the thunderstorm season of the company every year, lightning strikes cause damage The proportion of distribution transformers underwent overhaul accounted for more than 30%.


2. Ferroresonance occurs in the system, and the 10kV distribution line has the condition of overvoltage. When the system resonant overvoltage, the primary current of the transformer surges. At this time, in addition to the fuse on the primary side of the transformer, the transformer winding will also be damaged. In this case, the bushing of the transformer will flashover or explode.


2. Insulation damage


1. The short-circuit fault of the low-voltage line and the rapid development of the load increase, so that the current of the transformer can exceed dozens of times the rated operating current. At this time, the winding is affected by the large electromagnetic torque of the enterprise and shifts and deforms. The sharp increase in current causes a rapid increase in temperature changes, leading to accelerated aging of insulation technology.


2. The winding insulation is damp, which is caused by poor insulating oil or lowered oil level. First, in the storage, transportation or operation and maintenance of transformer insulating oil, water, impurities or other oil stains are accidentally mixed into the oil. The insulation strength is greatly reduced. The second is that the inner layer of the winding is impervious to impregnation, the drying is not thorough, the winding lead joints are poorly welded, and the insulation is incomplete, which leads to short circuits between turns and layers. When the surface area increases, accelerating the entry of moisture in the air into the oil will also reduce its insulation strength. When the insulation drops to a certain value, a short circuit will occur.


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Three, tap switch


1. The transformer leaks oil, so that the tap changer is exposed to the ambient air. After the insulation is damp, the performance of the insulation continues to decline, resulting in short-circuiting of different discharges and damage to the transformer.


2. The oil temperature is too high. The oil in the transformer is mainly used to insulate the windings, heat dissipation and moisture proof. 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 structural design, insufficient pressure management, unreliable contact, and the position of the external character wheel is not completely consistent with the actual working position inside, causing the star-shaped moving contact position to be incompletely contacted and misaligned Moving and static contacts make the insulation horizontal distance between the two taps gradually decrease, and various short circuits or ground discharges occur under the influence of the potential between the two taps. The short-circuit fault current can quickly turn the taps. Burnout may even damage the entire winding of the enterprise.


Fourth, oil seepage


Oil seepage is a common phenomenon of abnormal appearance behavior of transformer Z. Because the process of transformer body in our country is full of oil, there are rubber beads and rubber pads at each connection part to prevent leakage. The transformer needs to go through a long-term operation mode. Aging and cracking of the rubber beads and rubber pads can cause oil leakage. Of course, whether the screws are loose or the oil drain valve is not tightly controlled, the manufacturing enterprises have blisters or poor welding quality.


Five, the iron core is grounded at multiple points


It is not easy for us to find and test the multi-point grounding method of the transformer core of the 1.10kV distribution system. This is because the core grounding of the transformer design in my country’s power distribution system uses a thin copper sheet inside the core (silicon steel The other end is pressed on the iron core plywood and cannot be directly connected to the transformer shell.


2. There is a short circuit between the 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 the eddy current but cannot prevent the high-voltage induced current. If the insulating paint on the silicon steel sheet surface is naturally aging, it will A large eddy current loss is generated, and local overheating of the iron core is increased.


Six, overload


1. Uneven distribution of the three-phase load of the distribution transformer results in asymmetrical three-phase currents, and 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 of.


2. When the low-voltage side of the transformer is grounded or short-circuited between phases, it will produce a short-circuit calculation current that is significantly higher than the rated operating current by 20 to 30 times. Such a large current acts on the high-voltage winding, and the inside of the coil will generate a great This kind of developmental mechanical design stress will cause these coils to compress. After the short-circuit fault is relieved, the stress will disappear with China. If the coil is not reused and subjected to various mechanical engineering stresses, its insulating gaskets and backing plates There will be loosening and falling off; the iron core splint screws will also be loose, the high-voltage coil will be distorted or cracked, and the temperature of the enterprise will be several times higher than the allowable temperature rise, which will lead to the burning of our country’s transformers in a short time.


Seven, man-made damage


1. The lead wire of the transformer is a copper screw, and the length of the overhead line can generally be an aluminum core rubber wire. This can easily lead to corrosion of Chinese electrochemical equipment between copper and aluminum. Under the action of ionization technology, there is no one between copper and aluminum. The oxide film gradually increases the resistance of the contact and burns or fuses the screw, nut and lead at the lead.


2. The development of bushing flashover discharge is also one of the common abnormal appearance behaviors of transformers. Metal dust with conductive material properties in the air environment is attached to the bushing surface. In case of rain, snow, and humid weather, the power grid management system Resonance is achieved, and when overvoltage is struck by lightning, it will cause the casing flashover discharge or explosion.


3. In the process of tightening or loosening the lead nut of the transformer, the conductive screw rotates with it, causing the high-voltage coil lead in the transformer to break or the soft copper sheet on the low-voltage lead to short-circuit.


4. Failure to follow the maintenance work regulations and process technical standards during the core lifting maintenance, often accidentally destroy the insulation structure of the coil, lead, tap changer, etc. or forget the tools in the transformer, which can happen in the enterprise. Flicker, it will lead to short circuit protection grounding.


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