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First, core failure

In normal operation of a power transformer, the core must be somewhat reliably grounded. If there is no grounding, the suspension voltage of the core to the ground will cause intermittent breakdown and discharge of the core to the ground. After the core is grounded at one point, the possibility of forming the suspension potential of the core is eliminated. However, when the core is grounded at more than two points, the uneven potential between the cores will form a circulation between the ground points, and cause the fault of multi-point grounding heating of the core. The core grounding fault of the transformer will cause local overheating of the core. In serious cases, local temperature rise of the core will increase, light gas action will even cause heavy gas action and tripping accident. The short circuit fault between the local core pieces of molten iron increases the iron loss, seriously affects the performance and normal work of the transformer, so that it is not allowed to replace the core silicon steel sheet for repair. Relevant data statistics show that the proportion of faults caused by core problems is the third of all kinds of transformer faults.

Cause of failure:

1. Heavy negligence in the installation process. After completion, the positioning nails for transportation on the top cover of the transformer tank were not turned over or removed.

2. Heavy negligence in manufacturing or overhaul process. The support plate of the core clamp is too close to the core column, and the silicon steel sheet bulges and touches the support plate or yoke screw of the clamp.

3. The cardboard between the foot of the clamp under the iron core and the yoke falls off, causing the foot to collide with the silicon steel sheet or the transformer water board to be damp, resulting in short circuit grounding.

4. The bearing of the submersible oil pump is worn, the metal powder is deposited at the bottom of the tank, and the conductive bridge is formed under the influence of the electromagnetic force, so that the iron yoke is connected with the foot or the bottom of the tank.

5. Metal foreign bodies, such as copper wire, welding rod or core pieces, are accidentally dropped into the tank, resulting in multi-point grounding.

6. The wood pad between the lower clamp and the iron yoke ladder is damp or the surface is covered with a lot of mud, moisture and impurities, so that its insulation is destroyed.

7. The oil mud and dirt of the transformer block the longitudinal heat dissipation oil passage of the iron core, forming short circuit grounding.

8, transformer tank and radiator in the manufacturing process, due to the welding slag cleaning is not thorough, when the transformer is running, under the action of oil flow, impurities are often piled together, so that the core and the tank wall short.

Fault impact: local overheating or even burning of the core, resulting in short circuit of the magnetic circuit, which increases the loss of the core. Local overheating of the core causes the decomposition of transformer oil, resulting in the decline of transformer oil performance. The gas in the transformer is constantly increasing and precipitated, which may lead to the trip accident of gas relay operation.

Two, tap switch failure

The no-load tap switch is faulty. Circuit fault: From the perspective of affecting the change of transformer gas composition, it can be seen that the fault forms of no-load tap switch are often manifested in poor contact, increase of contact corrosion resistance heating, suspension discharge caused by grounding fracture of the fastening bolt on the insulation support of the switch. Mechanical failure: The failure of the no-load tap switch is reflected in the insufficient pressure of the switch spring, the insufficient pressure of the roller, the uneven pressure of the roller, the poor contact so that the effective contact area is reduced. In addition, the oil contamination at the switch contact increases the contact resistance, which will cause the connector contact surface burn during operation.

Structure combination: tap switch numbering error, disorderly, all levels of ratio is not regular, resulting in three-phase voltage imbalance, circulation and increase loss, cause transformer failure. Insulation fault: the interphase insulation distance of the tap adapter on the tap switch is not enough, the accumulation of mud on the insulation material is damp, when overvoltage occurs, it will also make the tap switch short-circuit fault between phases.

The mechanical failure of on-load tap switch includes the failure of switching switch or branch selector and the mechanical failure of operating mechanism, which is a serious failure and may result in the following situations:

(1) split selector with load conversion. This situation is similar to the load shut-off isolation switch, which will trip the main gas relay of the transformer body.

(2) The switching switch refuses or the switching is not in place. If the switch stops at an intermediate position for a long time in the middle of the switch, the transition resistance will be overheated due to long-term power-on, which may make the gas relay of the switch operate and trip the transformer.

(3) The contact of the switch or the split selector is not in good contact with overheating.

The transformer has a load tap switch three corresponding in the same position. The so-called "out of step", refers to the voltage regulation for some reason, so that the three-phase position is inconsistent. In this state, zero sequence voltage and zero sequence current will be generated due to the unbalance of the three phases of the secondary voltage. In the transformer voltage regulation process, short-term inconsistency is possible, if a long time is inconsistent, may make the transformer overheating or tripping.

Three, transformer leakage fault and oil flow live fault

The reason of transformer leakage: transformer solder joint, weld length: oil-immersed power transformer is a variety of welded fittings based on steel plate welded shell. A 31500KVA transformer has about 27 connection points with rubber seals, and the total length of weld is about 20m. Therefore, there may be many leakage paths. Inferior seal material: Inferior seal material and defect are the main reasons for leakage of transformer connection site.

Type of transformer leakage: air leakage, air leakage is a kind of invisible leakage, such as the head of the sleeve, the diaphragm of the oil storage tank, the glass of the safety airway and the weld trachoma and other parts of the air is invisible. However, due to the leakage caused by winding insulation moisture and oil accelerated aging of the great influence. Oil leakage: mainly refers to the leakage of oil in the casing or oil in the on-load pressure regulating tap changer room to the transformer body. The normal oil level of the oil-filled casing is higher than that of the transformer body. If the sealing part of the lower part of the casing is not tightly sealed, the oil shortage in the casing will be caused by the oil pressure difference, which affects the safe operation of the equipment.

When the transformer oil flow is charged, the partial discharge signal strength is equivalent to 2 ~ 3 orders of magnitude of the local discharge power of the transformer in normal operation. Strong discharge signals can also be measured on the small bushing of the transformer core grounding, and there is no definite relationship between the phase and the transformer operating voltage. When the transformer power supply is disconnected and only the submersible oil pump is started, the strong discharge signal can still be measured. When the submersible oil pump is stopped, the discharge signal will disappear.