Three-phase oil-immersed transformer knowledge

The core of a three-phase oil-immersed transformer consists of a closed core and a winding that is placed over the core. In addition, there are fuel tanks, oil conservators, casings, respirators, explosion-proof tubes, radiators, tap-changers, gas relays, thermometers, and oil purifiers.

(1) Iron core

The iron core is the magnetic circuit portion of the transformer. In order to reduce the hysteresis and eddy current loss in the iron core, the iron core is laminated from a silicon steel sheet having a thickness of 0.35 mm to 0.5 mm, and the surface of the silicon steel sheet is coated with an insulating varnish or a surface oxide film is used to insulate the sheets from each other. The upright part of the core of the three-phase transformer is called the iron core column, and the low voltage winding and the high voltage winding of the transformer are placed on the column; the horizontal part is called the iron yoke, which is used to form the closed magnetic circuit.

(2) Winding

The winding, also called the coil, is the circuit part of the transformer and is divided into the original and the secondary windings. The winding connected to the power supply is called the primary winding, and the winding connected to the load is called the secondary winding. Both the primary and secondary windings are wound with copper or aluminum wires wrapped with high-strength insulation. The primary and secondary windings of each phase of the three-phase transformer are cylindrically sleeved on the same core leg, and the low-voltage windings with a small number of turns are placed inside the core, and the high-voltage windings with a large number of turns are placed outside the low-voltage winding. This is placed because the low voltage winding is relatively easy to insulate the core. The sleeve between the low-voltage winding and the iron core and between the high-voltage winding and the low-voltage winding is insulated by a sleeve made of insulating material to reliably insulate them. In order to facilitate heat dissipation, a certain gap is left between the high and low windings as an oil passage to allow the transformer oil to circulate.

(3) Fuel tank

The fuel tank is the outer casing of the transformer. The iron core and windings are installed inside and filled with transformer oil. For transformers with large capacity, heat sinks or heat pipes are installed outside the fuel tank.

Transformer oil is a kind of mineral oil with good insulation performance. It has two functions: one is insulation, the insulation performance of transformer oil is better than air, and the winding is immersed in oil to improve the insulation performance and avoid contact with air. Prevent the winding from being damp; the second is the heat dissipation. Using the convection of the oil, the heat generated by the iron core and the winding is radiated to the outside through the tank wall and the heat pipe. Transformer oil is divided into three sizes, No. 10, No. 25 and No. 45, according to its freezing point. Their freezing points are -10 °C, -25 °C, -45 °C, respectively, which are generally selected according to local climatic conditions.

(4) Oil storage tank

The oil storage cabinet is commonly called a oil pillow. It is a cylindrical container. It is placed horizontally above the fuel tank and connected to the fuel tank of the transformer. The volume of the oil storage cabinet is generally about 10% of the volume of the fuel tank. The oil conservator is a capsule type oil conservator that separates the oil in the conservator from the outside air. When the transformer oil is inflated, the oil flows from the fuel tank to the oil conservator; when the transformer oil is cooled, the oil flows from the oil conservator to the fuel tank. The oil conservator has two functions: First, when the volume of the transformer oil expands or shrinks with the change of the oil temperature, the oil conservator plays the role of oil storage and oil replenishment, ensuring that the fuel tank is filled with oil and iron core and winding dip. In the oil; the second is to reduce the contact area between the oil surface and the air to prevent the transformer oil from moisture and deterioration.

The oil level of the oil conservator is displayed using a connecting rod type ferromagnetic oil level gauge to observe the level of the oil surface. When the oil quantity is insufficient due to leakage or the like, it should be replenished with oil in time. The oil level gauge is engraved with the oil level standard line at -30 ° C, +20 ° C and +40 ° C as the standard for oil loading. The oil level mark +40 °C indicates the maximum limit line of the oil level during full load operation of the transformer at the installation environment. The oil level must not exceed this line; +20 °C indicates the oil level at full load when the annual average temperature is +20 °C. Height; -30 °C indicates the lowest oil level line of the no-load transformer when the environment is -30 °C, not lower than this line. If the oil level is too low, it should be refueled. The oil pillow is equipped with a breathing hole to open the upper space of the oil pillow and the atmosphere. When the transformer oil expands and contracts, the air in the upper part of the oil pillow enters and exits through the breathing hole, and the oil surface can rise or fall to prevent the oil tank from being deformed or damaged.

(5) casing

The transformer winding lead wire is connected to the external circuit through a guide rod, which is an insulator between the guide rod and the cover, which functions as an insulating and fixing guide rod. The casing has two types of high pressure bushings and low pressure bushings.

(6) Explosion-proof tube

The explosion-proof pipe is installed on the transformer cover. When a sudden and sudden failure occurs inside the transformer, the pressure in the fuel tank will rise rapidly to prevent the fuel tank from exploding due to excessive pressure rise. After the action, the internal pressure of the box is released and the contact alarm or trip is turned on.

(7) Gas gas relay

The gas relay is installed between the oil conservator and the connecting pipe of the transformer fuel tank cap by using a flange. The gas relay in operation is full of oil. When a slight fault occurs inside the transformer and bubbles are generated, they first gather in the upper space of the gas relay. And forcing the oil level to drop, so that the upper open cup loses its buoyancy and its mass is aggravated, thereby deflecting in the opposite direction, causing the magnet to move close to the reed switch. The lower contact baffle is the same principle.

(8) Temperature measuring device

Oil surface temperature rise refers to the value of the oil level in the fuel tank that is allowed to exceed the ambient temperature when the transformer is working under rated conditions.

The main change body oil temperature temporary setting alarm is 80 ° C, and the trip is 100 ° C.

(9) Neutral grounding knife

The neutral point grounding method of China's 110kV power system mainly adopts the neutral point direct grounding method (including the neutral point via small resistance grounding method), that is, the large grounding current system. Because the system is in a single-phase ground fault, the ground short-circuit current is large.

When the transformer is powered off, the neutral point must be grounded. Because the transformer winding is semi-insulated (also known as grading insulation), that is, the main insulation of the near-neutral portion of the winding of the transformer, the insulation level is lower than the insulation level at the winding end. Therefore, in order to prevent the overvoltage from damaging the transformer insulation transformer, the neutral point must be grounded.

(10) On-load tap changer

When the oil conservator is used for an on-load tap-changer, a non-capsule switch oil conservator is installed at the bottom of the conservator.

The transformer voltage regulation mode is divided into two types: on-load voltage regulation and no-load voltage regulation:

On-load voltage regulation means that the transformer can adjust its tap position during operation to change the transformer ratio to achieve voltage regulation.

Transformer taps are generally tapped from the high voltage side, which is mainly considered:

(1) The high-voltage winding of the transformer is generally on the outside, and the tap-out connection is convenient;

(2) The current on the high voltage side is smaller, and the conductors of the current-carrying part of the lead-out line and the split-head switch are smaller in cross section, and the influence of poor contact is well solved.

In principle, the tap can be on which side, for economic and technical comparison, such as the 500kV large step-down transformer tap is extracted from the 220kV side, and the 500kV side is fixed.

When the voltage is too low or too high, it is necessary to adjust several taps of the on-load tap-changer to meet the requirements.

It should be adjusted one file at a time, that is, each time the N+1 or N-1 button is pressed, the middle pauses for 1 minute, and when a new number appears on the gear position indicator, press the button again. Repeat the above process in order to reach the final goal. When the electric operation is interlocked (ie, one operation will occur, one or more taps will be adjusted, commonly known as sliding), the second tap position should appear on the gear indicator of the main control panel, and immediately press the emergency. Stop the button and change to manual operation.

2 transformer operation and maintenance

In order to ensure the safe operation and reliable power supply of the transformer, when the transformer has abnormal conditions, it can be found in time, processed in time, and the fault is eliminated in the bud state to prevent the occurrence and expansion of the accident. Therefore, the transformer must be periodically operated. The tour is checked and the run record is made.

(1) Normal operation of the transformer

1 Rated operating mode. The transformer can be operated according to the nameplate specifications under the specified cooling conditions. The allowable temperature during operation of the oil-immersed transformer shall be checked according to the temperature of the upper oil. The upper oil temperature shall comply with the manufacturer's regulations, but the maximum shall not exceed 95 °C. In order to prevent the transformer oil from deteriorating too fast, the upper oil temperature should not exceed 85 °C.

The applied voltage of the transformer should generally not exceed 105% of the rated value. At this time, the secondary side of the transformer can carry the rated current. In some cases, the applied voltage may be 110% of the rated voltage after testing or with the consent of the manufacturer.

2 Allow overload. The transformer can be operated under normal overload or accidental overload conditions. Normal overload can be used frequently, and the allowable value is determined according to the load curve of the transformer, the cooling condition, and the load on the transformer before the overload. Accident overload is only allowed in the event of an accident (a transformer that can also be operated).

The allowable value of accident overload shall comply with the manufacturer's regulations; if there is no manufacturer's regulations, for the self-cooling oil-immersed transformer, it can be operated as required in the following table.

(2) Unusual operation and emergency treatment of the transformer

(a) Anomalies in operation. When the transformer finds any abnormality during operation (such as oil leakage, insufficient oil level in the oil pillow, abnormal heating, abnormal sound, etc.), it should be eliminated. If any of the following conditions occurs, stop repair immediately.

1 The internal sound is loud, very uneven, and has a popping sound.

2 Under normal cooling conditions, the temperature is abnormal and continues to rise.

3 Oil-filled fuel injection or explosion-proof pipe injection.

4 Oil leakage causes the oil level to drop below the limit on the oil level indicator.

5 The oil color changed too much and carbon appeared in the oil.

6 The casing has serious damage and discharge.

(b) Unacceptable overload, abnormal temperature rise and oil level. If the transformer overload exceeds the allowable value, the load of the transformer should be adjusted in time. When the rise of the transformer oil temperature exceeds the permissible limit, the cause should be determined and the method should be adopted to reduce it. Therefore, the following work must be performed.

1 Check the load on the transformer and the temperature of the cooling medium and check against the temperature that should be present at this load and cooling temperature.

2 Check the thermometer.

3 Check the ventilation of the transformer mechanical cooling unit or transformer room.

If the oil temperature is higher than 10 °C above the same load and cooling temperature, or the load is constant, the oil temperature will rise continuously. After checking that the cooling device, transformer room ventilation and thermometer are normal, it may be internal fault of the transformer (such as If the iron core is on fire, the coil layer is short-circuited, etc., stop repair immediately.

If the oil of the transformer has solidified, the transformer can be put into operation with load, but it must be noted that the upper oil temperature and oil circulation are normal.

When it is found that the oil level of the transformer is significantly lower than the oil level at the time of the oil temperature, it should be refueled immediately. If the oil level drops rapidly due to a large amount of oil leakage, it is forbidden to change the gas relay to only act on the signal, but it is necessary to quickly take measures to prevent leakage and immediately refuel.

(c) Processing when the gas relay is operating. When the gas relay signal is activated, the transformer should be inspected to find out the cause of the signal action, whether the air intrudes into the transformer, or because the oil level is lowered, or the secondary circuit is faulty. If the fault cannot be detected outside the transformer, the nature of the gas accumulated in the relay needs to be identified. If the gas is colorless, odorless and non-flammable, it is the air separated from the oil and the transformer can continue to operate. If the gas is flammable, the transformer must be stopped and the cause of the action carefully studied.

When checking whether the gas is flammable, special care must be taken not to bring the fire close to the top of the relay, but to be 5 to 6 cm above it.

If the gas relay action is not caused by air intrusion into the transformer, the flash point of the oil should be checked. If the flash point is lower than the previous record by more than 5 °C, it indicates that there is a fault in the transformer.

If the transformer trips due to the action of the gas relay and has been verified to be flammable, the transformer shall not be put into operation without special inspection and test.

The action of the gas relay depends on the nature of the fault. There are generally two types: one is the signal action without tripping; the other is the simultaneous action of both.

There are several reasons why a signal can be actuated without tripping.

1 Air leaks into the transformer due to oil leakage, fueling or a tight cooling system.

2 The oil level is slowly lowered due to temperature drop or oil leakage.

3 A small amount of gas is generated due to a transformer failure.

4 Caused by a through short circuit.

When the signal and the switch act at the same time, or only switch the actor, it may be due to a serious fault inside the transformer, the oil level drops too fast or the secondary circuit of the protection device is faulty. In some cases, such as after repairs, the air in the oil separates too quickly and may trip the switch.

(d) Transformer oil leakage treatment

There are two kinds of oil leakage: weld leakage and seal leakage. The weld oil leakage treatment is repair welding. When welding, the body should be lifted and the oil should be drained. Sealing oil leakage should identify the cause, such as poor operation (unstable gasket, uneven pressure, insufficient pressure, etc.), should be repaired as appropriate. If the gasket is aged or damaged (eg oil-resistant rubber is sticky, loses elasticity, cracks, etc.), the sealing material should be replaced.

(3) Inspection of oil-immersed transformers

The running transformer should be patrolled and monitored frequently to detect abnormal phenomena or faults in time to avoid serious accidents.

Items that should be inspected and monitored generally include:

(1) Whether the transformer has abnormal sound, such as uneven sound or discharge sound.

(2) Whether the oil level is normal, whether there is leakage or oil leakage.

(3) Whether the oil temperature is normal (the upper oil temperature should generally not exceed 85 °C).

(4) Whether the casing is clean and free of cracks, breakage and discharge.

(5) Whether the connector is hot or not.

(6) Whether the explosion-proof membrane of the explosion-proof pipe is complete.

(7) Does the gas relay leak oil and is filled with oil inside.

(8) Whether the respirator is unblocked, whether the oil level of the oil seal respirator is normal, and whether the silica gel in the respirator is saturated with moisture.

(9) Whether the cooling system is operating normally.

(10) Is the case ground wire intact?