Structure
The core part of the three-phase oil-immersed transformer is composed of a closed iron core and windings wrapped around the iron core posts. In addition, there are oil tanks, oil conservator cabinets, casings, breathers, explosion-proof pipes, radiators, tap changers, gas relays, thermometers, oil purifiers, etc.
(1) Iron core
The iron core is the magnetic circuit part of the transformer. In order to reduce hysteresis and eddy current losses in the iron core, the iron core is made of silicon steel sheets 0.35 mm to 0.5 mm thick. The surfaces of the silicon steel sheets are coated with insulating paint or surface oxide films are used to insulate the sheets from each other. The upright part of the iron core of a three-phase transformer is called the core column, and the low-voltage winding and high-voltage winding of the transformer are set on the column; the horizontal part is called the iron yoke, which is used to form a closed magnetic circuit.
(2) Winding
The winding, also called the coil, is the circuit part of the transformer and is divided into two types: primary and 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. The primary and secondary windings are made of copper or aluminum wire wrapped with high-strength insulation. The primary and secondary windings of each phase of the three-phase transformer are made into a cylindrical shape and are placed on the same iron core column. The low-voltage winding with a small number of turns is placed inside close to the iron core, and the high-voltage winding with a large number of turns is placed outside the low-voltage winding. This placement is because it is easier for low voltage windings to insulate the core. Sleeves made of insulating material are used to isolate the low-voltage winding and the iron core and between the high-voltage winding and the low-voltage winding 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 flow.
(3) Fuel tank
The oil tank is the outer shell of the transformer. The iron core and windings are installed inside and filled with transformer oil. For transformers with relatively large capacity, heat sinks or heat pipes are installed outside the oil tank.
Transformer oil is a mineral oil with good insulating properties. It has two functions: one is insulation. The insulating properties of transformer oil are better than air. Immersing the windings in oil can improve the insulation properties everywhere and avoid contact with air. Prevent the winding from getting damp; the second is the heat dissipation function, which uses the convection of oil to dissipate the heat generated by the iron core and winding to the outside through the box wall and heat pipe. Transformer oil is divided into three specifications: No. 10, No. 25, and No. 45 based on its freezing point. Their freezing points are -10°C, -25°C, and -45°C respectively. They are generally selected according to local climate conditions.
(4) Oil conservator
The oil conservator, commonly known as the oil pillow, is a cylindrical container placed horizontally above the oil tank and connected to the oil tank of the transformer by pipes. The volume of the oil conservator is generally about 10% of the volume of the oil tank. The oil conservator is a capsule-type oil conservator, and the capsule isolates the oil in the oil conservator from the outside air. When the transformer oil thermally expands, the oil flows from the oil tank to the oil conservator; when the transformer oil contracts, the oil flows from the oil conservator to the oil tank. The oil conservator has two functions: first, when the volume of transformer oil expands or shrinks with changes in oil temperature, the oil conservator plays the role of oil storage and oil replenishment, ensuring that the oil tank is filled with oil and the iron core and windings are immersed. in the oil; secondly, it can reduce the contact area between the oil surface and the air, preventing the transformer oil from getting damp and deteriorating.
The oil level display of the oil conservator uses a connecting rod ferromagnetic oil level gauge to observe the level of the oil level. When the oil level is insufficient due to leakage and other reasons, oil should be added in time to supplement it. The oil level gauge is engraved with the oil level height standard lines when the oil temperature is -30°C, +20°C and +40°C, as the standard for oil filling. +40℃ on the oil level mark indicates the maximum oil level of the transformer at the installation site when the maximum ambient temperature is +40℃, and the oil level must not exceed this line; +20℃ indicates the oil level when the annual average temperature is +20℃. Height; -30℃ indicates the lowest oil level line of the no-load transformer when the environment is -30℃. It must not be lower than this line. If the oil level is too low, add more oil. The oil pillow is equipped with a breathing hole to communicate the upper space of the oil pillow with the atmosphere. When the transformer oil expands with heat and contracts with cold, the air in the upper part of the oil pillow goes in and out through the breathing hole, and the oil level can rise or fall to prevent deformation or damage of the oil tank.
(5) Casing
The lead wire of the transformer winding is connected to the external circuit through the guide rod. The bushing is the insulator between the guide rod and the box cover. It plays the role of insulating and fixing the guide rod. There are two types of casing: high-pressure casing and low-pressure casing.
(6) Explosion-proof pipe
The explosion-proof pipe is installed on the transformer tank cover. When a severe sudden failure occurs inside the transformer, the pressure in the fuel tank will rise rapidly to prevent fuel tank explosion accidents caused by excessive pressure rise. After action, the internal pressure of the box is released and the contacts are connected to alarm or trip.
(7) Gas gas relay
Use the flange to install the gas relay between the connecting pipe of the oil conservator and the transformer tank cover. During operation, the gas relay is filled with oil. When a slight fault occurs inside the transformer and bubbles are generated, they will first gather in the upper space of the gas relay. And forces the oil level to drop, causing the upper open cup to lose buoyancy and increase its own mass, thereby deflecting in the opposite direction, causing the magnet to move closer to the reed switch. The principle of lower contact baffle type is the same.
(8) Temperature measuring device
Oil surface temperature rise refers to the value by which the oil surface temperature in the oil tank is allowed to exceed the ambient temperature when the transformer is operating under rated conditions.
The oil temperature of the main transformer body is temporarily set to alarm at 80℃ and trip at 100℃.
(9) Neutral point grounding knife
The neutral point grounding method of my country's 110kV power system mainly adopts the neutral point direct grounding method (including the neutral point grounding method through a small resistance), that is, a large grounding current system. Because when a single-phase ground fault occurs in the system, the ground short-circuit current is very large.
When the transformer is shut down for power supply operation, its neutral point must be grounded. Because the transformer winding is semi-insulated (also called graded insulation), that is, the main insulation of the transformer winding near the neutral point has a lower insulation level than the insulation level at the end of the winding. Therefore, in order to prevent overvoltage from damaging the transformer insulation, when the transformer is shut down for power transmission operation, its neutral point must be grounded.
(10) Tap changer
When the oil conservator is used for an on-load voltage regulating transformer, a capsule-free switch oil conservator is installed at the bottom of the oil conservator.
Transformer voltage regulation methods are divided into two types: on-load voltage regulation and no-load voltage regulation:
On-load voltage regulation means that the tap position of the transformer can be adjusted during operation, thereby changing the transformer ratio to achieve the purpose of voltage regulation.
Transformer taps are generally tapped from the high voltage side. The main considerations are:
(1) The high-voltage winding of the transformer is generally on the outside, and the tap leads out for easy connection;
(2) The current on the high-voltage side is smaller, and the conductor cross-section of the current-carrying part of the lead wire and tap switch is smaller, so the impact of poor contact can be easily solved.
In principle, the tap can be on either side. An economic and technical comparison must be made. For example, the tap of a 500kV large step-down transformer is drawn from the 220kV side, while the 500kV side is fixed.
When the voltage is too low or too high and several taps of the on-load tap-changer need to be adjusted to meet the requirements, the following situations need to be paid attention to:
It should be adjusted gear by gear, that is, every time you press the N+1 or N-1 button, pause for 1 minute. When a new number appears on the gear indicator, press the button again. Repeat the above process in turn until you reach the final goal. When there is interlocking in electric operation (that is, one operation will cause more than one tap to be adjusted, commonly known as sliding gear), you should press the emergency button immediately after the second tap position appears on the gear indicator on the main transformer control screen. Stop button and switch to manual operation.
