Oil Cooled Distribution Transformer Construction

The core part of the three-phase oil cooled distribution transformer is composed of a closed iron core and a winding set on the iron core column. In addition, there are oil tanks, oil storage cabinets, bushings, respirators, explosion-proof pipes, radiators, tap changers, gas relays, thermometers, oil purifiers, etc.

(1) oil cooled distribution transformer iron core

The iron core is the magnetic circuit part of the transformer. In order to reduce hysteresis and eddy current loss in the iron core, the iron core is made of silicon steel sheets with a thickness of 0.35 mm to 0.5 mm. The surface of the silicon steel sheets is coated with insulating varnish or the surface oxide film is used to insulate the sheets from each other. The upright part of the iron core of the three-phase transformer is called the iron 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) oil cooled distribution transformer winding

The winding, also called the coil, is the circuit part of the transformer, which is divided into 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 wire or aluminum wire wrapped with high-strength insulators. The primary and secondary windings of each phase of the three-phase transformer are made into a cylindrical shape and placed on the same iron core column. The low-voltage winding with a small number of turns is placed inside and 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 to insulate the low voltage winding from the core. The 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 insulate them reliably. 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) oil cooled distribution transformer oil tank

The oil tank is the shell of the transformer, in which the iron core and windings are housed 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 kind of mineral oil with good insulation performance. It has two functions: one is insulation. The insulation performance of transformer oil is better than that of air. Immersing windings in oil can improve the insulation performance of various places and avoid contact with air. Prevent the winding from getting damp; the second is the heat dissipation effect, using the convection of the oil to dissipate the heat generated by the iron core and the winding to the outside through the box wall and the heat dissipation pipe. Transformer oil is divided into three specifications: No. 10, No. 25, and No. 45 according to its freezing point. Their freezing points are respectively -10°C, -25°C, and -45°C. Generally, they are selected according to local climate conditions.

(4) oil cooled distribution transformer 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 with 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 heats up, the oil flows from the oil tank to the oil storage tank; when the transformer oil cools down, the oil flows from the oil storage tank to the oil tank. The oil conservator has two functions: first, when the volume of transformer oil expands or shrinks with the change of oil temperature, the oil conservator plays the role of oil storage and replenishment, ensuring that the oil tank is filled with oil and the iron core and winding are immersed In the oil; second, it can reduce the contact area between the oil surface and the air, and prevent the transformer oil from getting damp and deteriorating.

The oil level display of the oil conservator uses a connecting rod type ferromagnetic oil level gauge to observe the level of the oil level. When the oil level is insufficient due to leakage or other reasons, it should be replenished in time. The oil level gauge is engraved with the standard lines of the oil level when the oil temperature is -30°C, +20°C and +40°C, as the standard for oil filling. +40°C on the oil level mark indicates the maximum limit line of the oil level in full-load operation of the transformer at the installation site when the maximum ambient temperature is +40°C, and the oil level must not exceed this line; +20°C indicates the oil level at full-load operation when the annual average temperature is +20°C Height; -30°C means the minimum oil level line of the no-load transformer when the environment is -30°C, and it must not be lower than this line. If the oil level is too low, refuel. There are breathing holes on the oil pillow, so that the upper space of the oil pillow communicates with the atmosphere. When the transformer oil expands with heat and contracts with cold, the air on the upper part of the oil conservator enters and exits through the breathing hole, and the oil level can rise or fall to prevent the oil tank from being deformed or damaged.

(5) oil cooled distribution transformer Casing

The lead wire of the transformer winding is connected to the external circuit through the guide rod, and the bushing is the insulator between the guide rod and the box cover, which plays the role of insulating and fixing the guide rod. There are two types of bushings: high-voltage bushing and low-pressure bushing.

(6) Explosion-proof tube

The explosion-proof tube is installed on the transformer tank cover. When a violent sudden failure occurs inside the transformer, the pressure in the fuel tank will rise rapidly, in order 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 is connected to alarm or trip.

(7) Gas gas relay

Use the flange to install the Buchholz relay between the oil conservator and the connecting pipe of the transformer oil tank cover. The Buchholz relay is full of oil during operation. When a slight fault occurs inside the transformer and bubbles are generated, they will first gather in the upper space of the Buchholz relay. And force the oil level to drop, so that the upper opening cup loses its buoyancy and its own mass increases, so that the reverse deflection makes the magnet move close to the reed switch. The principle of the lower contact baffle type is the same.

(8) oil cooled distribution transformer temperature measuring device

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

The oil temperature of the main transformer body is temporarily set to alarm at 80°C and trip at 100°C.

(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, the large grounding current system. Because the system has a large ground short-circuit current when a single-phase ground fault occurs.

When the transformer stops power transmission, its neutral point must be grounded. Because the transformer winding is semi-insulated (also known as graded insulation), that is, the main insulation of the transformer winding near the neutral point, its insulation level is lower than the insulation level of the winding end. Therefore, in order to prevent overvoltage damage to the transformer, when the insulation transformer stops power transmission, its neutral point must be grounded.

(10) oil cooled distribution transformer tap changer

When the oil conservator is used for the on-load tap changer, a switch oil conservator without capsule is installed at the bottom of the oil conservator.