Oil Immersed Transformer Basic Introduction

1.1 Definition

Distribution transformer, referred to as "distribution transformer", refers to a static electrical device that transforms AC voltage and current according to the law of electromagnetic induction and transmits AC power in a power distribution system. In some areas, power transformers with voltage levels below 35 kV (mostly 10KV and below) are called "distribution transformers" for short. The place and place where the "distribution transformer" is installed is the substation. Distribution transformers should be installed on poles or on the ground in the open.

1.2 Structure

According to its structure, oil immersed transformer can be divided into body, oil conservator, insulating bushing, tap changer, protection device, etc.

1.2.1 Ontology

The body consists of three parts: iron core, winding and insulating oil. The winding is the circuit of the transformer, and the iron core is the magnetic circuit of the transformer. The two form the core of the transformer, that is, the electromagnetic part.

1.2.1.1 Core

The core is the main magnetic circuit part of the transformer. It is usually made of stacked hot-rolled or cold-rolled silicon steel sheets with a high silicon content, a thickness of 0.35 or 0.5mm, and an insulating paint on the surface. The core is divided into two parts: the core column and the iron yoke. The core column is covered with windings. The iron yoke is used to close the magnetic circuit. There are two basic forms of core structure: core type and shell type.

1.2.1.2 Winding

The winding is the circuit part of the transformer, which is generally wound on a winding mold with insulated flat copper wire or round copper wire. The winding is set on the transformer core column, the low-voltage winding is on the inner layer, the high-voltage winding is set on the outer layer of the low-voltage winding, and the sleeves made of insulating materials are used to separate the low-voltage winding and the iron core, and the high-voltage winding and the low-voltage winding. for insulation.

1.2.1.3 Insulating oil

The composition of oil immersed transformer oil is very complicated, mainly composed of naphthenes, alkanes and aromatic hydrocarbons. Transformer oil plays two roles in distribution transformers: one is to insulate between transformer windings, windings, cores and oil tanks effect. The second is that the transformer oil produces convection after being heated, which acts as a heat sink for the transformer core and windings. Commonly used transformer oil has three specifications: No. 10, No. 25 and No. 45. The label indicates the temperature when the oil starts to solidify below zero. For example, "No. 25" oil means that the oil begins to solidify at minus 25°C. Oil specifications should be selected according to local climatic conditions.

1.2.2 Oil conservator

The oil conservator is mounted on the top cover of the oil tank. The volume of the oil conservator is about 10% of the volume of the oil tank. There is a pipe communication between the oil conservator and the oil tank. When the volume of the oil immersed transformer expands or shrinks as the temperature of the oil changes, the oil conservator plays the role of storing and replenishing oil, ensuring that the iron core and windings are immersed in the oil; The contact surface with air reduces the rate of oil deterioration.

There is an oil mark on the side of the oil conservator, and beside the glass tube, there are standard lines for the oil level at -30°C, +20°C and +40°C, indicating the oil level that the transformer that is not in operation should reach; the standard The line can mainly reflect whether the oil quantity is sufficient when the transformer operates at different temperatures.

The oil conservator is equipped with a breathing hole, so that the upper space of the oil conservator communicates with the atmosphere. When the transformer oil expands with heat and contracts with cold, the air in the upper part of the oil conservator can come in and out through the breathing hole, and the oil level can rise or fall, preventing the oil tank from being deformed or even damaged.

1.2.3 Insulation bushing

It is the main insulating device outside the transformer box, and most of the transformer insulating bushings are made of porcelain insulating bushings. The transformer leads the lead wires of the transformer high and low voltage windings from the inside of the oil tank to the outside of the oil tank through the high and low voltage insulating bushings, so that the transformer winding is insulated from the ground (shell and core), and it is also the main part of the connection between the fixed lead wire and the external circuit. The high-voltage porcelain bushing is relatively tall, and the low-voltage porcelain bushing is relatively short.

1.2.4 Taps

The device for changing the taps of the high-voltage winding of the transformer and adjusting the tap position can increase or decrease the number of turns of the primary winding to change the voltage ratio and adjust the output voltage. After the oil immersed transformer is out of operation and disconnected from the grid, the method of manually changing the position of the tap changer to adjust the output voltage is called no-load voltage regulation.

1.2.5 Protection device

1.2.5.1 Gas relay

The gas relay is installed between the transformer oil tank and the connecting pipe of the oil conservator, and is connected with the control circuit to form a gas protection device. The upper contact of the gas relay and the light gas signal form a separate circuit, and the lower contact of the gas relay is connected to an external circuit to form a heavy gas protection. The heavy gas action trips the high-voltage circuit breaker and sends a heavy gas action signal;

1.2.5.2 Anti-riot pipe

Anti-riot tube is a kind of safety protection device for transformers. It is installed on the top of the transformer cover. The anti-riot tube is connected to the atmosphere. When a fault occurs, the heat will vaporize the transformer oil, which will trigger the gas relay to send an alarm signal or cut off the power to prevent the oil tank from bursting.