Distribution transformer, referred to as “distribution transformer”. It refers to a static electrical apparatus in a distribution system that transfers alternating current and alternating voltage according to the law of electromagnetic induction. Distribution transformers usually refer to power transformers operating in the distribution network with a voltage class of 10-35kV (most are 10kV and below) and a capacity of 6300KVA and below directly supplying power to end users.

Distribution transformer, referred to as “distribution transformer”. It refers to a static electrical apparatus in a distribution system that transfers alternating current and alternating voltage according to the law of electromagnetic induction. Distribution transformers usually refer to power transformers operating in the distribution network with a voltage class of 10-35kV (most are 10kV and below) and a capacity of 6300KVA and below directly supplying power to end users.

1. Introduction to power distribution

1.1 Definitions

Distribution transformer, referred to as “distribution transformer”, refers to the distribution system according to the law of electromagnetic induction to transform AC voltage and current and transfer AC electrical energy. In some areas, power transformers with voltage levels below 35 kV (mostly 10KV and below) are called “distribution transformers”, referred to as “distribution transformers”. The place and place where the “distribution” is installed is a substation. The distribution transformer should be installed on the column or on the floor in the open air.

1.2 Structure

This edition introduces the structure of the oil-immersed distribution transformer. According to its structure, the oil-immersed distribution transformer can be divided into the body, the oil storage tank, the insulation bushing, the tap changer, the protection device, etc. As shown below

 

1.2.1 Ontology

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

1.2.1.1 Iron core

The core is the main magnetic circuit in the transformer. It is usually composed of hot rolled or cold rolled silicon steel sheets with high silicon content, thickness of 0.35 or 0.5mm, and the surface is coated with insulating paint. The core is divided into two parts: the core column and the iron yoke. The core column is covered with winding, and 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 made of insulated flat copper wire or round copper wire wound on the winding die. The winding is set on the transformer core column, the low-voltage winding is in the inner layer, the high-voltage winding is set on the outer layer of the low-voltage winding, between the low-voltage winding and the iron core, between the high-voltage winding and the low-voltage winding, are separated by a sleeve made of insulating material to facilitate insulation.

1.2.1.3 Insulation oil

The composition of transformer oil is very complex, mainly composed of cycloalkanes, alkanes and aromatic hydrocarbons, transformer oil plays two roles in the distribution transformer: one is the insulation between the transformer winding and the winding, the winding and the core and the oil tank. The second is that the transformer oil generates convection after heating, which plays a heat dissipation role on the transformer core and winding. The commonly used transformer oil has three specifications, No. 10, No. 25 and No. 45, and its label indicates the temperature at which the oil begins to solidify at minus 25 ° C, for example, “No. 25” oil indicates that the oil begins to solidify at minus 25 ° C. Oil specifications should be selected according to local climatic conditions.

1.2.2 Oil Storage tank

The oil storage tank is mounted on the top cover of the tank. The volume of the oil storage tank is about 10% of the volume of the oil tank. There is a pipe connecting the oil storage tank and the fuel tank. When the volume of the transformer expands or shrinks with the temperature change of the oil, the oil storage tank plays the role of oil storage and oil replenishment to ensure that the iron core and winding are immersed in the oil; At the same time, due to the installation of the oil storage tank, the contact surface between the oil and the air is reduced, and the deterioration rate of the oil is reduced.

There is an oil mark on the side of the oil storage tank, and there is a standard line of oil level height when the oil temperature is -30℃, +20℃ and +40℃ next to the glass tube, indicating the oil level height that the transformer should reach when it is not put into operation; The standard line can mainly reflect whether the amount of oil is sufficient when the transformer runs at different temperatures.

The oil storage cabinet is equipped with a breathing hole to communicate the upper space of the oil storage cabinet with the atmosphere. When the transformer oil expands and shrinks, the air on the upper part of the oil storage cabinet can enter and exit through the breathing hole, and the oil level can rise or fall to prevent the tank from deformation or even damage.

1.2.3 Insulation bushing

It is the main insulation device outside the transformer box, and most of the transformer insulation bushings use porcelain insulation bushings. Through the high and low voltage insulating bushing, the transformer leads the high and low voltage winding from the oil tank to the outside of the oil tank, so that the transformer winding is insulated to the ground (shell and core), and is also the main part of the fixed lead and the external circuit connection. High pressure porcelain bushing is relatively tall, low pressure porcelain bushing is relatively short.

1.2.4 Tap

Transformer high voltage winding to change the tap device, adjust the tap position, can increase or reduce the number of turns of the primary winding part (public number: pump pipe), to change the voltage ratio, so that the output voltage is adjusted. After the transformer is out of operation and disconnected from the grid, the output voltage is adjusted by manually changing the tap changer position, which is called no-load regulation.

1.2.5 Protection Devices

1.2.5.1 Gas Relay

The gas relay is installed in the middle of the connecting pipe between the transformer oil tank and the oil storage tank, and is connected with the control circuit to form a gas protection device. The upper contact of the gas relay forms a separate circuit with the light gas signal. The lower contact of the gas relay is connected with the external circuit to form the heavy gas protection. The heavy gas action causes the high voltage circuit breaker to trip and sends out the heavy gas action signal.

1.2.5.2 Riot Pipe

The anti-riot tube is a safety protection device of the transformer, installed on the transformer cover, the anti-riot tube is communicated with the atmosphere, and the heat will vaporize the transformer oil when the fault occurs, triggering the gas relay to send an alarm signal or cut off the power supply to avoid the tank bursting.

Step 2 Classify

2.1 Classification by installation location

Distribution transformers are classified into indoor and outdoor transformers according to the installation location.

Outdoor installation is divided into platform 墎 type, tower type and floor type (including preassembled type).

2.1.1 Pole and tower type

The pole type is a frame on which the transformer is mounted on the pole. It is divided into single pole type and double pole type.

When the distribution transformer capacity is 30KVA and below (including 30KVA), a single-pole distribution transformer platform is generally used. Install the distribution transformer, high voltage drop fuse and high voltage arrester on a cement pole, the pole should be tilted 13°-15° in the opposite direction of the assembly of the distribution transformer.

When the capacity of the distribution transformer is 50KVA ~ 315KVA, the two-bar distribution transformer is generally used. The distribution transformer is composed of a main cement rod and another auxiliary rod. The main rod is equipped with a high voltage drop fuse and a high voltage lead down line, and the auxiliary rod is provided with a secondary reverse lead. The two-bar distribution transformer is sturdy by the single-bar distribution transformer.

The advantages of the tower installation: less land, no need for walls or fences around, the live part is high from the ground, and it is not easy to have accidents. Disadvantages: The bench with more steel, high cost.

2.1.2 Pier type

The pier type is a square pier built of 0.5-1m brick and stone under the transformer rod, and the transformer is placed on it. Generally, transformers above 315KVA are installed.

Pier transformer:

Pier transformer installation should pay attention to:

(1) The transformer should be installed around not less than 1.8m of solid fencing or building walls, the door should be locked and kept by a special person.

(2) The barrier and wall should have sufficient safe operating distance from the transformer.

(3) Warning signs such as “High pressure danger, no climbing” should be hung on poles or walls to prevent people and animals from approaching.

The advantages of pier installation: low cost, easy maintenance. Disadvantages: occupy a lot of land, to install barriers around, small animals easy to climb to the live part, easy to be damaged by external force accidents.

2.1.3 Floor type

The floor type means that the transformer is directly placed on the ground, and the high voltage lead down line, the drop type fuse and the arrester are all on the line terminal rod.

Floor type transformer installation should note:

(1) Reliable barriers must be installed around the transformer, and the door should be locked and kept by a special person.

(2) Warning signs such as “High pressure danger, no climbing” shall be hung outside the barrier.

(3) Because the live part of the transformer is very low from the ground, it must be cut off before entering the fence.

 

2.2 Electricity is classified according to cooling mode

According to the cooling method, it can be divided into oil-immersed and dry transformers.

Oil-immersed transformers rely on oil as a cooling medium, such as oil-immersed self-cooling, oil-immersed air cooling, oil-immersed water cooling, forced oil circulation, etc. Dry transformers rely on air convection for natural cooling or increase fan cooling, and are mostly used in high-rise buildings, high-speed toll stations for electricity and local lighting, electronic lines and other small capacity transformers.

2.2.1 Oil-immersed transformers are divided into:

1) Unenclosed oil-immersed transformer: mainly S8, S9, S10 and other series of products, widely used in industrial and mining enterprises, agriculture and civil buildings.

2) Enclosed oil-immersed transformer: mainly S9, S9-M, S10-M and other series of products, mostly used in the oil, chemical industry, more oil, more chemical substances.

3) Sealed oil-immersed transformer: mainly BS9, S9-, S10-, S11-MR, SH, SH12-M and other series of products, can be used for power distribution in industrial and mining enterprises, agriculture, civil buildings and other places.

2.2.2 Dry transformers are divided into:

1) Wrapped coil type dry transformer: mainly SCB8, SC (B) 9, SC (B) 10, SCR-10 and other series of products, suitable for high-rise buildings, commercial centers, airports, stations, subways, hospitals, factories and other places.

2) Non-encapsulated coil dry transformer: mainly SG10 series products, suitable for high-rise buildings, commercial centers, airports, stations, subways, petrochemical and other places.

2.3 Classification of voltage regulation modes

According to the voltage regulation mode can be divided into load regulation and no load regulation.

The so-called no-load voltage regulation and on-load voltage regulation both refer to the transformer tap changer voltage regulation mode. The difference is that the no-load voltage regulator does not have the ability to change the gear with the load, and the transformer must be cut off when the gear is adjusted. The on-load tap-changer can be equipped with a load switching gear.

2.4 Phase classification

According to the number of phases, it is divided into single-phase transformers and three-phase transformers.

Single-phase transformer A single-phase transformer is a transformer whose primary and secondary windings are single-phase. Single-phase transformer has simple structure, small size, low loss, mainly small iron loss, and is suitable for application and promotion in low-voltage distribution network with low load density.

Three-phase transformers are used to raise and lower voltages in three-phase systems. Three-phase transformer, the general primary has three windings, its connection is divided into triangle and star, extended triangle, etc., the voltage phase difference on the three windings is 120 degrees, that is, the common three-phase 380 volt connection mode, its traditional iron core is three-phase three-core column, three-phase five-core column, involute shape and other forms.