Distribution Transformer
A distribution transformer, commonly referred to as a "distribution trans", is a static electrical device in the distribution system that converts alternating current (AC) voltage and current based on the principle of electromagnetic induction to transmit AC electrical energy. Typically, it operates in the distribution network with a voltage level of 10-35kV (mostly 10kV or below) and a capacity of 6300kVA or less, supplying power directly to end-users.
1. Basic Introduction to Distribution Transformers
1.1 Definition
A distribution transformer, or "distribution trans" for short, is a static electrical device in the distribution system that converts AC voltage and current according to the principle of electromagnetic induction to transmit AC electrical energy. In some regions, power transformers with a voltage level below 35kV (mostly 10kV or below) are called "distribution transformers", abbreviated as "distribution trans". The location where a "distribution trans" is installed is a substation. Distribution transformers are preferably installed on poles or outdoors on the ground.
1.2 Structure
This section focuses on the structure of oil-immersed distribution transformers. Oil-immersed distribution transformers can be structurally divided into the main body, oil conservator, insulating bushings, tap changers, and protection devices, etc. (See the figure below: Decomposition of Distribution Transformer Structure)
1.2.1 Main Body
The main body consists of three parts: the iron core, windings, and insulating oil. The windings form the electrical circuit of the transformer, while the iron core constitutes its magnetic circuit. Together, they form the core electromagnetic component of the transformer.
1.2.1.1 Iron Core
The iron core is the main magnetic circuit part of the transformer. It is usually made by stacking hot-rolled or cold-rolled silicon steel sheets with a high silicon content (0.35mm or 0.5mm thick) and insulated with a surface coating. The iron core is divided into two parts: core columns and yokes. The core columns are sleeved with windings, and the yokes are used to close the magnetic circuit. The basic structural forms of the iron core include core-type and shell-type. (See: Iron Core Structure)
1.2.1.2 Windings
The windings are the electrical circuit part of the transformer, generally wound on a winding mold using insulated flat copper wires or round copper wires. The windings are mounted on the transformer's core columns. The low-voltage windings are placed on the inner layer, and the high-voltage windings are sleeved on the outer layer of the low-voltage windings. Insulation sleeves made of insulating materials separate the low-voltage windings from the iron core, and the high-voltage windings from the low-voltage windings to ensure insulation. (See: Winding Structure)
1.2.1.3 Insulating Oil
Transformer oil has a complex composition, mainly consisting of naphthenes, alkanes, and aromatic hydrocarbons. In distribution transformers, transformer oil serves two purposes: first, it provides insulation between the transformer windings, between windings and the iron core, and between windings and the oil tank; second, it circulates convectively when heated, dissipating heat from the transformer core and windings. Common specifications of transformer oil are No. 10, No. 25, and No. 45. The number indicates the temperature at which the oil starts to solidify at sub-zero temperatures. For example, "No. 25" oil means it starts to solidify at -25°C. The appropriate oil specification should be selected based on local climatic conditions.
1.2.2 Oil Conservator
The oil conservator is mounted on the top cover of the oil tank. Its volume is approximately 10% of the oil tank's volume. A pipe connects the oil conservator and the oil tank. When the volume of transformer oil expands or contracts with temperature changes, the oil conservator stores or supplements oil to ensure the core and windings are immersed in oil. Additionally, the oil conservator reduces the contact area between oil and air, slowing down the oil's deterioration rate.
An oil level gauge is installed on the side of the oil conservator. Next to the glass tube of the gauge, there are standard oil level lines corresponding to oil temperatures of -30°C, +20°C, and +40°C. These lines indicate the required oil level for transformers not in operation and mainly reflect whether the oil volume is sufficient when the transformer operates at different temperatures.
A breather is installed on the oil conservator, connecting the upper space of the conservator to the atmosphere. When transformer oil expands or contracts due to temperature changes, air in the upper part of the conservator can enter or exit through the breather, allowing the oil level to rise or fall and preventing deformation or even damage to the oil tank.
1.2.3 Insulating Bushings
Insulating bushings are the main external insulation devices of the transformer. Most transformer insulating bushings are porcelain insulating bushings. They lead the high-voltage and low-voltage winding leads of the transformer from inside the oil tank to the outside, insulating the transformer windings from the ground (tank shell and iron core), and also serve as key components for fixing the leads and connecting to external circuits. High-voltage porcelain bushings are relatively tall, while low-voltage porcelain bushings are shorter.
1.2.4 Tap Taps
Tap taps are devices for changing the taps of the transformer's high-voltage windings. Adjusting the tap position increases or decreases the number of turns in part of the primary winding, changing the voltage ratio to adjust the output voltage. Transformers that adjust the output voltage by manually changing the tap changer position after being shut down and disconnected from the power grid are called no-load voltage regulation transformers.
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 conservator. It connects to 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 connects to the external circuit to form heavy gas protection. When heavy gas is activated, it trips the high-voltage circuit breaker and sends a heavy gas activation signal.
1.2.5.2 Explosion Vent
The explosion vent is a safety protection device for the transformer, installed on the top cover of the transformer. It connects to the atmosphere. In case of a fault, heat vaporizes the transformer oil, triggering the gas relay to send an alarm signal or cut off the power supply to prevent the oil tank from bursting.
2. Classification
2.1 Classification by Installation Location
Distribution transformers are classified into indoor and outdoor types based on their installation location. Outdoor installations include platform-type, pole-type, and ground-type (including pre-installed type).
2.1.1 Pole-Type
Pole-type installation involves mounting the transformer on a frame attached to a pole, which is divided into single-pole and double-pole types.
When the capacity of the distribution transformer is 30kVA or less (including 30kVA), a single-pole distribution transformer platform is generally used. The distribution transformer, high-voltage drop-out fuse, and high-voltage arrester are installed on a single cement pole. The pole body should be inclined 13°-15° in the opposite direction to where the distribution transformer is assembled.
When the capacity of the distribution transformer ranges from 50kVA to 315kVA, a double-pole distribution transformer platform is typically adopted. The platform consists of a main cement pole and an auxiliary pole. The main pole is equipped with a high-voltage drop-out fuse and high-voltage lead wires, while the auxiliary pole has secondary return leads. The double-pole distribution transformer platform is more robust than the single-pole type.
Advantages of pole-type installation: small floor space, no need for surrounding walls or enclosures, and live parts are high above the ground, reducing the risk of accidents. Disadvantages: large steel consumption for the platform and relatively high cost.
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