Transformers are the most recognizable equipment in power systems. They are large, distinctive, simple in principle but require a lot of maintenance, making them seem much more substantial than 'cables'. On the power grid diagram, transformers are like the backbone nodes, while other equipment serves as their connections.

This article will introduce the components, principles, functions, classification, and application scenarios of transformers.

1.Composition of a Transformer

A transformer mainly consists of a core and windings.

The core serves as the magnetic circuit path of the transformer, while the windings are the electrical circuit part, made by winding a certain number of turns of enamel-coated wire.

The one connected to the power source is called the primary winding, also known as the primary coil. The one connected to the load is called the secondary winding, also known as the secondary coil, or the secondary side winding.

The basic forms of core structures are the Heart-shaped core type and the shell type.

• Heart-shaped core transformer

The columns of a core-type transformer are surrounded by windings. Simply put, the windings encircle the core, making the structure relatively simple and easier to assemble and insulate, which is why transformers often use a core-type structure.

• Shell-type transformer

In a shell-type transformer, the core surrounds the winding. Shell-type transformers have high mechanical strength and protruding corners, but their manufacturing process is complex and requires more materials. They are usually only used in low-voltage, high-current transformers or small-capacity power transformers.

2.Basic Working Principle of a Transformer

A transformer operates based on the principle of electromagnetic induction.

When a suitable AC power supply is connected to both ends of the primary winding, under the action of the supply voltage u₁, an AC current i₀ flows through the primary winding, generating a magnetomotive force in the primary winding. This excites an alternating magnetic flux ϕ in the core. This alternating flux ϕ links both the primary and secondary windings. According to the law of electromagnetic induction, induced electromotive forces e₁ and e₂ are generated in the primary and secondary windings, respectively. Under the influence of the induced electromotive force e₂, the secondary winding can supply power to the load, achieving energy transfer.

The ratio of the induced electromotive forces in the primary and secondary windings is equal to the ratio of the number of turns in the primary and secondary windings. The magnitude of the induced electromotive force e₁ on the primary side is close to the applied voltage u₁ on the primary side, while the magnitude of the induced electromotive force e₂ on the secondary side is close to the output voltage u₂ on the secondary side.

Therefore, by simply changing the number of turns in the primary or secondary winding once or twice, the output voltage u₂ can be adjusted. This is the basic working principle of a transformer, which uses the principle of electromagnetic induction to convert an AC power source of one voltage level into an AC power source of the same frequency but a different voltage level.

2.Basic Functions of a Transformer

The basic functions of a transformer include voltage conversion, current conversion, impedance conversion, isolation, and voltage regulation.

Voltage Transformation: Transformers can increase or decrease the voltage of alternating current to meet different electricity needs. For example, a step-up transformer is used to increase the voltage from a power plant to reduce energy loss during transmission, while a step-down transformer is used to lower high voltage to a safe usage voltage.

Current transformation: By changing the voltage, a transformer also correspondingly changes the current. According to the law of conservation of power, when the voltage increases, the current decreases, and vice versa. This characteristic makes transformers very important in power transmission, as they can effectively manage current loads.

Impedance transformation: Transformers can change the impedance of a circuit, making it more suitable for different load conditions. This is especially important in audio equipment and other electronic devices, as it can improve signal transmission efficiency.

Isolation: Transformers can provide electrical isolation, protecting the safety of equipment and users. This isolation can prevent high voltage from damaging low-voltage equipment, ensuring the safe operation of the equipment.

Voltage Regulation: Certain types of transformers (such as saturable reactors) can be used for voltage regulation, helping to maintain voltage stability and ensuring the reliability and stability of the power system.

4.Classification of Transformers

4.1Classified by capacity

• Small transformer: voltage below 10KV, capacity between 1 and 500KVA.

• Small and medium-sized transformers: voltage of 35 kV and below, capacity from 630 to 6300 kVA.

• Large transformers: voltage of 110 kV and below, capacity between 8000 and 63000 kVA.

4.2Classified by Use

• Power transformer: Used for stepping up, stepping down, distributing, and interconnecting in power transmission and distribution systems, or specifically used as transformers for power plants and substations.

• Instrument transformers: such as voltage transformers and current transformers, used for measuring instruments and relay protection devices.

• Power transformer: used for the control power supply, lighting, and indicators of general mechanical equipment.

• Electronic transformer: used in electronic circuits such as switch-mode power supplies, audio, pulse, and impedance matching.

• Test transformer: capable of generating high voltage to conduct high-voltage tests on electrical equipment.

• Special transformers: such as electric furnace transformers, rectifier transformers, voltage regulating transformers, etc.

4.3Classified by the number of phases of transformer windings

• Single-phase transformer: used for single-phase loads and three-phase transformer banks.

• Three-phase transformer: Used to step up or step down voltage in three-phase systems.

4.4Classified by Transformer Cooling Method

• Dry-type transformer: Cooled by air convection, generally used for small-capacity transformers such as local lighting and electronic circuits.

• Oil-immersed transformer: A transformer that uses transformer oil as the insulating and cooling medium, with the core and windings completely immersed in the insulating oil.

4.5Classified by Transformer Winding Connection Type

• Double-winding transformer: Used to connect two voltage levels in the power system.

• Three-winding transformer: Generally used in regional substations of the power system to connect three voltage levels.

• Autotransformer: The primary and secondary windings are combined into one, used to connect power systems of different voltages. It can also be used as a regular step-up or step-down transformer.

4.6Classified by Transformer Operating Frequency

• Power frequency transformer: its operating frequency is 50Hz or 60Hz.
• Intermediate frequency transformer: its operating frequency is 400–1000Hz.
• Audio frequency transformer: its operating frequency is 20Hz–20kHz.

• Supersonic Frequency Transformer: Its operating frequency is above 20 kHz, generally not exceeding 100 kHz.

• High-Frequency Transformer: A transformer with an operating frequency ranging from 20 Hz to over 100 kHz.

5.Application Scenarios of Transformers

5.1Power System

• Power plants: Transformers are used to step up the voltage generated by generators for transmission to the power grid, enabling long-distance electricity transmission.

• Substations: In substations, transformers convert high-voltage electricity into low-voltage electricity to meet the needs of various electrical equipment. At the same time, transformers can also perform functions such as reactive power compensation and voltage adjustment, ensuring the stable operation of the power system.

• Transmission lines: In transmission lines, transformers are used to increase the voltage to reduce energy loss, achieving efficient long-distance electricity transmission.

5.2Civil Sector

• Household electricity: Power transformers convert high-voltage electricity into low-voltage electricity suitable for home use, ensuring normal electricity consumption for residents.

• Charging batteries: Whether it's a laptop, phone, or electric vehicle, these devices require batteries to operate, and charging the batteries requires a transformer. The main function of a transformer is to regulate voltage and prevent leakage currents or surge currents from passing through the devices.

5.3Communication Field

Communication transformers are used in telephone terminal circuits and trunk line products to regulate the quality and condition of communication circuits. In addition, communication transformers are widely used in cable modems, network cards, hubs, xDSL broadband communication equipment, switches, fiber optic transceivers, routers, embedded systems, and VoIP network communication devices.

5.4Other Special Applications

• Audio Equipment: Audio transformers are commonly used to provide isolation for signals flowing through a circuit and help match the impedance values of the source and load. They can also eliminate unwanted or noisy signals and filter the input signal. These types of transformers are specifically designed to handle signals within the audible range, that is, signals with frequencies between 20Hz and 20kHz.

• Measuring Instruments: Current meters, voltage meters, and various other measuring tools and devices typically use transformers for general operation. For example, measuring current transformers provide necessary safety for the circuit by isolating the measuring device from the rest of the circuit and suppressing or stepping down large currents to optimal values before feeding them to the ammeter.

• Rectification: Rectifier transformers can convert AC to DC, with applications including motor control, mining, electric furnaces, R&D laboratories, high-voltage DC transmission, and more.