1. Types and functional characteristics of transformers

A transformer is an electrical component used to transform voltage, current or impedance. It is the main equipment for power transmission and distribution in a power system. Its physical appearance is shown in 1-1.

1-1　The physical appearance of the transformer

When transmitting power over long distances, a transformer can be used to increase the voltage sent by the power station to reduce losses during the power transmission process, so as to facilitate long-distance transmission of power; where electricity is used, the transformer reduces the high voltage for use Electric equipment and user use. There are many ways to classify transformers. According to the number of power phases, they can be divided into single-phase transformers and three-phase transformers.

11. Functional characteristics of single-phase transformer

1-2　Single-phase transformer structure characteristics

A single-phase transformer is a transformer whose primary winding is a single-phase winding. As shown in 1-2, the primary winding and secondary winding of the single-phase transformer are both wound on the iron core, the primary winding is the AC voltage input terminal, and the secondary winding is the AC voltage output terminal. The output voltage of the secondary winding is proportional to the number of turns of the coil.

Single-phase transformers can convert high-voltage power supply into single-phase low-voltage for use by various equipment. For example, it can change AC 6600V high-voltage into AC 220V low-voltage through a single-phase transformer to supply power to lights or other equipment, as shown in 1-3. Single-phase transformers have the advantages of simple structure, small size, low loss, etc., and are suitable for use in low-voltage distribution lines (below 60Hz) with small loads.

1-3　Single-phase transformer function schematic

Single-phase transformers are mostly used in rural power transmission and distribution systems, as well as in the power supply of some lighting or small motors. Examples of their applications are shown in 1-4. In addition, it can also be used as a power transformer in many electrical and electronic equipment.

1-4　 Application examples of single-phase transformer

2 Functional characteristics of three-phase transformer

Three-phase transformers are a kind of transformers that are widely used in power equipment. The three-phase transformer is actually a combination of three single-phase transformers of the same capacity. The primary winding (high-voltage coil) is three-phase, and the secondary winding (low-voltage coil) is also three-phase, as shown in 1-5.

Structure characteristics of 1-5　 three-phase transformer

Three-phase transformers are mainly used for step-up or step-down in a three-phase power supply system. The more commonly used one is to convert a high voltage of several thousand volts into a low voltage of 380V to provide power for electrical equipment, as shown in 1-6.

1-6　 three-phase transformer function schematic

Three-phase transformers have a wide range of applications, such as substations, industrial and mining enterprises, construction sites, irrigation and drainage equipment, post and telecommunications, textiles, railways, schools, hospitals, national defense, elevators, etc. It is also suitable for some low voltages with low power supply voltage and large fluctuations. Distribution lines. Its application examples are shown in 1-7.

1-7　 three-phase transformer application examples

2. The model and technical parameters of the transformer

Power transformers are the most widely used in electrical systems. Since there are many types of power transformers, their model identification and parameters are also different. Therefore, these models and parameters are often marked on the nameplate of the transformer for easy viewing during installation and maintenance. . 1-8 shows the nameplate identification of a typical power transformer.

1-8　Nameplate identification of typical power transformer

On the nameplate of the transformer, the type and technical parameters of the transformer are marked. The following describes the meaning and technical parameters of the transformer.

11. The meaning of the transformer model

There are many types of transformers. In order to facilitate the distinction between different types of transformers, letters or numbers are usually used to identify the model name of the transformer. The transformer model is usually composed of letters and numbers, which are used to indicate the number of phases, cooling methods, voltage regulation methods, winding core materials, winding connection methods, etc., as shown in 1-9.

1-9　Model naming method of transformer

Product name: The product name of the transformer is usually represented by letters, which indicate the coil coupling mode, number of phases, cooling mode, number of coils, coil wire material, voltage regulation mode, and special purpose of the product, as listed in Table 1-1.

Table 1-1 Product name meaning of transformer

Continued

Design serial number: It is also called technical serial number, which is represented by a number and indicates different varieties of similar products to distinguish the product's external dimensions and performance indicators. Sometimes it is omitted.

Rated capacity: The rated capacity of the transformer is expressed in "kVA" and can be read as "kVA". The transformer uses kVA as the unit. The reason is that when the load is not determined, the active power (symbol P, unit kW) and reactive power (symbol Q, unit kvar) cannot be obtained, and can only be expressed in kVA. Power, for example, the output power of a 1kVA transformer is about 1kW.

High-voltage side rated voltage: The high-voltage side rated voltage of the transformer is the rated voltage input from the input end of the primary winding, which is generally represented by the letter "kV". For example, 10kV means that the primary winding of the transformer can input a voltage of 10kV.

2 The main technical parameters of the transformer

The main technical parameters of the transformer are generally marked on the nameplate of the transformer, generally including rated capacity, rated voltage, rated current, cooling method, rated frequency, insulation resistance, winding connection group, number of phases, impedance voltage, etc., as shown in 1-10 Show.

1-10　The technical parameter identification on the transformer nameplate

In addition, the technical parameters of the transformer include no-load loss, load loss, temperature rise mode, no-load current, hoisting weight, oil weight, total weight, transportation weight, and tank withstand vacuum capability, as shown in 1-11.

1-11　Other technical parameters of transformer

Three, the symbol identification of the transformer in the line

Transformers are generally used in the control lines of electrical equipment and power distribution lines, and are one of the more commonly used equipment in electrical equipment control lines. Therefore, it is also very important to understand the symbol identification of the transformer in the line. Transformers are generally represented by the letter "T" in the circuit, and are identified by shape symbols, as shown in 1-12.

1-12　The symbol identification of the transformer in the circuit

There are two general symbols for transformers, which are used in power distribution circuits and electrical circuits respectively. The voltage input terminal is the primary winding of the transformer, and the voltage output terminal is the secondary winding of the transformer. In addition, transformers with magnetic cores or center taps are also indicated in their shape symbols, as shown in 1-13.

1-13　 the shape symbol of the transformer

1-14 　 shape symbol of three-winding transformer

Some three-phase transformers can also reflect the connection mode of the transformer in the symbol, as shown in 1-15.

1-15　 Three-phase transformer connection mode

In the actual circuit, there are two ways to connect single-phase transformers, namely series and parallel, as shown in 1-16. Transformers can be connected in series to get a larger rated voltage, while paralleling can get a larger rated current.

1-16　 Single-phase transformer connection in the line

There are many windings inside the three-phase transformer, and the most commonly used is the star (Y-shaped) connection method. This structure means that the primary winding of the three-phase transformer is connected in a Y-shape, that is, the end of each winding is connected to the neutral At this point, the other end of the winding is connected to the corresponding circuit, as shown in 1-17.

1-17　Three-phase transformer star connection

The most commonly used three-phase transformer in the circuit is the Y-Y connection, that is, the primary winding and the secondary winding are both Y-shaped connection, as shown in 1-18.