Internal Structure and Working Principle of Power Transformer

Transformer is a static electrical equipment used to transform AC voltage and current to transmit AC energy. It realizes electric energy transmission according to the principle of electromagnetic induction. Transformers can be divided into power transformers, test transformers, instrument transformers and transformers for special purposes: power transformers are necessary equipment for power transmission and distribution and power user distribution; The test transformer is used to conduct withstand voltage (step-up) test on electrical equipment; Instrument transformer is used for electrical measurement and relay protection of distribution system (PT, CT); Transformers for special purposes include furnace transformers for smelting, welding transformers, rectifier transformers for electrolysis, small voltage regulating transformers, etc.

Power transformer is a static electrical equipment, which is used to change the AC voltage (current) of a certain value into another voltage (current) with the same frequency or several different values. When the primary winding is connected with AC, alternating magnetic flux is generated. The alternating magnetic flux induces AC electromotive force in the secondary winding through the magnetic conduction of the iron core. The secondary induced electromotive force is related to the number of turns of the primary and secondary windings, that is, the voltage is directly proportional to the number of turns.

The main function is to transmit electric energy. Therefore, the rated capacity is its main parameter. Rated capacity is a customary value of power, which represents the size of transmitted electric energy, expressed in KVA or MVA. When the rated voltage is applied to the transformer, the rated current that does not exceed the temperature rise limit under specified conditions shall be determined according to it. The more energy-saving power transformer is amorphous alloy core distribution transformer. Its biggest advantage is that the no-load loss value is very low. Whether the no-load loss can be ensured is the core problem to be considered in the whole design process. When arranging the product structure, in addition to considering that the amorphous alloy core itself is not affected by external forces, the characteristic parameters of the amorphous alloy must be accurately and reasonably selected in the calculation.

A transformer is a static electric energy converter that transmits electric energy without changing its frequency.

Transformer is a large number of electrical equipment in the power system and plays a very important role. The total capacity of transformer is about 9 times that of generator. Its function is to increase or decrease the voltage of electric energy in the power system, so as to facilitate the rational transmission, distribution and use of electric energy.

In the power system, the higher the voltage, the smaller the current and the smaller the power loss on the transmission line; The cross-sectional area of the transmission line can also be reduced, which can reduce the metal consumption of the conductor.

Figure 1

Due to manufacturing difficulties, the generator voltage cannot be very high (below 20kV at present). Therefore, in the power plant, it is necessary to use step-up transformer to raise the generator voltage very high, so as to send a large amount of electric energy to distant power consumption areas, such as 35kV, 66kV, 110kV, 220kV, 330kV, 500kV, etc.

At the power load, the step-down transformer is used to reduce the voltage to an appropriate value for the user's electrical equipment. When the power transformer transmits electric energy, it also has some active power loss, but the number is small, so the transmission efficiency is very high. The efficiency of small and medium-sized transformers shall not be less than 95%, and the efficiency of large transformers can reach more than 98%.

2 Classification of power transformers

1. By function:

Power transformers are divided into step-up transformers and step-down transformers according to their functions. Step down transformers are used in plant substations. Step down transformer of terminal substation, also known as distribution transformer.

2. By capacity:

According to the capacity series, power transformers are divided into R8 capacity series and R10 capacity series.

R8 capacity series refers to that the capacity grade increases by R8 1.33 times. This series is adopted for the old transformer capacity grade in China, such as 100kVA, 135kva, 180KVA, 240kVA, 320kva, 420kva, 560kva, 750kva, 1000KVA, etc.

R10 capacity series means that the capacity grade is increased by multiple of R10 1.26. R10 series has dense capacity grade, which is convenient for reasonable selection and is recommended by IEC (International Electrotechnical Commission). This series is adopted for the new transformer capacity grade in China, such as 100kVA, 125kva, 160KVA, 200KVA, 250kVA, 315KVA, 400KVA, 500KVA, 630kVA, 800kVA and 1000KVA.

3. Divided by phase number:

Power transformers are divided into single-phase and three-phase according to the number of phases. Factory substations usually use three-phase power transformers.

4. According to voltage regulation mode:

According to the voltage regulation mode, power transformers are divided into on load voltage regulation (also known as off excitation voltage regulation) and on load voltage regulation. Most factory substations use no-load voltage regulating transformers.

5. According to winding structure:

According to the winding structure, power transformers include single winding autotransformer, double winding transformer and three winding transformer. Most factory substations use double winding transformers.

6. Classification by winding insulation and cooling mode:

Power transformers are divided into oil immersed, dry-type and inflatable (SF6) according to winding insulation and cooling mode. Oil immersed transformers include oil immersed self cooling type, oil immersed air cooling type, oil immersed water cooling type and forced oil circulating cooling type. Most factory substations use oil immersed self cooling transformers.

Figure 2

The so-called inflatable transformer refers to the transformer whose magnetic circuit (core) and winding are located in a shell filled with insulating gas. In the past, SF6 gas was generally used, so it is also called gas insulated transformer

7. By winding conductor material:

Power transformers are divided into copper winding transformers and aluminum winding transformers according to the material of winding conductors. In the past, aluminum winding transformers were mostly used in factory substations, but copper winding transformers with low loss have been more and more widely used.