Types and characteristics of power transformers
1. According to the number of phases
(1) Single-phase power transformer : Used for single-phase loads and three-phase power transformer sets.
(2) Three-phase power transformer: used to increase and decrease the voltage of the three-phase system.
2. Classified by cooling method
(1) Dry-type power transformer: relies on air convection for cooling, generally used for local lighting, electronic circuits, etc. Small capacity power transformer.
(2) Oil-immersed power transformer: relies on oil as the cooling medium, such as oil-immersed self-cooling, oil-immersed air-cooling, oil-immersed water cooling, forced oil circulation, etc.
3. Classified by purpose
(1) Power transformer: used for power transmission and distribution System voltage rise and fall.
(2) Instrument transformers: such as voltage transformers, current transformers, used in measuring instruments and relay protection devices.
(3) Test transformer: It can generate high voltage and conduct high-voltage tests on electrical equipment.
(4) Special transformers: such as electric furnace transformers, rectifier transformers, adjusting transformers, etc.
4. According to winding form
(1) Double-winding transformer: used to connect two voltage levels in the power system.
(2) Three-winding transformer: generally used in regional substations of power systems to connect three voltage levels.
(3) Autotransformer: used to connect power systems with different voltages. It can also be used as an ordinary step-up or step-down transformer.
5. According to the form of iron core
(1) Core transformer: a power transformer used for high voltage.
(2) Amorphous alloy transformer: Amorphous alloy iron core transformer is made of new magnetic conductive material, and the no-load current is reduced by about 80%. It is a distribution transformer with ideal energy saving effect, especially suitable for rural areas. Places with low load rates such as power grids and developing areas.
(3) Shell type transformer: special transformer for large current, such as electric furnace transformer, welding transformer; or power transformer for electronic instruments, TV, radio, etc.
Characteristic parameters of power transformer
1. Working frequency
The core loss of the transformer has a great relationship with the frequency, so it should be designed and used according to the frequency of use. This frequency is called the working frequency.
2. Rated power
Under the specified frequency and voltage, the transformer can work for a long time without exceeding the output power of the specified temperature rise.
3. Rated voltage
Refers to the voltage allowed to be applied to the coil of the transformer, which shall not exceed the specified value during operation.
4. Voltage ratio
Refers to the ratio of the primary voltage and the secondary voltage of the transformer. There is a difference between no-load voltage ratio and load voltage ratio.
5. No-load current
When the secondary of the transformer is open circuit, there is still a certain current in the primary. This part of the current is called no-load current. No-load current consists of magnetizing current (generating magnetic flux) and iron loss current (caused by core losses). For a 50Hz power transformer, the no-load current is basically equal to the magnetizing current.
6. No-load loss
Refers to the power loss measured at the primary when the secondary of the transformer is open circuit. The main loss is the core loss, followed by the loss (copper loss) caused by the no-load current on the primary coil copper resistance. This part of the loss is very small.
7. Efficiency
Refers to the percentage of the ratio of secondary power P2 to primary power P1. Generally, the greater the power rating of the transformer, the higher the efficiency.
8. Insulation resistance
Indicates the insulation performance between the coils of the transformer and between each coil and the iron core. The insulation resistance is related to the performance of the insulating material used, temperature and humidity.
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