Tap changers for 10kV transformers
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## Functions
They are used to change the number of turns of the high-voltage winding of the transformer, thereby adjusting the output voltage of the transformer to adapt to fluctuations in grid voltage or changes in load, ensuring that the output voltage remains near the rated value.
### Classifications
- **Off-circuit tap changers**: Adjustments can only be made after the transformer is powered off and disconnected from the power grid. Commonly, there are 3-position and 5-position types. For example, for a three-position transformer with a voltage ratio of 10kV/0.4kV, the voltage of the first position is 10,500V, the second position is 10,000V, and the third position is 9,500V.
- **On-load tap changers**: The position of the taps can be adjusted while the transformer is in operation. They are generally used for transformers with special purposes. In China, common ones have 17 positions, 11 positions, 9 positions, etc., and all are equipped with automatic and manual adjustment mechanisms.
### Working Principles
By changing the tap connections of the high-voltage winding of the transformer, the turns ratio between the high-voltage and low-voltage windings is altered, and then the output voltage is adjusted. For example, when the voltage on the low-voltage side is too high, the tap changer is adjusted to a higher position to increase the number of turns of the high-voltage winding, increasing the transformation ratio and reducing the secondary output voltage. Conversely, when the voltage on the low-voltage side is too low, the tap changer is adjusted to a lower position to reduce the number of turns of the high-voltage winding, decreasing the transformation ratio and increasing the secondary output voltage.
### Structural Components
Taking the common dry-type vacuum on-load tap changer as an example, it mainly consists of a switching switch, tap selector, energy storage mechanism, reduction gearbox, electrical control part, controller, etc.
### Application Scenarios
- **Off-circuit tap changers**: Widely used in small transformers for general power distribution purposes, such as rural distribution transformers.
- **On-load tap changers**: Applicable to places where higher voltage accuracy is required, where the load changes significantly or where voltage regulation without power outage is not allowed, such as 10kV substations in factories, hospitals, colleges and universities, large and medium-sized enterprises and institutions.
### Installation and Maintenance
- **Installation**: The tap changers of dry-type transformers are limited to indoor installation and use. The ambient temperature is generally between -25°C and 40°C, the altitude of the working point does not exceed 1,000 meters, the relative air humidity does not exceed 90%, and the installation site should be free of serious, corrosive gases and conductive dust.
- **Maintenance**: Regularly check the wear condition of the contacts of the tap changer, whether the contacts are in good condition, and whether the electrical connections are loose, etc. For on-load tap changers, it is also necessary to check whether the operation of the switching switch is normal and whether the transition resistance is damaged.
### Selection Points
Comprehensive consideration should be given according to factors such as the capacity of the transformer, load characteristics, voltage regulation range, and usage environment. For example, for transformers with large capacity, significant load changes and high requirements for voltage accuracy, on-load tap changers can be selected, and appropriate rated current, voltage regulation range and tap number can be chosen according to needs. For general distribution transformers, off-circuit tap changers usually can meet the requirements.
### Precautions for Operation
- **Off-circuit tap changers**: Before operation, it is necessary to disconnect the load on the low-voltage side of the transformer first, open the drop-out fuse on the high-voltage side with an insulating rod, take necessary safety measures, unscrew the protective cover of the transformer tap changer, and set the locating pin to zero position.
- **On-load tap changers**: During operation, it is necessary to strictly follow the operating procedures, conduct upshift and downshift operations through the controller or operating mechanism, and pay attention to observing the operation of the tap changer and voltage changes.
