A core-type transformer is defined as a transformer that has a magnetic core with a rectangular shape and two limbs or legs. The core provides a single magnetic circuit for the alternating flux. The windings are cylindrical and concentric, and they are distributed over both limbs of the core. The low voltage (LV) winding is placed near the core, while the high voltage (HV) winding is placed outside the LV winding. The core-type transformer is one of the main types of transformers, along with the shell-type transformer and the air core transformer.
Construction of Core Type Transformer
The core is made of laminated silicon steel sheets to reduce eddy current losses and hysteresis losses. The laminations are insulated from each other by a thin layer of varnish or paper. The core has a rectangular cross-section with two vertical limbs and two horizontal yokes. The cross-section of the core may be square, rectangular, or stepped, depending on the size and design of the transformer.
The windings are made of insulated copper conductors, which are wound on cylindrical formers. The LV winding is wound on the inner former, and it is insulated from the core by a layer of paper or cloth. The HV winding is wound on the outer former, and it is insulated from the LV winding by a layer of paper, cloth, or mica. The windings are divided into sections or coils, and each section is wound on one limb of the core. The sections are connected in series or parallel to obtain the desired voltage and current ratings.
The windings are also provided with taps or tappings to adjust the output voltage according to the load requirements. The taps are connected to a tap changer, which can be manual or automatic. The tap changer allows changing the number of turns in the windings without interrupting the power supply.
The core and windings are enclosed in a steel tank, which is filled with oil or gas for cooling and insulation purposes. The tank also acts as a shield against external magnetic fields and mechanical stresses. The tank has bushings on its top or side for connecting the terminals of the windings to the external circuit. The tank also has accessories such as a conservator, breather, radiator, thermometer, Buchholz relay, etc., for proper functioning and protection of the transformer.
Advantages of Core Type Transformer
The core type transformer has some advantages over other types of transformers, such as:
- It is economical and easy to manufacture for smaller sizes.
- It has better cooling efficiency due to the more exposed surface area of the windings.
- It has easier access to the windings for maintenance and repair purposes.
- It has lower iron losses due to a smaller core area.
- It has lower copper losses due to the shorter mean length of turn (MLT) of the windings.
Disadvantages of Core Type Transformer
The core type transformer also has some disadvantages, such as:
- It has a higher leakage flux due to a larger air gap between the windings and the core.
- It has a higher insulation cost due to the higher potential difference between the grounded core and the HV winding.
- It has a higher risk of inter-turn short circuits due to the closer proximity of the windings.
- It has lower mechanical strength due to less support from the core.
Applications of Core Type Transformer
The core-type transformer is widely used for power transmission and distribution applications. It is suitable for medium and high voltage levels, ranging from 11 kV to 400 kV. It is also used for generator step-up (GSU) transformers, which connect the generators to the grid. Some examples of core-type transformers are:
- Power transformers: These are used to step up or step down the voltage level between different stages of the power system.
- Distribution transformers: These are used to supply power to residential, commercial, and industrial loads at low voltage levels.
- Instrument transformers: These are used to measure voltage, current, power, energy, etc., in a power system.
- Isolation transformers: These are used to isolate two circuits or devices from each other for safety or noise reduction purposes.
- Autotransformers: These are used to transfer power between circuits with small voltage differences using a single winding.
Conclusion
A core-type transformer is a transformer that has a rectangular magnetic core with two limbs and cylindrical concentric windings. It is economical, easy to manufacture, and efficient for smaller sizes. It also has better cooling efficiency, easier access to windings, lower iron losses, and lower copper losses than other types of transformers. However, it also has higher leakage flux, higher insulation cost, higher risk of inter-turn short circuit, and lower mechanical strength than other types of transformers. It is widely used for power transmission and distribution applications at medium and high voltage levels.
