- Core Type Transformer Definition: A core type transformer is defined as a transformer with a rectangular magnetic core and cylindrical windings, primarily used for efficient electrical power transformation.
- Construction Details: It features laminated silicon steel sheets in the core to reduce energy loss and concentric windings to facilitate effective power distribution.
- Voltage Adjustment: Equipped with tap changers, it allows flexibility in voltage output to adapt to different electrical load demands.
- Cooling and Insulation: The transformer is housed in a steel tank filled with cooling and insulating materials like oil or gas, enhancing performance and safety.
- Operational Advantages: This transformer type is economical, offers better access for maintenance, and is efficient in cooling, making it preferable for smaller installations.
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:
- Its design allows for economical production, especially suitable for smaller sizes.
- It has better cooling efficiency due to the more exposed surface area of the windings.
- The windings are easily accessible for maintenance and repairs.
- The smaller core area results in reduced iron losses.
- Its simpler insulation layout suits high-voltage service, where the longer distance between windings and core is easier to manage.
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 sees wide use in power transmission and distribution, suitable for medium and high voltage levels 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
Rectangular core, two limbs, concentric cylindrical windings: that geometry defines the core-type transformer. It brings economical construction, good cooling, accessible windings and low iron losses, at the cost of higher leakage flux, pricier insulation, weaker mechanical bracing and some short-circuit risk. Power transmission and distribution at medium and high voltages is where those trade-offs pay off.
