Understanding the types of windings used in large transformers is crucial, especially when considering voltage levels like 11 kV to 33 kV for the low voltage side. Different winding configurations are suited for specific voltage, current, and power ratings, as well as mechanical stress considerations.

Transformer Winding Types for Low Voltage

Large transformers designed for transmitting power often handle significant current on their low voltage side. The winding type selected must be able to manage this high current density, minimize eddy current losses, withstand short-circuit forces, and provide adequate cooling.

Let's examine the suitability of the given winding types for low voltage (11 kV to 33 kV) applications in large transformers:

• Continuous Disc Winding: This winding is typically used for high voltage windings of large power transformers. It consists of a series of discs connected in series. While suitable for high voltage, it is less ideal for the very high current requirements of the LV side of large transformers below 33 kV due to challenges in current distribution and cooling for thick conductors.
• Crossover Winding: This winding type is generally used for small transformers and high voltage windings of medium-sized transformers. Conductors are wound in layers with crossovers between layers. It is not typically used for high current applications found on the low voltage side of large power transformers due to limitations in handling large conductor cross-sections and managing short-circuit forces.
• Spiral Winding: Spiral windings, or layered spiral windings, are sometimes used, but they are generally similar in application range to continuous disc windings for HV or tap windings, or less common for the main LV winding of very large units compared to helical.
• Helical Winding: This winding consists of one or more layers of rectangular conductors wound in parallel along the axial length of the core limb. The turns are arranged in a helix, similar to a screw thread.

Suitability of Helical Winding

Helical winding is particularly well-suited for the low voltage windings of large transformers operating in the 11 kV to 33 kV range for several reasons:

• High Current Capacity: It can easily accommodate multiple parallel conductors per turn, allowing it to handle very high currents without excessive temperature rise.
• Mechanical Strength: The robust structure of helical windings provides good resistance to the significant axial and radial forces experienced during short circuits in large transformers.
• Cooling Efficiency: The construction allows for relatively easy provision of cooling ducts between layers and turns, facilitating efficient heat dissipation.
• Even Current Distribution: By transposing the parallel conductors (e.g., using continually transposed cable - CTC), eddy current losses can be minimized, and current distribution improved.

Considering these factors, Helical winding is the most suitable type for the low voltage (11 kV to 33 kV) windings in large power transformers due to its excellent high current handling capability, mechanical robustness, and efficient cooling characteristics.