Transformer Windings: Concentric and Overlapping Types
In power transformers, there are two primary winding configurations: concentric and overlapping. The majority of power transformers produced in China use a core-type transformer structure, and their windings are typically designed in a concentric configuration.
A concentric winding means that in any cross-section of the transformer's core column, the winding forms a cylindrical shape around the core. Typically, the low-voltage winding is placed closest to the core, while the high-voltage winding is located on the outside. An insulation gap, as well as a cooling channel (oil duct), must be maintained between the high-voltage and low-voltage windings, and between the low-voltage winding and the core column. This gap is necessary for insulation and to allow proper heat dissipation. The size of this insulation distance is determined by the voltage level of the winding and the space required for heat dissipation.
When the low-voltage winding is placed closer to the core, it requires less insulation space, allowing the winding size to be smaller, which in turn reduces the overall transformer size. The concentric winding comes in several forms:
Single-layer cylindrical
Double-layer cylindrical
Cylindrical Windings
A cylindrical winding is a spiral formed by winding flat wire tightly together. It can be wound in a single layer or double layer. Generally, double-layer windings are preferred over single-layer windings. This is because single-layer windings, due to their elastic deformation, tend to loosen, causing the coils at the ends not to be tightly packed. A double-layer winding is much more stable and less likely to loosen.
When large currents are involved, each winding coil can be formed by several conductors arranged in parallel along the axis. However, the number of parallel conductors is typically no more than 4 to 5. Cylindrical windings provide the largest surface area for cooling, which improves cooling performance. However, they are mechanically weaker and are generally used in low-voltage windings of small-capacity transformers.
Spiral Windings
For transformers with a slightly larger capacity, the low-voltage winding may consist of only a few turns (20-30 turns), but the current is much higher, so a larger cross-sectional area for the windings is required. This necessitates using multiple conductors (6 or more) wound in parallel. In cylindrical windings, it is not suitable to use many conductors in parallel because the conductors are tightly packed in a single layer, which leads to large coil pitches and makes the coil unstable. To address this issue, spiral windings are used. Spiral windings are wound radially with one conductor pressed against another, leaving a gap between each turn.
When more conductors are used in parallel, the conductors can be split into two groups, forming a double-layer spiral winding. When temperature rise and insulation conditions allow, the spiral winding can be designed with a combination of normal and smaller oil ducts, creating a semi-spiral winding. A single semi-spiral winding is referred to as single semi-spiral, while a double spiral winding is referred to as double semi-spiral. Semi-spiral windings are widely used in medium and large transformers due to their high space utilization.
Switched Conductor Windings
To further minimize additional losses in non-fully shifted spiral windings, switched conductors are used. Switched conductors are formed by twisting multiple strands of wire in a specific pattern, resulting in a single thicker conductor that is insulated with paper. This type of winding is primarily used in the mid- and low-voltage windings of high-capacity transformers.
When using switched conductors, the strands must be an odd number, and if the height of the conductor (A) is ≥12mm, a layer of cable paper (0.12mm thick) must be inserted between the strands. Additionally, the conductor used must be continuous, with no joints. The length of the switched conductor must be 3 to 5 meters longer than the length required for the coil.
The minimum inner diameter of a coil with switched conductors is governed by the following formula:
D_min ≥ n * pitch / π
Where:
D_min is the minimum inner diameter of the coil,
n is the number of conductors used in the switch.
pitch refers to the distance between the conductors.
This formula ensures that all conductors must shift positions at least once within a single coil turn, which prevents the coil diameter from being too small.
Continuous Windings
Continuous windings do not have welded joints and can only be wound using flat wire. The conductor arrangement, shown in the diagram, is specially wound so that the joints alternate between the inside and outside of the coil. The key advantage of continuous windings is that they do not require welding, as the conductors are naturally connected during the winding process.
If the conductor's cross-sectional area is large, multiple conductors can be wound in parallel, but no more than 4 conductors are typically used. The conductors are alternately switched as the winding progresses.