Transformer windings are coils of copper or aluminum wrapped around a transformer’s core (Figure 1). They determine what voltage is produced and whether that voltage is stepped up or down. This article looks at the parts of a transformer’s windings as well as specific transformer winding designs.
PARTS OF TRANSFORMER WINDINGS
Transformer coils have two main parts: a primary winding and a secondary winding (Figure 2). These windings are next to each other but are not electrically connected.
Primary Winding
The primary winding receives voltage from the supply source. When the supplied voltage is higher or lower than the nominal rating, adjustment taps on this winding ensure the output voltage remains within acceptable tolerance.
Aluminum or copper cables attach the tap leads at the winding to a tap-changing device. Conductor leads terminate each winding to its respective transformer bushing. The primary winding makes up the outer section of a coil.
Secondary Winding
The number of turns on the secondary winding determines the voltage induced at the output coil. This value may be higher or lower than the supply voltage, depending on the turn-to-turn ratio between the primary and secondary windings. This secondary winding provides voltage to loads such as buildings and equipment.
For distribution step-down transformers, this winding comprises the lower voltage. This section of the coil is closest to the core, requiring less insulation. Cables or bus bars connect the winding’s terminations to the transformer’s secondary bushings.
HOW WINDINGS WORK
Transformer windings operate on Faraday’s law of induction. Both windings use alternating current (AC) to produce an electromagnetic field. This field interacts with the electric circuit at the coils to produce the designated output voltage.
Applied Voltage
To get voltage at the secondary coil, an AC voltage is first applied to the primary coil. The alternating current, which changes direction 60 times per second (or 50 times per second for 50 Hz power systems), creates an electromagnetic field around the primary and secondary windings.
Induced Voltage
The electromagnetic field generated by the primary windings induces a voltage in the circuit of the secondary windings.
Ratio
The applied voltage and induced voltage values are determined by the ratio of turns between the two windings. For example, if the primary voltage is 12,000 volts and the secondary voltage is 480 volts, that’s a ratio of 25 to 1, meaning for every 25 turns in the primary coil, there’s one turn in the secondary coil.
Each individual turn in the coil will measure the same voltage (or volts per turn). This is one key principle behind the electromagnetic induction which happens between the primary and secondary windings.
Core
The iron core strengthens the electromagnetic field produced by the coils. To reduce stray flux and maximize the voltage change, the primary and secondary windings are typically wound around the same core limb (Figure 3).
WINDING TYPES
We will focus on two winding types: rectangular and disc windings. Rectangular layer windings are the most common for distribution transformers. For larger power class units, disc windings are used.
Rectangular Windings
Most medium-voltage distribution pad-mount and substation transformers use rectangular windings. Starting from the inside of the coil and working our way out, the innermost section of the coil is made up of the low-voltage conductor. With its need to carry higher current, the low-voltage conductor is usually wound in large sheets.
Sheet/Strip Windings
Full-width sheet conductor windings began being used in the 1950s. Using them in coil design improved short circuit strength, voltage regulation, and efficiency. They are commonly used for low-voltage coils in distribution transformers. Each layer of the winding is made up of a whole copper or aluminum sheet. Several smaller strips (Figure 4) can also be used to make up a full sheet.
The shape of the coil is determined by the form around which the turns of a conductor are wound (Figure 5). Each strip/sheet is interleaved with thermally upgraded kraft paper insulation. The paper is coated with an adhesive that is activated when heated. This keeps the coil’s turns tight and compacted. When the winding is finished, it is removed with the form for fitting onto the core. The turns are numbered from inside out, starting with the turn closest to the core.
Sheet windings offer two great advantages:
- They offer significant cost savings.
- They reduce axial short circuit forces at the coils.
For this reason, vertical bracing is minimal on rectangular designs with sheet wound coils, relegating most of the short circuit strength to the core clamp securing the winding assembly.
Layer Windings
The high-voltage windings are called layer-wound (Figure 6). The conductor for the HV winding is much smaller than the large sheets used for the LV winding. Conductors for layer windings can be round or rectangular. The common choice for distribution units is enamel-coated magnet wire.
The insulated magnet wire is wound around a rectangular form from top to bottom. Once at the bottom, the conductor is then wound over the first layer back to the top. Some designs will use a run-back, so the start of the next turn begins again from the top of the coil. A run-back allows each layer to be wound from the top down, rather than alternate top-to-bottom and bottom-to-top. Vertical ducts (Figure 7) are inserted between layers for cooling.
Disc Windings
Many large power transformers are built using circular disc windings. Disk windings are usually built in a special, positive pressure-regulated room separate from the rest of the factory. Disc windings are made to handle the higher short circuit forces and larger impulse ratings of high-voltage transformers (above 69 kV).
The conductors in disc wound coils form a spiral pattern, making individual discs. Once one disc is formed, a drop-down (or crossover) is used to start the formation of the second disc. For a continuously wound disc winding, the disc is wound from the inside out. Then the turns of the conductor are turned over manually. This allows the cross-over to the next disc to be made without splicing or brazing (Figure 8).
As Figure 8 shows, the turns of each disk run in parallel with each other. Adding radial spacers between each disc minimizes circulating currents between these parallel turns. For disc wound transformers, the location of the HV and LV can vary between various design types. This discussion is outside the scope of this article.
Special tools bend the conductor for each cross-over (Figure 9). This is done by hand or by automated machines. Figure 10 shows disc wound coils under construction at the factory.
You may notice that each disc alternates the way the turns number from outside in (or inside out). Inside means nearest the core. Outside means the outside surface of the coil. The turn number counts from outside in with the top (first) disc. The second disc down counts its turns from inside out.
This configuration plays a key role in the coil’s short circuit withstand capability. Where the turns number from outside in, the short circuit forces will push inward. Where the turns number from inside out, the short circuit forces (Figure 11) will push outward. In the event of a fault, mechanical stress displaces in two opposite directions across the coil.
COIL AND CORE ASSEMBLY
After the winding process, each coil is fitted onto a limb of the transformer’s core. The core style will depend on the winding type. Figure 12 shows an example of a large power transformer coil lowering onto a three-limb core.
PHOTO COURTESY OF TRANSFORMER CONSULTING SERVICES INC.
For the smaller distribution unit in Figure 13, a distributed gap five-limb core is being stacked around the finished coils.
WINDING CONNECTIONS AND LEADS
During the coil winding process, all connection points are brought out to be terminated. These connection points include the primary and secondary winding leads and the voltage adjustment tap leads. The tap connections and coil leads are labeled and prepped for termination later.
Once assembled around the core, the makeup of all termination points begins. The connections may be made with cable or bus. Lower voltages with higher current will typically use bus. Higher voltage applications with lower currents utilize cable connections.
Making Connections
Copper or aluminum conductors make up these connections, depending on the winding conductor. The primary and secondary leads brought out of the coils are welded or brazed (Figure 14). Due to the higher temperatures required for welding copper, braising is the more common option.
PHOTO COURTESY OF TRANSFORMER CONSULTING SERVICES INC.
Aluminum welds together more easily at lower temperatures. Welded terminations are more common with aluminum.
Terminations to Devices and Bushings
The primary winding leads are brought out to make permanent connections to the bushings. The HV leads are usually cable-connected. The secondary winding leads terminate at the low-voltage bushings. For distribution transformers, connections terminate with a hard (or flexible) bus or cable.
Connections between the winding leads and devices are made with bolted or crimped connections. Once terminated, the tap leads run to the tap-changing device.
For larger transformers with bushings requiring field installation, a draw lead may be brought out. Figure 18 shows three draw leads for the primary bushings. The tap leads run to a load tap changing (LTC) device.
CONCLUSION: CIRCULAR VS. RECTANGULAR WINDING TYPE
When choosing between rectangular and circular transformer windings, several factors come into play, including cost, efficiency, and voltage capacity.
- Rectangular. One of the most significant benefits of a rectangular design is cost. The typical five-leg distributed gap-wrapped core design with rectangular layer HV and sheet LV wound coils is simple to construct, lower cost to build, high efficiency, and offers a reliable service life. This makes it the obvious choice for distribution applications. The reduction of axial short circuit forces from the sheet wound LV coils also allows this design to be used in larger applications as well. Design improvements and short circuit testing show their usefulness at sizes up to 20 MVA (69 kV and below).
- Circular Disk. Although they cost more, circular disc designs offer larger short circuit withstand capability. This is why they are used exclusively on larger power units above 20 MVA. Transformer windings above 69 kV use disk windings as a rule. The unique features of disc-type windings make them suited for voltage requirements up to 550 kV BIL.
However, winding the coils of a circular disk design is far more labor-intensive and costly. The build time is also longer. The lower cost, quicker build time, and high-efficiency ratings available for layer-wound rectangular designs make them ideal for medium-voltage transformers 35 kV and below. For higher voltage applications above 69 kV, disc wound units are the go-to choice.
Ben Gulick began working for Maddox Industrial Transformer at its South Carolina location in 2016 and is currently the Technical Sales Engineer. He received his BA in music from Indiana University before starting his career in the electrical industry with a contractor in Indianapolis, Indiana.