Three phase transformers take the high voltage power coming out of the transmission lines and reduce it to a safe level for consumer use. They are commonly used in power distribution to residential, commercial and industrial settings.
Aside from having three windings on a single core, these devices are also insulated to prevent temperature rise. This is important because elevated temperatures can lead to degradation of insulation materials and even failure.
Core
The core of a three phase transformer is a closed magnetic iron structure that provides a path for the magnetic flux. This is a significant factor in the efficiency and performance of a three phase transformer.
The primary windings of each phase of a transformer are connected to their respective legs of the core, creating three distinct magnetic circuits for each phase. This arrangement reduces the leakage flux and improves voltage regulation.
Typically, the three separate single phase transformers are connected in either a wye or delta configuration to form a three-phase bank of transformers. This allows for flexibility in power distribution to meet specific electrical requirements. In core-type transformers, the secondary and primary windings are evenly split between the limbs of the core. The central leg of the core carries the combined fluxes produced by the three legs, which allows the circulating currents to dissipate without affecting the other two legs. This results in reduced copper and stray losses.
Windings
The windings in three phase transformers are insulated to protect them from voltages. There are several different types of insulation used, but oil is usually preferred as it acts as a coolant and provides an electrical insulator.
Each leg of the core type three-phase transformer induces its own magnetic flux. When balanced sinusoidal line voltages are applied to the primary, these currents will induce the corresponding fluxes r, y and b in the legs. The center leg carries the sum of these fluxes. When the three legs are combined, their induced magnetic fluxes will mix and more or less annihilate each other. Depending on the connection configuration of the windings, the three phase transformers will react differently to balanced positive or negative-sequence voltages. It will also react differently to zero-sequence currents, which depend on the configuration of the transformer’s magnetic circuits. A wye-connected transformer will respond differently to these voltages than a delta-connected one.
Insulation
The insulation on a three phase transformer provides a barrier system to separate the windings from the core and each other. There are different types of insulation, such as oil or paper, but they all serve the same purpose: to reduce heat losses and electrical noise.
In addition, they offer better stability in voltage. This is especially important in industrial settings where electricity needs to be transmitted over long distances to consumers at appropriate voltage levels. A three phase transformer can be constructed by connecting together three single-phase transformers in a bank or it can be manufactured as a single unit. The three primary and secondary windings can then be connected in a variety of configurations to meet any application. These include the wye, delta, star-star, and delta-delta connections. The wye connection is usually used for step-up transformers. It eliminates third harmonic voltages in phases or lines but does not prevent them from passing through the neutral wire.