EE 8 - Three-Phase Transformers

Three Phase

Transformers used to step-up or step-down high voltages in power transmission systems are called _ transformers.

Iron Core

A three-phase transformer consists of three sets of primary and secondary windings wound around an _ assembly.

Amperage

The electric grid uses a three-phase system because it allows for higher transmission at lower _.

Labor

Using higher gauge (thinner) copper wire in three-phase systems significantly reduces material and _ costs.

Core Type

The type of construction where the core has three limbs in the same plane and each leg carries both LV and HV windings

High Voltage

In Core Type construction, the low voltage windings are insulated from the core rather than the _ windings.

Shell Type

The type of construction where each phase has its individual magnetic circuit, similar to three single-phase transformers stacked

Magnetic Fluxes

When the primary is excited with a three-phase source, currents flow through individual windings producing _.

Star-Delta

The connection type commonly used to step-down voltages in transmission end substations

Delta

In a Star-Delta connection, the primary is connected in star and the secondary is connected in _.

Delta-Star

The connection type used to step-up voltage, commonly employed at the sending end of high tension transmission systems

4 (or Four)

In a Delta-Star connection, the secondary is connected in star fashion so that a _ wire system is possible.

Delta-Delta

The connection type generally employed for three-phase power loads like motors where the load needs single voltage with high current

Star-Star

The connection type where both primary and secondary are connected in star and there is no phase difference between voltages

Operating Frequency

In the formula for Turns per Volt $1 / (4.44 \times f \times B \times A)$, 'f' stands for _.

Magnetic Flux Density

In the formula for Turns per Volt, 'B' represents the _.

Iron Losses

Losses caused by the alternating flux in the core, also known as Core loss

Copper Losses

Losses that occur due to the transformer winding resistance ($I^2R$ loss)

Hysteresis Losses

Losses occurring because of the variation of magnetization in the core of the transformer

Eddy Current Losses

Losses caused by EMF circulating currents within the body of the conducting core material

Space

Compared to three single-phase units, a three-phase transformer is cheaper and occupies less _.

Whole Unit

A major disadvantage of three-phase transformers is that if one phase is defective, the _ must be shut down.

Capacity

Because three-phase transformers are typically self-cooled, their _ is reduced simultaneously.

Step Up

In the generation side of applications, the transformer is used to _ the voltage level.

Steps Down

In the distribution side for commercial use, the transformer _ the level of voltage.