Step-Up Vs. Step-Down Three-Phase Transformers

Electrician checking the status of a step up transformer.

A transformer is a device used to adjust current and voltage levels in an alternating current (AC) circuit. When used as part of an AC power transmission circuit, transformers are employed to step-up the transmission voltage to a higher value to reduce power loss, and then again to step-down the voltage to safe levels for use by customers and equipment. This transformation of voltage is achieved through electromagnetic induction.

A simple single-phase transformer has two coils, a primary and a secondary, wound on a common magnetic core. When electric power is applied to the primary, a voltage is induced in the secondary. The ratio between primary and secondary voltages will be the same as the ratio between the number of turns in the primary coil winding and the secondary coil winding, which is called the turns ratio. Voltage and current share an inverse relationship, hence the current in the secondary coil winding will be reduced by the same ratio for which the voltage was increased, thus conserving overall power in the primary and secondary windings.

In power transmission systems, however, three-phase transformers are most commonly used. In balanced three-phase circuits, the AC voltage in each phase has a phase difference of 120 degrees with respect to the next phase. Three-phase transformers consist of three primary and three secondary windings, wound either in a star or delta configuration. This type of transformer operates on the same basic principle as single-phase transformers.

Below, we’ll delve into the differences between three-phase step-up and three-phrase step-down transformers.

Step-Up Transformers

A step-up transformer is used to increase the transmission voltage to reduce line losses. By increasing voltage, line current proportionally decreases, and power loss from cable resistance is reduced. The smaller current also allows for the downsizing of conductors to smaller diameters. Typically, electricity is generated at 11 kV and transmitted at 22 kV, or 44 kV and higher voltage levels. Step-up transformers are employed at this stage to increase the transmission voltage to those levels. Another application for step-up transformers is in motor circuits, where they’re used as starters for electric motors, as the higher voltage is able to initiate motor rotation and overcome the starting resistance.

Because of the relatively high value of primary current, these transformers have primary windings constructed of thick, insulated copper. In addition to the number of primary and secondary turns, the secondary voltage of a three-phase circuit also depends on the type of winding configuration used. For example, a delta-star connection, in which the primary windings are connected in delta, and the secondary windings are star-connected, results in an increase in the line voltage by a factor of 1.732, or the square root of 3, times the turns ratio. This factor must be considered when calculating the required turns ratio of a three-phase transformer to achieve the desired value of step-up voltage.

Step-Down Transformers

At the end of a transmission line, the high transmission voltage must be reduced to lower values as power distribution and ultimately, power consumption, occurs at much lower voltages. Since most electrical appliances work at voltages on the order of 240 V, it’s essential that the kV-range transmission voltage is stepped down drastically. Three-phase step-down transformers are employed in such cases.

Using the same principles as a step-up transformer, these devices transform the high-voltage, low-current power in the primary to high-current, low-voltage power in the secondary. Typical secondary voltages are of the order of a few hundred volts, and these transformers are equipped with thick copper windings in the secondary coils to accommodate the higher secondary currents resulting from the stepped down voltage. In a star-delta configuration, the voltage is automatically stepped down by the same factor of 1.732, or the square-root of 3, times the turns ratio.

The Versatility of Three-Phase Transformers

Three-phase transformers are used universally in a wide range of power systems, in both transmission and distribution circuits. These transformers may also be employed in any industry using specialized three-phase equipment. Since they provide electrical isolation between two parts of a circuit, they can greatly improve safety while allowing for lighter, more compact solutions than those offered by three single-phase transformers delivering the same amount of power. Plus, a balanced three-phase transformer delivers power with improved electrical characteristics over what is possible using three independent single-phase transformers. For these reasons, three-phase transformers serve as integral components in all types of modern-day electrical installations.





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