What is the difference between a power transformer and a distribution transformer?

Power transformers and distribution transformers are essential components in electrical power systems, each serving distinct roles in the transmission and distribution of electricity. Understanding the differences between these two types of transformers is crucial for designing and maintaining efficient power networks. This article aims to provide a comprehensive comparison between power transformers and distribution transformers.

What is a distribution transformer?

A distribution transformer is a device that transfers power from the electrical grid to your home, businesses, and commercial buildings. They play a crucial role in the power distribution system by stepping down the voltage from higher levels, typically found in transmission lines, to lower levels suitable for consumer use.

Distribution transformers are commonly installed on poles in residential areas or on the sides of buildings where there is a significant amount of foot traffic. In some cases, they may also be located underground in areas where overhead lines are impractical or not permitted. These transformers are designed to be easily accessible for maintenance and inspection purposes.

The primary function of a distribution transformer is to reduce the voltage from the higher levels present in the transmission lines to a lower level that is safe and compatible with the electrical appliances and devices found in homes and businesses. This voltage reduction is necessary to prevent damage to consumer equipment and ensure the efficient and reliable delivery of electricity.

Inside the distribution transformer, you will find large coils of copper wire wrapped around an iron core. These coils, known as windings, are responsible for carrying the electrical current. The iron core serves as a pathway for the magnetic field generated by the current, facilitating the transfer of energy from one coil to another without significant loss or breakdown of voltage.

Unlike distribution feeder lines, which connect multiple facilities or buildings, distribution transformers are designed to connect to a single building or a set of buildings. They supply power to a specific area or a localized network of consumers, ensuring that the electricity is distributed evenly and reliably.

In terms of structure, a distribution transformer shares similarities with an autotransformer. Both types of transformers consist of windings and a core. However, the windings in a distribution transformer are connected in such a way that they can supply current to both sides of the load. This arrangement allows for efficient voltage reduction and distribution to consumers.

Types of Distribution Transformer

Since distribution transformers are used for voltage regulation and load distribution, they come in several sizes, rated by voltages and currents. And these can be classified into four types based on their application.

Single Phase

This distribution transformer is designed for single-phase power systems. The primary winding of this transformer is connected to an incoming 3-phase supply line through 3 separate terminals. It has one secondary winding for each phase and is connected to the load through different terminals. The secondary voltage is equal to the sum of all three phases multiplied by their respective turn ratios of the primary windings.

Three Phase

On the other hand, this type of distribution transformer is intended for three-phase power systems. It has three secondary windings connected in a delta connection with a shared neutral wire (ground). Each of these three secondary windings has a different voltage depending on their turn ratio relative to one another. For example, a three-phase transformer with four poles will have three secondary windings with 120 degrees and one voltage winding connected to all three others equally (in parallel).

Pad-Mounted

These transformers are installed directly on concrete pads or pad foundations. It can be used in place of conventional pad-mounted transformers by using a direct burial cable, which allows you to bury power lines underground without worrying about corrosion or breakage.

Typical applications include small commercial buildings, apartment complexes, and industrial plants where the need for more than one phase is minimal or not required (for example, one phase may be used for lighting while another is used for heating). Or it can also be used in areas with no overhead wires, or safety may be an issue due to high voltage lines being present near an electrical substation.

Pole Mounted

These are designed to be mounted on poles above ground level, so they can be easily accessed when necessary. They are also used for pole-mounting special purpose transformer banks, such as capacitors or recloser banks. Pole-mounted transformers may have an internal grounding system or an external grounding connection point for use with a separate grounding electrode system. This type of distribution transformer may also include other features such as bushings, lightning arrestors, and surge arrestors.

What is a power transformer?

A power transformer, also known as a grid transformer or a large power transformer, is a crucial component of electrical power systems. It is responsible for transmitting electrical energy efficiently and reliably across long distances at high voltage levels. Power transformers are typically used in electrical substations and are essential for power generation, transmission, and distribution.

The primary function of a power transformer is to step up or step down the voltage levels of electrical energy. Step-up transformers increase the voltage from the generation level to a higher voltage suitable for long-distance transmission over transmission lines. Step-down transformers, on the other hand, decrease the voltage from the transmission level to lower distribution voltages suitable for consumer use.

Power transformers are designed to handle high power ratings and operate at high voltage levels, typically ranging from hundreds of kilovolts (kV) to several hundred kilovolts. They consist of a laminated iron core and two or more sets of windings. The windings are made of copper or aluminum conductors and are insulated from each other and the core to prevent electrical short circuits.

The core of a power transformer is constructed from thin, insulated laminations of iron or steel. The laminated core reduces energy losses by minimizing eddy currents and hysteresis losses. The windings, which are wound around the core, are responsible for transferring electrical energy from one voltage level to another.

Power transformers operate on the principle of electromagnetic induction. When an alternating current (AC) flows through the primary winding, it creates a magnetic field that induces a voltage in the secondary winding. This process allows for the efficient transfer of electrical energy from the primary side to the secondary side, with minimal loss.

Power transformers are designed to be highly efficient and reliable. They are equipped with cooling systems, such as oil or air-cooling, to dissipate heat generated during operation. The cooling system ensures that the transformer operates within acceptable temperature limits and prolongs its lifespan.

In electrical power systems, power transformers are used at various stages, including in power plants, electrical substations, and distribution networks. They play a critical role in voltage regulation, power transmission, and maintaining the stability of the electrical grid.

What is the difference between a power transformer and a distribution transformer?

Power transformers and distribution transformers are used for high voltage, low current applications. However, there are some differences between the two:

Transformer Size / Insulation Level

Power transformers have a higher insulation level than distribution transformers. Power transformers have a voltage rating of more than 1 kV and can withstand short circuit currents up to a few hundred amperes. In comparison, distribution transformers have a voltage rating of less than 1 kV and can withstand short circuit currents up to tens of amperes.

Iron Losses and Copper Losses

Power transformers lose energy as heat due to eddy current losses in their steel laminations and hysteresis losses in the core material. Distribution transformers don’t have any iron parts, so they have lower iron losses than power transformers. They also lose energy due to hysteresis losses in the core material. Still, these are much smaller than power transformers because distribution transformers are smaller, so they don’t generate as much heat as power transformers do when carrying heavy currents.

Maximum Efficiency

Power transformers are generally less efficient than distribution transformers because of their larger size and heavier currents. A typical distribution transformer might have an efficiency rating of 90% or better than 80% for a standard power transformer. However, there are types of power transformers with efficiencies varying from 97 to 99 percent.