Electrical Transformer: Its Function and Types

Kathmandu : In this week's 'Urja Gyan' column, which is published every Friday by Urja Khabar, we aim to provide information about the working of electrical transformers, their types, and other related topics. Last week, we provided insights about relays used in electrical circuits and their types.

1. What does a transformer do?

A transformer changes voltage from high to low or low to high. It generally comes in two types: power transformers and distribution transformers. A large transformer, typically connected to power stations or substations, is called a power transformer, while smaller ones found in towns and villages are known as distribution transformers. There are some structural differences between these two types.

A transformer has two sides: one is the high-voltage side, and the other is the low-voltage side. The high-voltage side has large insulators. High-voltage wires, such as 11kV, 66kV, or 132kV, are connected to this side. On the low-voltage side, there are typically four insulators: one for neutral (on the far right) and three for phases. The insulators on the high-voltage side are called HT bushing, and those on the low-voltage side are called LT bushing.

Inside the transformer, there is transformer oil, coils (windings), and a large iron core. The coils are wrapped wire turns, and just like humans breathe, transformers also breathe. This is done through a breather, which contains fine silica gel (white/blue grains), which absorbs moisture in the air, preventing water from entering the transformer. If water enters, the transformer may catch fire.

Transformer oil cools the transformer and prevents electrical components from touching each other. The oil is stored in a conservator tank, which is usually half-filled with oil. The level of oil can be checked through a window in the tank. In modern transformers, a meter indicates the oil level.

2. Types of Transformers

Depending on the application, transformers come in various types:

Distribution Transformer: Used to supply electricity to homes or factories.

Step-Up/Step-Down Transformer: A step-up transformer increases voltage (e.g., for transmission lines), while a step-down transformer reduces voltage (e.g., for use in homes and industries). For example, the electricity generated in power stations is usually at low voltage, which is stepped up using a step-up transformer, transmitted to substations, and then stepped down for safe usage.

Power Transformer: Used in power stations and substations to manage voltage levels. They are crucial for the transmission and distribution of electricity.

Autotransformer: This type of transformer has a single winding that acts as both the primary and secondary coil. Unlike regular transformers, where the primary and secondary windings are separate, this type shares part of the winding between them.

Variable Voltage Transformer: Has a fixed primary voltage but a secondary voltage that can be adjusted. It's commonly used in testing laboratories.

Oil-Cooled/Dry-Type Transformer: Most transformers are oil-cooled, but certain small transformers, such as welding transformers, use air cooling.

Core/Cell Type Transformer: Refers to the internal structure of the transformer. Core-type transformers, like those used by Nepal Electricity Authority, have windings that wrap around a core. Cell-type transformers have cores that typically cover the windings. Core-type transformers are easier to maintain than cell-type transformers, but cell-type transformers have advantages like more secure windings.

Outdoor/Indoor Transformer: Indoor transformers are placed inside buildings, while outdoor transformers are installed outdoors. There are differences in cooling mechanisms and bushings between the two.

Multiple-Choice Questions

What is core loss?

(a) Hysteresis loss
(b) Eddy current loss
(c) Copper loss
(d) Both (a) and (b)

Answer: (d) Both (a) and (b)

What does a transformer primarily change?

(a) Power
(b) Power factor
(c) Voltage
(d) Frequency

Answer: (c) Voltage

What happens to core loss if the frequency is increased while keeping the voltage constant?

(a) It increases
(b) It decreases
(c) It remains constant
(d) None of the above

Answer: (a) It increases

What is the function of the silica gel in the transformer's breather?

(a) Absorbing water particles
(b) Absorbing oil particles
(c) Absorbing air
(d) None of the above

Answer: (a) Absorbing water particles

What unit is used to measure the capacity of a transformer?

(a) VA/kVA/MVA
(b) Volt
(c) Ampere
(d) Kilowatt

Answer: (a) VA/kVA/MVA

What does the tap changer on a transformer control?

(a) Current
(b) Voltage
(c) Power
(d) Both current and power

Answer: (b) Voltage

Which equipment is protected by Buchholz relay?

(a) Circuit breaker
(b) Transformer
(c) Capacitor bank
(d) Alternator

Answer: (b) Transformer