ELECTRICAL TRANSFORMERS

A transformer is a static device having coils coupled through a magnetic medium connecting two ports at different voltage levels in an electric system allowing the exchange of electrical energy between the ports in either direction via magnetic field. Simply speaking, transformer is a device used to step up or step down the voltage in circuits. 

The various types of transformers in use are discussed here. If you think that transformer is a just a metal box mounted over the poles in transmission and distribution network, it is not correct. It is a very important electrical equipment that one can find it every where from transmission and distribution network to small electronic circuits. But the only thing is they difference just in size and shape not in their basic principle of operation.

1. Power transformer

You can find this type of transformer in distribution networks, substations, industries etc. It is used to step up and step down voltages based on the requirement. It is also used in the interconnection of two power systems. The main purpose of using power transformer is to reduce I2R loss causes in the transmission lines by increasing voltage level and reducing current flow. In industries power transformers are used to supply loads at its rated voltage.

2. Instrument transformer

Devices cannot be directly connected to the line carrying high current or the line at extremely high voltages, since it causes damage to the equipment. Hence in order to facilitate the measurement of electrical parameters such as voltages, current, power, energy etc. instrument transformers are used.

There are two types of instrument transformers:

I. Potential / voltage transformer

II. Current transformer.

I. Potential / voltage transformer:-

Potential transformers are used for the measurement of high voltages and to provide protection against under and over voltages. It is similar to power transformers. Most of the measurement devices can measure voltages on the range 0 – 120V. But power system consists of voltages as high as 750 kV. Potential transformers are used to step down this high voltage equivalent to low and measurable voltage level. More over it is also used in the measurement of low voltages by stepping up to higher measurable voltage levels. The potential transformers must transform the input voltage accurately to output voltage both in magnitude and phase.

II. Current transformer:-

It is the current ratio transformer used to step down current levels and provide it to measuring instruments like an ammeter. It is also used along with protection relays. It is always connected in series to the circuit. The measure devices or protective relays are connected to the secondary winding of the current transformer.

3. Autotransformer:-

Auto transformer consists of a single winding that acts as both primary and the secondary. When the primary and the secondary are electrically connected so that a part of winding is common to both is known as auto transformer. When compared to a power transformer an auto transformer has lower reactance, lower losses, smaller excitation current and better voltage regulation. It is used in the starting of induction motors, in the interconnection of two HV systems and variable voltage power supplies.

4. Audio frequency transformer:-

Audio frequency transformers are used in audio frequency electronic amplifiers for matching load to the output impedance of the power amplifiers. The operating range of audio frequency transformer is 20 Hz to 20 kHz. These transformers are used in the electronic circuits for stepping up the voltage or impedance. They are small in size and have iron cores. In order to match the high output impedance of amplifiers to he low impedance input of loud speakers, audio frequency transformers are used. These transformers prevent the flow of DC bias current from the amplifier to the loud speaker, hence protecting the loud speakers from DC bias currents.

5. Grounding transformer:-

When delta – delta transformers are used in the power system, neutral grounding is not possible. In such cases a special star- delta transformers are used. These transformers are called grounding transformers. The star connected primary is connected to the supply and the secondary delta is left unloaded. Grounding transformers draw only a small exciting current from the supply during balanced (normal) conditions. Under fault conditions large current may flow through them.

6. Welding transformer:-

To limit high short circuit currents during welding, welding transformers are used. These transformers have very high reactance both at the primary and the secondary. Normally the primary and the secondary winding are placed in separate limbs or in the same limb with sufficient gap in between. The presence of gap in between the winding increases the

7. Isolation transformers:-

Isolation transformers are used to isolate any equipment from an AC power source. The turns ratio of any isolation transformer is unity. Isolation transformer are used to maintain electrical safety. It is used to transfer power between two circuits that must not be connected. It provides protection against electric shock.