Transformer

A transformer is a static device. Through a transformer, electrical power can be converted from one circuit to another without changing the frequency. The main function of a T/F is to receive power at one voltage level and then supply it at another. It works based on mutual induction between two circuits. There are two main types of T/F used in electrical power systems.

Power Transformer/Two Winding Transformer.

Auto Transformer.

Power Transformer Construction and Working

A Power T/F has two inductive coils. These inductive coils are magnetically connected through low magnetic resistance and electrically isolated from each other. If one coil is connected to a voltage source, an alternating flux is produced in the laminated core and most of it links with the other coil.
An emf is induced in the other coil due to the expansion and contraction of flux from one coil to another by Faraday’s law and the principle of mutual induction. If the circuit of the other coil is closed, current will start flowing in that circuit. Thus, electrical energy is transferred magnetically from one coil to another. The first coil, which is connected to the source of alternating voltage, is called the primary winding, and the second coil, which supplies the energy to the load, is called the secondary winding.

Auto Transformer Construction and Working

It is a single winding T/F. This winding is used jointly as both primary and secondary windings. Primary and secondary winding are connected magnetically and electrically. It works based on self-induction between two circuits and consists of a single winding, it uses less Copper. It is used where the T/F ratio is slightly different from unity.

In some cases, a very small change is required in voltage levels. For example, 125kV to 132kV or 11.9kV to 11.5kV is necessary. A two-winding T/F is expensive in these situations. In such places a special T/F is used, called auto T/F.

Types of Transformers

Step-up T/F

A T/F that converts low voltage to high voltage is called a step-up T/F. A step-up transformer has lower primary voltages and higher secondary voltages. It has lower primary turns and higher secondary turns. It is installed at the point of generation to step up the voltage and send it to other areas.

Step-down T/F

A T/F that converts high voltage to low voltage is called a step-down T/F. A step-down transformer has higher primary voltages and lower secondary voltages. It has higher primary turns and lower secondary turns. Step-down transformers are used in substations and distributions to step down the voltage to make it usable.

One to One/Isolation T/F

Transformers whose primary to secondary ratio is 1 to 1 and their voltages are equal are called one-to-one/isolation transformers. It protects against electric shock and provides galvanic isolation between load and source. It eliminates noise from the input signal and provides a smooth signal without changing the voltage.

Transformer Parts

Following are the parts used in the T/F.

• Winding
• Core
• Steel Tank
• Conservator Tank
• Breather
• Thermometer
• Pressure Relief Pipe or Vent
• Valves
• Bushings
• Buchholz Relay
• Tap Changing Switch
• Oil Gauge
• Oil Pumps
• Cooling Fans
• Radiator
• Transformer Insulation

Types of Transformer w.r.t Core

Following are the types of transformers depending on the core.

• Core Type
• Shell Type
• Berry Type

Types of Transformer w.r.t Uses

Following are the types of T/F w.r.t uses.

• Large Power T/F
• Small Power T/F
• Distribution T/F
• Instrument T/F
• Sign Lighting T/F
• Series T/F
• Bell Ringing T/F
• Constant Current T/F
• Control and Signal T/F

Tests of Transformer

The following electrical and mechanical tests are required on the transformer to ensure the performance of the T/F in all respects.

• Polarity Test
• On Load or Short Circuit Test
• No Load or Open Circuit Test
• Back to Back Test
• T/F Turn Ratio Test (TTR)
• Insulation Resistance Test
• Winding Resistance Test
• Capacitance and Dissipation Factor Test (C & DF)
• Sweep Frequency Response Analysis Test (SFRA)
• Dielectric Strength Test (DES)
• Dissolved Gas Analysis Test (DGA)
• Tan Delta Test
• Buchholz Relay Test
• Function Test (Winding & Oil Gauge, Flash Point, Pour Point, Slug, Interfacial Tension Test)

Cooling Methods of Transformer

The following cooling methods are used in dry type T/F.

• Air Natural Cooling Method
• Air Forced/Blast Cooling Method

The following cooling methods are used in Oil Immersed type T/F.

• Oil Natural Air Natural (ONAN)
• Oil Natural Air Forced (ONAF)
• Oil Forced Air Forced (OFAF)
• Oil Forced Water Forced (OFWF)

Applications of Transformers

It is used to step up or step down the voltage in AC transmission and distribution systems.
It is widely used to control the voltage of a power system.
It can also be used as an impedance matching.
It can also be used to derive the phase-shifted voltage from the mains power supply.
It is used to provide low voltage to motors during starting.
It is widely used in electronic and audio systems devices.