Transformer

What Is a Transformer?

A transformer is an electrical device that transfers electrical energy from one circuit to another by electromagnetic induction without changing power and frequency (also called transformer action)

Basic Parts of a Transformer

These are the basic components of a transformer.

1.    Laminated core: The core is used to support the windings in the transformer

2.    Windings: There are two windings wound over the transformer core that are insulated from each other.

Primary windings - These are the windings to which the input voltage is applied.

Secondary windings - These are the windings to which the output voltage is applied.

3.    Insulating materials: insulating paper and cardboard are used in transformers to isolate primary and secondary windings from each other and from the transformer core

4.    Transformer oil: Transformer oil or insulating oil is an oil that is stable at high temperatures and has excellent electrical insulating properties. It is used in oil-filled transformers, some types of high-voltage capacitors, fluorescent lamp ballasts, and some types of high-voltage switches and circuit breakers.

Chemical name of transformer oil is " Hydrotreated Light Naphthenic Distillate "

Trade names, the most common of which were Askarel® and Pyranol®

5.    Tap changer: The output voltage may vary according to the input voltage and the load

6.    Conservator: The conservator conserves the transformer oil. It is an airtight, metallic, cylindrical drum that is fitted above the transformer. The conservator tank is vented to the atmosphere at the top,

7.    Breather: The breather controls the moisture level in the transformer

8.    Cooling tubes: Cooling tubes are used to cool the transformer oil. The transformer oil is circulated through the cooling tube

9.    Buchholz Relay: The Bochholz Relay is a protective device container housed over the connecting pipe from the main tank to the conservator tank

10.  Explosion vent: The explosion vent is used to expel boiling oil in the transformer during heavy internal faults in order to avoid the explosion of the transformer.

Transformer safety device

1.    Buchholz (Gas) Relay

2.    Pressure Relay

3.    Oil Level Monitor Device

4.    Winding Thermometer

Of the above, the first four are found in almost all the transformers, whereas the rest are found only in transformers that are more than 50 KVA.

1. Buchholz (Gas) Relay

The Buchholz protection is a mechanical fault detector for electrical faults in oil-immersed transformers. The Buchholz (gas) relay is placed in the piping between the transformer main tank and the oil conservator. The conservator pipe must be inclined slightly for reliable operation.

Often there is a bypass pipe that makes it possible to take the Buchholz relay out of service.

The Buchholz protection is a fast and sensitive fault detector. It works independent of the number of transformer windings, tap changer position and instrument transformers. If the tap changer is of the on-tank (container) type, having its own oil enclosure with oil conservator, there is a dedicated Buchholz relay for the tap changer.

A typical Buchholz protection comprises a pivoted float (F) and a pivoted vane (V) as shown in Figure 1. The float carries one mercury switch and the vane also carries another mercury switch. Normally, the casing is filled with oil and the mercury switches are open.

2. Pressure Relay

Many power transformers with an on-tank-type tap changer have a pressure protection for the separate tap changer oil compartment. This protection detects a sudden rate-of-increase of pressure inside the tap changer oil enclosure.

Figure 3 shows the principle of a pressure relay.

3. Oil Level Monitor Device

Transformers with oil conservator(s) (expansion tank) often have an oil level monitor. Usually, the monitor has two contacts for alarm. One contact is for maximum oil level alarm and the other contact is for minimum oil level alarm.

The top-oil thermometer has a liquid thermometer bulb in a pocket at the top of the transformer. The thermometer measures the top-oil temperature of the transformer. The top-oil thermometer can have one to four contacts, which sequentially close at successively higher temperature.

4. Winding Thermometer

The winding thermometer, shown in the figure below, responds to both the top-oil temperature and the heating effect of the load current.

The winding thermometer creates an image of the hottest part of the winding. The top-oil temperature is measured with a similar method as introduced earlier. The measurement is further expanded with a current signal proportional to the loading current in the winding.

This current signal is taken from a current transformer located inside the bushing of that particular winding. This current is lead to a resistor element in the main unit. This resistor heats up, and as a result of the current flowing through it, it will in its turn heat up the measurement bellow, resulting in an increased indicator movement.

Types of Transformer

Transformers can be categorized in different ways, depending upon their purpose, use, construction etc. The types of transformer are as follows,

1.  Step Up Transformer & Step Down Transformer - Generally used for stepping up and down the voltage level of power in transmission and distribution power system network.

2.  Three Phase Transformer & Single Phase Transformer - Former is generally used in three phase power system as it is cost effective than later. But when size matters, it is preferable to use a bank of three single phase transformer as it is easier to transport than one single three phase transformer unit

3.  Electrical Power Transformer, Distribution Transformer & Instrument Transformer - Power transformers are generally used in transmission network for stepping up or down the voltage level. It operates mainly during high or peak loads and has maximum efficiency at or near full load. Distribution transformer steps down the voltage for distribution purpose to domestic or commercial users. It has good voltage regulation and operates 24 hrs a day with maximum efficiency at 50% of full load. Instrument transformers include C.T & P.T which are used to reduce high voltages and current to lesser values which can be measured by conventional instruments.

4.  Two Winding Transformer & Auto Transformer - Former is generally used where ratio between high voltage and low voltage is greater than 2. It is cost effective to use later where the ratio between high voltage and low voltage is less than 2.

5.  Outdoor Transformer & Indoor Transformer - Transformers that are designed for installing at outdoor are outdoor transformers and transformers designed for installing at indoor are indoor transformers.

6.  Oil Cooled & Dry Type Transformer - In oil cooled transformer the cooling medium is transformer oil whereas the dry type transformer is air cooled.

7.  Core type, Shell type & Berry type transformer - In core type transformer it has two vertical legs or limbs with two horizontal sections named yoke. Core is rectangular in shape with a common magnetic circuit. Cylindrical coils (HV & LV) are placed on both the limbs. Shell type transformer: It has a central limb and two outer limbs. Both HV, LV coils are placed on the central limb. Double magnetic circuit is present. Berry type transformer: The core looks like spokes of wheels. Tightly fitted metal sheet tanks are used for housing this type of transformer with transformer oil filled inside.

Types of Instrument Transformers

Instrument transformers are of two types –

1.  Current Transformer (C.T.)

2.  Potential Transformer (P.T.)

Current Transformer (C.T.)

Current transformer is used to step down the current of power system to a lower level to make it feasible to be measured by small rating Ammeter (i.e. 5A ammeter). A typical connection diagram of a current transformer is shown in figure below. Primary

of C.T. is having very few turns. Sometimes bar primary is also used. Primary is connected in series with the power circuit. Therefore, sometimes it also called series transformer. The secondary is having large no. of turns. Secondary is connected directly to an ammeter. As the ammeter is having very small resistance. Hence, the secondary of current transformer operates almost in short circuited condition. One terminal of secondary is earthed to avoid the large voltage on secondary with respect to earth. Which in turns reduce the chances of insulation breakdown and also protect the operator against high voltage. More ever before disconnecting the ammeter, secondary is short circuited through a switch ‘S’ as shown in figure above to avoid the high voltage build up across the secondary.

Potential Transformer (P.T.)

Potential transformer is used to step down the voltage of power system to a lower level to make is feasible to be measured by small rating voltmeter i.e. 110 – 120 V voltmeter. A typical connection diagram of a potential transformer is showing figure below.