Transformer- Classification. Important aspects –for being adept. Part-04

Description :

Transformers can be classified into different types which are used in the electrical power system for various purposes.

A. On the basis of voltage level that are no turns in the primary winding & secondary winding.

  a. Step-up transformer: Primary winding which is connected to the power supply has less no of turns than secondary winding of the load side. So, it transforms low voltage (V1)& high current into the high voltage (V2), low current.

  b. Step-down transformer: Primary winding which is connected to the power supply has more no of turns than secondary winding of the load side. So, it transforms high voltage (V1)& low current into the low voltage (V2), high current in the secondary side.

B. On the basis of usage

 a. Power transformers: They are used for high voltage (33 KV & more)power transfer & large in size. They are placed in generating station & transmission system and designed for maximum efficiency of nearly 100%.

  b. Distribution transformers: They are placed at the consumer end to supply a relatively small amount of power at low voltage(<33KV) to residential, commercial & industrial purposes. They are small in size & operate at low efficiency ( 60- 70%) and not always fully loaded.

C. Based on transformer core construction :

  a. Core type transformer: Winding is placed on two limbs of the core, winding coil is surrounded considerable part of the core. It has only one magnetic flux path. The core type uses concentric cylindrical winding.

  b. Shell type transformer: Winding is placed on the mid-arm of the core, the core is surrounded considerable part of the winding coil. It has two magnetic flux paths. Sandwiched winding is used in shell-type transformers.

D. On the basis of no of phase :

 a. Single-phase transformer: It has one primary & one secondary winding. It receives single-phase AC power from the source & transforms (output) single-phase AC with different voltage levels.

  b. Three-phase transformer: It has 3 sets of primary & secondary windings. These sets of winding configuration between primary and secondary may be : Δ(delta)-Y(wye), Δ(delta)- Δ) (delta), Y(wye)- Δ(delta), or Y(wye)-Y(wye). It receives three-phase AC power from the source & transforms (output) three-phase AC with a different voltage level than primary.

E. On the basis of primary & secondary winding connection.

a. Two winding transformers: Primary & secondary windings are electrically isolated from each other. A two winding transformer is generally used where the ratio between the high voltage and low voltage side (N1/N2) is greater than 2.

 b. Autotransformer: The auto transformer’s Primary & secondary windings are both electrically & magnetically interconnected. It is one kind of power transformer. So, it consists of a single continuous winding. Winding is tapped in one side to get secondary voltage which may be step-up or step-down. Autotransformer is more cost-effective when turns ratio (N1/N2)<2 and has better voltage regulation & higher efficiency.

F. On the basis of cooling system

a. Oil cooled transformer. The cooling medium is transformer oil. The heat generated inside the transformer tank is conveyed by transformer oil to the radiator. Then the radiator dissipates heat by way of radiation or convection.

b. Dry-type transformer: Air cooling is used in a dry-type transformer.

G. Instrument transformer:

It is used to measure high voltage & current and power. It’s also used for the protection of the transformer against different faults. Instrument transformers are classified as potential transformer (PT) & current transformer (CT).

a. Potential transformer: It is a step-down transformer. The primary winding is connected across the high voltage line whose voltage is to be measured, and all the measuring instruments and meters are connected to the secondary side of the transformer. Usual transformation ratios are 10:1, 30:1, 40:1,80:1, 100:1, 120:1 etc. A potential transformer deals with a very low amount of power & very similar to a standard two winding transformer.

 b. Current transformer: It is used for measuring as well as protection purposes. When the current in the circuit is high enough to apply directly to the measuring instrument, the current transformer is used to transform the high current into the desired value of the current needed in the circuit. The primary coil of the current transformer has one or more turns of heavy wire & always connected in series in the circuit in which the current is to be measured. The secondary coil has many turns which must always be connected (closed circuited) across the measuring instrument to prevent the core from being fully magnetized. Otherwise, instrument accuracy will be declined.
For example, if the CT ratio is 3000:5, it means a CT has an output of 5 amps when the input current is 3000 amps on the primary side.