How to select suitable transformer?

How to select suitable transformer?

1.  Rating (MVA):

Rating of transformer determined by the total loads it will supply.

What is distribution transformer?

Transformer used to distribute power for customers (home , factories, workshops, etc) , & has rating less than or equal 5 MVA.

I.e. It is step down transformer.

What is power transformer?

Transformer used to transmit power from generation station to sub stations to reduce losses, & has rating < 5 MVA.

I.e.  It is step up transformer.

2.  Common types of transformers

3.  Neutral earthing of transformer

4.  Percentage impedance :

The full load percent impedance (FLPI) of a transformer is an important parameter in power-supply system design. FLPI is determined by the construction of the core and physical spacing between the primary and secondary windings. Typical FLPI values range from 1 to 5%. FLPI is a measure of the ability of a transformer to maintain its rated voltage with a varying load. The lower the FLPI, the better the voltage regulation.

 

FLPI also determines the maximum fault current that the transformer can deliver.

 For example, if a 5% FLPI transformer supplying 5 A nominal at the secondary is short-circuited, the device can, theoretically, supply l00 A at full voltage. A similar transformer with a 10% FLPI can supply only 50 A when short-circuited.

 

How to calculate short circuit level?

Example: consider a 2 MVA transformer with an impedance of 5%. The maximum fault level available on the secondary side is:

2 MVA x 100/5 = 40 MVA

and from this figure the equivalent primary and secondary fault currents can be calculated.
Note: The figure calculated above is a maximum. In practice, the actual fault level will be reduced by the source impedance, the impedance of cables and overhead lines between the transformer and the fault, and the fault impedance itself.

The impedance of a transformer has a major effect on system fault levels. It determines the maximum value of current that will flow under fault conditions

5.  K factor

It is the factor that refers to the ability of the transformer to withstand high percentage of non linear loads .

Example: K 4 transformer used when non linear loads are 50 % of total loads supplied by transformer.

6.  Cooling the transformer

There are 4 letters refers to the way that transformer being cooled.

Example: ONAN cooling method:

O: first letter refers to the fluid used to cool the coils of transformer (i.e. inside transformer ) , in this example the fluid is oil .

N: 2^nd letter refers to the way that internal fluid flowing, in this example it is natural way .

A: 3^rd letter refers to the fluid used to cool transformer body (i.e. outside transformer), in this example the fluid is air.

N: 4^th letter refers to the way that external fluid flowing, in this example it is natural way.

8.  Transformer construction